Is my battery math correct?

[kingofbanff]

Sorry that was not clear at all. I was referring to your point that 96 volts would be the best configuration but I don't think my Outback Radian can handle that as input. I will have 13kw of SunPower panels. My question was instead of the FullRiver dc224 which battery would you suggest?

[Sunking]

hmmm...the mav vdc input for my inverters is 64. So I could still run it in at 48 volts but the batteries are just higher capacity? I'm wondering if there is still time to change the order. Do you have a suggested battery? Thanks

Not sure I follow you. I thought you were running 48 volts?

A 48 volt inverter should operate from 42 to 64 volts as that is the range of a 48 volt battery.

There is one other catch and I don't think it should be a problem. To run 48 volts means your total panel voltage needs to be at least 72 volts, and your controller capable of 48 volts. You did not say what kind and how many panels you have, or what kind of controller.

So I am not sure what you are asking.

[kingofbanff]

hmmm...the mav vdc input for my inverters is 64. So I could still run it in at 48 volts but the batteries are just higher capacity? I'm wondering if there is still time to change the order. Do you have a suggested battery? Thanks

[Sunking]

Thanks for the quick reply. I was working in kwH but all the batteries are quoted in aH so I was using lazy cackle to convert.

It is a 48 v system. So 448AH at 48 volts is 21kwh. 30% is 6Kwh. Interestingly my estimated usage in a grid down situation is 6kwh per day. (TV, cold beer and some fans).

You did right by using Kwh. Amp Hours is just the end result. So if you calculated the battery size to be 21 Kwh and ran 48 volts then, 21,000 watt hours / 48 volts = 437.5 AH.

Where you went off track is using 224 AH cells. You want to use only 1 string of batteries. So you should have been looking for 400 to 450 AH batteries. Would have been half the hardware and about doubled the life of the batteries. Would have been a lot less expensive in the end.

[SunEagle]

Thanks for the quick reply. I was working in kwH but all the batteries are quoted in aH so I was using lazy cackle to convert.

It is a 48 v system. So 448AH at 48 volts is 21kwh. 30% is 6Kwh. Interestingly my estimated usage in a grid down situation is 6kwh per day. (TV, cold beer and some fans).

While 30% may be a little high to get the most out of those batteries your math is correct about it being 6kWh.

Now remember what you take out of the battery system you have to put back and more due to efficiency and losses. So depending on where you live, a 5 hour insulation time period would require about 1800 watts of solar panels to generate that 6kWh.

[kingofbanff]

Thanks for the quick reply. I was working in kwH but all the batteries are quoted in aH so I was using lazy cackle to convert.

It is a 48 v system. So 448AH at 48 volts is 21kwh. 30% is 6Kwh. Interestingly my estimated usage in a grid down situation is 6kwh per day. (TV, cold beer and some fans).

[Sunking]

No one can answer correctly because you have not said how the batteries are configured. There are 5 possibilities. Which one do you have.

6 volts @ 3584 AH: All 16 batteries in parallel which would be very stupid
12 volts @ 1792 AH 2 x 8.Extremely poor configuration
24 volts @ 896 AH 4 x 4. Extremely poor configuration
48 volts @ 448 AH 2 x 8, Doable but poor configuration
96 volts @ 224 AH all in series. Correct configuration.

Lastly you do not use Amp Hours to calculate usage. You use Watt Hours. Amp Hours means nothing and is just an end result.

To answer your question, most likely your math is way off. Please tell us you are not operating at 12 volts.

[SunEagle]

Your math is off.

How are you wiring those 16 x 6v batteries?
If they are wired for 48volt you would have a 448Ah system. (2 sets in parallel of 8 wired in series)
If they are wired for 24volt you would have a 896Ah system. (4 sets in parallel of 4 wired in series)
If they are wired for 12volt you would have a 1792Ah system. (8 sets in parallel of 2 wired in series)

When you wire batteries in series the voltage adds up but the Ah stays the same
When you wire battery strings in parallel the voltage stays the same but the Ah adds up.

Unless you have a 6volt system (all 16 batteries wired in parallel) then you would get 3584Ah system but at 6 volts.

As for how much you will use a day, do not do the math in Ah but in Watt hours. Using Ah can result in the wrong usage amount.

So what is your calculated daily (24hour) watt hour usage?

[kingofbanff]

My system will have 16 Full River DC 224 6 volt batteries. Total energy stored 3584aH. So if I ran the batteries down only 30% I would have 1075aH. My inverters seem to max out at 65amps. Dividing 1075/65 gives me 16.5 hours of battery if I was maxing my draw.

Roughly correct?

Edit: I don't care if I'm off by a factor of 20% because of inefficiencies. I'm wondering if I am off by factor of 4 or 5 because I don't get the basics. There I just saved the nitpickers from some keystrokes.


Thanks