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Sunday, June 21, 2015

Battery Ratings

Dear RV Doc, your article on batteries was informative but gave information that is in conflict with information in two other articles I've read. You say the Amp-Hour (AH) rating is a 20-hour rating that indicates the number of amps a battery can deliver for 20 hours. From this one would expect a 100 AH battery to deliver 5 amps for 20 hours. The other articles I've read stated that the AH rating is based on a 20-amp drain. That at 80 degrees F., a 100 AH battery can deliver 20 amps for five hours before the voltage (12-volt battery) drops to 10.5 volts. The results may seem to be the same but one article went on to say that the length of time is not a direct inverse proportion to the amp drain. In other words, a 100 AH battery cannot be expected to last 10 hours at a 10-amp drain or 20 hours at a five-amp drain. What is correct? Keith L., (Swansea, IL)
Keith, I think it may be a matter of semantics mostly, but the 20-hour mention is the common time frame usually referenced for rating batteries, according to the Battery Council. Peukert’s Law notwithstanding! The 20-hour time frame is simply the usual benchmark. 

Here's how one major battery manufacturer states it:

"The amp-hr rating for an automotive battery is usually based on a 20hr discharge at 80° F, at a particular rate (determined by the amount of active material within the battery), until a minimum voltage of 1.75V/cell (10.5 volts for a 12 volt battery) is reached. For example, if a 12-volt battery at 80° F delivers 4 amps for 20 hours before its voltage drops below 10.5 volts, its amp-hr capacity will be 4 amperes x 20 hours = 80 ampere-hours.

Scientific measurements/data will be different. Obviously, to be 100% correct, the Peukert formula would have to be factored in. But to explain that would simply further confuse most readers not holding a degree in engineering.



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