Trawler Blogs

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.

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.

We installed our Lithium Ferro Phosphate (LFP) battery bank in August 2021 (Out with the Old, in with the New).  It consists of 10 Battleborn GC2 12V-100 Ah batteries arranged as two serial banks of five paralleled batteries giving us 500 Ah at 24V.  We also installed a Victron Cerbo to help control and monitor our Victron equipment (chargers and battery monitor).  I subsequently installed a Raspberry Pi single board computer running Signal K to record and display data being generated on board (Boat Data).

I’ve now been collecting the data for three years and I haven’t been doing much more than displaying real time data while on board.  As a winter project, I’ve tried to organize and analyze it more carefully.

While at the dock and connected to shorepower, the batteries are not doing very much. The AC needs of the boat (e.g., water heater, toaster oven) are supplied directly from the shore (mediated through the isolation transformer and inverter/charger). The DC needs (e,g, refrigerator, lights) are handled by batteries working with the DC charger half of the inverter/charger.  While actively cruising and underway, the alternator driven by the propulsion engine provides the current for DC loads while the AC loads are taken care of by inverter part of the inverter/charger powered through the alternator.

It is only while at anchor (or occasionally at a dock), with no shore power connection, that the batteries must do work and discharge some of their stored power.  In the three cruising seasons for which I have data (2023, 2024 and 2025), I identified 247 depletions of significance.  I ignored the small discharges that occur when transitioning from shore power to engine power when leaving the dock, and the reverse situation, engine power to shore power, when arriving at a dock. I also ignored short duration discharges associated with events where the engine is off and you aren’t on shore power (e.g., visiting a fuel dock, waiting in a temporary anchorage for currents to subside).

The first chart shows the distribution by duration of the 247 discharge cycles.  Our cruising style is one of motion and we only spend multiple nights at anchor in one spot a dozen or so times a year.  When we do, we run our generator daily to recharge the battery banks. A lot of the 20+ hour discharge cycles are probably associated with multiple nights at one site. Being a slow boat, cruising around 6-1/2 to 7 knots, we tend to put in long days to cover the same distance that faster boats do. The short duration discharge cycles often represent a 7 PM arrival at a destination followed by a 5 AM departure the next morning.

The next chart shows depth of discharge (as measured in amp-hours, Ah) distribution for those same 247 discharge cycles. The same comments as above about deeper discharges being associated with multiple nights at anchor and smaller discharges representing longer days underway apply to the distribution.

The last chart shows the power consumed by hour of the day. There are certain items on board that once they are turned on are rarely turned off.  The big examples are refrigerators and freezers (we have two of each) and all our monitoring equipment (e.g., the NMEA2000 bus). 

The data show that we consume 8-9 amps as a baseline. Because we are primarily operating on DC, while at anchor, we tend to turn on the AC inverter part of the Inverter/Charger only when we need it (e.g., using the Starlink antenna). Turning on the inverter tends to increase our usage by an additional 4 amps (we rarely use the inverter to run a large AC load like a toaster oven or electric kettle).

The last major DC load is the Kabola furnace.  On cold mornings, we often heat the boat up with the Kabola.  It will easily use 8 amps as the pumps and blowers kick on but after things warm up, the load usually drops to around 4 amps. There is a temperature dependence to our usage.  Cold weather will result in longer and more frequent operation of the Kabola while hot weather causes higher duty cycles in our refrigerators and freezers. The highest usage is in the early evening when we would have AC power on in order to watch streaming TV via the Starlink antenna and set the Kabola thermostat up to keep the boat comfortable.

When comparing our amp or amp-hour numbers, remember that we are operating at 24v DC.  The equivalent numbers for a 12v DC system would be double.