I bought the 12v 180Ah units from Battery Evo, which is the same company as Big Battery. For whatever reason, the Battery Evo models are all listed as being able to work in series 2max. I have mine at 24v with a series cable from the Battery Evo website. Before I put them together, I individually charged them with a 20amp smart charger (Amazon)that has a lifepo4 setting. It's a cheap charger I bought for my car but it works just fine for the Evos.
Once they are fully charged, blinking led lights at the connector port. I connected them to my 24v MPP Solar 2.4kW all-in-one. They normally stay within .1v of each other, regardless of the load. You say one battery discharged first which will shut down the inverter. It needs 21-22volts to operate the inverter. Do you have the MPP 01 setting at SBU or SOL? and at what voltage do you have setting #12? Default at 23v or less? I reset mine to 24v. Is your MPP hooked up properly to AC line voltage?
I had the same issue as you, where the inverter shut down when one of the batteries discharged first. I went back to the settings and I had SBU which gives priority to solar to power loads, and then if solar isn't enough or isn't available then the battery takes over the loads until it hits the low voltage set in setting 12, then utility licks in, which may not be soon enough if one battery passes below the low voltage cutoff. I reset mine to SOL which is solar first and then when solar isn't enough or available, (the sun goes down, cloudy day), the utility will then charge and on bypass, power the loads. This keeps the inverter from discharging the battery to its low voltage warning cutoff.
Since then, I have had no problems with them.
