While I am generally pleased and optimistic about my Lithium Iron Phosphate (LFP) battery upgrade on the solar launch, the nature of Battery Management Systems (BMS) has been a learning experience.
Last time I mentioned the new, hopefully more capable inverter that I had installed. The idea was that I was going to use the boat’s solar power system when on dry land. I have two battery farm vehicles and a Zero motorcycle that need charging and they can drag down the main system when it is cloudy and hot. I figure my a/c is my power priority when the heat index is 119. Hey, it’s Florida. It’s July!
Initial tests looked ok. The inverter fit the old spot with minimal cable stretching required. It ran the coffee maker and the microwave, so next day I loaded up a 48v golf car charger and brought the Yamaha G19-based truck alongside the boat. I plugged in, the charger made its usual buzz so I went off to do some other chores. Checking on it a little later there was a beeping noise, which I traced to the inverter. The inverter has some onboard diagnostics and its chirping can tell you what is wrong. The voltage was out of range.
Indeed it was! The only voltage was coming directly from the charge controllers and was swinging wildly without a proper load. BOTH batteries had tripped their 100 amp breakers. The breakers were intentionally undersized as the old inverter could not be stressed much, anyway. Everything had worked fine with the lead acid GC8 batteries, but now something was amiss. The buggy was charging at around 1500 watts. 100 amps times 26 volts leaves power to spare, so what the heck?
One of the previously reported quirks with the LFP battery BMS units was that only one will discharge until it is down about 25% and then the other chips in. There is always some wiggle room in electronic component specs and I speculate that is what is causing this. Perhaps I can sit down and contemplate the programmable features and throw in some sort of fudge factor that will get them on even footing, but that is not where we are now. What apparently was happening, was the one battery was comfortably cranking out 80 amps until it got to the threshold where the other battery kicked in. Unlike the lead batteries, which were directly connected in parallel, the one battery was all fresh and it not only took over the 80 amps the inverter needed, it began charging the other battery at a high rate. The batteries can safely deliver over 300 amps continuously, though I think I set the BMS to limit at 200 amps. Still, this was enough to trip both breakers. What a revolting development. I ordered new 200 amp breakers and so far so good. The issue seems to present itself when encountered from a full charge. I will let the system run back up to 100% and the plug the buggy back in. And the motorcycle. That will give a continuous load test at around 3kw. If that holds, I’ll let you know.
Meanwhile, I need to finish some other repairs so I can get back on the water.