What if with one switch you can go from serial to parallel.

[jasonhc73]

Dewalt did it with a 3S battery, the 20/60VMax Flexvolt.

Why not an 8S, or 16S setup?

WHY would you want to change from serial to parallel?
So you can use the natural balancing of a parallel circuit. Consider a low cell in a 16S battery. Now consider it parallel to 15 other cells like itself, not just the one it is next to. Look at how much faster it would balance.

Every series balancer/bms that has a lead between one cell and the next can only do one type of balancing, which is top peak resistance balancing. It can only attempt to reduce the voltage level of the top.

Do you know of a balancer that can send current to a low voltage cell?
Outside of putting 16 (or 8) separate cell level chargers on a battery, is there another option?

This does have a drawback of course. You cannot use the battery when it is a parallel state balancing itself. You would certainly have to have another set.

Test scenario:
1 4s 12 battery(like any 12v LiFePO4), with 4 benchtop chargers, one per cell. Who has 4 chargers to give it a test?
You get to keep the battery in series without a complicated "S2P switch", but 4 chargers per 12V is kind of pricey. But hey, this is experiment land.
Oh, and power the desktop chargers from an inverter that derives its' power from solar panels!
If you really think you are cool, put wheels on it.

 

[Sgt Raven]

From what I have read and watched on Youtube. Some of the Hobby chargers. when you're done flying your drone or something like that and want to bring those batteries down to 40% for storage. Instead of turning that juice into heat, they can move the power to another battery. These are some of the chargers that have a battery in and out circuit. If we applied the same idea to a 4-8-16-32S battery, then the BMS could actively take power from the high cells and put it in the low cells.

 

[Craig]

Unless I am missing something here. You are talking about passive balancing. Active balancing is supposed to take energy from high cells and give to low cells.

I actually charge my 2s 12v batteries like this from 2 12v SCC each to its own battery then I series the batteries to get to 24. Look at the 2 small SCC in my avatar

 

[SolarMuppet]

Active balancers take from the high cell and move down to the low cell. They can do this at any state of charge but, due to the flat ‘curve’ of lithium only actually do it at the extreme ends of the cycle as that is when imbalance actually shows enough to trigger them. They also Have the advantage over passive balancing in that they can do it for a lot more of the exponential part of the curve than passive ones do and can shift more current. Passive ones rely on very full charges being given to make them kick in and then you don’t have much time to balance before the charge is cut off. You run the risk that packs run only to a max of say 90% for longevity actually fail to balance and appear to lose capacity as they are cutting out when the most out of balance cell reaches full charge while the others are behind and the balancer can’t cope, you then need to do a proper top balance to restore capacity. That said, in a well matched pack, balancing will be less of an issue.
Oh I went off on a tangent. Parallel balancing is ok but only really when the pack is being charged to full capacity. the flat curve again being the issue as with voltages very close for widely differing states of charge, current flow is minimal but on the exponential part at the top end voltages vary a lot more and so decent balancing current can flow as the lower cells hold the higher ones down by taking the available current as they appear as lower resistances to the charging circuit that the almost full one. The important thing in any series / parallel switch is you absolutely must ensure break before make on all contacts.

 

[jasonhc73]

Unless I am missing something here. You are talking about passive balancing. Active balancing is supposed to take energy from high cells and give to low cells.

I actually charge my 2s 12v batteries like this from 2 12v SCC each to its own battery then I series the batteries to get to 24. Look at the 2 small SCC in my avatar

Ya got another idea for 64 cells?