Effect of Current on Amp Hour Capacity - SailNet Community
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Effect of Current on Amp Hour Capacity

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.

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Last edited by Maine Sail; 03-11-2012 at 06:37 PM.
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Old 03-11-2012
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Re: Effect of Current on Amp Hour Capacity

Thanks for that, MS.. where does one typically find the 'Peukart factor' for any particular battery? I didn't get any documentation with the last bank I bought last year..

Ron

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Re: Effect of Current on Amp Hour Capacity

Xantrex battery monitor manuals have a list of some the major brand's Peukert factor listed. Not sure about Victron.

Brian
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Re: Effect of Current on Amp Hour Capacity

Quote:
Originally Posted by Faster View Post
Thanks for that, MS.. where does one typically find the 'Peukart factor' for any particular battery? I didn't get any documentation with the last bank I bought last year..
You generally call the manufacturer if it is not posted in their on-line manuals...

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Re: Effect of Current on Amp Hour Capacity

How much does the P factor vary by brand or battery type?

And, what do you multiply or divide by the factor in order to use it?

I called Deka/East Penn recently, because their AGM's showed a maximum charge at 13.7 volts (at a set temp point) and the charger was preprogramed at 13.8, several tenths above "optimum" and one tenth above "maximum". I asked, would it make any difference in long term life? In...anything?

And was somewhat baffled when they said no, don't worry about it. I dunno...why publish a maximum, to the tenth of a volt, and then say don't worry you can exceed it. If I can exceed it, it isn't the maximum, is it?

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Re: Effect of Current on Amp Hour Capacity

A few things to put Peukert in perspective.

Flooded lead-acid batteries have a self discharge rate of about 0.5% per day. At most discharge rates that's fairly trivial. But, for a large bank with a small load, self discharge can have more of an effect than Peukert, resulting in a lower capacity than Peukert would predict.

At the other end of the spectrum, very high discharge rates can also lower the capacity more than Peukert would predict (in part this is due to temperature effects).

Finally, Peukert's constant, k, is not constant through time; it will increase as the battery ages. If a battery company states that k = 1.25 (or whatever), that is undoubtedly the number they have empirically derived for a brand new battery. However, a four or five year old battery (particularly on that hasn't been treated very well) can have a significantly higher k. In other words, the batteries apparent capacity will fall off faster with increased load than would have been predicted by using the company's stated k.

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Re: Effect of Current on Amp Hour Capacity

Quote:
Originally Posted by SlowButSteady View Post
A few things to put Peukert in perspective.

Flooded lead-acid batteries have a self discharge rate of about 0.5% per day. At most discharge rates that's fairly trivial. But, for a large bank with a small load, self discharge can have more of an effect than Peukert, resulting in a lower capacity than Peukert would predict.
Battery monitors do not and can not account for self discharge. This is why a know full charge and manual reset as often as possible can help as can solar or wind..

Quote:
Originally Posted by SlowButSteady View Post
At the other end of the spectrum, very high discharge rates can also lower the capacity more than Peukert would predict (in part this is due to temperature effects).
Temperature affects both capacity and Peukert. Capacity is at 77F anything above or below needs to be temp compensated. Sadly not all battery monitors have the ability for a temp sensor...

Quote:
Originally Posted by SlowButSteady View Post
Finally, Peukert's constant, k, is not constant through time; it will increase as the battery ages. If a battery company states that k = 1.25 (or whatever), that is undoubtedly the number they have empirically derived for a brand new battery. However, a four or five year old battery (particularly on that hasn't been treated very well) can have a significantly higher k. In other words, the batteries apparent capacity will fall off faster with increased load than would have been predicted by using the company's stated k.
Same as capacity diminishes with age the Peukert number climbs.. Even at a 5A load a 100Ah battery at 5 years old is not going to deliver 100Ah's due to age and condition..

I load test my batts so I know actual capacity over time but most don't have the means to do this...

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Re: Effect of Current on Amp Hour Capacity

Quote:
Originally Posted by Maine Sail View Post
Battery monitors do not and can not account for self discharge. This is why a know full charge and manual reset as often as possible can help as can solar or wind..
Actually, they could. It wouldn't be easy, and it would probably be a bit messy, but they could.

Quote:
Originally Posted by Maine Sail View Post
Temperature affects both capacity and Peukert. Capacity is at 77F anything above or below needs to be temp compensated. Sadly not all battery monitors have the ability for a temp sensor...
True. But I wasn't referring to just the battery's temperature. I was also referring to the chemical kinetics involved in the battery.

Quote:
Originally Posted by Maine Sail View Post
Same as capacity diminishes with age the Peukert number climbs.. Even at a 5A load a 100Ah battery at 5 years old is not going to deliver 100Ah's due to age and condition..

I load test my batts so I know actual capacity over time but most don't have the means to do this...
I've never taken a close look at the "load test" devices used commercially. However, I've always been a little dubious as to their accuracy. I have no idea exactly what you use, but if it's one of those little gizmos the oil change place uses on my Honda when they try to sell me a new battery every visit (apparent my car's battery has been "about to die" for years), I wouldn't but much stock in its readings.

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Last edited by SlowButSteady; 03-12-2012 at 12:34 AM.
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Re: Effect of Current on Amp Hour Capacity

Quote:
Originally Posted by SlowButSteady View Post
I've never taken a close look at the "load test" devices used commercially. However, I've always been a little dubious as to their accuracy. I have no idea exactly what you use, but if it's one of those little gizmos the oil change place uses on my Honda when they try to sell me a new battery every visit (apparent my car's battery has been "about to die" for years), I wouldn't but much stock in its readings.
In the field I use pulsed load, capacitance and also have a 500A carbon pile. None of these will tell you the actual capacity. They can give you a good basis for state of life and are actually quite accurate if you have a good one. I find very consistently that when a battery can no longer put up at least 80% of its cranking amp rating, via pulsed load or capacitance, they usually fail shortly there after.. If CA tests at 90% or better I usually suggest they could possibly go another season if they watch them closely.

My capacity tester is a home made unit that uses a Victron monitor and takes advantage of the built in relay with programmable voltage cut off. I charge the batts to full, reset the monitor then apply the 20 hour load for the batteries. When they hit 10.5V the load is disconnected via the monitors relay and an external DPST relay and the Ah's used are captured. Home made rig but works okay. I would like to come up with a more "stable" load but it is close enough and far from the 3-4k they want for "professional ones"....

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Re: Effect of Current on Amp Hour Capacity

Thanks, Maine, for this excellent post. It should help clear up some misunderstandings re: AH capacity.

There's one more factor which in many situations with multiple batteries in a single bank can make a difference: load balancing.

If you have multiple batteries in a single bank; and

If they are not connected together in such way as to balance the loads on each one; then...

Each battery's contribution to the total load being drawn will differ, sometimes by a very large amount. Obviously, if one battery in the bank is contributing significantly more or less than the others, it's Peukert factor will also differ.

The effect will be present both on discharge (under load) and charge (when charging from an external source).

Bill

Last edited by btrayfors; 03-12-2012 at 08:39 AM.
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