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Discussion Starter #1
Hi,

I'm powering my ME1003 motor with 4S 5Ah battery packs (http://goo.gl/cbvTm). The packs will be wired in parallel in threes, so I get 15Ah. And 5 of these paralleled packs will be put in series to make 14.8Vx5 = 74V (I know the nominal voltage is higher, but let's assume the battery packs are actually at 14.8V).

My question is, when it comes time to charge this, what do I do? The only solution I can think of is to have disconnects at the series connections and further parallel the paralleled packs so I have a 14.8V @ 75Ah battery pack I'm charging.

Does this make sense? This is going on a Kart. I'm trying to find some sort of solution where I can charge my entire pack without having to charge each of the paralleled packs one after another.

Any comments / suggestions / help would be awesome.
 

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See the dealer links on the RHS of your screen, some of which carry battery chargers. Available for a few volts to hundreds of volts.
 

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Discussion Starter #3
See the dealer links on the RHS of your screen, some of which carry battery chargers. Available for a few volts to hundreds of volts.
I guess my question is more about the actual wiring rather than the charger to use. Unless the charger comes into play somehow. Thinking of my 4S batteries as a single 14.8V 5Ah cell, my entire battery system is wired in a 5S3P configuration. My question is how the hell do I charge something like this? Can I just treat the 5S stuff as a total of 20S and find a charger that does that? I've heard charging batteries in series isn't as safe as parallel.

The cheapest way I've found for the disconnection of the serial parts is to just have a nut/bolt through connectors and remove the nut/bolt when I charge the 3P paralleled packs.

Maybe my rambling isn't being very clear...
 

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They're not 14.8V cells, they're 4 single cells in series, and you'll need to keep the lipo balanced. You need to treat the individual packs as 4 cells inside.

You could treat the entire thing as a 20-cell charger, but you also need balancing for each pack. I wouldn't use the balance leads to parallel the individual cells, as the balance leads might actually share current while discharging and fry those wires.
 

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Discussion Starter #5
They're not 14.8V cells, they're 4 single cells in series, and you'll need to keep the lipo balanced. You need to treat the individual packs as 4 cells inside.

You could treat the entire thing as a 20-cell charger, but you also need balancing for each pack. I wouldn't use the balance leads to parallel the individual cells, as the balance leads might actually share current while discharging and fry those wires.
I know I was more saying that to explain how the 14.8V are wired externally to make my battery pack for the Kart. I'm trying to figure out how people have systems like this but also easily charge them when it comes time. It seems like you just have to dismantle the system and charge smaller packs and put it all back together.
 

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I know exactly what you're doing and what you're asking.

You're trying to series first, then parallel, which is not a good idea. It's best to parallel at the cell level, then put in series. If I were you, I'd buy single cells and make the pack yourself. Parallel 3 cells, then put those in series.

If you've bought them already, take the packs apart, and parallel at the cell level with large gauge wire.

If you don't want to do that, You'll need to balance each pack individually, i.e. balance boards for each one, or charge/balance each pack individually.


May I ask how you're going to achieve LVC detection on a cell-level basis? i.e. how will you know when one cell is low? How will you keep from overdischarging?
 

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Discussion Starter #7
I'm obviously not super familiar with these things other than my limited usage of them, but I was going to treat the 14.8V packs as a single entity. Lets say we ignore the series-ing to get to 74V, so we only have 3 14.8V packs in parallel to give 14.8V with 15Ah. Can't I just use something like this http://goo.gl/scJ7b to charge them in parallel? Assume I will disconnect the balance cables after charging is done. Or is that not safe enough?

As for LVC, I don't have a plan right now. I was just going to measure the voltage across a set of the 14.8V 3P packs individually (So I'd have 5 voltage measurements). Could the cells in each 14.8V pack really go out of whack when discharging over short periods of time?
 

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(Bad link above, it links to this thread)

Always parallel at the cell level. Also, don't connect the balance leads together, a surge of current will fry those leads and if they melt, the hanging wire could short other things. Each pack could have a different state of charge, so connecting them would cause current to flow from higher SOC to lower SOC, which could be many more times the rating of the balance leads. If you're NOT going to parallel at the cell level, then you need to individually balance all 20 packs.

Also, don't measure voltage at the pack level, do so at the cell level. You want to immediately detect if a cell goes low and cut off discharge. If not, you risk overdischarging that cell.... and with Lipo that's not a great idea, they're volitile.

Everything at the cell level IMHO. You're just asking for trouble with a lipo pack doing it hodge-podge.


Do it right, take the packs apart and re-build the packs, or order single cells to begin with.
 

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Discussion Starter #10
Actually I could also use the balance leads to monitor the voltage of the individual cells right? As in, when I charge, I charge battery by battery (battery here assumes a single 14.8V 5AH) and when I use, I plug in the balance leads to some sort of voltage measurement board I make that can monitor the cell voltages?
 

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Yes, but you need to monitor while charging AND discharging. While charging, if one cell goes to HVC, stop the charger. While discharging, if one cell goes LVC, stop Discharging.
 

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That board still doesn't keep high surges of current from going between cells during discharge. It might work during charge, if and only if the packs are all the same SOC.
 

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$22.59 for a hardcase 20C lipo pack
$8.98 for a single 20C lipo, $35.92 for 4

Diffference of $13.33 and you'd need to assemble, but you wouldn't need fancy balance boards and complicated wiring scheme.

Some sort of Monitoring BMS is required though. It's up to you whether to balance the pack or not, but I'd suggest some sort of BMS that can monitor and balance 20S of lipo.
 

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Discussion Starter #14
Thanks for being patient, but to clarify, you are saying that in a configuration like http://i.imgur.com/3Juss.jpg where each rectangle is really 4 3.7V cells in series, Ia might be much higher than Ib or Ic in some cases causing the wires that are putting the cells in series to burn up?
 

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No problem, just don't wanna see you go down a path that goes awry.

I'm not saying that at all, but lets look at that case:
Lets say you get 200A total, and 125A from Ia, 100A from Ib and 25A goes into Ic. 200A total, but current can flow in two directions. Is that Ia cable rated for 125A?


What I was talking about, is:
Imagine the balance wires connected between all three packs.... you now have Ia1, Ia2, Ib1, Ib2, Ic1, Ic2 etc.... where current can flow between Ia1 and Ib1/Ic1. If that tiny little balance wire isn't enough, it'll act like a fuse and overheat and I've seen it done.

You could use that balance board you showed me, but you better be sure you don't have a ton of current flowing from a balance wire into another series string.
 

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Discussion Starter #16
So given the following conditions:

1. Balance wires aren't paralleled while discharging (I'll be charging at MUCH MUCH lower currents).

2. Cable that's connected batteries in series is rated for a good margin above theoretical current leg (i.e. Ia, Ib or Ic)

I should be ok right?

Thanks again for all the help,
Aditya
 

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but you still would have to individually balance each pack separately and have twice as much cell-level monitoring to detect LVC and HVC.
 
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