The 30Ah Lithium battery I bought is advertised as Trolling battery, so low speed silent (fishing) boat operation. Not a great fit for lead acid replacement in a car, but you can make it work. Cor.
On Sun, Feb 15, 2026, 11:39 PM Cor van de Water <[email protected]> wrote: > 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/ce3a8a77/attachment.htm> _______________________________________________ Address messages to [email protected] No other addresses in TO and CC fields HELP: http://www.evdl.org/help/
