Wiring up Batteries

[redhot]

Hi all,

I have just purchased 3 x 110AH Leisure batteries for my boat and I have a quick question.

Is it correct to wire all the +'s together from 1-2-3 and all the -'s together in the same way?

Ok, its 2 questions,
Also, am I correct to wire the charger on the + or - terminal of battery 1 and the other connection onto the - or + (depending on what I ahve done on batt 1) of battery 3?

Hope you can understand my question.

Thanks
Mike.

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[snowleopard]

yes, link the batteries with heavy wire (like on your car's starter motor) with proper terminal clamps. that makes them the equivalent of one big battery. then connect a cable from the +ve side to a big fuse (i use 150A) using the shortest possible run of cable.

connect all loads and charger inputs to the -ve side of the batteries and the other end of the fuse. that way there's little risk of a short setting the boat on fire!

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[Evadne]

Although in theory wiring up batteries in parallel is allowed it is potentially dangerous in practice and I would always have a diode on the output (+) of each before joining them. You can't charge through a diode so you either have to have a 3-way switch to connect the alternator directly to each in turn (but then you can't switch over while the engine is running, so the other two will run down,) or fit a battery management unit that should incorporate the diodes and some sort of automatic charger circuit. Or you could charge the other two using a pair of DC battery chargers running from No. 1, but that is starting to get needlessly complicated. For your set-up the single unit is the method I'd recommend, although having only one battery on my boat I can't tell you which one is best from experience.

The diode stops a current sink on one from draining the whole bank. If a cell were to go short circuit in one battery, the effect could be like dropping a spanner across the terminals of all three. There's a lot of stored energy in those 3 batteries. It's also needed as, unless your batteries are matched (i.e. have the same internal resistance, which is unlikely) you will not drain or charge them equally if they are simply parallelled up. The ideal diode conducts your maximum (i.e. starter) current without getting hot and drops as little voltage across it as possible. They are expensive (even if you use one from a car) which is one reason why the aforementioned management units are also expensive.

There are lots of books and magazine articles on the subject as well.


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[Gunfleet]

I suggest you get one of the books on boat electrics. To be honest, your question really implies you should get an auto electrician to do the job.

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[oldsaltoz]

G'day Mike,

You can save the problem of the whole bank going flat due to cell failure by installing a battery change over switch,
the three position type, marked battery one-two-both-and off, just connect one battery to battery one, and use this as your starting engine battery, connect the other two batteries + to + and - to - and connect as battery two.
This will be your supply when at anchor or when the engine is not running / charging.
Don't panic about not being able to switch banks when the engine is running, because you can, just purchase a "make before break" type, it connects to the next bank before it is isolated from the first bank. The alternator output wire also goes to this switch.

It's good practice to install volt and battery condition gauges, this will tell you if and at what rate the alternator is charging, and what bank needs charging the most.

An isolator switch also means the boat is a bit safer when left unattended and power to all circuits is turned off.

I hope this helps

Andavagoodweekend. Old Salt Oz /forums/images/icons/cool.gif

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[pvb]

Yes, you\'re correct...

I'm assuming these 3 batteries are for your domestic bank, and that you have a separate starting battery. If so, you can safely hook the 3 batteries together in parallel, as you've described. The potential problem of a bad cell in one discharging the others isn't worth worrying about, and you certainly don't need to consider complex diode/switching arrangements.

Wiring the charger as you've described is good, because it helps to ensure that each battery gets the same charging voltage (by cancelling out some of the tiny connection losses). Similarly, if you take your main positive & negative feeds from opposite ends of the bank, this will be better.

You should ideally take this opportunity to wire a high-amperage fuse in the + lead, as close to the battery bank as possible, to protect the wiring to the main switch/panel.

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[snowleopard]

1 bank or 2?

let's say your daily load is 20AH and you have 2x100AH batteries.

wire them as a single bank and you discharge them by 10% each day.

wire them as 2 banks used alternately and you discharge each by 20% every other day.

at a 10% discharge level, battery life is 2000 cycles, at 20% it's 700 (data by nigel calder)

in other words batteries last longer as one bank than as separate banks.

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