Lithium battery charging from alternator


Well-Known Member
IMO not a surprising result, alternators put out voltage, different batteries require different voltage for optimum life and charge, alternators do not provide the correct voltage for every battery chemistry and do not have different charge rates to suit battery condition


Well-Known Member
i has a chat with a battery sales guy and basically what he had to say was unless someone was buying or had suitable chargers he would not sell lithium because of the headache it causes when the BMS dies.

with what he was telling me and a little more reading up I think this is close to what goes on.
In most cases lithium likes a nice solid flat voltage. No multi stage just a solid say 14.5 volts or whatever the optimum voltage is for a given battery. Some manufacturers will say that no specialised charger is needed and that’s because the BMS can manage to correct that voltage for the lithium cells. I would say by disconnecting the cels at below a suitable voltage and by burning a bit of energy at higher voltages. If your battery runs a very good BMS it might see out the life of the battery without failure but if it can’t manage the heat or the extra switching cycles in the BMS it could die an early death leaving a battery that probably still has plenty of life in the lithium cells dead in the water.


Active Member
Hi folks and like many subjects relating to automotive electrics, there are lots of OPINIONS but very little real knowledge about what ever subject is being covered.

This one is a perfect example.

Most lithiums can be charged directly from an alternator and if it’s the auxiliary battery then it’s not likely to be a problem, even in vehicles with SMART alternators.

How you set up a vehicle to be able to separate the lithium auxiliary battery from any lead acid batteries ( including the cranking battery ) when the motor is not running, that is the only real issue.

But there is a MAJOR potential problem when using a lithium battery as a cranking battery.

Again, while charging the lithium from the alternator is not likely to pose a problem for a lithium cranking battery, but there is a very high possibility of damaging both the alternator and the vehicles electronics.

The the problem is not from overloading the alternator but the exact opposite.

There have been a number of issues where the lithium battery’s BMS has shutdown while the lithium cranking battery is being charged. Be it from over voltage, or the battery reaching a fully charged state, or the lithium battery’s BMS failing.

Regardless of what causes the sudden shutdown, you now have a free-wheeling alternator, and in any situation, an alternator running without a battery connected to it, will allow the alternator to generate very high voltage spikes.

These spikes are being generated by all alternators at all times, but the cranking battery, because of its very size, acts as a massive spike suppressor.

With a lead acid cranking battery, even if it’s stuffed, it will still suppress these high voltage spike, and thus, protect the vehicle’s electronics.

With the lithium cranking battery removed from the alternator, these high voltage spikes will quickly destroy a vehicle electronics, including the alternators voltage regulator.

This is the REAL potential problem of using a lithium battery as a cranking battery.


Active Member
Problem 2 only applies to cranking lithium battery and can probably be solved by an alternator protection device or a good dropin like lithionics, (but why bother when a $250 80AH flooded does a perfect job). but Problem 1 more applies to large AGM or lifepo4 auxilary batteries and is solved by getting a dcdc to limit current & avoid taxing your alternator to death & potential overloading the bms. Nothing has been done for years and the problem will never be fixed by factory because they refuse to give us real 250A alternators. All that matters is politics with emssions.
A reality check.

There are 3 main reasons why a BMS exists, 1: cheap dirty way to sell lithium batteries to the masses coz you can't without protection, 2:because the user balances all the time it means the makers can simply grab any mismatched cells out of the bin to save money, and slap that 2year warranty on the battery, always balancing each cycle is also harmful,- there is alot more to it, but it's a joke and the warranty reflects that. and 3: just another product to sell. the bms is an underengineered joke, a good bms will usually cost more than the cells them self & can still fail, some very $$$ dropins do have well matched cells & a good bms but their warranty is prorated by the time a bms fails. Also if a bms shuntsoff because of a bad cell there is not alot you can do to actually fix it.

Charging your standard lithium off the alternator is a silly idea and may overload the bms & is a poor way to budget your vehicle amperage. A good dc-dc is over 90% efficient and is made to handle load dumps and a 60A unit will fully charge a 200AH lithium from empty in just over 3hrs, all while having amps to spare off a 100A stock alt. how can you possible complain about that? I spent 150bucks on glue lined heatshrink alone on my setup so no, a dc-dc is lunch money in the overall scheme of things.

Here is why you get a dc-dc charger for lithium aux battery:
Standard alternators:
* To overcome voltage drop in wirering, even 8M of 4ga wire & a fuse can drop the voltage bad enough with a temp compensating alt. My starter terminals are just ~13.75V once engine is warmed up, with the above resistance it simpy can't charge properley.
* Since they are constant voltage sources the battery will quite literally suck them dry & maybe overload the bms and what about air con? driving lights? your average good LED setup will consume about 27amps & thats if their good brands.
* Alternators dont drop the cv voltage to a safe float level & you will overcharge. Yes you can muck around with a relay or something, but why when we have such a device that is easy to install?
This is the beauty of a constant current hard power source dc-dc with dedicated volt sensing & absorb timer setting, you can charge to whatever SOC% you want, 80,90, 95 etc without risk of overcharge/overabsorbing. FWIW you cant do that at all with solar. Also some units like enerdrive will go to float at tail current value that is usually higher than any loads so no issue there either.
* Finally ask these people who claim that a 100A rated alt can sustain 100A or over 60%, to actually give realworld testing of large 300AH+ battery & long charge times routinely that confirms this. Nothing exists.

Smart alternators:
* You cannot reliably charge a lithium battery, lfp needs at least 13.7-13.8V AT THE TERMINALS to get it charged past the 50% soc in any fast time. Although lfp has a 99%+charge efficiency all the way to full & low internal resistance, it still needs a voltage difference to push large current. Drop down to 13.5- 13.6 it accepts much less current in the mid SOC range. 13.2-13.4 is going to be painfully slow.
As they say know your CV physics or get lied to.
you will spend far more money on the red tape getting the thing disabled & finding a way to kill the charge once full, vs just buying a dc-dc.

If your really wanting to charge direct off the alternator your far better off with well matched bare prismatics which only requires proper fusing, LV/HV cutouts. Top balance once in a blue moon with a DMM & a ~15W 2.7ohm resistor. It couldn't be simpler.
Pretty much backs up what I was told by my lithium battery supplier. My system will simply be a Low Voltage Disconnect straight off the battery and a DC/DC charger to handle everything else. He said it doesn't need a BMS if you use a quality DC/DC. He balanced the cells prior to purchase.