Estimating Total Capacity in an Old Battery

A controlled discharge test can tell you just how much life is left.

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TEMPERATURE VS. VOLTAGE

VOLTAGE VS. DRAW

USING A HYDROMETER

ESTIMATING AH CAPACITY

STATE OF CHARGE 120 F 110 F 100 F 90 F 80 F 70 F 60 F 50 F 40 F 30 F 
100 % 13.1V 13V 12.90V 12.80V 12.70V 12.60V 12.50V 12.40V 12.30V 12.20V 
90 % 13V 12.90V 12.80V 12.70V 12.60V 12.50V 12.40V 12.30V 12.20V 12.10V 
80 % 12.92V 12.82V 12.72V 12.62V 12.52V 12.42V 12.32V 12.22V 12.012V 11.02V 
70 % 12.82V 12.72V 12.62V 12.52V 12.42V 12.32V 12.22V 12.12V 11.02V 11.92V 
60 % 12.70V 12.60V 12.50V 12.40V 12.30V 12.20V 12.10V 12V 11.90V 11.80V 
50 % 12.56V 12.46V 12.36V 12.26V 12.16V 12.06V 11.96V 11.86V 11.76V 11.66V 
40 % 12.40V 12.30V 12.20V 12.10V 12V 11.90V 11.80V 11.70V 11.60V 11.50V 
30 % 12.25V 12.15V 12.05V 11.95V 11.85V 11.75V 11.65V 11.55V 11.45V 11.35V 
20 % 12.08V 11.98V 11.88V 11.78V 11.68V 11.58V 11.48V 11.38V 11.28V 11.18V 
10 % 11.81V 11.71V 11.61V 11.51V 11.41V 11.31V 11.21V 11.11V 11.01V 10.91V 
0 % 11V 10.9V 10.80V 10.70V 10.60V 10.5V 10.4V 10.30V 10.20V 10.10V 
Darrell Nicholson
Darrell Nicholson is Director of Belvoir Media Group's marine division and the editor of Practical Sailor. A lifelong thalassophile, he grew up sailing everything from El Toro dinghies to classic Morgans on Miami's Biscayne Bay. In the early 90s, he left a newspaper job to sail an old gaff-rigged ketch across the Pacific and has been writing about boats and the sea ever since. His weekly blog Inside Practical Sailor offers an inside look at current research and gear tests at Practical Sailor, while his award-winning column,"Rhumb Lines," tracks boating trends and reflects upon the sailing life. He sails a Sparkman & Stephens-designed Yankee 30 out of St. Petersburg, Florida.