Fitting a new battery
Need more electrical power? Duncan Kent explains the pros and cons of adding an extra battery to your service bank
Duncan Kent on what to think about before adding a battery
With more and more electrical gear going on board the modern cruising yacht there comes a time when the battery bank needs expanding to cope with the rising energy demands.
It’s still quite common for new boats to come with a small engine start battery and an equally minimal capacity service battery – the sort of thing that will only just run a small fridge for 24 hours before it will need recharging. Add to this the occasional use of an electric anchor windlass, lighting, navigation instruments and an autopilot and you’ll be needing to run the engine every six hours or so.
Increasing the capacity of your battery bank will allow you to go longer between charges, or to dig deeper into your reserves if necessary, but there is more to consider than just the cost of an extra battery: it’s vital to consider the method of charging and whether you need to upgrade your shore power charger, alternator or alternative power generators.
Understanding capacity
An appropriate time to consider adding another battery is when you’re about to replace the existing one. That way you’ll be starting afresh with all new batteries, which is always the ideal – an older battery can otherwise drag down a new one as it reaches the end of its life.
Also, when installing a two-battery (or more) domestic bank it makes sense to buy batteries of the same capacity. The Ah rating most commonly indicated on leisure or deep-cycle batteries is called its C20 rating and refers to its theoretical capacity when discharged over a 20-hour period.
Engine start batteries have thinner plates for coping with brief high-current surges and are more commonly rated using their Cold Cranking Amps ability (CCA). These aren’t suitable for use in a service bank as they rapidly die if
frequently deeply discharged.
The best batteries for domestic use will be labelled ‘deep-cycle’, which means they’ll have thick plates designed to deliver their energy slowly and repeatedly.
Adding an extra battery
In a 12V system adding an extra battery is simply a case of mounting it as close as possible to the existing batteries and then connecting in parallel, connecting ‘alike’ terminals (positive to positive, negative to negative) using large diameter cable (usually 70mm² diameter) and properly crimped battery terminals. Unless you have the tools and some hefty cable hanging around I would suggest you measure up and have the cross-links professionally-made. You could buy a crimper (hydraulic ones are undoubtedly the best) and terminals to do it yourself, but the investment for such a small job will usually be prohibitive.
When connecting two batteries in parallel it’s important to note that the output voltage of the bank will remain the same, but your available capacity (Ah) will increase. There is often confusion with amps and amp hours. Put simply, an amp is a measure of current flow, whereas an amp hour is a measure of current flow every hour. So, in theory a 100Ah (C20) battery could provide a 20A current for five hours before becoming flat. It won’t actually, for a number of complex reasons, but for simplicity I’ll let it stand.
If you were to join the two 12V batteries together in series (positive to negative, taking the output from the second +ve and -ve terminals), then you’d have a 24V output, but no additional capacity. Two 12V/100Ah batteries connected in series will still provide 100Ah capacity, but at 24V. Some boats use a 24V system for heavy load devices such as windlasses, winches, water makers and big bilge or shower pumps because doubling the voltage halves the current draw for the same power rated device.
Protection
Battery banks should always be protected with high-current fuses (c. 200A) on both the positive and negative output terminals, and as close to the terminals as possible, with no power take-offs until after the fuse. Special fuse blocks are available for this purpose, which are designed so that nothing can be connected directly to the battery without going through the fuse. This gives maximum protection against battery short-circuits, which can cause a fire and/or explosion if left unprotected.