The “yang” to the discharge cycle’s “yin”, is the charge cycle. We have three ways to restore the batteries after they have been depleted.
Shore Power – We have a Victron Multiplus 24 3000/70 inverter/charger that when provided AC power can recharge the battery. While our boat is wired to accept 240V-50A, for simplicity, we only connect to shore power via 120V-30A
Alternator – We have two alternators on our Lugger 1066T diesel engine, one for the start battery and the other for the house bank battery. The house bank alternator, a Leece-Neville 4740JB (24V-200A), is regulated by a Balmar MC-624.
Generator – We have a Northern Lights OM773LW2 – 9KW generator. This provides AC power to the Multiplus Inverter/Charger but, in addition, we have a Victron Skylla-I 24/100 charger attached to the output of the generator.
The “State of Charge” (aka SoC) is often the metric used to determine the status of the battery. It runs from 1 (or 100%), when the battery is “full” to 0 (or 0%), when there is no power available. For our battery bank, each 1% change in the SoC is about 5 Ah. With a 24V battery bank, that 5Ah of energy would power the base load of our boat (8-9 A) for about 35 minutes.
For this analysis, I wanted to see the charging characteristics all the way until battery full and so I eliminated cycles that didn’t go to at least 99% SoC. I wasn’t quite as concerned about the starting level of the discharge, though. I ended up with 219 charge cycles that went to completion, 7 for shore power, 27 for generator and 185 for engine. The small number of shore power charging is not surprising since upon arriving at a dock, in most circumstances, would already be full.
The chart below shows the aggregate charging profile for each method of charging. While the monitoring system collects data continuously, it only preserves the data for the long term once every six minutes (one-tenth of an hour), taking an average of the values since that last time increment. To allow the comparison of charge cycles with different durations and different battery depletions, I used the charge cycle’s end point as a reference. At any given time point, I averaged SoC for those charge cycles that extended to it.
A characteristic of LFP batteries is that they accept high levels of charging current until nearly complete. My observation is that the charging is nearly constant, limited by what the charging source can provide, until about 98% SoC. At that point, the current acceptance rate decreases rapidly. At 99% SoC, the battery monitor, a Victron BMV-712 in our case, may say it is “close enough” and report a 100% SoC.
On the chart, I’ve had Excel compute the linear regression line for each category with the Y-intercept being set to a 100% SoC. The X- coefficient in the equation represents the slope of the charging curve. An average hourly charge rate can be computed from the coefficient by multiplying it first by 500 (the number of Ah in our full battery bank) and then again by 60 (the number of minutes in an hour). For shore power that calculation suggests a 48 amps per hour charge rate. For the engine it is 96 amps per hour and for the generator 129 amps per hour. These charge rates are lower than the actual output of charging source because they are net of the charging source’s output and whatever DC loads the boat’s system requires.
