Cold Cranking Amps and Amp-hours

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Bill Peacock:
How can you calculate the capacity of a battery in AH when you only know the capcity in cold cranking amps? I want to use a group 27 deep cycle battery to power my laptop during emergencies and would like to figure out how long it will last.

Bill Peacock

Michael L. Rains:
I don't know of a simple conversion for CCA to Ah, but if my memory serves me correctly, a group 27 battery has about 500-600 cca, so if you were using the battery to supply dc to the laptop directly, I would guess that the battery would supply power for at least 3-4 days (based on the fact that our laptop at the shop will run 4-6 hours on a fully charged 3Ah battery).   If you are using an inverter supply, the current drain on the battery will be a little greater, so I would guess 2-3 days useable time.

If someone else is willing to share their experience or knowledge, please do so.  My numbers are just educated guesses based on my experience running a cb transceiver on a used car battery.
Mike, k0rfi

Charles E. Harris:
Engine starting batteries are rated in cranking amps. The standard BCI CCA rating is at 0 degs. F (about -18C) whereas Marine Cranking Amps are at 32 degs. F (0 degs.C). Engine starting batteries are designed to provide a heavy surge current of as much as 200 amps for a period of 5-10 secs.  There is no direct correlation between CCA and amp-hours, because starting batteries are not designed for slow periods of deep discharge.

One high quality deep cycle battery will generally outlast four similar sized engine starting batteries in parallel, when they are used in a deep cycle application, such as powering your HF rig.

Deep cycle batteries are generally rated in amp-hours at a C/20 discharge rate, which means 1/20 of the battery capacity for 20 hours until the battery reaches 1.75 volts per cell.  A Group 27 battery rated at 100 amp-hours will maintain a 5 amp load continuously for 20 hours.  Actual capacity will vary with temperature, the size of the load and the rate of discharge.  A quality Group 27 battery rated for 100ah at the 20 hour rate may approximate 135ah at the 48 hour rate and only 80 ah at the 10 hour rate.

A faster discharge rate or a heavy load which exceeds C/10 reduces the capacity much more quickly.  In determining the battery capacity needed good engineering practice is to sum all equipment loads, times operating duty cycle in percent, times a 150 percent safety factor.

Deep cycle batteries used in UPS and telecommunication applications are rated in reserve capacity, which is the number of minutes the battery will maintain a constant 25A load at 80 degs. F until voltage drops to 1.75 volts per cell.  To provide an approximate conversion to amp-hours, multiply reserve capacity by 0.6.  A Group 30 Battery rated for 225 minutes RC approximates 135 amp-hours. Assuming a 20% SSB duty cycle, such a battery powers a 100watt HF rig for 12-hour duty shift in typical contest or net control operation including a 50% safety factor.  

If what you have is really an engine starting battery, you should give serious consideration to replacing it with a deep cycle type.

Virginia RACES has several training handouts on batteries and auxiliary power which I'd be happy to provide to anyone upon request for nonprofit, educational and public safety use.

73 de KE4SKY
Virginia RACES State Training Officer

You can't compute one from the other.

Scott Compagno:
I put together a battery page some time ago.  Please take a look and let me know if there are errors:


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