What are the factors that go in to deciding how big a battery charger to install on a sailboat?
If you have a generator on board you should probably have the biggest charger the batteries can support. This would make the best use of your generator and charge your batteries in the least possible time. Generators like to be run at close to full output so this arrangement will give the maximum life as well as the greatest efficiency.
So what is the biggest charger the batteries can support? Well the standard calculation is to take the amp hour capacity of the battery bank and divide by four, and that gives you the maximum charging rate in Amps that the batteries can support. Its only a guideline, there are no hard and fast rules and different battery types will behave differently. As an example, if you had a 400 amp hour battery bank then you could support a 100 Amp charging rate.
This picture shows the display panel of a Blue Sea Systems battery charger
If you don't have a generator on board then there is another way to look at this problem. What rate of charge will get your batteries up to full charge overnight. For this calculation I picked 8 hours as being overnight. So in this example a 400 Amp hour battery bank that was depleted by 50% would need 200 amp hours so in 8 hours that would be 25 amps. In reality the charge rate is not constant and so the last part of the charge will go slower, but in reality overnight is more like 12 hours than 8, so that is why I chose that time for my example.
There is no hard and fast rule, if you have a smaller charger it will take longer, but they will all get there in the end. Available space and cost are of course other factors to be taken in to account. And finally you have to consider the load you are putting on the batteries while you are charging. In the example above if you had lights and refrigeration on all night that consumed an average of 10 amps, then you would have to increase the size of the charger by 10 amps to compensate.