Recently I've noticed a few posts where folks will state;
"At the rated 1A draw you'll get X amp hours." or "At the rated 5A draw you get X amp hours."
The "rated load" is only directly proportional to a specific battery's Ah capacity.
The Ah capacity used for deep cycle batteries in marine applications is a 20 hour Ah rating. Most all battery monitors need this 20 hour rating to be programmed correctly. 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 properly 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.
100Ah Battery / 20 = 5A
So a 100 Ah battery can support a 5A load for 20 hours before falling to 10.5V which is considered dead for the 20 hour capacity test. It should be noted that this is what they test the batteries to not what you draw your bank down to...
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. A 60Ah battery can not be applied the same load as a 160Ah battery and hit it's rated capacity over 20 hours.
But, there is a GOTCHA, always is......
Here's the catch, it is called the Peukert Effect. In very simplistic terms it means that any load applied to the battery above the 20 hour rating (the divide by 20 number) will result in less Ah capacity. On the other hand any load below the 20 hour rating will result in more Ah capacity.
I think looking at the math helps. 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, as I mentioned above.
As you can see any load above the rated capacity at 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..
This is why I almost always cringe when I see people wanting to use large inverters with 80A+ draws on a relatively small bank. It can cut your available capacity, and without a properly programmed battery monitor you'll not know it.
It is also another reason why a larger bank with smaller loads can last longer & survive better.
Take a parallel bank of four 100Ah batteries. You now 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 503Ah bank.
If you add just one more battery and make the bank 500Ah's and 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 an average of an 8A load.
Conversely, size your bank small at 100Ah, which would have a 5A support for 20 hours, 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. Shallow discharges are good for the battery bank and deep discharges are bad.
This should help explain 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.
A battery monitor will make all these calculations for you internally and then represent them as a % of bank capacity. This of course only works well if it has been programmed correctly. 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.
"At the rated 1A draw you'll get X amp hours." or "At the rated 5A draw you get X amp hours."
The "rated load" is only directly proportional to a specific battery's Ah capacity.
The Ah capacity used for deep cycle batteries in marine applications is a 20 hour Ah rating. Most all battery monitors need this 20 hour rating to be programmed correctly. 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 properly 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.
100Ah Battery / 20 = 5A
So a 100 Ah battery can support a 5A load for 20 hours before falling to 10.5V which is considered dead for the 20 hour capacity test. It should be noted that this is what they test the batteries to not what you draw your bank down to...
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. A 60Ah battery can not be applied the same load as a 160Ah battery and hit it's rated capacity over 20 hours.
But, there is a GOTCHA, always is......
Here's the catch, it is called the Peukert Effect. In very simplistic terms it means that any load applied to the battery above the 20 hour rating (the divide by 20 number) will result in less Ah capacity. On the other hand any load below the 20 hour rating will result in more Ah capacity.
I think looking at the math helps. 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, as I mentioned above.
As you can see any load above the rated capacity at 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..
This is why I almost always cringe when I see people wanting to use large inverters with 80A+ draws on a relatively small bank. It can cut your available capacity, and without a properly programmed battery monitor you'll not know it.
It is also another reason why a larger bank with smaller loads can last longer & survive better.
Take a parallel bank of four 100Ah batteries. You now 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 503Ah bank.
If you add just one more battery and make the bank 500Ah's and 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 an average of an 8A load.
Conversely, size your bank small at 100Ah, which would have a 5A support for 20 hours, 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. Shallow discharges are good for the battery bank and deep discharges are bad.
This should help explain 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.
A battery monitor will make all these calculations for you internally and then represent them as a % of bank capacity. This of course only works well if it has been programmed correctly. 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.