No responsibility can be accepted for anything you build from this or other sites! Use your own common sense and take some responsibility for once! There may or may not be errors.
There is so much FUD (fear, uncertainty, and doubt) spread around the internet regarding lithium batteries.
It's dangerous! The cells are junk! You'll get scammed! It's complicated! It's nothing like lead-acid! You need to compress your cells just right!
A lot of people are scared off. They just buy an expensive pre-assembled lithium battery. And get taken to the cleaners by the huge prices!
But it's really pretty easy.
If you're running lead-acid, you'll know it's far from perfect. Some of the problems with lead acid:
If it's working fine for you, don't dick with it! But if those are problems, consider lithium. It fixes nearly all of the above!
There are a few extra problems lithium introduces which lead acid doesn't have:
For a very simple small system (less than 100Ah and no inverter), lead acid works out more economical. Otherwise lithium wins out.
The types of lithium-ion batteries used in mobile devices are deadly dangerous. But Lithium-Iron-Phosphate (also known as LFP, LiFePO4, or just lithium in the van life world) is one of the safest batteries you can get.
All batteries can be dangerous if mis-handled or shorted out. But a LiFePO4 will usually just vent and if you're very unlucky, it might burn slightly. Just like burning plastic. A lead acid can burn like this too, but it also explodes and sends acid everywhere. Worse yet, a lead acid battery has no BMS (battery management system) to protect it!
I feel much safer using LFP than lead acid. Handling lead acid batteries puts me on edge.
Handling the raw cells is more dangerous than handling a fully assembled battery pack with BMS protection. But it's no different to handling a lead acid battery. Yes the lithium battery can deliver higher currents if shorted out. But if you're dropping a spanner on the terminals, the current is more likely limited by the poor contact at the terminals. It will eventually turn into a burning mess with either type of battery! Don't drop spanners on the terminals.
If you have a higher voltage pack, 24V, 48V etc., then you need to be extremely careful. That's a high enough voltage to push some serious fault current if you short it out. But for a 12V pack? Treat it with the same respect you treat lead acid and you will be fine.
You should wear eye protection when working with any batteries. You don't have to worry about acid, but sparks can happen. A lot of molten metal could fly towards your eyes if you short something.
At the current prices, there is no question that you should build a lithium battery yourself. Yes there are some cheap-ish pre-assembled Chinese batteries now, but you don't know what quality parts you're getting. It depends what's available to you including shipping costs. And also how much risk you're willing to take buying from China versus a UK supplier (with either cells or a pre-assembled battery).
Some people advocate server rack batteries or portable power stations. I don't like either, they're extremely overpriced. They also have limitations like unusual voltages (48V for a server rack battery) or current limits (10A max for the 12V socket on a portable power station).
You can buy Chinese cells from a UK supplier for twice the cost of ordering from China. But bear in mind there are difficulties with China right now. And if your cells arrive damaged? Too bad. Your credit card company won't refund you unless you ship the cells back. And you can't ship them back because you don't have free shipping like the Chinese do. It will cost you thousands to send them back.
But on the flip side, you can afford to buy a second set if the first set is defective.
You can also buy non-Chinese cells but they are more expensive still. I don't see a good reason to use them yet.
The cells most commonly recommended are the EVE 280Ah cells. They are the go-to cells for almost all DIY builds. There are some other cell manufacturers too, of variable quality. But bear in mind that if you buy a cheap pre-assembled battery, you don't even know what cells they're putting in there. At least you can pick and choose if you build one!
It can be overwhelming to see all the complicated bits and pieces people use for lithium battery builds. But here I keep it simple.
The minimum you need:
Optionally, you can add these items:
This is the part which is most alien to lead acid users. It does six things, most of which would be useful on lead acid too (but they never bother):
Technically you can run an LFP battery without a BMS, but it's easier to damage it compared to lead acid. Over-discharge it just once, it's probably ruined. You also won't know how much charge is left in the battery, voltage readings don't work at all for LFP. For all practical purposes, you need a BMS.
At the time of writing there's three affordable Chinese BMSs on the market. And a few name-brand ones. The Chinese BMSs I'm aware of are the Daly BMS, the JK BMS, and the JBD BMS. Each have ups and downs and Youtube has more information. But really you can pick any of them and it'll work fine.
You need to look at the current rating for your BMS and choose according to your needs. This will set a limit on how fast you can charge and discharge the battery. Many BMSs have a lower rating for charge than discharge.
It's a good idea to use fibre tape to firmly secure all four cells together into a single battery-shaped block. This prevents the cells moving in relation to each other, which reduces stress on the terminals.
Some people also claim that the pressure exerted by the tape on the cells will somehow improve the battery lifetime. In any case, it can't hurt.
There's just one thing to watch out for: the cells are made of metal, and the metal case is connected to the positive terminal!
The cells are wrapped in a protective plastic wrapping to avoid them shorting when assembled together. But it's a very thin wrapping which could wear through with vibration. Especially in a mobile application, it's strongly recommended that you put some sturdier plastic sheet between each cell. I cut pieces of damp-proof course to the size of each cell and put it in-between. It's very robust stuff.
You should also put this plastic material on the bottom of your pack. If the cells sit on a damp surface or piece of wood (and moisture is everywhere), electrically-accelerated corrosion can occur between the cell bottoms via the damp surface! The plastic is non-porous so prevents the problem.
Before you start wrapping, you should cut two pieces of wood to the size of a cell and "book-end" your pack with them. This prevents the tape from putting pressure on the corners of your cells and possibly distorting them.
Now assemble the cells together with alternating + and - terminals. Put the plastic sheet in between each cell, and gently push them all together. Add the wooden book-ends. When they're all lined up nice and even (including the book-ends), start wrapping! Go around several times at the top, middle and bottom for a nice secure pack.
Plastic separators
+---------------+---------------+
| | |
V V V
_ _____ _____ _____ _____ _
//| / O+ /| /| / O- /| /| / O+ /| /| / O- /| //|
// | / / | / | / / | / | / / | / | / / | // |
// | / O- / | / | / O+ / | / | / O- / | / | / O+ / | // |
// | /___ / | / |/___ / | / |/___ / | / |/___ / | // |
|| | | | || || | || || | || || | | || |
|| / | | / | / | | / | / | | / | / | | / || /
|| / | | / | / | | / | / | | / | / | | / || /
|| / | | / | / | | / | / | | / | / | | / || /
||/ | |/ |/ | |/ |/ | |/ |/ | |/ ||/
++ +---+ +---+ +---+ +---+ ++
^ Cell #1 Cell #2 Cell #3 Cell #4 ^
| |
Wood panel Wood panel
Gently squeeze together:
_ _____ _____ _____ _____ _
/// O+ // O- // O+ // O- ///|
/// // // // /// |
/// O- // O+ // O- // O+ /// |
///___ //___ //___ //___ /// |
||| ||| ||| ||| ||| |
--------> ||| ||| ||| ||| ||| <--------
||| ||| ||| ||| ||| /
||| ||| ||| ||| ||| /
||| ||| ||| ||| |||/
+++---+ +---+ +---+ +---+++
Wrap snugly with tape:
_ _____ _____ _____ _____ _
/// O+ // O- // O+ // O- ///|
------ /// // // // ///#| --------
/ <---- /// O- // O+ // O- // O+ ///# | <---- /
/ / ///___ //___ //___ //___ ///# #| / /
/ / ############################ # | / /
/ / ||| ||| ||| ||| ||| # #/ / /
/ / ############################ #/ / /
/ / ||| ||| ||| ||| ||| #/ / /
/ / ############################/ / /
/ / +++---+ +---+ +---+ +---+++/ / /
/ / / /
/ --------_ >---->---->---->---->----- /
------ / _ \ ----------------------------
| | | |
| |_| |
\____/
Fibre Tape
Tape some plastic sheet under the pack:
_ _____ _____ _____ _____ _
/// O+ // O- // O+ // O- ///|
/// // // // ///#|
/// O- // O+ // O- // O+ ///# |
///___ //___ //___ //___ ///# #|
############################ # |--
||| ||| ||| ||| ||| # #/ /
############################ #/ /
||| ||| ||| ||| ||| #/ /
############################/ /
/+++---+ +---+ +---+ +---+++/ /
/-----------------------------/ <-- Plastic sheet
All the cells in a battery need to be at the same voltage. Otherwise one cell will go flat before the others, and you won't be able to pull the full capacity.
For lead acid, this is done by deliberately over-charging the battery slightly. It forces all the cells to be fully charged at the expense of some water loss in the ones being over-charged.
That won't work for lithium batteries. They can't tolerate any over-charging.
Under normal operation, the BMS takes care of balancing the indiviual cells. But when you first buy your cells, they might require an initial balance to get them close enough.
The safest and most reliable way to balance the cells is to buy or borrow an adjustable bench power supply with a current limit feature.
You can get a suitable Chinese power supply on the internet for about £30. It can be used to charge any battery as well as power projects. Well worth it. Look for one with separate knobs for voltage and current, as well as a digital meter for voltage. It needs to go down to 3V and up to 14V.
The higher the current your power supply is rated for, the faster the balancing will happen. Four 280Ah cells will balance in about two weeks with a 2A power supply. You can cut that down to about 3 days with a 10A supply!
It is often recommended to perform balancing only once you've taped your cells together. The reason is that the cells swell slightly when charged. They are supplied from the factory about half charged. If you were to balance all your cells first (i.e. fully charge them), they will be at their largest. Then when you discharge the cells, they will be loose inside the tape wrapping.
That means all the positive (+) terminals are connected together. Then separately, all the negative (-) terminals are connected together.
Since you will have taped your cells together in their final configuration, you will have alternating + and - on each side. So you'll need to use crocodile clips to connect the terminals together in a zig-zag fashion. Check and double-check your connections! Otherwise you'll find out how well a crocodile clip works as a fuse! (tip: very well, if you don't mind burned fingers and burning plastic)
The power supply should be set to exactly 3.65V, and the current to the maximum it allows (or just a bit below if it's a cheap power supply - don't want to burn it out). Once set, it's a good idea to verify the voltage is correct using a multimeter. Some cheap power supplies can be a bit off, and you need to be exact here.
You should notice the cells are drawing the maximum current and the voltage drops down to somewhere between 2.5V and 3.6V. If it drops lower, you probably made a wiring error.
Now you just need to wait! Several days to a couple of weeks.
It's a good idea to periodically check the power supply. If it's a cheap Chinese one, it might malfunction and put out too high a voltage. Eventually this will over-charge your cells. Checking once every few hours is enough.
Remember, you only have to do this once. Never again. So be patient!
Top view of cells:
+---------------+-----------------+
+-----+ +--|--+ +-----+ +--|--+ |
|+ O +| |- O -| |+ O +| |- O -| |
| | | | | | | | | | +----+ |
| \__|_|_ _|_|__/\_|_|_ _|____|+ -|____+
| | | \ / | | | | \ / | +----+ |
|- O -| |+ O +| |- O -| |+ O +| Power supply|
+--|--+ +-----+ +--|--+ +-----+ |
+---------------+-------------------------+
Cell #1 Cell #2 Cell #3 Cell #4
Disconnect the power supply and all the cells from each other.
Your BMS should come with the cables it needs. If not, you might have to get hold of some heavy cabling for the power connections.
There are three connections on the typical BMS:
Balancing+power leads
|||||
|||||
+----------------+
| |
| |
=====| Xi's BMS |=====
^ | | ^
| | | |
| +----------------+ |
| |
To To
Battery Van
Negative Negative
Here we'll be looking at the balancing leads.
You'll have a plug which fans out into 5 or more thin wires. These wires are as follows:
Depending on your BMS, you might also have these additional wires:
These wires allow the BMS to measure the voltage of each cell, and keep the cells balanced. The BMS will also power its internal electronics from these wires.
Most people simply terminate these leads with ring terminals and screw them straight to the battery terminals. I don't recommend this! If something goes wrong inside the BMS, there is no fuse to protect those thin wires. You don't really want a bunch of melting and burning wires under your bed!
A much better idea is to put a small fuse (5 to 10 amps) in series with each of the wires. The fuse should be as close to the battery terminal as possible. That way the full length of the wire is protected.
Here are some options you can use:
Or you could live dangerously and not fuse it. Probably thousands of vans on the road like that now. Even many pre-built battery packs lack the fuses! Blooooody ell...
At least you won't need an electric hob and inverter. Just cook over the battery.
Remember the golden rule of all low voltage electrical systems: whenever you go from a thicker wire to a thinner wire, put a fuse close to that location suitable for the thinner wire! This applies not just to the BMS. Your whole van wiring should follow this rule. Battery terminals to thin BMS wires is just an extreme example.
Keep the connector unplugged from the BMS while you're connecting it up. That way a mistake won't wipe out your BMS. Don't plug it in until the pack is completely assembled!
Clean the cell terminals and bus bars with some isopropyl alcohol. Some people advocate polishing them to get the best connection. But if you do the polishing wrong, you can end up with a slightly domed terminal which makes a worse connection! Unless you know what you're doing, a simple cleaning with isopropyl alcohol will be fine.
Now drop the bus bars into the correct places on your pack. Be very very careful! Put one in the wrong place, or drop one, and there will be sparks everywhere.
Top view of cells:
Bus bar Bus bar
| |
V V
+-+=========+-+ +-+=========+-+
|+|O O|-| |+|O O|-|
| +---------+ | | +---------+ |
| | | | | | | |
| | | +---------+ | | |
|- O -| |+|O O|-| |+ O +|
+-----+ +-+=========+-+ +-----+
^
|
Bus bar
Cell #1 Cell #2 Cell #3 Cell #4
Do not install any washers or terminals under the bus bars! The bus bars must rest directly on the battery terminals.
Next, put the battery positive cable over the cell positive terminal that doesn't have a bus bar. And connect the battery negative lead to the negative terminal without a bus bar. Be careful not to short the loose ends of the battery positive and negative leads!
Finally, we install the balance lead ring terminals. Three of them go on top of the bus bars, and the other two go on top of your main positive and negative leads. They should always go on top, so that the high currents in your positive and negative leads have the most direct connection to the battery terminals.
Now we can put the nuts on all the terminals. If you have a torque wrench, it's recommended to torque them up to about 8N-m. They need to be reasonably tight but not over-tightened. Otherwise you could damage your cells!
The nuts they supply should have teeth (ridges) on the bottom to help lock them. If they don't, you may need to add a toothed washer directly below the nut. Otherwise you risk it shaking loose over time.
Your battery positive lead goes off to the rest of your van system like normal. But not the battery negative!
The battery negative lead must connect to the BMS - and only the BMS. It connects to the terminal marked battery or B-. Do not connect anything else to this wire! It just goes between the battery and the BMS. Nothing else!
Then you connect your van's negative to the BMS load negative, not the battery negative lead!. The BMS load negative may be marked P- or L-.
In other words, all current flowing back to the battery negative must pass through the BMS on its way there.
Top view of cells:
+-+=========+-+ +-+=========+-+
|+|O O|-| |+|O O|-|
| +---------+ | | +---------+ |
| | | | | | | |
+------------+ | | | +---------+ | | |
+====| P- B- |========O -| |+|O O|-| |+ O======== Positive to
| | | ^ +-----+ +-+=========+-+ +-----+ van stuff
| | Xi's BMS | |
| | | +--- Nothing else may connect to this wire!
| | | (except one of the BMS balance wires)
| +------------+
|
|
Van's body (negative)
and the negative for
every accessory in
your van.
A more detailed view showing the balance wires:
Top view of cells:
+----------------------------------------------+
| |
|+--------------------------------+ | <- Fuse each
|| | | of the
||+---------------------------+ | | balance
||| | | | leads close
||| +-+=========+-+|+-+|========+-+ | to the
|||+--------------O O|-|||+|O O|-| | battery
|||| | +---------+ ||| +---------+ | | terminals
||||+-------------+ | | ||| | | +----+
+------------+ | | | | +----|----+ | | | |
+====| P- B- |========O -| |+|O---+ O|-| |+ O--|==== Positive to
| | | ^ +-----+ +-+=========+-+ +-----+ van stuff
| | Xi's BMS | |
| | | +--- Nothing else may connect to this wire!
| | | (except one of the BMS balance wires)
| +------------+
|
|
Van's body (negative)
and the negative for
every accessory in
your van.
At this point, you still have the plug with all the thin balance wires disconnected from the BMS. Don't connect it yet! You need to do some tests.
If the wiring of the balance wires is wrong, you might blow your BMS up. So you need to use a multimeter to double-check before you plug it in.
Offer up the connector to the BMS to see which way it plugs in (but do not actually make contact). Use the markings on the BMS to determine which pin on the connector does what. You may find they're labelled something like B-, 1, 2, 3, 4, B+.
Hold the multimeter negative probe against the B- pin on the connector. You can usually poke it into the side of the connector to make contact with some metal. If not, you may need a thin piece of wire poked in the end of the connector.
Now set the multimeter to voltage (not current!!!), take the multimeter positive lead and touch it to the terminal marked cell #1 or just 1. You should read very roughly 3 volts.
Now move the probe along to #2. You should read roughly 6-7 volts. Along to #3, it should read about 9-11 volts. Then finally along to #4, 12-14 volts. If your BMS has a B+ pin, then check this too. Again it should read 12-14 volts.
All good? Now you can insert the plug into the BMS.
Some BMSs will power up as soon as you insert the plug. Others may require a button to be pressed to wake it up. If a button is included in the package with the BMS, be sure to plug it in. The BMS will cut all the power to your van off until you turn it on!
If you have a really crappy BMS, it may still appear dead. It may need "waking up". To do this, simply connect the van negative and battery positive leads to a charger with about 14.6 volts. Once woken up, it should stay on unless there's a fault which causes it to trip out.
Most BMSs are generic and are designed for different types of cells and voltages. Do not use the default settings!!!
So we need to change the settings in the BMS to sensible values to protect the battery. How do we do this?
APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APP APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS AP S APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS A PS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS PPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APP APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS AP S APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS A PS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS PPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APP APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS AP S APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS A PS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS PPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS APPS
Fucking internet of shit. But it is what it is.
Download the Chinese Special Fried App for your BMS. Bear in mind it will almost certainly have Chinese Government spyware inside. Ideally use a burner phone or something like GrapheneOS. Otherwise at least try and deny it network permissions.
With the BMS turned on / woken up, you should be able to connect to it. Figure out the default password (probably 0000, 000000, 1234, 123456, or 4thjune), and change it to something much more secure.
Now you need to find the following settings and change them:
Cell over-voltage protection cut-off: 3.65V Cell over-voltage protection, re-enable (if present): 3.60V Cell under-voltage protection cut-off: 2.55V Cell under-voltage protection, re-enable: 2.60V Battery low-temperature shutoff (if present): 3C Battery low-temperature re-enable (if present): 6C
There may be an on/off switch in the app. If so then switch the bloody thing on! Otherwise you won't get any power out. This is the equivalent of your big battery isolator switch on a lead acid system.
Make sure the settings are saved! Disconnect, switch the BMS off and on again, and reboot your phone. Then check it's saved them.
The BMS should ideally come with some temperature probes/sensors. Put it low down in the middle of the battery. If there's a second probe, put that one at the top of the battery. Use tape to secure it firmly to the battery, wrap the tape all the way around.
The temperature probes will shut the charging / discharging off if the temperature gets too high. Most BMSs will also shut down charging if the temperature is below freezing too.
Some models of Daly BMS do not have working low temperature charging cutoff! If there's any possibility your battery could drop below freezing, you'll need to install a separate thermostat which somehow cuts off charging. Most DC-DC chargers have an enable wire which you connect to the ignition switch. You can put a thermostat in line with this. If you have solar panels too, you need to figure out a way of cutting solar charge off too.
If you want to charge below freezing, you need to install heating pads on your battery. You can get pads which are thermostatically controlled to prevent water tanks freezing. These may be a good choice, they shouldn't overheat the battery and will only turn on when it's below freezing. Be sure to connect it to the ignition circuit of your van. You don't want the heaters running all the time. Only when you're charging! You must use a BMS with working low-temperature cutoff too. It will keep the charging disabled until the battery gets up to temperature. Be sure to put the temperature probe some distance from the heat pads. Otherwise the heat pads will heat the probe faster than the battery!
Some types of BMS have an output for controlling a heating pad. That can be a good option too.
There are lots of terminals involved. To make sure they're all making good contact, hook up an inverter. Connect up a fan heater (not exceeding the rating of your inverter and BMS!) and run it for half an hour. Have a feel around every single terminal in the system to check for heat. Warm is fine, hot is sometimes OK, but searing hot is bad. Especially look out for one terminal that's much hotter than other similar terminals.
This is the biggest difference you'll notice between lead acid and lithium. It will either delight you or write your van off.
For the mains and solar chargers, you can get away with using intelligent chargers designed for lead acid. But it's a much better idea to use chargers with lithium settings. If your charger has a custom voltage setting, set it to 14.0V to 14.2V for all bulk and absorption. Float should be somewhere around 13.6 volts. Lithium doesn't need floating, but the objective is to be sure small loads don't drain the battery much. 14.4V to 14.6V will work for bulk/absorption too, but it's a bit on the high side. Nobody seems to know if this actually shortens battery life.
Charging from the alternator though? Very different story.
If you have a newish van with a smart alternator, you're going to need a DC-DC charger (sometimes called a battery-to-battery charger). Even for lead acid! The voltage of these varies all over the bloody place.
If you have a less tech-encrusted and more sensible van, then it's tempting to use a split charge relay like you would with lead acid.
But beware!
First, the voltage of most alternators is set a bit high, around 14.6V. See if you can adjust it down a bit. It may vary with temperature, so check on a cold day in the winter (where voltage is highest). You might get away with 14.6V if you have to.
The biggest problem? Alternator burn-out.
An alternator is not designed to work at 100% output for a prolonged period. Especially so if the engine is idle, because the alternator is turning slower. The built-in fan won't be as effective. And you're not moving, so no cold air coming into the engine bay.
If you connect a dead lead acid battery to an alternator, the current is limited by the shitty lead acid battery itself. You'd have to be very unlucky to burn your alternator out with lead acid. Sometimes the charging current is an absolute joke. Can be as low as a few amps! Not even worth having a split charge relay at that point.
Lithium batteries will take whatever they can get from the alternator and wiring. The result? Your alternator will go to 100% load (if not over 100% of its rating!) and will burn out fast. If your alternator can put out more current than your BMS can handle for charging, your BMS will trip out and no charging will happen.
There are four ways around this.
The least invasive and most popular solution. You don't have to dick with your van's alternator, and the warranty isn't voided. It charges the lithium battery perfectly, and you won't burn your alternator out (if DC-DC converter is properly sized!) It also works with newer vans and their stupid smart alternators.
The downside is that DC-DC chargers are expensive and bulky. If you have an older van with a small alternator, a DC-DC charger can burn your alternator out even faster than a split charge relay! Small DC-DC chargers like 30A may be OK. Larger will probably burn it out. Newer vans with big alternators are OK.
You connect one end of the DC-DC charger to your starter battery. The other end connects to your lithium battery. Switches (or APPS APPS APPS) on the charger are used to set it to lithium mode.
Then you run one small wire from a switched circuit in your van (fag lighter socket, radio power, etc.) to the trigger input of the charger. This ensures it only runs when the ignition is on.
That's it!
If you can't find a DC-DC charger large enough for your needs, you can put more than one identical unit in parallel.
But there are smaller and cheaper options...
Connect the two batteries via the split charge relay using a short cable. Make sure the BMS (and cable) can handle a bit more than the full output of the alternator.
Then attach a temperature sensor to the alternator to switch off the relay when the temperature gets too hot.
There is a risk the temperature sensor might not act fast enough. But if you do it right, it will protect the alternator.
Charging might be slower than you expect. Travelling at speed it may be OK. Lots of cold air moving over the alternator. Otherwise it will cycle on and off as it gets too hot.
Because the alternator is heated by the square of the delivered current, it will be off for longer than you expect. Say the alternator can handle 80% of its rating continuously. You might only get an average of 50% of the rating into the battery. That might be enough for you, and it's cheap.
Some companies make external alternator regulators with fully adjustable voltage and temperature sensing. Normally they are paired with their own alternators, but you can fit one to any alternator. All you need is to disconnect the field winding (the brushes) from the alternator's internal regulator and connect them to the fancy regulator. May require stripping the alternator down.
This will charge the lithium battery perfectly. You can set the right voltage. It will gradually dial back the current as it warms up. Since it's not cycling on and off, you actually get close to the alternator's true rating.
The downside? The starter battery may not be charged optimally. During most of the charge, the lithium battery will pull the alternator down to about 13.6V. That's enough to stop your starter battery running down. But it will put back the charge you used to start the engine very slowly.
You could fit a timer to the split charge relay, to give the starter battery a good 5 minute charge before the lithium battery turns on.
Alternatively you could use a DC-DC charger to charge the starter battery from the lithium battery (the reverse of how you'd normally do it!)
Rather than putting the lithium battery near the starter battery, put it at the back of your van. Run long cables from the starter battery (not the alternator!) via the split charge relay to the lithium battery.
The length and diameter of these cables can be calculated to not burn your alternator out.
First we need to figure out how much current the alternator can handle. Many people suggest no more than 80% of the rating of an alternator for continuous use. If you're idle and stationary, you might even go a bit lower (65%?). So if you have a 100A alternator, you want to keep the load below 65A.
Many alternators will experience a voltage drop when under load. So increase engine to 2000rpm, then put a 65A load on the alternator (turn all accessories on, then add additional loads to reach 65A if needed). Now measure the voltage at the starter battery. Let's say you measure 13.8V.
13.8V at the starter battery, 13.0V at the lithium battery with 10% state of charge. That's a difference of 0.8V.
Using Ohm's Law, we divide 0.8V (the voltage difference between batteries) by the current we want to charge with (65A). The result is 0.012 ohms (12 milliohms). That's the resistance of cable we need.
Install a circuit breaker rated about 25% greater, 80A. Then use a cable rated about 25% greater still, 100A. Using reference tables online, you can see a 35mm2 cable will easily handle this current. Depending on how it's installed (amount of airflow around it), 16mm2 may be sufficient. Let's go with 25mm2 since the cables aren't buried in insulation but not in free air either.
A copper 25mm2 cable has a resistance of about 0.65 milliohms per metre. So we need about 18 metres of cable to limit the current to 65A.
Now you have a choice. You could use two runs of 9 metres, positive and negative. But then the negative terminal at the lithium battery will be floating half a volt above the van chassis. May or may not be a problem depending on whether your van life accessories connect to the chassis as well.
Or you could run a single length of 18 metres for the positive, and use a much heavier cable (and the chassis in parallel) for the negative.
Just make sure you don't coil the excess cable up, it'll get too hot.
The more accessories you turn on while driving (blower, headlights etc.), the more the voltage sags and the charging current reduces accordingly. So the alternator never gets overloaded in this situation.
Now let's consider what happens if your alternator doesn't have a voltage drop under a 65A load. Still reading the same as when 14.5V!
Now you have to be more careful. It means the load presented by accessories like the headlights and blower are on top of your charging current. You can burn your alternator out!
In this case subtract all the vehicle loads (don't forget the radiator fan) from the charging current. Let's say all your loads came to 35A. That leaves you 30A for charging your battery. So size the cable to limit the current to 30A accordingly.
In general if your alternator has a "stiff" supply like this, don't bother. A DC-DC charger may be a better choice.
The final thing you need to consider is end-of-charge cutoff. Allow the starter battery to charge for 5-10 minutes. Then measure the starter battery voltage with the engine at 2000rpm and all accessories switched off.
If it's 14.4V or less, you should be OK. The split charge relay can remain engaged while you drive, the lithium battery won't be destroyed.
But if it's over 14.4V, you will damage your lithium battery by over-charging it. The BMS will normally shut off before damage happens. But the BMS is supposed to be a last resort protection! Don't rely on it!
In this case you need to disengage the split-charge relay when the lithium battery voltage (not starter battery voltage) reaches about 14.2V. The simplest way to do this? Use a good (programmable) solar charge controller (with or without solar panels) and two relays:
+-------------------------+
| Solar charge controller |
| |
| Batt Load PV |
| - + - + - + |
+-------------------------+
| | | | |
+---+ | | Solar panels (optional) Split-charge relay
| | | /
| +------- | -------------------------------O O------------------+
| | | : |
| | | Change-over relay +-\/\/\---+ |
| | | N.C. | | |
| | | /O------+ | |
| | | +---O/ | |
| | | Relay | : O | |
| | | / N.O | : N.O. | |
| | +------O O--------\/\/\----------------+ |
| | : | | |
| | +--\/\/\-----+ | |
| | | | | |
| | | | | |
| +---------+ | | | +---------+
| | + | | +12V | | + |
| | Lithium | | ignition | | Starter |
| | battery | | switch | | battery |
| | - | | feed | | - |
| +---------+ | | +---------+
| | | | |
+------+-----------+------------------------------------+--------------+
Note: fuses not shown!
Set the solar charge controller to turn on the load at 14.2V. Turn off the load at about 13.4V
When the load output is switched off, the change-over relay does not activate. Current passes from the ignition switch to the split-charge relay and switches on charging. When the battery is fully charged, the load output turns on. The change-over relay pulls in, breaking the circuit to the split-charge relay. Charging stops. Finally a third relay cuts power to the change-over relay when the ignition is off. Otherwise the change-over relay would drain power from the battery.
The cables between the solar charge controller and the lithium battery should be as short and thick as possible. Mount the solar charge controller right next to the battery. The cables should also go direct to the battery (via a dedicated fuse). Not via an existing fuse box, cut-off switch or the inverter's wiring. This lets it sense the battery voltage accurately.
Victron solar charge controllers can be configured to these voltages using their app. Use a Smart Solar one. Otherwise you need to buy the separate bluetooth dongle or USB cable to configure it. Many other solar charge controllers will work, but some shitty ones may not have fully configurable voltages.
The battery may not charge completely to 100%, but it gets close. To fully charge a lithium battery it needs about 30 minutes to "soak" at 14.0-14.2V. But if you have solar panels, they will take care of the rest. If you don't, hey! At least the battery will last longer not getting all the way to 100%.
Another advantage of lithium: they run at a slightly higher voltage than lead acid. Even if your lithium battery is around 10% charged, it will still charge a lead acid battery when connected in parallel.
To prevent your starter battery running low when parked for long periods, use this circuit:
Schottky diode
1N5818 etc. 0.5A polyfuse (PTC)
|\| /
+------| |--+-----\///\/----+
| |/| | __/ |
| | |
| / | |
| |\| | |
+------| |--+ |
| |/| |
| / |
| Zener or TVS diode |
| ~20V, 1W |
| |
+---------+ +---------+
| + | | + |
| Lithium | | Starter |
| battery | | battery |
| - | | - |
+---------+ +---------+
| |
+---------------------------+
The starter battery will be held in float around 12.8 to 13.4 volts. If you have some small charge coming into the lithium battery (solar, mains etc.) both batteries remain charged. The zener diode protects the schottky diode from voltage spikes. You want a low voltage (~25V) schottky diode as they have lower voltage drop. Put it in a metal box to protect against fire.