Question: Battery Reserve Capacity

Red31, thanks for uploading the chart. Very helpful. Gives me a good idea of the different discharge rates based on load.

Westend, yes, I did miss the formula. Thanks for pointing it out.

BFL13, you're definitely one of the battery gurus around here--lol! Appreciate you clarifying the 20 hr rate.

otrfun wrote:
Really appreciate everybody's input. I thought the reserve capacity would give me some idea of how many amp hours of current I could expect to get out of a given battery. Apparently that's not the case.

Interstate says their SRM-24 battery is capable of producing 5 amps for 17.1 hours or 15 amps for 4.9 hours. However, without knowing what voltage they're discharging to, it's hard to determine what this battery is really capable of.

Is there somekind of battery spec that I can use to get a very general idea (nothing exact) of how many amp hours of current I can expect to get out of a new battery (24, 27, or GC2) when it's discharged from 12.7 volts (100 percent) to 12.2 volt (50 percent)?

Thanks!

The 20 hr rate is just divide the AH by 20 to get the current, so a 100 AH battery means they used 5 amps for the draw down to 10.5v

A 232AH battery bank has a 20 hr (current) rate of 11.6 amps.

So if a battery will be down in 17 hrs at 5 amps that means it is less than 100AH at its 20 hr rate. However, that 5 is more than its 20 hr rate so Peukert happens that reduces the time to run the battery down even more--exponential not linear.

otrfun wrote:
Really appreciate everybody's input. I thought the reserve capacity would give me some idea of how many amp hours of current I could expect to get out of a given battery. Apparently that's not the case.

Interstate says their SRM-24 battery is capable of producing 5 amps for 17.1 hours or 15 amps for 4.9 hours. However, without knowing what voltage they're discharging to, it's hard to determine what this battery is really capable of.

Is there somekind of battery spec that I can use to get a very general idea (nothing exact) of how many amp hours of current I can expect to get out of a new battery (24, 27, or GC2) when it's discharged from 12.7 volts (100 percent) to 12.2 volt (50 percent)?

Thanks!

You may have missed this sentence over at Battery University: "No accurate RC to Ah conversion exists but the most common formula is RC divided by 2 plus 16. A short method is dividing RC by 1.9."

does your 17.1 hrs @ 5 amp show up below? 20 hr rate is what to use (well 1/2 of the 20 hr rate when discharged 1/2). Note there are group 24, 27 are just size of the container and their capacity can vary. GC2, 2 of them would (12v) be ~220ah @ 20hr. More important is to FULLY charge regularly!

Really appreciate everybody's input. I thought the reserve capacity would give me some idea of how many amp hours of current I could expect to get out of a given battery. Apparently that's not the case.

Interstate says their SRM-24 battery is capable of producing 5 amps for 17.1 hours or 15 amps for 4.9 hours. However, without knowing what voltage they're discharging to, it's hard to determine what this battery is really capable of.

Is there somekind of battery spec that I can use to get a very general idea (nothing exact) of how many amp hours of current I can expect to get out of a new battery (24, 27, or GC2) when it's discharged from 12.7 volts (100 percent) to 12.2 volt (50 percent)?

Thanks!

a group 24 RV battery has a RC of 140 minutes. It may also have a 20 hr rating of 80 ah, discharged @ 25A over 2.3333hrs is 2.33hr*25A=58ah. Discharge it over 20 hrs @ 4A and it will provide 80ah.

A better question may be how much RC or 20 hr rating capacity might ya need for one, two etc days without utilities, without discharging below 50% and how to get it recharged FULLY after <50% discharges.

20ah a day, 50ah a day?

I would make no assumptions based on Reserve Capacity.. Though it is related in part to battery capacity.. The key words in that phrase are "in part" there are other factors that affect it.. So it's not a good indication of anything useful when it comes to house power.

Engine starting it may or may not matter.

time2roll wrote:

otrfun wrote:
If it is 0 percent, is it generally correct to assume that 70 minutes at 25 amps would discharge the battery to roughly 50 percent?

battery will be a little less than 50%. It is constant amps not constant power. Actual power extracted diminishes as voltage tapers off. More power is extracted during the first 70 minutes vs the second 70 minutes.

They are not the same measurement, but when you do a draw -down at the "20 hr rate" based on the rated AH, you have the same question.

" Is it at 50% capacity when you run the 20 hr rate for 10 hours?"

I have done this often to test my batteries as they age to see how much capacity they really have by now. I run them down at their 20 hr rate (eg 5 amps for a 100AH battery) and see if it takes 10 hrs to get to 50% (as measured by hydrometer SG, AND confirmed by resting voltage per battery specification.) If it only takes 9 hours to get to 50% then they are 90% as good as new.

The usual blurbs on batteries show that it is supposed to be linear for discharging, but it is not linear for charging back up. So linear discharge should mean you are half -way down at the 10 hr mark out of 20 hours.

Same would be true using RC if you draw down at that RC constant rate.

However, in real life it seems to be a bit more complicated, where they seem to run down a little faster after the first 10 hours (if you over-shoot) which suggests it is not so linear.

Therefore it could be that 10 hours is more than half way out of 20, which seems silly, but we are talking batteries here! :)

Somebody who has more experience going below 50% might be able to confirm or deny that, but that is what I have (maybe) noticed--and I do pay attention while I am doing the tests.

otrfun wrote:
If it is 0 percent, is it generally correct to assume that 70 minutes at 25 amps would discharge the battery to roughly 50 percent?

battery will be a little less than 50%. It is constant amps not constant power. Actual power extracted diminishes as voltage tapers off. More power is extracted during the first 70 minutes vs the second 70 minutes.

RC is calculated to 10.5V. You can read more about this here.