Attaching a fuse to a new set of batteries?

DrewE wrote:
A short would presumably be a single shorted cell, not a dead short across a single 12V battery.

Well ... current flow rate is all about the potential (voltage difference) driving it and the capacity of the potential source to supply the rate.

I worry about one battery going bad and having one shorted cell such that it becomes downgraded to only a 10 volt battery. Hence (with, say, two originally 12 volt batteries in parallel) you would then have a 12 volt good battery with a capacity for supplying maybe 1000+ instantaneous amps across the 2 volt potential difference created by the 10 volt bad battery "load" in parallel with it.

I wonder how much heat and/or gas pressue a 1000+ amp current flow driven by 2 volts can create?

If my fears are justified, I guess it might be a good idea to fuse between paralleled RV batteries ... just in case.

Hi Phil,

Keep the interconnecting cables under ten inches so they can not short out to the frame.

Don ... I keep the cables well insulated: I embed the positive battery cables in flex-tubing ... in addition to the normal heavy automotive type insulation on them.

pnichols wrote:

DrewE wrote:
A short would presumably be a single shorted cell, not a dead short across a single 12V battery.

Well ... current flow rate is all about the potential (voltage difference) driving it and the capacity of the potential source to supply the rate.

I worry about one battery going bad and having one shorted cell such that it becomes downgraded to only a 10 volt battery. Hence (with, say, two originally 12 volt batteries in parallel) you would then have a 12 volt good battery with a capacity for supplying maybe 1000+ instantaneous amps across the 2 volt potential difference created by the 10 volt bad battery "load" in parallel with it.

I wonder how much heat and/or gas pressue a 1000+ amp current flow driven by 2 volts can create?

If my fears are justified, I guess it might be a good idea to fuse between paralleled RV batteries ... just in case.

A 1000A current with a 2V drop is 2kW of power being dissipated, so under those conditions there would indeed be a lot of heat.

However, I'm pretty sure you won't get anywhere near 1000A, as the battery with the shorted cell won't charge at that rate with a 2V supply above nominal (and the voltage in the good battery would also sag more than 2V quite quickly with that amount of current). Exactly what charge rate and hence current flow you'd see depends on lots of things, such as the batteries state of charge and the ambient temperature. It seems to me it should in general not be all that far off from an equalization charge, which of course doesn't generally result in extreme collateral damage.

If you generally see these levels of current readily from a two volt difference between batteries, we'd see jumper cables get vaporized, or at least overheat, on a fairly regular basis.

Drew, what you say above makes solid electrical sense regarding what is most likely to happen.

What prompted my (conservative) decision to add a fuse between our two paralleled 12 volt RV batteries was both a small but possible safety concern ... plus a vague memory of reading someplace about it being a recommended practice.

I just ran across this article today. In paragraph 7.3.3 of this article, the author states "For safety, a DC fuse in series with each parallel battery is highly recommended.":

http://jgdarden.com/batteryfaq/carfaq7.htm#better

Hate to be the source of dumb questions, but for safety, should I fuse the positive ends of the batteries, or both ends?

What fusing are you referring to ... fusing the whole battery bank's capacity between it and the RV ... or the extra "inter-battery" fusing that goes in between any paralleled batteries within the bank?

Typically all the fuses go at the positive end of the battery (or batteries). There's no real need to fuse both ends, at least in any ordinary application, as interrupting the current at one pole will interrupt it at the other one as well (thanks to Kirchoff's laws and corollaries). For main fuses, it's perhaps not quite so important which end is fused; but for branch circuits, with lower and varying capacities, shorts to ground (as in the framework in general) are one of the big things that is being protected against, and fusing the ground lead of the branch circuit does nothing to protect against them while fusing the positive lead does. For consistency, if no other reason, fuse the positives.

Of course, adding additional fuses is harmless (from a safety standpoint), but it does add more cost and connections that may work loose and things to check when stuff stops working.

Intra-battery-short safety fuses placed between each paralleled battery and it's neighbor can be in either the lead connecting the positive terminals between each battery and it's neighbor or in the lead connecting the negative terminals between each battery and it's neighbor, whichever is more convenient. Intra-battery-short protection is the same with each fuse in either lead between batteries.

However, as others have pointed out the chance of a massive short inside a battery is very remote ... but the danger if it ever did happen could be large.