I wonder what might occur if the lead acid 12v battery in a 2024 Prius Prime were replaced with a drop in LiFePO4 12v battery ?
Drop in LiFePO4 batteries have a Battery Management System (BMS). The System protects the LiFePO4 battery (isolates battery from charge/discharge) pending: Low charging temp, Max discharge current rate, Max charg current rate, Min discharge voltage. Max charging voltage. If the BMS protection points are not in agreement with the Prius electrical system operating parameters their might be a problem. The LiFePO4 has a much lower internal battery resistance than Flood Lead Acid. At the same charging voltage, the LiFePO4 will accept more charging amps than the Flood Lead Acid. If 12v battery charging during Traction battery charging is based solely on chaging voltage set by the Prius the charging current could be extreme (may even cause the LiFePO4 max charge current protection to isolate the battery from the Prius 12v electrical system).
This is the first time I've read about the possibility that the Prius 12 volt charging system might pump more current into the 12 volt battery than it could handle, whatever chem that battery might be. I personally think Toyota programmed the 12 volt system to charge the battery at a really low amperage. Way lower than any car using an alternator, at least any that I've owned. But until we get somebody to actually get Amp readings across a full spectrum of use cases, we wouldn't know for sure exactly how many amps the charging system actually hits the battery with at any one time.
Im also thinking because the battery internal resistance is so low, the charging current could be really high when the battery SOC is low. LiFePO4 batteries can accept a large charging current. The BMS protects the internal wiring and components in the BMS by limiting this current, generally.
I'd love to know how to get accurate internal resistance reads from batteries while both discharging and charging, since most times I read about it, I see only one reading and not too much else about when the reading was taken or under what ambient conditions the battery was exposed to. Anyways, here's a chart for 12 volt lead acid and a few other chems. Battery Internal Resistance Chart | Battery Tools Again, the chart is a basic generalization for what healthy batteries should look like under ideal conditions and at nominal voltage. Take from that what you may.
So has anyone actually tried this? I recently replaced my SLA with a LiFePO4 in a UPS. I was surprised to see it cut off at a much lower load than my SLA. The problem is the BMS. While an SLA can provide surge current, the BMS will simply shutoff at its rated amps. I'd hate to learn that the BMS couldn't take the drain or charge current while you're on the road! In my case it was a 6AH battery with a 6AH BMS so it's a pretty low limit. You'd probably have to go with something like a 100AH battery with a 100AH BMS in which case probably nothing would knock it off its feet but then you're talking $200 which is a lot more than a lead acid... still probably cheaper than a gel/SLA and save you money in the long run. But it's curious if this was a good idea, why wouldn't Toyota have done it? A LiFePO4 weighs a fraction of the Lead Acid. How much money do they put into each car to just save a few pounds?
Dealership price of a replacement Toyota TrueStart lead acid battery that fits the gen 5 Prius is $206 (before discounts, can be found for cheaper depending on the dealership). 00544-H4052-470 - Truestart H4 (En) Longo Toyota Parts Truestart H4 (En) 00544-H4470-TS | Parts Molle Toyota
