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Go Back   SailNet Community > General Interest Forums > Gear & Maintenance > Electrical Systems
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Old 04-09-2010
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Battery Confusion - Amp Hours vs. Reserve Capacity

A number of times on this and other forums, but twice just in the last two days, I have come across situations where a battery owner is confusing reserve capacity (RC) with amp hours (Ah) and wanted to quickly address this.

Battery life/longevity is a constant topic of frustration for sailors. What makes this even more frustrating is the confusion between Reserve Capacity (RC) and Amp Hours (Ah). There is much more to batteries but I do not want to drag that out here.

RC and Ah are NOT one in the same as I often see people use them interchangeably. Usually, most batteries that do not have an amp hour rating are also not usually a purpose designed deep cycle battery. Some still are, but have just not been Ah tested.

A safe bet is to always by a battery with a Ah capacity rating. This ensures that the battery has been tested and rated for such. You still can use the RC/Reserve Capacity number but this number should generally be divided by two to arrive at a close Ah capacity.

For example the Trojan AGM 4D battery is rated at 325 RC and 165 Ah. If you divide by 2 you would get 162.5 Ah's. This is usually a close approximation to the Ah capacity of a given battery.

RC is generally almost DOUBLE what the usual Ah rating is. On another forum, a rather astute and knowledgeable owner stated that their battery had XXX Ah's. The reality is that this battery was never rated in Ah capacity and only in RC so this bank was nearly HALF the size it was thought to be, which could prove to be an expensive miscalculation.

Mistaking RC for Ah can be a dangerous and costly mistake. You could kill your batts, and be left dead in the water. This is especially true if after removing 150 AH from a 300 RC bank, thinking you are only at half discharge, when you are actually closer to flat dead.

There are a few theories on how to determine Ah capacity from RC but none that I have found spot on. Some say to divide RC by 2 then to add 16 to that number while others just say to divide by 2.

Personally I prefer to buy batteries rated for Ah capacity rather than to attempt a guess, as any guess could throw off a battery monitor or energy budget or could leave you depleting your bank beyond 50% DOD which can considerably shorten its life..

If you buy a specific group size battery eg: group 24, 27, 29/31, 4D, 8D etc. and the one you're buying has a significantly higher "rating" than others of the same physical size that should be a red flag that you are looking at an RC measurement rather than an Ah capacity measurement..

Just a head up.
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Last edited by Maine Sail; 04-09-2010 at 07:58 PM.
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Old 04-09-2010
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Old 04-19-2010
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So if I have a 79Ah house battery and my GPS, VHF, instruments and running lights all draw 3 amps together, in 26 hours its completely dead, or 13 hours to half dead. If I understand correctly, I don't want to go below half, so 13 hours is my limit.

Correct?

What is the purpose of the "reserve capacity" rating?
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Old 04-19-2010
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Reserve capacity is the number of minutes a battery can maintain a useful voltage under a 25 ampere discharge. This link explains this and more.
Frequently Asked Questions (FAQ) - DC Battery Specialists
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Old 04-05-2011
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They may not be the same number but they do measure similar things. Reserve Capacity is usually the number of minutes a battery can deliver at a '25-amp rate' ... so multiply the number of minutes by 25A (and divide by 60) and you have a measure of Ah (@25A).

Amp Hours is usually the number of Ah a battery can deliver at the '20-hour rate'. The key difference between the two is (other than the obvious of having different units) the discharge rate that the capacity is measured at. A number is meaningless without the associated unit.

An RC of 325 min:
325min * (1 hr/60min) * 25amps = 135Ah (@25A)

An Ah rating of 165Ah:
165Ah = Xamps * 20hrs => 8.25A

So, your Trojan has an Ah rating of 165Ah (@8.25A discharge rate) and one of 135Ah (@25A discharge rate). They aren't mystical numbers that require some 'theory' to relate ... it's just unit conversions and a little understanding.

You are further feeding the misunderstanding by not carrying appropriate units with RC and Ah ratings. 'removing 150 AH from a 300 RC bank'? There is no such unit as an 'RC'. If you would call it 300 min Reserve Capacity then it should be obvious to all but the most daft that you can't compare minutes to amp-hrs (like that apples to oranges thing...). Having someone design an electrical system who doesn't understand units is probably the root cause of this 'dangerous and costly mistake'.

It's science. The details matter.
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Last edited by scraph; 04-05-2011 at 11:51 AM.
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Old 04-05-2011
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Quote:
Originally Posted by scraph View Post
So, your Trojan has an Ah rating of 165Ah (@8.25A discharge rate) and one of 135Ah (@25A discharge rate). They aren't mystical numbers that require some 'theory' to relate ... it's just unit conversions.
But when you leave out our good friend Peukert you don't get the same ratings. Most all battery monitors are programmed off a 20 hour Ah capacity rate not an RC rate. I have seen customers program in the RC number as Ah's and kill their banks in no time.

Still, even extracting the Ah's produced with an RC rating you get 135Ah's vs. 165 Ah's due to Peukert effect.

The 20 hour rate is usually a lot closer to real world consumption per hour than 25A is and with a 25A load you get 30 less Ah's out of the battery than with an 8.25A load so trying to program a monitor off that will confound the calculations on most monitors unless they offer an RC programing menu. I have yet to find any customer who calculated the Ah's from RC. They usually just plug in the RC minutes as Ah's.

My point in posting this was to prevent folks from confusing Ah's at the 20 hour rate and reserve capacity ratings as I have seen done many times..

Take the DEKA / WM DeepCycle 105. It has 105 AMP HOURS at the 20 hour rate of 5.25 amps 80f to 10.5 volts and 185 MINUTES at the RC load of 25 amps at 80f to 10.5 volts. Plugging in 185 as your Ah capacity will kill your banks in no time as have witnessed quite often.

I had a customer with two DEKA 105's and 370 Ah's plugged into his battery monitor. He was regularly drawing the bank down to about 20% SOC and could not figure out why his bank was lasting just two years and he had trouble starting his motor on occasion. When I explained that he only had 210 Ah's vs. 370 Ah's he was rather surprised..
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Quote:
Originally Posted by jarcher View Post
So if I have a 79Ah house battery and my GPS, VHF, instruments and running lights all draw 3 amps together, in 26 hours its completely dead, or 13 hours to half dead. If I understand correctly, I don't want to go below half, so 13 hours is my limit.

Correct?

What is the purpose of the "reserve capacity" rating?
You're correct above.

The purpose of "reserve capacity" is essentially for automobile applications. It is a measure of how many minutes a car battery should be able to maintain above 10.5V if the alternator isn't charging. It's how long you have to get to the mechanic for a new alternator, basically. It is a capacity measurement same as the Ah rating, just shown differently and measured at a different discharge rate (batteries have 'lower' capacity when you discharge them faster).
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Originally Posted by scraph View Post
You are further feeding the misunderstanding by not carrying appropriate units with RC and Ah ratings. 'removing 150 AH from a 300 RC bank'? There is no such unit as an 'RC'. If you would call it 300 min Reserve Capacity then it should be obvious to all but the most daft that you can't compare minutes to amp-hrs (like that apples to oranges thing...). Having someone design an electrical system who doesn't understand units is probably the root cause of this 'dangerous and costly mistake'.

It's science. The details matter.
Like your 0.225A draw on your Fusion or your 10W bulb reading closer to a 5 watt bulb.. You got all the details worked out and all the answers, we know.. Thanks for educating us...
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Quote:
Originally Posted by Maine Sail View Post
But when you leave out our good friend Peukert you don't get the same ratings.
I'm still curious who these genius theorists are who claim 'you can just divide it by 2 ... or maybe add 16 also' ... what on earth?!

I heard this theory that you can divide inches by 12 to get feet ... some people think you should also add 0, though.

No theory necessary, Peukert's is a law. In fact, Peukert's law is an equation that lets you directly convert between RC ratings and Ah ratings. The Peukert Constant for your Trojan is approximately 1.178. See if you can manage anything with that.

I'll take replacing a meter any day. It's much better than knowing just enough to get into a discussion but not enough to get back out (without, of course, getting personal). (and that was irony at the end)

My Fusion draws 0.453A and the 10W consumes 13W ... I just wish it would draw exactly 10W so I'd know as much as you.
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Originally Posted by scraph View Post
I'm still curious who these genius theorists are who claim 'you can just divide it by 2 ... or maybe add 16 also' ... what on earth?!
Perhaps you can email them for clarification...

Battery University / Cadex Electronics

" The RC to Ah conversion formula is as follows: RC divided by 2 plus 16."
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