Calculation test with GC2s

JoshK · February 2016

Ok, I am rusty on this so I need some help. Please double check this math carefully.
The Sam's Club $84 GC2 is 6v. And I think 208AH.

So 6 of these batteries could provide a grand total 7.488 KWH over 20 hours.
And assuming 50% Depth of Discharge, I can use 3.7 KWH over 20 hours.
So 187 watts are available for 20 hours. (3740 Wh total)
And assuming an inverter with 77% efficiency I could run a 144 watt load continuous for 20 hours and only drain the batteries 50%.

So 6 GC2 are perfect for a 144 watt continuous load for 20 hours. At that time they are at 50% DOD and need a recharge.

Photowhit · February 2016

Well sorta...

More of a system sort of thing, and typically you wouldn't design a system to discharge to 50% with out recharging. Particularly on a less than reliable charging source, such as most renewable energy sources...

You parameters are screwed up if your talking solar, charging is typically done at 10-13% rate for bulk and less to top it off, so 4 hours even under optimum conditions would leave the batteries under charged...

It would be better if you told us what you wanted to do?

JoshK · February 2016

I just want to get this straight in my head. It's meant to be a simple example I can remember easily for years. That's all. I chose 20 hours because it matched the 20 hour ratings.

So what I would remember would be:
"For every 140 watts continuous, I need a minimum of $500 batteries."
and if I'm lucky enough to remember more details:
"That's six GC2 batteries and 50% Depth of Discharge"

Photowhit · February 2016

...of course I just told you it won't work for solar....

If you give us a real situation, we can give you real answers...

JoshK · February 2016

Charging and solar issues aside, I just want to get confirmation I manipulated all the numbers correctly.

Raj174 · February 2016

The math seems correct, I assume the hypothetical configuration is 3 strings of 2 for a bank of 12 volt 624 AH

marxman · February 2016

JoshK,

I understand your question as when it comes to battery and loads there is a lot of variables.

Now there is no set rule to the math of batteries as the reason why is that most batteries sold are overrated and made differently. So its best to get at least 20% more amp hours then required. Plus do some research on the behavior of GC2 type of batteries.
The type of load is important too as how many volts and amps and how long. As this has a huge factor in your math estimating your battery bank. As nobody can estimate this but you, as that is why people are asking for more information to help you.
Next topic is hardware, charger type and size of wiring etc. all places a factor in your question.

I don't agree with everything this link I am suggesting below recommends as it is their theory. But I do believe their knowledge shared when it comes to their real life experiences. As see how adding a larger battery bank can actually be a great benefit to the life of the batteries and how to wire them all together properly. Good luck

http://www.smartgauge.co.uk/technical1.html

vtmaps · February 2016

JoshK said:
So 6 of these batteries could provide a grand total 7.488 KWH over 20 hours.
And assuming 50% Depth of Discharge, I can use 3.7 KWH over 20 hours.

Yes, but only if you are drawing 187 watts. If you draw less watts, you have more than 3.7 kwh and if you draw more than 187 watts you will have less than 3.7 kwh. This is because of Peukert's Law.

Also, your battery will have less capacity when it is cold or old... when designing a system you should take that into consideration.

JoshK said:
So 187 watts are available per hour for 20 hours.

No, 187 watts are available for 20 hours.
Watts is a rate of energy use. It can be expressed as energy per time (energy ÷ time). When you multiply energy per time by time, you get energy. Thus 187 watts X 20 hours = 3700 watthours which is an amount of energy.

When you say 'watts per hour' you are saying energy per time per time. It's not what you mean, but 'watts per hour' does have a meaning....

Suppose you are drawing 75 watts at noon, and then drawing 150 watts at 1 PM, and then drawing 225 watts at 2 PM, and then drawing 300 watts at 3 PM (etc). You rate of energy use is increasing at 75 watts per hour.

Raj174 said:

The math seems correct, I assume the hypothetical configuration is 3 strings of 2 for a bank of 12 volt 624 AH

I also assume this is three parallel strings in a 12 volt configuration. Having 3 parallel strings will work for awhile, but it is not an optimal configuration. Ideally the OP would have a single string of batteries.

--vtMaps

JoshK · February 2016

Thanks vtMaps, I really appreciate the details. I corrected post #1.
And everyone is right that this is not enough to design a system based on. For those already running a battery based system, they have a 'feel' for correct answers. I don't have that yet, so I question my math skills. But this clears up the basics. Thanks.

JoshK · February 2016

Sorry I missed you Raj174. Thank you too.

Raj174 · February 2016

JoshK,

I recommend going to Batteryuniversity.com where you will find a lot of information on battery technology. How to connect and maintain them, including all of the various chemistries with the advantages and disadvantages of each type.

Good luck

JoshK · February 2016

Thanks. I actually like that website a lot and read a lot of it a year or two ago. I recommend it to.

Calculation test with GC2s

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