Battery Management Tips

One of the fundamental things when spending time on cruising yachts is battery management and there are some basics which will help you to better understand the day to day running on a yacht.
The first principal is to understand just what capacity you actually have available to you depending on the size of your batteries, their type and how they are charged. As an example, if you have the normal sealed lead-acid batteries and have for example 3 x 80A batteries in the house bank and your yacht is kept on a mooring, then the batteries will normally be charged by your standard engine alternator.
Charging this way becomes inefficient above 90% typically, and your batteries are depleted with the voltage heading down towards 11.6 volts at which point most 12v fridges will stop to function with the batteries heading down under 60%. So, in this scenario the useable range will be from 60% to 90% of your total capacity of 240 Amp hours which is therefore 72Ah.
If you have a yacht with halogen lights, then through an evening you can expect to use 10Ah, and the fridge will use an average of 2.5Ah x 24 hours a day (60Ah), so assuming that you use nothing else you’ve just depleted your capacity in 24 hours and will need to re-charge quickly. There are higher performing batteries such as glass matt batteries (AGM) which take charge at a higher rate, so can be charged more quickly, but you are still limited to the same capacity.
This same principal of the useable range (being 30% in real terms of your total capacity) is a very good rule of thumb. If your yacht is kept on a marina and has shore-power, then that slower rate of charge by a battery-charger will probably take your batteries close to 100%, so thereby increase your range to close to 40% of your capacity. But having the boat on a marina with shore-power will also probably mean that you have the fridge on and therefore it will already been down to temperate before you leave and therefore use less power initially.
Yachts like Jeanneau are supplied with LED internal lights, LED navigation lights, and the instrumentation is back-lit with LED lights, so these only use a fraction (approx. 1/20th) of halogen lights. A common boost to your system on board that has become extremely popular in the last two years is high capacity flexible solar panels. One 140A flexible panel can easily be fitted to your existing bimini, as they are available with zips, and this one unit alone with run your fridge 24 hours a day.
Another development from Jeanneau for this coming model year is that every model across the range now comes standard with a boosted 120A alternator, meaning less charging time under engine to bring your batteries back up to charge, and in general the battery capacities have also been significantly increased in recent years. A battery monitor that shows you what percentage your batteries are at is a very useful tool in helping you to manage your charging and these are widely available and easily fitted (see photo).
So, with the combination of increased alternator, larger battery capacity, lower power consumption from LED lighting and availability of easily mounted flexible solar panels, it’s possible to enjoy extended time on board without the constant worry of not running your batteries flat.