Hey guys,
I have a question about charging and discharging with series parallel.
If I have cells in parallel I get more amp hour, at the same voltage.
If I have cells all in series, I get the total amp hours of each cell but the voltage goes up per cell added.
Right now I am at 11, 8.4v Nissan leaf batteries, totaling about 92 volts
If I add a battery 8.4v in series I will get a nice ~100v for my pack voltage.
So....
My question is, does it matter for cell balancing during charging and discharging what specific ORDER the batteries are in?
Can I safely put 6 batteries back to back in parallel then have my regular series pack of 12 tied in or is it better, safer, necessary to add a parallel connection THROUGHOUT my series pack?
I don't want the pack to become unbalanced since I don't use a BMS and charge well below 4.1v per cell.
Since the pack will be the same voltage and capacity regardless of if the parallel cells are all clumped together or spread evenly through the module, I assume that it shouldn't make a difference, but I'd like to hear what you all think since I'm still learning about the mystery of electricity.
Can you draw out the 2 different wiring diagrams of the battery you're speaking about?
Quick drawing basically a clump of parallel cells in line with a clump of series cells
I just want to make sure that each cell charges at the same rate, I was worried that if they were clumped up like that, then the parallel cells would not get the same voltage as the series cells
Quick drawing basically a clump of parallel cells in line with a clump of series cells
I just want to make sure that each cell charges at the same rate, I was worried that if they were clumped up like that, then the parallel cells would not get the same voltage as the series cells
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What is it you're trying to accomplish? Just get a charger that suits the entire pack. Also, don't have different parallel groups of cells (different Ah cell groups) in series. That will cause unbalancing of the cells during charge and discharge. They will charge/discharge at different rates, and they will potentially get damaged.
The short answer is that you can’t do either of these. All of your series groups “cells” ( groups of parallel cells) need to be matched in V and Ah so the same number of parallel cells are grouped in each “series cell” to prevent uneven discharge.
Your 2 module cell will discharge at half the rate of you single module cells, creating a voltage imbalance between your series cells, this will cause your single modules to drain faster, and try to keep up when they have nothing left. Those cells will overdischarge, and you will kill them.
Dude I'm very glad you came on here, and I hope you've done so before you attempted anything like what you've drawn.
I reckon some serious 'book time' would be a very wise investment before you do too much more tinkering as, and I don't mean any offence here, I'm genuinely concerned that you might wind up doing yourself some serious damage.
What you've drawn here in itself isn't actually very likely to lead to injury but it does highlight that the fundamental comprehension of the systems you're working with simply isn't there.
To address your questions:
Parallel means more charge at the same voltage - yes.
Series mean more voltage at the same charge level - yes.
But
Parallel involves having multiple cells connected positive to positive, and negative to negative - ie: BOTH poles are connected between adjacent cells
Series involves multiple cells connected positive to negative at ONE pole only.
and
Parallel cells MUST be of equal voltage.
Series cells MUST be of equal charge.
Hence
YOU CANNOT MIX AND MATCH BETWEEN THESE TWO ARRANGEMENTS!
The stack voltage in your left hand drawing will wind up being 0V - you've basically stacked pairs of modules in opposition with each other. This isn't actually dangerous unless you attempt to charge them, at which point you'd be in for a VERY bad day!
The image on the right however will lead to the failure of the lower 2/3rds of the arrangement if you attempt to charge or discharge to the full extent of the capacity in the parallel bank on top - how dramatic the failure would depend on a number of external factors, but failure *is* assured.
There are only two combined arrangement options which will result in a pack configuration which is both safe to operate and will last past one charge-discharge cycle:
You can have an arrangement of 7 (arbitrary number) banks connected in series, where each bank is comprised of 20 (arbitrary number) cells in parallel.
or
You can have an arrangement of 20 parallel strings or 7 individual cells in series.
Either of these arrangements requires a total of 140 identical cells, and have the same voltage and charge.
The former requires voltage monitoring for balance at only 7 locations; whereas the latter requires voltage monitoring at all 140 cells.
Finally then I feel the need to point out that 8.4V is NOT one cell, it's TWO - they just happen to be in one MODULE, and the voltage at the center tap should be monitored by whatever mechanism you feel is appropriate to observe your pack balance. I will also add that this SHOULD ABSOLUTELY BE A BMS! I understand that this makes for additional expense, and I'm fully aware that it is possible, with the appropriate manual intervention, to operate a pack without one - but I would very STRONGLY advise against it given your apparent knowledge base.
In close, once again, my intent here is not to offend - I am merely genuinely concerned for you.
I echo the above ^^^^
I learned to always work at the individual cell level. So think of series first to get the total voltage that you want... then, if space is adequate, you can double the pack size by adding another cell to each series cell, parallel at the cell level. And don't mix different chemistries, different manufacturers, different aHr ratings, different anything.
My 2 centavos
Current rides: '96 Honda Ohlins VFR, '03 Cannondale C440R, '03 Cannondale Cannibal, '06 Yamaha 450 Wolverine 4x4
Current builds: WORX.VOR.v3.2
I hope this helps clear things up a bit. This is my battery pack goal
I misunderstood the nomenclature about series parallel. 12s12p would be 144 batteries. Physically improbable with Leaf cells on a motorcycle frame.
I just wanted to make my pack voltage at 100.8, then double the batteries in parallel to double to AH without changing the voltage. I BELIEVE this would be 12s2p. I hope the picture helps compared to my crappy attempt on the white board at work.
Likes: 15
Originally Posted by frodus
WOW!
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