Parallel paradox

[willman]

My current set up is as Fig 1; correctly balanced pair of 12v batteries in parallel, I believe.

Considering adding a third battery. Due to physical layout constraints, and tight wiring looms etc, I want to add the third battery north of the existing pair as in Fig 2. Ideally leave the positive connections as is to battery 1, negative connections as is to battery 2.

Would I get away with just one lead from the new positive and the new negative battery as illustrated, to maintain a correctly balanced triplet of batteries in parallel, or have I messed up the parallel/balancing?

Seemed like a very simple conundrum when I was sketching it, but me feels it's not that simple!

Many thanks

 

[Tombola]

not quite. The way I always think of it is just imagining Im making one large battery. As long as all batteries are the same age (ish) and type/size, and cable lengths same between each.

 

[willman]

Thanks for replying.

Existing batteries 1 & 2 are under drivers seat right next to each other and with very tight and clustered wiring looms. I could fit the new (third) battery approx 20 cm behind the seat, but alas, looks like I'll need to take 2 leads off the new battery then as in Fig 3!

Thanks again

 

[L' Hobo]

Could you just replace leads of the same length on ALL batteries as suggested by Tombola?

The newer age of the extra battery might cause problems?

 

[Tombola]

The ideal scenario is same age same type same size batteries,same size and thickness of leads from each and same cables to feeds/bus bars.
but obviously each thing is more important.

May be a pain but like L'hobo says, same length cables on all is probaly the most critical. (usually trhe shorter the better)
If the new battery is the same as the others and the other havent been bashed then you could get away with that.
But also, it depends how much "loss of life" you can cope with of them as the balance will wear one battery more than the other.

Id get stuck in and add the same length wires and give it a shot if it was my van. Sometimes advice can be too precise and if it meant over the life of the batteries you lose a few days/weeks use then is it worth it?

I didnt think too much difference in one of my cables leading to a bus bar from my 400ah of lithium (2 batteries), one was about 8 inches longer, but it did, I noticed the charge of one dropping quicker than the other an done charging quicker . Moved the busbar and shortened the cable...sorted.

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[willman]

I'm leaving the factory batteries next to each other under the seat. The new will have to be 20cm behind them. To achieve the Tombola sketch, I'd have to move all the positive terminals onto the new battery behind my seat (not ideal) and from memory there were about 8 connections with very little slack (unless I start unwrapping all the looms and coils etc).

The existing batteries were new just before Christmas and I haven't been anywhere yet! Figured a new battery (same make/model) would get along with the others. Although I was thinking 80ah new (existing 95ah x2), but same make and variety.

I could certainly make the battery leads all the same length in my Fig 3 ?

 

[autorouter]

The idea of connecting the load wires to different batteries will only work for two batteries. There is a similar arrangement, but more complicated, for 4, 8 or 16 batteries. For any other number, including 3, the only way to equalise the voltage drops is to have equal length wires from a common load point.

For example, on the positive side there will most likely be a big fuse very near the batteries. Equal length wires from the fuse to each of the battery positives will give equal voltage drop.

On the negative side, it may be necessary to use a common connection or busbar, and run equal length wires from that. If there is a shunt on the negative wire that is an ideal point to run the equal wires from.

 

[Burgo]

Just wondering if the extra cost and weight of a third battery and the fitting faff is worth the candle. Would it not achieve nearly the same to dump the old two and put in just one lithium, and save a load of weight and cabling??

 

[willman]

Just wondering if the extra cost and weight of a third battery and the fitting faff is worth the candle. Would it not achieve nearly the same to dump the old two and put in just one lithium, and save a load of weight and cabling??

Yes I agree, jumping to lithium would seem the sensible route, I'm just not ready to go there...yet....(£170 for 285ah, all virtually new, seemed like an immediate cheap easy fix for the rain in Spain days)...

 

[Rapido925M]

I had 2 batteries of different make, different Ah and different age. They lasted 7y and 5y without any difficulty . Is this such a huge problem that it requires re-wiring, special connections, things moving around, (sleepless nights?)?

How much battery power do you need in Spain during the summer? Last time I went in April with one 95Ah battery and paid for EHU for 5 nights in the month. We don’t have solar nor any other alternative supply. OTOH we don’t watch tv

Gordon

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[willman]

I had 2 batteries of different make, different Ah and different age. They lasted 7y and 5y without any difficulty . Is this such a huge problem that it requires re-wiring, special connections, things moving around, (sleepless nights?)?

How much battery power do you need in Spain during the summer? Last time I went in April with one 95Ah battery and paid for EHU for 5 nights in the month. We don’t have solar nor any other alternative supply. OTOH we don’t watch tv

Gordon

Oh I'm not losing sleep over it. And you're probably right - I probably won't run out of power with just the two!

Just an itch I want to scratch, sorry

 

[willman]

The idea of connecting the load wires to different batteries will only work for two batteries. There is a similar arrangement, but more complicated, for 4, 8 or 16 batteries. For any other number, including 3, the only way to equalise the voltage drops is to have equal length wires from a common load point.

For example, on the positive side there will most likely be a big fuse very near the batteries. Equal length wires from the fuse to each of the battery positives will give equal voltage drop.

On the negative side, it may be necessary to use a common connection or busbar, and run equal length wires from that. If there is a shunt on the negative wire that is an ideal point to run the equal wires from.

Interesting.

May I ask - my Fig 3 above - if all 4 leads were 450mm long (16mm2) and all batteries same, and loads where marked, what would be the shortcomings on this set up?

If it's 'rocket science', the short answer will do.

Thanks

 

[mitzimad]

in your first drawings you have confused the way a battery runs its out on the positive and back in via the negative if the battery is only 200 mm away i wouldnt wory about equal length cables all you need is two wires from the extra battery to the existing one (one + one -) if your that worried about the cable restistance use the next size up

 

[autorouter]

May I ask - my Fig 3 above - if all 4 leads were 450mm long (16mm2) and all batteries same, and loads where marked, what would be the shortcomings on this set up?

Yes the configuration in Fig3 would work. For each battery, the cable length would be the two load cables, plus two lengths of link cable. If those link cables were the same length, as you say, then the voltage drops would be equal.

In the diagram, the links are drawn as unequal lengths. You would have to make the actual link cables all equal, even though two of them would be unnecessarily long.

 

[willman]

Yes the configuration in Fig3 would work. For each battery, the cable length would be the two load cables, plus two lengths of link cable. If those link cables were the same length, as you say, then the voltage drops would be equal.

In the diagram, the links are drawn as unequal lengths. You would have to make the actual link cables all equal, even though two of them would be unnecessarily long.

Yes, exactly. 450mm is the longest stretch, so make them all 450mm - I wouldn't have thought to do that (counter intuitive)!

Thanks