Reserve capacity minutes is the minutes a newish healthy fully charged battery can maintain a steady 25 amp load before voltage falls to 10.5v which is considered 100% discharged.
Basically how long one might be able to expect to keep driving after charging system failure.
a 100 amp hour battery can provide 5 amps for 20 hours before voltage falls to 10.5v, but due to Peukert's law cannot provide 20 amps for 5 hours
Performing an accurate 20 hour capacity test requires a constant current load, of 1/20th the battery's 20 hour amp hour rating.
Many loads will decrease with battery voltage.
the test to be accurate needs to keep the battery at 77f. The pros use a water bath. and a load which remains precise as battery voltage drops.
Charging mode: constant current load. If the temperature of the heat sink has reached 87â, the load is automatically reduces the powerï¼if the temperature continues to rise, it will continue to reduce the power, when the internal temperature reaches 91â, it will stop.
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Some, myseff included, knowing the difficulty of getting a true capacity remaining percentage, instead do a 10 hour test around the 20 hour load rate, and see what the battery voltage rebounds to. if it rebounds to teh 12.2v range I consider it still pretty healthy.
Even if this method is not going to nail down a number, it is comparable to the same test on the same battery at a later date or the same test on a new battery and can give a good indication of remaining battery capacity, and also give a good indication whether any hail mary charging/equalization attempts at capacity restoration charges were beneficial.
I'd enjoy the load tester above, but likely will not own it unless it will pay for itself.
Also I am considering a impedence tester just for quick resistance measurements and another piece of data to compare.
I consider my starter to be a load test every engine start, as I watch a voltmeter drop during engine cranking.
There's all sorts of ways to gauge battery capacity, but a standardized test so you can compare results to others, requires either one apply a steady 25 amp load fand watch a clock and a voltmeter, or apply 1/20th the amp hour rating, precisely, and hope it can do so for 20 hours before 10.5v is reached.
The standard load test is good, for a starter battery, but gauging the health of a battery that will see longer slower discharges is a bit more involved, as a battery which fails a CCA test, can still power ones overnight DC loads 'just fine' for another year or 3, or, NOT.
Apply load, watch voltage drop...recharge, repeat, compare.
