I wanted to keep an eye on my auxiliary 12V battery, so I installed this device: Vgate Battery Assistant Tester BLE4.0 Not all that useful, but it does the job of showing the charge level. My only peeve is that some of the screen messages are in Chinese.
I can monitor my main 12V battery through a device plugged into the lighter socket. I needed a way to monitor the aux battery as well.
Just to keep things interesting, Toyota themselves use the term "auxiliary battery", by which they mean the 12-volt battery that comes in the car.
I also noticed that the current drain by the device in the original post is 15 mA. That is unacceptably high. The BM2 battery monitor has only a 1-mA current drain, which is very good, and the BM2 app is written in English. BM2 battery monitor on Amazon
It's coming in a little handy so far. It shows changes in the voltage level to two decimal places. Today I took a road trip of a few hours, and I saw the voltage creeping down slowly, after an initial slow climb. I still don't know why it went dead the other day, but based on the slow decline in voltage today, it's a step in the right direction for understanding what happened. I am running a 12V cooler, rated at 5 amps, but the car's 12V system should be able to handle that. The main purpose for the auxiliary battery is to keep the drinks cooler going while I'm parked. I checked the connections and they seem OK. But for some reason, the voltage will creep up slowly for about an hour, and then slowly decrease until the battery is dead. I suspected the isolator may be bad, but it does allow the battery to charge for the first hour or so. I do have a workaround; I can plug my 5 amp battery charger into the inverter socket if I have to, to charge it as I drive, but I'm still unraveling this mystery.
I may have something... I was looking at isolators on Amazon, and some of them say, "not suitable for lithiun batteries, which is my auxiliary battery. The isolator I currently have makes no mention of suitability for lithium so I ordered a different one that does say it's compatible. It may have something to do with incorrect triggering voltages. This one is adjustable, so that may be the answer.
I've got the new smart isolator arriving here today. I only ordered it yesterday. I'm gonna miss Amazon, once the trade war boycotts hit.
As I mentioned many times, the Toyota 12-V battery-management system (BMS) is not compatible with an LFP (LiFePO₄) lithium-ion battery. It will not charge it properly. Those LFP batteries are intended for dumb charging systems without a BMS, like the old alternator/IC voltage regulator systems.
Relax, it's been doing a good job for over a year now. I just need to tweak some switching voltages and it will be perfect. My problem is that the lithium battery is rated at 12.8 volts, which is the cut out voltage of the isolator. My system runs at about 12.4-12.6V most of the time for some reason, so that explains why my lithium auxiliary battery went dead on that long road trip. The drinks cooler was drawing 5 amps continuously. During the winter, I often saw voltages of 14.1 which is above the cut-in voltage of 13.3 of the isolator, which explains why it never went dead over the winter. I was also not running the cooler, so I never noticed a problem until the other day on my 15 hour road trip. I believe the charging regulator sets a higher charging voltage of 14.1 in colder temperatures, like in winter for better longevity and performance. This summer, I'm never seeing it go over 12.6, which is better for the lead-acid battery in warmer temperatures.
