How can I calculate maximum "healthy" amp draw on a battery bank?

[Sunking]

Here is discharge chart for 50AH AGM battery (page 2, upper right). At 100A discharge rate it stays above 11V for about 8 minutes.

Very good academic theory. Now add in wire and connection resistance and it will not work.

[Shmel]

Here is discharge chart for 50AH AGM battery (page 2, upper right). At 100A discharge rate it stays above 11V for about 8 minutes. This means, that you could plug a microwave into this battery, but it would leave you only 13AH of capacity. And I bet, that the battery would become really hot.

45977.pdf

[Handmade Matt]

No problem from a 200 AH battery. Only time it might be a problem is when the battery is already deeply discharged. Stay above the 50% rule and no problem.

Awesome, and seeing as how I have a 250ah bank I'm in an even better position!
Thanks again bud.

[Sunking]

I can be happy pulling 29amps from my batteries occasionally in short bursts on sunny days.

No problem from a 200 AH battery. Only time it might be a problem is when the battery is already deeply discharged. Stay above the 50% rule and no problem.

[Handmade Matt]

Ah, brilliant.
I understand now.
Thanks again.
I can be happy pulling 29amps from my batteries occasionally in short bursts on sunny days.
Normally however I'll be pulling 3-8amps in the evenings with a maximum possible of 10amp which will give me my full capacity.

[Sunking]

The capacity will be reduced for that particular discharge then... Charge it up again and discharge back at C20 and you'll get your full capacity?

Basically yes. Look at this battery spec it clearly demonstrates Peukert Law. Look at the various discharge rates. The same battery is rated 72 Amp Hours at the 1 hours discharge rate, and 266 AH at the 100 hour rate.

[Handmade Matt]

It is called Peukert Law

The capacity will be reduced for that particular discharge then... Charge it up again and discharge back at C20 and you'll get your full capacity?

[Sunking]

Is that for that particular discharge, or for the battery in general for it's future? Damage to the battery?

It is called Peukert Law

[Handmade Matt]

You can pull more but the AH capacity of the batteries will be diminished.

Is that for that particular discharge, or for the battery in general for it's future? Damage to the battery?

[Sunking]

All the issue is related to the battery technology internal resistance and the voltage drop that is incurred. Voltage = Current x Resistance. For argument sake the internal resistance is a fixed constant. So the variable is Current. So as the current increases the voltage drop increases proportionally.

For example if you ever look at starting batteries you see a spec called Cold Cranking Amps or CCA. This tells you the internal resistance of the battery. CCA is how many amps a new full charged battery can deliver at a specified temperature at 7.2 volts. Note I said 7.2 Volts. So let's say the battery can deliver 550 CCA. So a 12.6 volt battery internal resistance is [12.6 - 7.2 volts / 550 amps = .001 Ohm's of resistance

With a FLA battery the maximum recommended charge discharge current is C/8. Go beyond that and the voltage at the terminals start to drop off significantly. The battery can certainly go beyond that but at the cost of voltage. For example I can take a 50 AH battery and connect it to a 1000 watt inverter. As long as I operate at 120 watts and less life is good. But now I hook up say a 800 watt microwave oven, the inverter immediately trips off when it tires to pull 67 amps from the battery. At 67 amps even though the battery is fully charged up, the voltage at the terminals falls below 10 volts, and the inverter is designed to trip off at 11 volts. Not going to work.

Of here are some rules to live by.

FLA max C/8
AGM max C/4
NiCd 1C
LFP 5C

There are some exceptions with specialty hybrid batteries that can bo beyond this general rule of thumb.

[DBaldwin]

You can pull more but the AH capacity of the batteries will be diminished. how much is dependent on the amount of draw.
Google Peukerts law on wikipedia there is a very good explanation and a calculator there. It helps if you can find the Peukerts factor from the manufacturer.
the default in the calculator is 1.3 which is a general idea for FLA batteries.

Very helpful, thanks!

[Naptown]

I see what you mean. By 100 hours at 1amp the voltage is just under 12v.
At 10amp discharge rate the battery voltage will drop under 12v just before ten hours us up.

I've just emailed them to make sure they sent me the correct data sheet.
However, either way, we are looking at a maximum of 10-12amps being a healthy maximum current to pull from my bank?

You can pull more but the AH capacity of the batteries will be diminished. how much is dependent on the amount of draw.
Google Peukerts law on wikipedia there is a very good explanation and a calculator there. It helps if you can find the Peukerts factor from the manufacturer.
the default in the calculator is 1.3 which is a general idea for FLA batteries.

[Handmade Matt]

It looks to me like this diagram is for 100AH battery, and maximum recommended continuous current is 10A.

I see what you mean. By 100 hours at 1amp the voltage is just under 12v.
At 10amp discharge rate the battery voltage will drop under 12v just before ten hours us up.

I've just emailed them to make sure they sent me the correct data sheet.
However, either way, we are looking at a maximum of 10-12amps being a healthy maximum current to pull from my bank?

[Shmel]

It has this diagram:

It looks to me like this diagram is for 100AH battery, and maximum recommended continuous current is 10A.

[Handmade Matt]

Thanks everyone.

For longest life of cells and the charge I would go not more than a 20 hour rate.
This will get almost the most (or at least the rated) AH from the batteries. Higher than that and Peukerts Law starts to bite you.

So for a 250ah 24v bank that would 12.5amps?

Is not this stated in battery manual?

It has this diagram: