AGM battery question

[lligetfa]

I am old school where amp hours were expressed in simple terms so 200 AH could be 1 amp for 200 hours or 2 amps for 100 hours, etc. Now I realize that does not scale all the way to 200 amps for 1 hour hence why I think the new school nomenclature has changed. Anyway, I have an automotive AGM battery that went dead on me and I am trying to determine its AH capacity as to how long it should deliver low amps.

The battery and specs are as follows. Note that AH is spec'd as 20 amp hour rate 55. So how many low amps for how many hours?

[Dave]

Les, ya, it is a pain, because not all battery's follow a linear curve....usually, if lucky, battery spec might mention it.....

[lligetfa]

Well... I already knew that much which was why I said it doesn't scale to 200 amps for one hour. From the bit of reading I've done, I think the "20" is the baseline for charge rate which is also the discharge rate.

[lligetfa]

As a point of comparison, here is a horking big battery that costs around $1200 as compared to my $200 battery.
https://www.napacanada.com/en/p/NAB8A8DT716

[lligetfa]

I found the following article, https://www.batterystuff.com/kb/article ... mean-.html which states:

For deep cycle batteries the standard rating is 20 hours. So, if a battery has a rating of 100AH @ 20Hr rate, then that battery was discharged over 20 hours with a 5 amp load.

So then is the formula for my battery 55 / 20 = 2.75 amps?

[lligetfa]

According to this http://oldschooltech.info/2016/11/off-g ... rinciples/ site, their answer is confusing since they give the answer in AH but the time is fixed at 20.

As an example, the GC2 golf cart battery is described with the following amp-hour ratings: a) 20 amp hour rate: 215, and b) 5 amp hour rate: 157. This means that if discharged over 20 hours, the battery can supply 215 amp-hours

Is the answer for that example 215 / 20 = 10.75 amps? It does seem to scale.

[Charlie]

If you need to get an amp hour rate, then i would say your looking at the wrong battery. Though thats doesnt mean the battery doesnt have an amp hour rating - its just never published because they are normally so low.

So my understanding is if your 55 hour rate is the "C" rating of the battery, then if discharged over C20 hours it would deliver 55ah

Our batteries we use on solar sites are normally 120ah @ C10 hours or if you discharge it slower over 100 hours, you get 150ah @ C100 hours

[Charlie]

So to clarify your battery has 55ah over 20 hours
So it can be discharged at a constant rate of 2.75amps and will last 20 hours before going flat.