A Practical Approach to Batteries On Board

How Our Batteries and Chargers are Sized on Celebrate!

Posted: January 28, 2017


This article is in response to Facebook questions about some of the decisions I made in revamping our batteries prior to our Northwest passage.

I assume you understand about multistage chargers and the general calculation of boat power loads, etc.

I look at battery bank/charging system design from a different viewpoint than many boaters. This is partly due to the specific situation on our boat and partly due to my personal bias as an Electrical Engineer that I think many explanations are more complicated than necessary. Although my approach is different, I am in agreement with most other explanations.

Our new batteries on Celebrate are these AGMs in J125 cases. We could squeeze more capacity in less space with the stand-up style!


Let’s assume for the moment that we only charge our batteries in “bulk” mode and that the discharge load is small relative to battery capacity. The first assumption simplifies the math (and we can remove the assumption later), and the second eliminates Peukart effects and we can assume that the batteries are always 95% efficient.

We have a 100A 24V Mastervolt charging system and the boat normally draws ~10A of 24V load. If we run the charger for 1 hour (in bulk), we’ll get 9.5 hours of boat operation—independent of the bank capacity. So I have an operation/charge ratio of 9.5:1.

Battery capacity doesn’t change the ratio, only how long we can go between charges—and who wants to be obliged to run the charger every few hours?

Let’s say that the useful battery capacity is 30% of the rated capacity (bulk charging) so my ~1000Ah bank has a 300Ah capacity. Now I can run the boat for 30 hours and charges take 3.2 hours.

If I had lithium batteries, the above bulk charge assumptions would be almost true as lithiums can accept full charging current during virtually the entire charge cycle.

Should I choose to charge the batteries longer, I can increase the effective capacity but reduce the operation/charge ratio. Perhaps I can run the boat for 40 hours and recharge in 5 hours with a hypothetical ratio of 8:1. Here battery capacity comes into play because the amount of bulk charging time is limited by the battery capacity. A larger bank will accept higher charge currents for a longer time before the maximum voltage is reached and charging current begins to reduce.

Couldn't we improve the ratio with a bigger charger? Unfortunately increasing the charger capacity doesn’t improve the ratio as much as we’d like. If I put in an additional 100A charger so I can charge at 200A, the bulk charging time will be reduced as it is limited by the amount of time that the bank can take 200A. So instead of reducing the charge time from 5 hours to 2.5 hours, it might only be reduced to 3.75 hours to reach the same state of charge. Note that the charge current should not exceed Capicity/4 (for AGM) so I should never try to put in more than 250A.

Practical Considerations

Now for the practical considerations aboard our boat. At anchor, the solar panels can keep ahead of the load so we don’t consider it. For an ocean passage, we’d want to run the watermaker every day to top off the tanks so if the watermaker fails, we have a full load of water. The watermaker runs on AC so I run the genset every day for water and this is usually enough to charge the batteries as well. The genset is more fuel-efficient than the propulsion engine. Because the watermaker and charger together only need about 5,000 watts, there’s no need to fire up the 160hp Yanmar to supply less than 10hp for electric needs.

When we use the engine, it will typically run for many hours at a stretch (or days) so we don’t need a big alternator. We have an 80A 24V alternator which only puts out about 50A once it warms up. The 12V alternator is larger but the genset-based charger is only 25A. I also have a panel switch so I can charge the 12V bank from the 24V bank by running the 12V charger on the inverter. But I only use this in special situations such as noticing that the 12V is low at 2am and not wanting to break the silence of a beautiful sail by starting the genset.

The house batteries are 24V so the wire-size for the windlass and electric winches, etc., can be smaller. The boat was built with a separate 12V battery/charger/panel for instruments and radios and it’s not a bad idea to isolate the electronics from the industrial loads. I put an oscilloscope on the 24V batteries and could see the voltage drop several volts when big motors are starting up. On the other hand, the autopilot runs on 24V and its electronics have never been bothered by the electrical noise on the 24V circuits.

When I revamped the batteries I decided not to put in a 24V-12V converter as this allows my to run the 24V batteries harder without concern about losing the electronics. Adding the converter would have increased the size of the job and somewhat reduced the viability of the system. If the system were being put in a new boat, I can’t say whether the separate 12V batter bank is worthwhile.