Begginers Guide to Processing 18650 Cells | Cell Database


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all cells parallel balancing
#1
Hey guys, 

i've been reading (and studying) for 2 years now on the topic but I have one question I can't get my head around...

the parallel cells should even themselves out within the pack, the series packs individually might drift off a little,... 

Why not simply put a few relay switches (ok they might be a bit expensive and cabling would need to support the low voltage but more "simple" compared to BMS's) between the packs and make em switch between series and parallel. Parallel mode they form one big pack (in my mind) and when balanced out you can switch it back to series mode. 

I know there is probably more to it, but that is my question, what is the difference? 

I know the basics of the bms, but what is wrong in the above mentionned system? 
i'll probably facepalm myself after your comments, but i couldn't really find the answer. 

thanks and good luck in your DIY projects.
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#2
Biggest issue is the pack will have to be offline in order to balance that way.
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#3
Well, yes, in theory you could do that. DeWalt actually uses something like this for their powertool batteries. They have a 18V range like all powertool makers, but they also have a 54V system they call FlexVolt. The batteries are equipped with a mechanism, at least I think it is a mechanical thing, to change their configuration between 18V 6Ah and 54V 2Ah. Obviously they do this for higher voltage and not balancing, but basically this is similar to what you are thinking of.

The problem is that this isn't easy to do, especially not for bigger batteries. Usually you build you parallel packs first and then take as many as you need to put them in series to get the desired voltage. Like this:+1-+2-+3-. The cells within packs 1, 2 and 3 will always have the same voltage. However, now the entire packs start to drift. To balance them you want to disconnect the connection between the packs and connect them in parallel. To do that you need to power down your system because you don't have the needed voltage anymore and then connect them in parallel. For this 3S example you need five switches. For any xS system you need 2x-1 switches, I think. That is 27 switches for a bigger 14S system. And if only one of them fails...
Also, if the packs differ quite a bit there will be a huge current evening out the voltage difference. You don't want that. And if you balance them while they don't then you don't need to balance them at all. I think this is completely the wrong approach. You don't want to balance your packs once they started to drift. You want to stop them from drifting in the first place.

In short:
- Downtime needed
- Complex cabling
- New points of failure
- Potentially huge balancing currents
- "Cures the symptoms, but not the cause", you don't want the packs to drift in the first place
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#4
Search for flying capacitor balancing, and replace the capacitor with a pack, and you can have your cake and eat it also.

So S14+1 packs, and the +1 pack is all the time flying, ie. being connected to one of the packs in series.
Minimum relay/switches are N+5 DPST ie. 19.

We discussed it here quite recently.
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#5
A proper BMS also gets you alot more data and information of the cells in terms of how they are doing. Like what pack is getting out of balance more frequently.. Are you getting same DOD out of them still (Shunt needed) and so forth.
So by having a proper BMS system you will be able to spend less time on your battery bank and be able to maintain and get them to live longer if done properly.
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#6
Hey I know its expensive but the way i do it works pretty well but im not sure if its a good solution for large packs.

I have 2 12p packs and use them in series for 8.4v
each cell has a diode from the my 12v PSU to a 1amp 5v regulator then to a to2056 charger board then out through a lm317 current limiter set at 500ma so no battery can discharge at more than 500 ma in theory
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