New to Forum for posting have a question on voltages

DsullDsull Registered Users Posts: 3
Quick question as I'm not 100% knowledgable on 12 volt systems beyond what I read to install my solar, and Automotive/Heavy Truck systems.

First, the system.
I have the 400 Watt Kit from Renogy on my RV. Running 8 AWG between panels to Junction box, and then 4 AWG to controller (per the Tables for distances.) 40 AMP breakers on either side of the controller to my buss Bar. System is running voltages and amps at each junction as per system specs during full sun. 
2/0 cables to my 4 6-volt batteries
80 amp breaker to Inverter, 300 Amp Catastrophic fuse between battery bank and Bus bar. Inverter running 2/0 Gauge wires to 1000 watt Aims inverter.
Meter is a Victron BMV-712 with 500 Amp shunt. Shunt is wired through 2/0 Gauge between bus bar and battery ground.

During the summer I rarely saw it drop below 12.2 volts even with the kids watching movies at night, Mini fridge and my CPAP. Now that it's cold, I'm seeing voltages under load around 11.6-11.7 but the meter only showing about 120 amp hours used. This is with the Furnace running pretty heavy (5-7 amp Draw), a minifridge for the coldies (2-3 amp draw intermittent) and my CPAP going all night (it's a hog, 7-10 amps before I turned down the humidity and heated hose settings to lowest). I got paranoid and ran my little honda 650 generator direct 12 volt to batteries the second night as it charges fastest that way (15 amps output) to keep voltages up in the mid 12's.

I guess my question is should I be scared of the voltage or is it normal with the amount of load hitting the batteries. The meter showed full voltage by around 5 PM after charging around 24 amp hours during the day during full sun, and the tail in and tail out periods as the sun went up and down. Never lost inverter power to A/C system, and furnace was good all night. Just not used to seeing the meter drop that far and throw on low voltage alarm.

Do I need to run 2 more batteries or is this normal?


  • DsullDsull Registered Users Posts: 3
    Forgot to add, the batteries are 4 GC-2 Interstates.
  • EstragonEstragon Registered Users Posts: 4,495 ✭✭✭✭✭
    4 x GC2 would be rated ~450ah.  Depending how cold they are though, this will be reduced.  At 10°C, for example, they'll have an apparent capacity of roughly 15% less. 

    Discharging at a rate of more than 5% per hour will reduce this further, as they're typically rated at a 20hr rate.  Using 400ah at 10°C, 5% would be ~20a/hr, so if the 120ah was used in less than 6hrs, apparent capacity would be reduced further.

    120ah is about 1/3 of 400ah, and I'd expect a resting voltage of about 12.4ish at that state of charge.  Under load though, it could be much lower.  It may not be practical to let the bank rest for several hours to get a good resting voltage, but you may be able to get some idea by removing loads for a few minutes and watch how voltage recovers.  Assuming flooded, getting a specific gravity reading would be helpful.

    It may be it's just a combination of cold batteries, and higher loads that's causing the drop.  It may also be they've suffered some sulfation and permanent loss of capacity, or some combination of both.  Before adding batteries, I'd suggest figuring out which it is.

    Also, with the small honda generators, you may be better to charge with a higher amp (eg [email protected]) AC charger if possible, as the DC outputs tend to be small.  Not sure if this is the same as yours, but this specs 8.3adc output.
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • DsullDsull Registered Users Posts: 3
    Awesome thank you I tested them today and they are back to full charge via solar with Specific Gravity test. Do you think they will be ok, or should I add 2 more batteries because the load will be around the same if not a little more around this area, Temps are around -6 C. We live in southern utah, and we camp mainly from September to June to avoid the heat.
  • EstragonEstragon Registered Users Posts: 4,495 ✭✭✭✭✭
    That they've apparently recovered per SG testing is good news.  Do note though that SG readings should be adjusted for the cold.

    At -6°C, I'd expect an apparent capacity of roughly 370ah.  You could confirm this by applying a known load of say [email protected] for a few hours and then testing again.

    Assuming the existing are about 370ah, adding 2 more would give you something like 540ah at that temp.  A couple of things to consider; when adding a new string to old, the new tends to age with the old.  If the old actually haven't really recovered, the new will tend to lose capacity to match the old.  Second, with 3 parallel strings, you should really add individual fuses on each string (to prevent a shorted cell in one string from being fed by the other two).

    540ah taken to 50% charge isn't likely to be fully rechargable with existing solar, especially with shorter days and lower sun.  My guess is 75-100ah may be a good average winter day.  If you could add a ~40-50a AC charger and run the genny for a couple hours in the morning, then let solar do what it can, depending on loads, you may be able to keep the bank in an acceptible range.  Going under 50% regularly is hard on a lead acid bank.  Getting to 100% every day isn't necessary, but getting to 85-90% or so should be a goal.
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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