Ask PYS: Power

Battery Charging Sweet Spot and Maximum Depth of Discharge– David, on Dec 21, 2014 asked:

Working through the booklet that came with the battery monitor, how low as a % should I or can I go before needing to charge up the house; starter batteries? We left H Bay at approximately 85% and it climbed up to 93% during our crossing. Obviously the alternator is doing its work nicely. Then the monitor went to 100% after we were connected to shore power. What % would you think I should use as my low point? Before I worry about needing a charge (go for a cruise or plug in to shore or use the genset).

With flooded lead acid batteries your maximum depth of discharge should be 50%. Also, remember that your batteries are going to go in absorption charge mode around 80-85%. This means, pass 80-85%, it won't be as efficient (not Bulk charging anymore) to charge them via the charger with the generator. We call the 50% and 85% values, your battery charging sweet spot. While away from the dock and under generator power alone, you will want to oscillate between 50% to 85% for maximum charge efficiency.