Battery advice please

[jfm]

Thanks. That's complex stuff. The shorepower might well charge all 3 banks plus gen, but that would take a lot of dioding!

Remeber those cross over switchs can give a false sense of security. If the coil is energised by the flat battery, they dont work. Ideally you need a selector so you can energise the coil from any battery bank, but that's often omitted in production boats.

Mine is much simpler. Two banks. One bank is dedicated to one engine, with a trickler charger to top it up by shorepower over winter. The other (bigger) bank does t'other engine, domestics and genset start, and is charger by big shorepower charger

Then I have a crossover switch to link the banks, and it can be enrgised by either battery bank. That's it - lots simpler than yours, phew!

 

[John100156]

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1. such a short is very rare, and all systems are designed to make the chances of it happening near zero. If it happens, for sure excessive current will flow, could start a fire, even in OP's present set up. Adding a couple of extra batteries isn't going to change it from safe to unsafe. The only way to deal with such shorts is to make sure they never happen.

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1) Well it may be rare but when I am out to sea I do not want to find I am the exception. Although the possibility of a short is rare, I can see no logical argument for not placing a fuse/breaker as close to the battery as possible, as I suggested, to minimise this risk.

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2. Anyway, the extra batteries on the bank will not increase the current flowing through the short. The current could run for longer, but it'll be the same amps

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2)Mmmm I do agree with this, fault resistance will be the same so therefore current flow. But a larger capacity will increase the length of time the cable is heated and therefore I can see no harm in increasing a short length of cable in size to a fuse/breaker placed close to the battery.

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3. Really, no-one sizes cables so they can handle a short across the terminals of a battery like these. The boat would need several tonnes of copper to do that!

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3) No, they should just protect the cable with a suitably sized breaker/fuse as close to the bank as possible - If a fuse is placed close to the battery it will protect the cabling system and you can keep all cabling from then on to the original size!
/forums/images/graemlins/ooo.gif

 

[John100156]

Hey jfm, regarding 2) above I have just read my original reply, I never said extra batteries on the bank will increase the current flowing through the short! I said:
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It is possible by increasing battery capacity that, should a short occur in the main cable between the battery bank and main fuse, excessive overheating will occur due to excessive fault current flowing.

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[jfm]

I don't get your logic. He already has circuit breakers (= fuses, in effect). Several of them I hope, like one for domestics, one for bowthruster, one for each engine start, and so on. There's no need for any more of them. Adding further batteries to increase capacity, upstream of the circuit breakers, is perfectly fine and there is zero benefit in adding any more breakers imho

 

[John100156]

OK, I hear what you say - but I have seen several boats where unprotected (by fuse/breaker) heavy battery cables are run quite a long way before any fuse link is installed. And I note you have said 'I hope', I dont know the OP's set-up hence the advice.

I suppose my point is by installaing additional battery capacity any fault that develops could permit fault currents to flow for longer, hence lengthen the time the cable is heated.

I hear of lots of boats catching fire! I just dont know why a main fuse link sized with adequate discrimination to other breakers could not be installed close to the battery terminal.

OK, let me think - could it be that any fuse melting could potentially ignite battery gasses? Surely this is unlikely if properly contained/ventilated.

Is it because the risk is low and its a rare or an unlikely occurrence - because the +ve cable needs to short with metal that is negatively grounded and the length of unprotected cable is generally short? Perhaps. But in any case a fuse would IMO in my case protect the main cable.

I seriously would be interested to know if there are any other reasons/opinions why a main fuse-link should not be installed very close to the battery with a heavy link cable! /forums/images/graemlins/smile.gif