Two 12s10p In Series vs Two 24s5p in Parallel

[The Mighty Volt]

Hi folks.

I am putting together these 18650 A123 cells and it seems a straight-up, one-piece 24s10p battery is just going to be too heavy and awkward to go into a ruck-sack like the much smaller batteries I have been using so far.

Its for my BMX and so what I wanted to do was make up some plywood boxes and mount one on each side of the frame, as near to the head-stem./handlebars as possible. I'd probably run a brace of threaded bolt between each one to secure them, I might use polyurethane bushes if the bar is going thru the frame itself.

Should I make two 72v5Ah {24s5p} packs and parallel them, or two 36v10Ah {12s10p} batteries and series them?

To be honest, I prefer to series two packs, that way I have two smaller complete units of 12s10p which can be charged and handled so much easier. It also gives me two 12s10p batteries I can use on a different project should I so feel the need......a 72v5Ah battery really isn't worth squat, on its own, is it??

Anyways I just wanted to check that I am not making any major errors here. It's been a PITA to solder and weld these suckers up with copper shim and I don't want to get it wrong.

What are the rules for connecting individual strings in series and then connecting completed packs in series?? Is it simply a question of making sure they have equal SOC's before connecting?

Are there any such rules which apply for paralleling?

Many thanks.

 

[E-bike4life]

I would go with the later, two 36v10ah. I always try to parallel cells at the lowest voltage possible for various reasons- management/current sharing.

When connecting batteries/packs in parallel they must be at same State Of Charge or you risk sharing current at very high rates. The higher charged battery will push current to (charge) the lesser battery as fast as it can.

When connecting batteries/packs in series they need not be at the same SOC although it would be ideal.

 

[The Mighty Volt]

I would go with the later, two 36v10ah. The reason is for easier management.

When connecting batteries/packs in parallel they must be at same State Of Charge or you risk sharing current at very high rates. The higher charged battery will push current to (charge) the lesser battery as fast as it can.

When connecting batteries/packs in series they need not be at the same SOC although it would be ideal.

Thanks mate.

 

[E-bike4life]

No problem. Are you using a BMS?

If not, I'm sure you are already on top of this but be sure to wire up some balancing leads. Will come in handy to manage the pack.

 

[The Mighty Volt]

No problem. Are you using a BMS?

If not, I'm sure you are already on top of this but be sure to wire up some balancing leads. Will come in handy to manage the pack.

I won't be using an onboard BMS. My BMS will be balance taps, probably two sixes, and a cell-log. The packs are 10Ah nominal, basically that means coming in around 7.5Ah to keep everything sweet. Regular balancing and gentle 5A charging should ensure everything is cool.

Cheers.

 

[cell_man]

2 blocks of 12S as already suggested is the way to go, if you have 2 * 24S in parallel you should really link them at the cell level also and with reasonably high current cable too, at least short cable runs, or you can run into problems.

 

[dogman dan]

I'd go with the 36v blocks. Mostly so you can charge two blocks with two chargers twice as fast. A lot depends on the charger, perhaps even 4 18v blocks. Whatever is going to make charging fast and easy.

 

[The Mighty Volt]

Cheers chaps.

18v charging would indeed be very very handy.

 

[999zip999]

A question please. If you run 4-18v10p packs for 72v in series. Is this harder on each cell for draw and balance as opposed to a 24s10p pack. How do they like to live together ? Meaning small high voltage lower ah packs are easier to place in a build. But is it more demanding on the cell ? On cell pur cell ?

 

[dogman dan]

I don't get your question. If a battery is say 10 ah, the drain on each cell is likely to be the same or nearly so, no matter how you configure it.

Say you make it 2p 24s. paralelled cells, 24 of em in series. draw 10 amps. 5 amps will flow from each cell. IF, all the cells have the same internal resistance.

Say you make it 1p 24s, then paralell it with 1p,24s. draw ten amps, 5 amps will flow from each cell. IF, all the cells have the same internal resistance.

If you have a runt cell, it will produce less leaving the remainder to be provided by the other, either way you string em.

BUT, if you paralell them first, the weak cell can be supported by the other, making overdischarge a little bit less likely.

 

[cell_man]

A question please. If you run 4-18v10p packs for 72v in series. Is this harder on each cell for draw and balance as opposed to a 24s10p pack. How do they like to live together ? Meaning small high voltage lower ah packs are easier to place in a build. But is it more demanding on the cell ? On cell pur cell ?

It makes no difference to the cells. The only difference is that you will likely get a little more voltage sag on the pack due to the losses in the cable that link the small packs together and dealing with the additional wiring between packs and the balance wires can be a bit of a headache.