Battery load tester?

MrWizard wrote:
#2..no, impedance is battery impedance, not system load resistance

I was thinking of this approach:



V is just the no load potential on the battery. I see the method is very sensitive to the accuracy of the load resistor; probably too difficult to determine as it heats up.

I had the "left walking" experience with this old vehicle last winter and just assumed it was a battery problem - it turned out to be a corroded connection between the battery cables and their battery ends. Now a friend has given me a failed one year old battery from his motorhome, which was sitting with 11.5 volts on it. I equalized and reconditioned (desulphated) it and am using it in the old van but wondering if it is good enough for winter.