How many batteries is a bank?
My marina’s mechanic said that I should not have more than five batteries: I was wondering why. FIONA, via CB website
TONY REPLIES: This might look like nonsense but it could well be correct for your boat and your use – and he would be reluctant to fit another if he thought you would destroy it as fast as the rest by misuse. Some possible reasons are: • No physical space for another battery • Wiring system sub-optimal, meaning work needed. See diagram; but bear in mind that if more batteries means putting them on both sides of the boat a more complex system will be required); • Statistically greater chance of failure (it only takes one internal short-circuit to ruin all the batteries) • That your problem is lack of charging capacity or excess consumption rather than battery capacity. In the last case, do a power audit of all the electrical appliances and how long you use each, between recharging (see tb-training.co.uk/16elect. htm#bmn68). The total you arrive at should be about 25% of the bank capacity. However because of charging inefficiencies, you will then need to put in between 10% and 50% more than you took out (I like to work to 30%).
But once you’ve come up with a figure for how much charge you need to put back in the battery, don’t just divide this by alternator power to calculate the recharging time needed. Alternator output is only at its rated maximum for maybe 20 minutes on a well set-up and properly used system; from then on it gradually drops to a very low figure. Experience shows that over a twoto four-hour period the average alternator output on a well designed and used system would only be about 50% of the maximum. Allowing for this in your calculations could mean that recharging would take well over the 12 hours moored up engine run time allowed by CRT regulations (8am to 8pm) or the length of time you are cruising daily, if you have a small alternator and a large battery bank.
Correct wiring for batteries
The number of batteries depends on your boat