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Effect of Current on Amp Hour Capacity

3K views 18 replies 8 participants last post by  hellosailor 
#1 · (Edited)
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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#2 ·
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..
 
#5 ·
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?

Monty Python?
 
#6 ·
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.
 
#7 ·
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..

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...

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...
 
#10 · (Edited)
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
 
#11 ·
Intrabank imbalances can definitely be a problem and MANY banks are connected to the boat incorrectly. Connecting the bank so loads are drawn "through" it, rather than "off" one end can make a measurable difference which can be seen & measured on a good analyzer..

It's why I post these often...

Creates a much better balanced bank


Creates an imbalanced bank..
 
#12 · (Edited)
Maine-
"Battery monitors do not and can not account for self discharge. "
Oh, come on. If you can stick a number on it, the itsy-bitsy computer can compute it. If it was a simple "the self-discharge rate is %% per day" then the battery monitor could be set up to subtract that amount from the charge state every day. (Or a bit of it hourly, because Daylight Slavings Time gives us one 23-hour day and one 25-hour day.)

I think it is simply a matter that self-discharge rates are not readily available, and the overall accuracy is not expected to be that high, so the monitor makers ignore it.

On battery condition and load testing, I was under the impression that some of the pricey new testers that the distributors are using actually measure the internal resistance of the battery, something you and I aren't going to do with a handy multimeter. A simple brute force load tester is the kind of thing it used to be easy to DIY, with a voltmeter and some "loads" scavenged from a broken toaster or other heating elements. Not a problem until you start dealing with larger batterie, larger loads, and larger fires when things get too hot. (Ooops.)
 
#13 ·
Maine-
"Battery monitors do not and can not account for self discharge. "
Oh, come on. If you can stick a number on it, the itsy-bitsy computer can compute it. If it was a simple "the self-discharge rate is %% per day" then the battery monitor could be set up to subtract that amount from the charge state every day. (Or a bit of it hourly, because Daylight Slavings Time gives us one 23-hour day and one 25-hour day.)
Without a temp sensor it would be difficult. Batts self discharge at a dramatically slower rate in cold temps and accelerate in hot temps..

On battery condition and load testing, I was under the impression that some of the pricey new testers that the distributors are using actually measure the internal resistance of the battery, something you and I aren't going to do with a handy multimeter.
Yes and I own one and they are NOT cheap... You can not do these tests with DVM's...

A simple brute force load tester is the kind of thing it used to be easy to DIY, with a voltmeter and some "loads" scavenged from a broken toaster or other heating elements. Not a problem until you start dealing with larger batterie, larger loads, and larger fires when things get too hot. (Ooops.)
Even my 500A carbon pile is too small for some batts to do a proper load test... This is where conductance or pulsed load analyzers come into play...
 
#14 ·
"Without a temp sensor it would be difficult."
You know Raymarine is putting everything on WiFi and Bluetooth. So...Yo oculd put a single BT temperature sensor on EACH battery, net cost $5 landed in the US, retail $50, and pass that information on to the chargers, the monitor, the low battery alarm...frightening simple and feasible these days.
Or they could just use a built-in temp sensor. "Cabin" temp would beat nothing, set the program a little higher or lower to compensate.

"Yes and I own one and they are NOT cheap"
Oddly enough, I was looking into that and Schlumburger sells one for $80 for "car" size batteries, wet lead only on the one I saw. Not cheap but nowhere near the $900 models either. I wonder what happens inside those boxes.

"Even my 500A carbon pile is too small for some batts" Never s surplus antiaircraft carbon arc rod around when you need one surplus, is there? :)
 
#15 ·
Internal battery resistance can be measured...

From: How to calculate internal resistance of a 12V/ 12 Ah battery? - Yahoo! Answers

What you need to do is measure and record the open circuit voltage of the battery, Then you need to load it with a resistor. Use one that draws a good bit of current (50 ohm, 100 ohms, etc) and measure and record the voltage again. The difference between the two readings is the number we need for the math below. (You might have to do this second measurement fast so as not to burn up your resistor unless you have a big power resistor. Remember that P=V^2/R which for 50 ohms would be 12^2/50 or almost 3 watts; so be fast and careful if you use a resistor with a smaller power rating).

OK now the math. The internal resistance of the battery actually froms a resistive voltage diveder with the external resistor you load it with. This is what causes the voltage drop. The standard equation for this voltage drop is:

Vdrop = Vbatt*(Rbatt/(RL+Rbatt))

Where:
Vdrop is the difference in your two readings above
Vbatt is the open circuit voltage measured above
RL is the resistor you load the battery with
Rbatt is the battery internal resistance you're looking for

If you do a bit of algebra and solve the equation above for Rbatt, you get the following equation:

Rbatt = Vdrop*(RL/(Vdrop+Vbatt))

Just plug the numbers you measured above into this equation and you'll have your answer.

Here's an Example:

Vbatt = 12V
When loaded with 500 ohms the battery voltage drops to 11.9 V ... or Vdrop = 0.1V

Plug the numbers in:

Rbatt = 0.1*(500/(0.1+12))

Rbatt = 4.1 ohms

Hope this helps,
Dave

P.S. There is no way to calculate it without taking a measurement. Knowing the battery voltage and capacity is not enough.
Source(s):
No reference. I'm an associate faculty in the EE department at UWM. This is basic circuit theory.
Regards,
Brad
 
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