The 12V replacement batteries often have another side effect, depending on which type you bought: I deliberately bought a 30Ah type for my EV as replacement for the dying aux battery, which is more than sufficient as no cranking is involved. It specified a max charge Voltage as well as max current. I noticed that immediately as I expected something like this to happen: as soon as I started the car and the 100A dc/dc converter kicked on, the Lithium aux battery detected too high current and was switched off by the BMS. You do not notice this until you turn the car off and it dies instantly, as soon as the dc/dc turns off. This also means the battery does not charge and it will lose charge with every time you turn you car on again, until it dies. So, I added a pair of parallel 0.2 Ohm resistors, so that the Lithium battery needs to discharge to 11V before the 14V from the dc/dc will kill it, so it will somewhat slowly charge with every use until it remains close to the dc/dc voltage. I also tricked the dc/dc into always deliver 14V, as its normal behavior is to measure aux charging current and when that drops below a threshold, the voltage also drops to 13V which is too low for the Lithium battery. I discovered by disconnecting the sensor, the car thinks the battery is always seeing 150 Amps and the voltage stays 14V. I also added a 30A diode in parallel to the resistors so that in discharge direction I can draw a current peak during starting without the voltage dropping too low.
I your application of using this type of Lithium batteries with BMS, if the BMS turns the charging off due to either the cells reaching max voltage OR seeing too high charging current, then the voltage across the BMS transistors can be dozens of Volts, blowing every BMS on all batteries, unless you give each battery its own 12V charger, but then you STILL run that risk during discharge when you drive the car and one of the BMS detects a reason to disconnect, it gets the full pack voltage across its transistors, which likely will overload and blow up... You can use Lithium, but the system needs to be designed for high voltage series operation. Good luck! Cor. On Sun, Feb 15, 2026, 6:56 PM Lee Hart via EV <[email protected]> wrote: > One precaution: Dooley, if you have so-called "racing" lithium batteries > to replace 12v lead-acid batteries, they are *NOT* suitable for use in > series to power an EV. Two reasons: > > 1. Their amphour capacity is much lower than the lead-acid battery being > replaced. That means a lot less range in an EV. The manufacturers do this > to make it lighter (for racing), and so they can use fewer smaller > (cheaper) cells. The 12v battery in a normal car is only used to start the > ICE. This only need high "cranking" amps -- not amphour capacity. Even very > small lithium cells can have high cranking amps, but with much lower > amphour capacity. > > 2. These battery have (or should have) an internal BMS (Battery Management > System). The BMS will disconnect the battery to prevent it from being > overcharged or run dead to prevent fires. This disconnect is only built to > interrupt 12v. If you put these batteries in series and one of these > disconnects opens, it will see the ENTIRE series pack voltage, and so is > likely to fail and cause a fire! > -- > Excellence does not require perfection. -- Henry James > But it *does* require attention to detail! -- Lee Hart > -- > Lee A. Hart https://www.sunrise-ev.com > > _______________________________________________ > Address messages to [email protected] > No other addresses in TO and CC fields > HELP: http://www.evdl.org/help/ > > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20260215/1d5f8000/attachment.htm> _______________________________________________ Address messages to [email protected] No other addresses in TO and CC fields HELP: http://www.evdl.org/help/
