some questions about my system/agm replacement

[Muaddib1116]

I've been diagnosing a drain on my battery system and I could use some advice.

My new boat (new to me) has a 600ah 24v lithium house bank that was installed by the last owner about a year ago. Naturally, this powers most of the boat's systems. It also charges two 60ah 12v AGM starter batteries. One is the starter battery for the generator and engine. The other acts as a power source for the few 12v systems the boat has, primarily some of the instruments (raymarine mfd's).

A couple weeks ago, when I got to the boat, the whole system was dead, even the mastervolt and victron battery monitors were down. I put up the solar panels, and after a few minutes, the battery monitors came back on. The starter battery was around 12.6v. The other 12v AGM is not monitored for some reason. The 24v lithium bank was down around 13.5v, but the both monitors were claiming that the bank was at 100% charge. I don't know why they read that, maybe since the power dropped so low that they shut off, they just did some weird reset when the solar kicked them on?

I started the generator and started charging the batteries via two mastervolt chargers (one apparently charges at 90amps and one at 70amps). After a few hours, I turned the chargers off and the 24v bank read 26.35V on the battery monitor, the starter agm read 12.95v. I checked the batteries across the terminals with a multimeter and the lithiums all read in the mid 13v's and the starter agm read 12.95v. The other 12v agm read 12.4. I turned the chargers back on and let it run for another half hour and then turned the chargers off again, and everything read the same, so I turned off the generator.

With all the breakers on for the stuff I normally leave on, there was a 11amp draw on the house bank. I went through each breaker to figure out what was doing that, and the biggest draw was the breaker for the circuit that charges the 12v agm's from the 24v lithium bank. To run a test, I left the 24v panel on, with most of the breakers for normal stuff on (bilge pumps, auto watermaker backflush, lights, toilets, etc.), but I turned off the breaker for the 12v charging circuit. I turned the 12v battery switch off, and the generator/engine battery switch off, and I left the boat for 6 days.

Today I went back to the boat and checked everything. The lithium bank was down to 88% capacity at 25.6v. At least I know that the monitor is not stuck at 100%, but this seems like it may be a lower voltage than it should be. The starter battery read at 12.8v and when I tested it with the multimeter at the battery it read 12.8v. The 12v agm for the instruments read at 4.3v though. So I think I have a smoking gun.

Unless there is something hooked up to the 12v instrument battery upstream of the battery switch, then I think it is safe to say that this battery is bad and can't hold a charge. And the automatic charger was draining the lithium bank to keep it charged. The problem is, I don't know the boat well enough to know if there is something between the switch and the battery that could be draining it.

I pulled the battery out and brought it home. Is there a way I can get the battery tested before I buy a replacement?

The battery seems to be a proprietary size (D52) for Varta (the starter battery still on the boat is the exact same battery). I can probably find a common size that will fit. The question here is that it is some sort of start/stop battery for automatic start/stop cars, is this a necessity for this type of setup?

It also says it's BMS and IBS compatible. Is this a specific thing that I have to be careful to have in its replacement? or is this just marketing a feature that all batteries have?

As far as my lithium bank, how can I be sure that they are full? what should the full voltage be for a 24v lithium bank?

I appreciate any and all advice, especially after that text wall.

[sailjumanji]

Charge it to float, let it sit for at least 24 hours to stabilize, measure the voltage. If less than 12 volts, you have a bad cell.

If you take it into a battery shop, they are going to charge it fully, then run a short load test on it and can verify the same thing.

[smac999]

Battery monitors will reset when they lose power.

Your 24v house bank was probably down 800ah.., If it was at 12v. (Charging from this low I’m pretty sure is dangerous as hell, and a bms should have prevented that). The agms down too. Maybe 40 between both. The 160a of chargers would be running for over 6h to charge them all again. Not just a few hours.

The lithuim Likly was not at 100% when you stoped charging. . So the 88% is also too low. You are simply 12% lower the. When you stoped charging

[Muaddib1116]

Ok, thanks, I will charge the system again, for longer. What voltage for the 24v lifepo4 bank should I see at full charge?

[smac999]

you look at amps not voltage. charge untill the amps drop to 0.

[Muaddib1116]

Do you mean the positive amp "draw" that is shown on the battery monitor while it's charging?