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I am aware it's best to use cells of the same capacity, same age, etc. However, until I can replace the cells I have I need to make use of these.

The batteries I have are 2V flooded lead-acid cells, 6x 400Ah and 12x 225Ah (both at C10). I need 12V at the highest possible capacity with the cells available (without destroying/overcharging any cells, obviously).

What I want to accomplish is parallel the 12x 225Ah to create a 12V 450Ah battery and then parallel this with the 6x 400Ah in series (=12V 400Ah) to yield 12V ~800Ah. This should not result in overcharging any cells, since only like voltages are paralleled.

It only should result in a slight underuse of the capacity of the 12x 225Ah cells (~11%), but that seems acceptable (especially since the 225Ah cells are a little older than the 400Ah cells, so their capacity is probably a little nearer to 200Ah anyway).

What would be the best way to connect the 12x 225Ah cells for this? I can think of two ways to connect the cells:

  1. Create six pairs of 2x 2V 225Ah in parallel and connect these 6 pairs in series, or,
  2. Create two strings of 6x 2V 225Ah in series, and parallel these two strings.

Am I correct in thinking that option 2 only yields 3x225Ah=675Ah in practice, since I am effectively paralleling 400Ah, 225Ah, 225Ah (and the 400Ah battery will perform as if it were a third 225Ah battery), while only option 1 will yield the desired 800Ah; paralleling 400Ah and (225Ah+225Ah=450Ah) (where the 450Ah battery will perform as a 400Ah)?

Also, am I perhaps missing another possible (better) way to connect these cells? Thanks!

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There's only one sensible way to do this.

  • Make a 400Ah 12v battery with a series string of your six 2v 400Ah cells.
  • Make two 225Ah 12v batteries, with two series strings each of six 2v 225Ah cells.
  • Put all three 12v batteries in parallel to make one 850Ah battery.

Each series string has the same capacity cells in series (you must use the same capacity in series, or bad things happen (to the cells)).

The three paralleled batteries have the same voltage (you must use the same voltage in parallel, or bad things happen (to the cells, and to you)).

The capacity of all cells is used. You will be able to get the full capacity of all the batteries in parallel, 400+225+225 = 850.

You may have described this configuration in your OP, but it was tl;dr.

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  • \$\begingroup\$ When putting different capacities in parallel, would the greater capacity not behave as if it were the lesser capacity when charging? In this case I think the 400Ah would behave as if 225Ah and the result would be 675Ah. Yes, if I would fully charge all batteries before putting them in parallel they would discharge as 850Ah once, but when they are in parallel the 400Ah string would only partly charge (as if a 225Ah) and the set would then re-charge (and thereafter discharge) as 675Ah. I don't think this issue would be there if the 225Ah's were first paralleled by two. \$\endgroup\$ – Terrence Koeman May 31 '18 at 11:46
  • \$\begingroup\$ @TerrenceKoeman I find it difficult to make a reply, I can't fathom your logic, so don't quite know what to refute. It's obvious that as all the batteries rise to 13.8v, all will be charged to their full capacity, and as they all drop to 11v, they will all release their full capacity. We do understand the same thing by 'parallel' don't we? That all the + terminals are connected, all the - terminals are connected, so all batteries see the same voltage? Think of the hydraulic analogy, several cisterns in parallel, and let water flow in and out. How much flows, individually, together? \$\endgroup\$ – Neil_UK May 31 '18 at 12:48
  • \$\begingroup\$ Yes, we both understand parallel correctly. The problem isn't the voltages though, but the capacities under charge. I'll simplify: Imagine two 12V batteries, one 100Ah the other one 200Ah, both fully charged. When these are put in parallel, the combined capacity for a discharge is then 300Ah. However, since the internal resistance of the batteries is not proportional to the capacity, when charging this system the larger capacity battery will behave as if the lesser capacity (because of self-balancing in an ideal system), so the system will charge (and discharge) as if one 200Ah battery. \$\endgroup\$ – Terrence Koeman Jun 6 '18 at 20:08
  • \$\begingroup\$ @TerrenceKoeman No. Repeat after me. Each battery swings between 13.8v (charged), and 11.5v (discharged). The 100AH battery absorbs and releases 100AH when its terminal voltage does this. The 200Ah battery absorbs and releases 200Ah when its terminal voltage does this. The terminal voltage of both batteries is the same. Each behaves the same whether they're in parallel with another battery or not. The 200Ah battery needs to absorb 200Ah for its terminal voltage to change from 11.5v to 13.8v, whether it's put in parallel with a 100Ah battery, a 5Ah battery, or a 1,000,000 Ah battery. \$\endgroup\$ – Neil_UK Jun 6 '18 at 21:10
  • \$\begingroup\$ @TerrenceKoeman OK, I've just fully read your comment. What 'ideal system'? What 'self balancing'? Self balancing means each battery absorbing and releasing its own rated capacity when its terminal voltage swings over its full voltage range. Perhaps you'd better spell out the properties you think an 'ideal system' has that creates the 'neighbour sensing' behaviour that your two batteries have. Does the 100Ah battery magically acquire 200Ah capacity because its neighbour is 200Ah? If not, then why should a 200Ah battery drop to 100Ah because its neighbour is 100? \$\endgroup\$ – Neil_UK Jun 6 '18 at 21:14

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