Measuring Battery Capacity

[tom52]

I know that the capacity of deep cycle batteries is usually measured as amp hours
i.e. a 100 amp hour battery when new will give one amp for 100 hours in theory.
Sometimes their capacity is given as Reserve Minutes which I understand to be how many minutes it will give 25 amps. My batteries are rated at 200 reserve minutes.
So I thought that 180 reserve minute rating equated to 75 amp hours ( 3 hours X 25 amps)
But looking at batteries where both ratings are given I see that this is not the case
e.g 105Ah or 185 reserve minutes : 75Ah or 130 reserve minutes:97Ah or 180 reserve minutes. The source of these equivalents is the excellent West Marine Catalogue.

What is the relationship and is there some formula for comparing battery capacity when they use different measures? Anyone know what my 200 reserve minutes equates to as Ahours.

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[jamesjermain]

I'm not surprised it has taken a while for someone to reply to your question, Tom. This is a very complex subject.

I can't give you a technical explanation, maybe someone else will. The problems arise because both means of measuring battery capacity use different criteria depending on the type and use of the battery. Ah ratings define the number of amp-hours that can be pulled from a battery, but the answer differs depending on how fast these amps are drawn. British batteries are usually rated at 10 hours to discharge (10.5 volts) which is called the C10 rate. US batteries are rated at 20 hours (C20). Deep cycle (traction) batteries are often rated at C5. The faster a battery is discharged the fewer Ah is can deliver.
Reserve minutes is the way car batteries have been rated and this is becoming increasingly common in marine batteries. It rates the number of minutes a battery will last (before reaching 10.5 Volts) at a given rate of discharge. The normal load for this rating is apparently 25amps. But when comparing batteries is is important to check this figure.

Frankly I don't know how you compare an Ah rating and a Reserve Capacity rating so not much help I'm afraid.

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[pvb]

Peukert...

Short answer is that there isn’t a simple way of equating reserve minutes ratings to Ah ratings.

About a hundred years ago, a scientist called Peukert showed that if you discharge a battery more quickly, it’s effective capacity temporarily decreases. Say you have a battery rated as having 100Ah capacity at the 20 hour rating (C20). If it’s fully charged, it should theoretically deliver 5A for 20 hours, before the voltage drops below 10.5v. However, if you were to draw 10A from it, it wouldn’t last for 10 hours, but only for around 8.5 hours - so its capacity would have dropped to 85Ah. And if you were to draw 50A from it, it would be down to 10.5v in about 1 hour - so its capacity would then have dropped to only about 50Ah. It’s the same battery, but its effective capacity changes according to how much current is drawn from it.

From this, it’s easy to see why a 100Ah battery can only supply 25A for maybe 3 hours, and hence the difference between Ah ratings and Reserve Minutes ratings.

Peukert developed a mathematical formula to define this phenomenon, and it includes a factor known as “Peukert’s Exponent”. The exponent varies according to battery construction.

The better-quality battery monitors take Peukert’s Exponent into account when calculating the amp-hours used. Probably the best value battery monitor is the Link 10, and this has sophisticated and powerful software to use Peukert’s Exponent to give highly accurate results. However, if you don’t need that analysis on your boat, one of the most cost-effective electrical improvements you can make is to add one or more additional batteries to your domestic bank.


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[tom52]

Thanks chaps. I may not have the answer to my question but at least I have a better understanding and have confirmed the depths of my own ignorance !

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[Talbot]

<<have confirmed the depths of my own ignorance >>

You are lucky, I have not got half way with plumbing the depths of mine (SWMBO reckons its a bottomless pit) /forums/images/icons/crazy.gif

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[Evadne]

Everything you wanted to know ...

... about batteries but wer afraid to ask. I did a search on Peukert's coefficient and found <A target="_blank" HREF=http://www.boatelectric.com/ElectricityonBoard20030212.pdf> this</A> little handbook that someone has published. I've only skimmed it so far but it seems to have the basics right, and goes into some subjects quite deeply from a yachtsman's point of view.

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[tom52]

Re: Everything you wanted to know ...

Dave
Thanks for the link. It looks useful. I think you should post a new thread drawing it to everyones attention.
Thanks again.

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[discoduck]

Hi PBO published a couple of months ago on testing battery capacity. Connect a resitive load ie a 55 watt car bulb to a fully charged battery. Measure the time it takes to reduce the battery voltage to say 11 volts.
Now some sums.Divide 55 by 12 to obtain current. Then multply this by the time it takes to discharge to 11 volts. the gives your Ah. If it below about 70 % get a new battery.

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