Today I think I found the lower limit on battery voltage for my 150 E-Tec to start! Boat has been sitting for about a month since last run. Voltage on my old No. 1 battery was 12.07 volts, and it didn't light off on first revolution like it normally does! (Took 2 revolutions!
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A typical battery capacity chart says 12.1V = about 25% of capacity, so looks like that's about the lower limit for a successful start. Battery is a group 27, at least 5 years old; don't remember CCA rating, but it's at least 600, more likely 700-800. Might be able to go a little lower on starting voltage, but probably not very much. This battery is definitely ready for the recycle bin! Will replace it as soon as I finish bottom painting project. Denny