On researching about car batteries, I found that the Battery Council
International and the battery manufacturers publish application guides that
contain the CCA (cold cranking amperage) and the RC (Reserve Capacity)
specifications.
Cold Cranking Amperage: In warm weather, starting an engine will typically
consume less than 5% of a car battery's capacity.
Reserve Capacity: It's what's available for the car and accessories after
the engine is started. It's the number of minutes a fully charged battery at
80 degrees F (26.7 degrees C) can be discharged at 25 amps until the voltage
falls below 10.5 volts.
QUESTION: Why 10.5 volts? What's magical about that number? Will the MG not
run below 10.5 volts?
REASON for question? I have added equipment that exceed the generator's
capability at night. I'm concerned that my amperage usage during winter at
night with the headlights on and the heater fan blower on High (plus
Radio/CD, etc.) is greater than the generator can match. I'm discharging
(exceeding the generator) approximately 5 amps but I don't think I have a
problem. If my calculations are right, I can drive under those conditions
for 7.5 hours before the voltage would get to the 10.5 volt level, more than
enough to get across Florida from weekend car shows.
CALCULATIONS: I'm assuming my Sear's Diehard group 34/78 battery has a RC of
90 minutes (cannot find the actual specs on the Sears Diehard, only others
of the same group which range from 1.5 hrs to 2 hrs).
If I assume it takes 90 minutes to discharge my battery at 25 amps until the
voltage falls below 10.5 volts, then if I am only discharging at 5 amps, I
should get (25/5 x 90) or 450 minutes or 7.5 hours. Does this look right?
Again, what's magical about the 10.5 volt level?
Stu Keen
1951 MGTD MkII
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