How many bat­ter­ies is a bank?

Canal Boat - - Back Cabin: Experts -


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-train­ 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

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