Recently I've read a few posts where folks state;
"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 is a 20 hour Ah rating. Most all 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 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 at 77F, for 20 hours, before falling to 10.5V which is considered end of test for the 20 hour capacity test.
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 all while at a specific temperature of 77F. A 60Ah battery can not be applied the same load as a 160Ah battery and hit it's rated capacity over 20 hours.
So if your battery can deliver C/20 for 20 hours at 77F you have a battery with 100% of its rated capacity.
But there is a GOTCHA always is......
Here's the catch, it is called the Peukert Effect or Peukerts Formula though sometimes referred to as Peukerts Law, which it really is not.. In very simplistic terms it means that any average loads applied to the battery ABOVE the 20 hour rating can result in less usable Ah capacity before hitting your low voltage threshold. On the other hand any load BELOW the 20 hour rating can result in slightly more Ah capacity.
I think looking at the math will help. This is the mathematical formula on a 100Ah battery.
100 Ah Battery With A Peukert 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
Note: Increases in capacity, at slow rate discharge shown above, are from mathematical formula and usually do not = actual chemical capacity. Gains at slow discharge rates can range from 105% to 120% of capacity (thick plate batteries) but I've not seen much more...
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 slightly more available Ah capacity..
This is why I almost always cringe when I see people wanting to use large inverters with 80A+ draws on the battery or bank. It can change your available usable capacity and without a properly programmed battery monitor you'll not know it.
It is also another reason why a larger bank with smaller applied loads tend to 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 might have 503Ah bank, in mathematical theory.
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, 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.
Peukerts formula is waaaaay more complicated than I have explained it here, and is ever changing as your batteries age, but having your battery monitor set close to the manufacturers stated Peukert can result in an Ah counter that will be more accurate than one that is not properly programmed. Ah or Coulomb counter accuracy is a topic that could span pages and a topic for another day..
"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 is a 20 hour Ah rating. Most all 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 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 at 77F, for 20 hours, before falling to 10.5V which is considered end of test for the 20 hour capacity test.
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 all while at a specific temperature of 77F. A 60Ah battery can not be applied the same load as a 160Ah battery and hit it's rated capacity over 20 hours.
So if your battery can deliver C/20 for 20 hours at 77F you have a battery with 100% of its rated capacity.
But there is a GOTCHA always is......
Here's the catch, it is called the Peukert Effect or Peukerts Formula though sometimes referred to as Peukerts Law, which it really is not.. In very simplistic terms it means that any average loads applied to the battery ABOVE the 20 hour rating can result in less usable Ah capacity before hitting your low voltage threshold. On the other hand any load BELOW the 20 hour rating can result in slightly more Ah capacity.
I think looking at the math will help. This is the mathematical formula on a 100Ah battery.
100 Ah Battery With A Peukert 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
Note: Increases in capacity, at slow rate discharge shown above, are from mathematical formula and usually do not = actual chemical capacity. Gains at slow discharge rates can range from 105% to 120% of capacity (thick plate batteries) but I've not seen much more...
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 slightly more available Ah capacity..
This is why I almost always cringe when I see people wanting to use large inverters with 80A+ draws on the battery or bank. It can change your available usable capacity and without a properly programmed battery monitor you'll not know it.
It is also another reason why a larger bank with smaller applied loads tend to 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 might have 503Ah bank, in mathematical theory.
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, 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.
Peukerts formula is waaaaay more complicated than I have explained it here, and is ever changing as your batteries age, but having your battery monitor set close to the manufacturers stated Peukert can result in an Ah counter that will be more accurate than one that is not properly programmed. Ah or Coulomb counter accuracy is a topic that could span pages and a topic for another day..
