Hey everyone, I'm having a bit of a head-scratcher with my solar battery system. I've recently added to my setup and now I'm noticing some odd discharge readings. Here's what I have: I'm using two GSL 0511A-B-GBP2 5.1kWh wall-mount lithium batteries. The first battery is around six months old, and the second one is brand new but the same model. They're hooked up in parallel, and my inverter is a Voltronic Expert VM3.
The problem arises when I look at the discharge during usage. For instance, when my house pulls about 2kW, the individual battery management system (BMS) displays show the following: the older Master Battery discharges around 1.7kW, and the new Slave Battery discharges about 1.0kW. So, together, that's 2.7kW, which is noticeably more than the 2kW shown by the inverter. I'm left wondering where that extra 0.7kW is disappearing to. I get that it's not always a perfect load share between batteries, but this difference is quite puzzling!
1 Answer
It sounds like you're on the right track with your setup. Ideally, batteries in parallel should discharge pretty evenly. Check to make sure the cables between them and to the inverter are of the same length and type. Also, ensure all connections are properly torqued down. While your batteries may be the same model, double-check if they are indeed the same version, with matching BMS firmware, and that they started at the same State of Charge (SOC). Another thing to consider is your inverter's parasitic draw; it needs power to operate, and that could be pulling from your batteries. While this likely doesn't account for all of the extra load, you might see a parasitic drain of around 150-350W, depending on the inverter's size.
I looked into other lithium battery brands like Pylontech and LG Chem, and it seems like the master battery's display typically shows data for the entire bank, not just for itself.
Inverters usually don’t draw more than 100W for operation, so it might be worth rechecking everything!