6V Batteries

[CJH]

Can somebody please advise on one specific aspect of using 6V batteries in pairs? We're about to fit two pairs of Trojan 6V batteries into a vehicle at work, to give us a nice big auxiliary power supply. I've read up on the optimum wiring configuration (positive to one pair, negative to the other, to equalise charging and draining), but one thing I can't find is how to spec the cable that links two 6V batteries as a pair.

I know that the practical answer is that this cable should be as thick as possible, to minimise resistance, but from a theoretical point of view does this cable carry the same current as the rest?
Is it just a case of 'current is the same everywhere in a circuit'? Or is there something different about this cable since it is effectively sitting in the middle of a 12V battery?

[kevtherev]

I know that the practical answer is that this cable should be as thick as possible

.......a case of 'current is the same everywhere in a circuit'

[CJH]

Haha - so I answered my own question then. Cheers Kev.

[1664]

I know that the practical answer is that this cable should be as thick as possible

.......a case of 'current is the same everywhere in a circuit'

Provided the circuit is ckosed with no branches. The second battery is another branch.

If I'm understanding correctly and the batteries are connected in parallel (+ve to +ve, -ve to -ve) with charger connected directly to one of the batteries. Both batteries in an equal state of discharge, say both demanding 10 amps a piece (nice round number) from the charger. The cables from the charger to the batteries will have to pass 10+10=20 amps, the cables from the first battery to the second will only have to supply the demand of the second battery, 10 amps.

Practically though you might just as well use same size cables anyhow

E D I T: On the other hand if they're connected in series to make 12v (+ve to -ve, -ve to chager, +ve to charger) then that is one closed loop and the above is bollox

[1664]

I have been up since 5am y'know

[CJH]

If I'm understanding correctly....

You're not, but you're forgiven, it's past your bedtime. And maybe my description wasn't clear.

E D I T: On the other hand if they're connected in series to make 12v (+ve to -ve, -ve to chager, +ve to charger) then that is one closed loop and the above is bollox

Now you're getting it.
Four 6V batteries. First pair connected in series to make a 12V pair. Second pair connected in series to make another 12V pair. The two 12V pairs are then joined in parallel to make a big 12V supply. We can forget the second pair for the sake of this question. Within one pair, I'm asking about the cable that goes from the +ve of one battery to the -ve of the other. But I think you've already answered that - it's a closed loop so it's carrying the same current as the cables that then connect the 12V pair to the charger/load.

[California Dreamin]

Hi guys....isn't this a case of wiring to the 'max' possible current flow?

Talking about 'charge current' doesn't seem correct because this would be relatively low compared to what these batteries could be moving....

Unlikely scenario's but: what happens if one of the pair was say, 30% discharged, what sort of current would be flowing when the batteries are connected (the batteries effectively equalising)
Just throwing it out there!


Martin

[CJH]

Hi guys....isn't this a case of wiring to the 'max' possible current flow?

Talking about 'charge current' doesn't seem correct because this would be relatively low compared to what these batteries could be moving....

Unlikely scenario's but: what happens if one of the pair was say, 30% discharged, what sort of current would be flowing when the batteries are connected (the batteries effectively equalising)
Just throwing it out there!


Martin

An interesting point - do batteries that are in series equalise like parallel batteries do? I have no idea, but clearly the mechanism would be different if they do.

But in this case we've bought 4 identical new batteries that are going to live and work together permanently, so with the correct wiring they should age at similar rates and therefore always be at virtually the same level of charge. (The 'correct' wiring I believe involves connecting the positive of one pair and the negative of the other pair to the load/charge cables)

The question of rating the cable for maximum flow is correct though, and we're onto that - my question was about the short bit of cable between two 6V series-connected batteries, just in case it needed to be twice as big as the rest, or half as big maybe. It looks like it just needs to be the same as the rest though.

[California Dreamin]

Hi Chris.......think of the two batteries as ONE.....in fact, 6 batteries as each CELL (3 cells in each 6 volt case) is its own battery contributing approximately 2.1 volts towards the whole. Each cell is then internally connected to the next with very robust flat plate rods (for want of a better description).

A 6 volt battery is 3 separate batteries where as a 12 volt one is 6....so you see putting the two together achieves the same results but with two physical cases '6 cells' instead of 3.....Yes each cells equalizes with the rest irrespective of whether they are in two cases....

And...if you read that article as I did, you will see how important it is to minimize the charge disparity between the cells..(your connection would be the best) but this also means minimal voltage drop to achieve as equal a charge to each cell as possible....big fat connections!

Martin

[CJH]

Thanks Martin

Yes, I'm sure we're thinking of the same article - the one that showed optimum cabling configurations for 2, 3, 4 batteries connected in parallel. In my case, since we're only talking about 2x 12V 'batteries', the configuration is quite simple.

When batteries are connected in parallel it's easy to see how they equalise eventually - a higher voltage in one will tend to charge the other one - but when we're talking about 6x 2V cells in series it's not clear to me what the mechanism is that causes them to equalise. Does a cell with a higher voltage give up its charge before a cell with a lower voltage? Does a depleted cell take more of the charging current? But I'm happy to take your word for it. In fact I was happy to accept the conventional wisdom on this when we ordered the 6V batteries in the first place.

[California Dreamin]

High mate......personally I wouldn't get too bogged down with this but would be looking at proper battery cable and terminals to connect the two batteries.....obviously not the charge/discharge side.

Martin

[CJH]

Agreed - I have a good idea what to tell the workshop now. But that doesn't stop me being curious about the theory behind it.

[Ian and Lins]

I used to have a 1950's Austin FX3 taxi that had two hefty 6v batteries connected in series to make a 12v battery. It was original Austin fitment. The cables connecting them were the same gaugue/size as the earth and live that would connect a single 12v battery.

[California Dreamin]

Interesting that many Lorries also use a 2 X 12volt battery setup (24 volts)...good thick cable joining the two, however servicing a very heavy cranking ouput, so only to be expected.

Martin