Bank Capacity

[douglasjett]

Hi All,

I've been in the process of updating my 24 volt system for the past few months. I have added a MidNite 150 controller, re-configured my 9 255 watt 20v nominal panels to 3 sets of 3, and I have added (8) 12volt 110ah LTH solar batteries. Everything seemed to be working fine for a while. In the AM my voltage would be between 24 and 24.2. I'm not overly happy with that result but thats what I had and I was OK with it. Well, my old inexpensive inverter then went out... While I was shuffling around trying to keep the house lit up I wired 4 of the batteries in parallel and used a 700 watt car inverter to run the house.. So, them I'm at 440 watts * 12 volts correct? Well, funny thing in the AM I'm still sitting 12 to 12.1 volts. The 4 12 volt batteries are yielding the same result powering the house overnight... I have since gone back to 24 volt configuration with a new inverter, 8 batteries in series/parallel for 440 watts * 24 volts... Same result 24 to 24.2 volts in the morning. How can this be? Shouldn't I be getting 2x the watts from the 24 volt set up?? Help!

[Sunking]

Hi All,

I've been in the process of updating my 24 volt system for the past few months. I have added a MidNite 150 controller, re-configured my 9 255 watt 20v nominal panels to 3 sets of 3, and I have added (8) 12volt 110ah LTH solar batteries. Everything seemed to be working fine for a while. In the AM my voltage would be between 24 and 24.2. I'm not overly happy with that result but thats what I had and I was OK with it. Well, my old inexpensive inverter then went out... While I was shuffling around trying to keep the house lit up I wired 4 of the batteries in parallel and used a 700 watt car inverter to run the house.. So, them I'm at 440 watts * 12 volts correct? Well, funny thing in the AM I'm still sitting 12 to 12.1 volts. The 4 12 volt batteries are yielding the same result powering the house overnight... I have since gone back to 24 volt configuration with a new inverter, 8 batteries in series/parallel for 440 watts * 24 volts... Same result 24 to 24.2 volts in the morning. How can this be? Shouldn't I be getting 2x the watts from the 24 volt set up?? Help!

No, you do no understand the difference between power (watts) and energy (watt hours).

A battery energy or Watt-Hour Capacity = Battery Voltage x Amp Hours.

Watts (aka power) = Voltage x Current.
Watt Hours or Energy = Watts x Hours

So lets say you have 2 x 12 volt 100 AH batteries.If you wire them in series you have a 24 volt @ 100 AH battery or 2400 watt hour capacity. Wire them in parallel and you have a 12 volt @ 200 AH battery or the exact same 2400 watt hours.

Now for the load nothing changes either with respect to power. Say the load is 100 watts which means either way you configure the batteries, on paper you can run the 100 watt load for 24 hours.

12 volts x 8.33 amps = 100 watts.
24 volts x 4.167 amps = 100 watts.

[douglasjett]

Exactly Sunking... I should have more Watt Hour Capacity at 24 volts, I do not...

[Sunking]

Exactly Sunking... I should have more Watt Hour Capacity at 24 volts, I do not...

You got me confused. Capacity does not change with voltage. With batteries Power and Energy add in parallel and series circuits. If you have say 4 x 12 volt 100 AH batteries, you have 4800 Watt Hours of capacity. Does not matter how you configure them

4 in Parallel (4P) is 12 volts x 400 AH = 4800 wh
2 in series, 2 parallel strings (2S2P) is 24 volts x 200 AH = 4800 wh
All 4 in series (4S) is 48 volts x 100 AH = 4800 wh

Makes no difference how you configure them, watt hour capacity does not change.

[douglasjett]

I understand... I wired (4) in parallel 12v x 440 = 5280 wh.....Then I wired (8) in series ie, 24v x 440 = 10560 wh.... That is very clear to me.

Same overnight load = the voltage drop is the same... at 0600 the 12 volt configuration has a voltage of 12.0v... or about 42% charge. Also at 0600, the 8 battery 24 volt series configuration has a voltage of 24v.... or about 42% charge.

How can 4 batteries at 12v be equal to 8 at 24v

[Sunking]

OK my bad, I misunderstood.

1st issue you cannot use voltage as SOC indication accurately on a working system. Second issue just because you doubled the capacity of the batteries does not mean you increased the capacity of the system. Think of it like a car. You replaced a 10 gallon tank with a 20 gallon tank does not mean you can go further if you can only put in 10 gallons at a time. Sure you doubled the tank size, but you did not increase the gas pump size.

[douglasjett]

1st issue. I have sealed batteries so early in the morning would be the best time to use voltage as SoC right? 2nd... What could be keeping my tank 1/2 full? Batteries at Absorb state by noon and Absorb for 4 hours... I'm not sure how I get the tank any more full? Could there be some resistance in the house wiring or something that is sucking me down? I mean I'm very disappointed right now. I spent $2000 to upgrade my system and I can't even run a fan overnight...

[Sunking]

1st issue. I have sealed batteries so early in the morning would be the best time to use voltage as SoC right? 2nd... What could be keeping my tank 1/2 full?

Not enough panel wattage. You doubled battery capacity, I get it, but you did not double panel wattage. The panels have to be able to replace what power you used the day before. It is like money. If it cost you $100 per day to live, and you only make $50 per day buying a bigger wallet will not fixya.

Here is why you cannot use battery voltage on a working system. The battery needs to be disconnected and rested for a day to use voltage as a relative indicator. A battery underload or discharging will read lower than the SOC, and a battery while charging will read much higher than SOC. So I think you have 1 of 2 things going on, or both.

1. Putting faith in using voltage as SOC indication.

2. Using more power in a day than the panels can replace. Now stop and think about that. If you are using more energy than the panels can produce in a day, was likely true before and after you added battery capacity. I would expect a smaller size battery to have equal or even higher voltage in the morning vs a larger bank. A smaller bank, at least the panels would put them at a much higher state of charge at the end of the day than a battery twice the size. They ended the day with a higher voltage than a larger bank would.

If you used your hydrometer you would know more about what is going on.

[douglasjett]

Well, I hope that 1 is true and that my state of charge in the morning is better than the voltage reading... As for 2 I'm charging (8) 110ah 12v batteries with 2300 watts and a MidNite 150. Like I said they are at Absorb by noon. There is no issue with the panel output. I added up my nightly load... Worst case scenario; I make microwave popcorn, I watch TV for 5 hours, it's hot and the fridge runs all night, I have 2 fans going etc.. My nightly usage is in the neighborhood of 2500 watts. With 8 new fully charged 110ah batteries wired in series I should have 5280 wh to 50% discharge.. Something is wrong... I think I might have a phantom load in the house wiring

[Sunking]

Phantom's are easy to find. But if you use 50% capacity per day is an issue.