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.