Recently I've read a few posts, here and elsewhere where folks will say;
"At the rated 1A draw you'll get X amp hours."
or "At the rated 5A draw you get X amp hours."
The Ah capacity used for deep cycle batteries, in marine
applications and other industries, is very often a 20 hour Ah rating. There are also 5 hour and 10 hour ratings. Most battery monitors need this 20 hour rating to be programmed correctly and most all reputable battery manufacturers of deep cycle batteries can supply you with the 20 hour Ah rating. They will also supply you with the Peukert factor for correctly programming a battery monitor.
To figure the load your battery can support to deliver the same Ah's as the 20 hour rating you divide the rated 20 hour Ah capacity by 20. (Same as post above)
100Ah Battery / 20 = 5A
So a 100 Ah battery can support a 5A load for 20 hours before falling to 10.5V which is what the industry considers dead for the 20 hour capacity test.
Same math of different sized batteries:
60Ah Battery / 20 = 3A
So a 60Ah battery can only support a 3A load for 20 hours before hitting 10.5V.
130 Ah / 20 = 6.5A
And a 130 Ah battery can support a 6.5A load for 20 hours before hitting 10.5V.
As you can see the "rated load" is entirely dependent upon the Ah capacity of the specific battery in question and is not a simple 1A or 5A or 10A rating because it depends on the specific hour rating and Ah capacity of the battery.
Simply put a 60Ah battery can not have the same load applied to it as a 160Ah battery and still hit it's rated capacity over 20 hours.
But there is a gotcha, always is......
It is called the Peukert Effect. To dumb it down it means that any load applied to the battery above the 20 hour rating will result in less Ah capacity than the rating. On the other hand any load below the 20 hour rating will result in more Ah capacity.
I think looking at the math will help. This is the math on a 100Ah battery.
100 Ah Battery With A Peukert Factor of 1.25:
100Ah Battery - 80 Load = 50 Ah Capacity
100Ah Battery - 50A Load = 56.23 Ah Capacity
100Ah Battery - 40A Load =59.5 Ah Capacity
100Ah Battery - 30A Load = 63.9 Ah Capacity
100Ah Battery - 20A Load = 70.7 Ah Capacity
100Ah Battery - 10A Load = 84 Ah Capacity
100Ah Battery - 5A Load =100 Ah Capacity
100Ah Battery - 3A Load = 113.6 Ah Capacity
100Ah Battery With - 1A Load = 149.5 Ah Capacity
I highlighted the 5A load in red because that is exactly what the "divide Ah capacity by 20" gets you too..
Any load above the rated capacity of the 20 hour Ah rating results in less Ah capacity. Any load below the 20 hour capacity rating and you have more available Ah capacity..
It also helps to explain why a larger bank with smaller loads will survive better. It is not being cycled as deeply as you think if you are thinking in terms of the "face value" rating.
Take a parallel bank of four 100Ah batteries. With four you have a 20 hour rating that can support a 20A load, or 5A per battery, X 4 = 20A. When you run this bank at an average load of say 8A you'll really have a 503Ah bank not a 400Ah bank..
If you add just one more battery and make the bank 500 Ah you'll have a 25A support load, BUT, apply the same 8A load and you have a bank that can deliver 665 Ah's using the same average 8A load.
Conversely, size your bank small, at 100Ah, which would have a 5A support, and still apply the same 8A load and you really only have an 89 Ah bank. Bank size vs. load matters and the bigger the bank and the lower the load the less capacity you use and thus the shallower the discharge cycle. If you know your rough "average load" this can be calculated. Shallow discharges are good for the battery bank and deep discharges are bad so a bigger bank with the same load will be cycled less deeply, the way most people do it.
This is why we humans, unless perhaps you're Stephen Hawking, can't keep track of Ah capacity by simply watching the amp screen
on a simple ammeter. I much prefer my Peukert calculated state of charge or % charged screen
A battery monitor will make all these calculations for you internally and then represent them as a % of bank capacity or SOC. This of course only works well if it has been programmed correctly. The vast majority of battery monitors I see are not properly programmed. Better than 95% are still on the default Peukert #.. For proper programming, at a minimum, you need the banks total Ah capacity, at the 20 hour rate, and the Peukert factor for your specific batteries. A battery monitor can be a great tool but it is only as good as the person using it..