Question About State Of Charge

[AL1952]

Have you disconnected each of the 6 batteries and performed a voltage test well after they have been under no load. You might find a bad apple in the bunch which is causing the problem.

Thank you Sun Eagle for your suggestion. It was very helpful. I tested all batteries individually and each one was exactly 13.3 volts. They are however 8 years old and about due for replacement. I don’t want to make the same mistake I did with these less than ideal batteries. I want something better that has less possibility of trouble. You are obviously very knowledgeable so if I may, I’d like some advice from you about your recommendation for new batteries given these parameters. My space is VERY limited. I can currently fit up to 8 smaller 35 Amp batteries, like my current ones, in the space I have, which is on a shelf 4 feet up from the floor. I now have 6 batteries. The space is not ventilated very well and somewhat close to an ignition source, which is why I chose sealed AGM batteries in the first place. I am also old and partially disabled so lifting and moving larger 50+ pound batteries isn’t possible for me. Can you suggest any batteries (6 or 12 volt) and how to set them up (series or parallel), that would give me the same (210 AH system) or better, up to the best capacity of my solar panels, that won’t break my bank account? I currently have 4, 100-watt panels on the roof of a shed next to my house, with just enough space and budget for 1 more. If you think I need it if you recommend a bigger battery storage capacity let me know. I’m open to any of your informed suggestions. Thank you for your help. It is much appreciated.

[SunEagle]

I have 6 batteries. Wired in series. All equal length # 2 wire. PMW 30 amp controller. When you say "a battery that shows less than 12V is being over discharged" do you mean at rest or when under load?

First off those 6 batteries create a 210Ah 12v system (6x35Ah = 210Ah). It needs about 21amps of charging which 4 100 watt panels going through a PWM CC yields a little over 20 amps which is good.

What I meant was that a 12V battery that measure less than 12V at rest we being hurt. But few people wait the hours with no loads connected to perform that measurement.

I do not know why your system is showing low voltage under load except that it might be too much for the batteries. I will say that 6 batteries wired in parallel end up charging and discharging unevenly so you might have one or more bad batteries in the system which is lowering the overall voltage measurement.

Have you disconnected each of the 6 batteries and performed a voltage test well after they have been under no load. You might find a bad apple in the bunch which is causing the problem.

[AL1952]

I have 6 batteries. Wired in series. All equal length # 2 wire. PMW 30 amp controller. When you say "a battery that shows less than 12V is being over discharged" do you mean at rest or when under load?

[SunEagle]

Well measuring the voltage is not the best way to know what the SOC status of your battery system but with AGM you do not have a lot of options.

I guess my question is how many of those 35Ah batteries do you have an how are they wired? Also what type of CC are you using? A PWM type will maybe get your 67% of the panel wattage while an MPPT can get you up in the 90% of the panel wattage.

So before I give my opinion if you should continue to use your batteries as you mentioned I really need to know how they are being charged but I will say that a battery that shows less than 12V is being over discharged and may not provide the number of cycles the manufacture has stated.

[AL1952]

Before I knew any better many years ago I bought some crappy 12 volt, 35amp AGM batteries to use with my 4, 100 watt solar panels. I have always been puzzled by the fact that when under full continuous load the system draws the batteries down from 12.7 resting volts to 10.3 volts (measured at the batteries) under load, over a period of 4 hours of continuous operation. The moment I remove all load the voltage goes up to 12.3 volts and stays there until I turn on the solar panels to recharge them. I was told that the state of charge should be measured at full rest, no load, no charging. If the state of charge is not supposed to go below 50%, or 12.1 volts for these batteries at rest, is it safe for me to continue operating at full load until the voltage drops to 12.1 at rest, even if the operating voltage under load drops as low as 10.0 volts or less?