Why parallel packs?

Imagine you have a 18s2p lipo pack. Now imagine you put it in series and parallel the series groups. Now balance them and put the balance connectors in parallel.

People will say, "oh the weak cells will pull tons of current through the balance connectors potentially melting them." But will they?

Lets say you have a good cell with an Ri of .002 ohms and a weak cell with a resistance of .004 ohms. Lets say you hit them for 100a. The voltage sag difference between the two cells will be (.004 ohms *100a)-(.002ohms*100a) = .2v

.2v/ (.002 ohms + .004 ohms + .004 ohms for the wires) = 20a flowing through the balance harness for a short while.


Now given that most of our lipo packs are pretty well in balance I have to wonder if paralleling them is worth the effort. Cause it's a lot of work! The main argument for it seemed to be keeping them in balance, but simply putting them in parallel with the balance taps is easy and will take care of that nicely.

Is my math really wrong or what?

20A through 28AWG wire? The AWG standard says if you're intending to run that spec, you should be running at least 14AWG. I don't know how long it will take to balance, but if it's more than a few seconds, I'm not sure how good your balance cables are going to be after that.

That would make for really subpar performance and cycle life. if we use the example of some 20-30C lipo, one part of the 5AH will try to supply all the power in this 100A situation, and will get very hot, while the other one will probably stay just warm to the touch, that's because only so much current can flow through those little 20-24AWG wires.

You will actually get a severe voltage disbalance between the packs, which will slowly correct itself as one battery tries to charge the other - but the balance connectors will try to transfer more amps they can; they'll act as a resistor and waste heat from copper resistance.

There is a reason why large packs containing many cells in parallel have to have optimal current sharing.
You need fat wires between the two so that the load is equal on both.

Also i'm not sure what you mean by it being a pain in the ass to parallel things. Once you make a proper harness, you're done with it. I run 20AH of turnigy 20C stuff, it's great.. 1 plug charges it.. i only have to worry about checking the balance a few times a year..

The thing is - in practice - the packs should only have a voltage difference under extremely heavy loads - and only for seconds. I really can't see the balance harnesses passing much current.

I'm not sure what type of math you'd need to calculate that though. differential equations? calculus?

I've got 6 x 5.8Ah 8 cell Turnigy long packs hooked up as 3P series'd into a 16Cell pack.

One of the cells is a bit weak, but I had soldered all the balance cables together in each 8Cell pack. No problems so far after 450 cycles and about 5,000 miles later. I balance every week or two with my iCharger 10B+.

The thought of having to balance charge 6 times instead of just two each week would make me want to peddle!

I just didn't like adding all that wire to the pack, while on the bike. So I just use a jst paralelling harness only when I want to balance charge a pack. Like at most every 10 cycles. So my jst paralelling harness stays on the charger, and I only need two of them, instead of four.

I just don't feel like it's needed to keep em paralelled at cell level for riding. Anytime I've seen any cell weak enough to give a damn about it, it's been on a pack that has a cell so bad it's going in the recycle pile anyway.

Actually thinking about it Dogman I reckon your way is best. Will do that next time.

I hook up my ballance wires in P, makes monitoring a large number of packs easier.. There are situations for both methods..

After a few years here, and especially after getting into lipo, I began to realize there really are a million ways to go about this.

Some are wrong, usually indicated by a flash of plasma. But there are lots of right ways to use the batteries. It just all depends on your needs, what equipment you may have, etc.

The way I handle my lipo is entirely dependent on the fact that I use it on several bikes, a lawnmower, a weedwhacker. So nothing is ever permanently installed on a device. So that makes my doing it the hard way, not so hard. It's coming off the bike or the mower or whatever for charging anyway.

If built one bike, and always ran it on one set of batteries, I'd be much more inclined to be installing it permanently on the bike, with permanent balance harness built into a hatch on the box, etc.

I just do it the hard way, because it makes it easy to move the batteries from bike to bike. Or rotate sets of packs through the bike at the track. The track bike kind of set the tone. I wanted to be able to slap in fresh cells in less than 90 seconds if I needed to.

It may cost me money I don't have lots of, but I have better things to do than check voltage on every cell daily. Honestly, not paralelling my packs at the balance wires is mostly pure laziness. Once you are only looking at em once a month or so, it's not that much more work to just work through all the packs with a cellog 8, and then balance only the packs that need it.

auraslip said:

The thing is - in practice - the packs should only have a voltage difference under extremely heavy loads - and only for seconds. I really can't see the balance harnesses passing much current.

I'm not sure what type of math you'd need to calculate that though. differential equations? calculus?

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Only for seconds - if your heavy load only lasts seconds..

If you have a 100A load, and the balance wires are only transferring 20A (if they even could do that), you've got the other lipo pack putting out 80A and taking all the stress. You've got one pack sagging and making heat, losing watt hours, then you've got another pack making heat as it tries to push more juice through 20-24awg wires

Congratulations, you have the worst parallel current sharing configuration possible. You've managed to make a 20C battery into one capable of 11C that will wear your batteries unevenly and provide the lowest watt hours possible.

This pack saw 90 cycles without a balance, no puffing, no loss of amp hours in over 1 year. Why?

1) The parallel balance harness keeps the individual cells in check 100% of the time.
2) All packs have been charged and discharged on a balancing charger to weed out the duds; some packs were set aside and not used in this pack.

This is, bar-none, the easiest way to live with lipo. Want to check the balance? there's 2 plugs. Want to charge it? you already know the balance is good, so just plug it into your bulk charger, charge to 4.15-4.17v/cell, and get on with your life.

It doesn't need to be more complicated than that, and setting it up properly in the from the get go is not hard.

I guess that is kind of why I don't paralell at the balance wires most of the time. I'd rather let a bad pack die, than strain a good one supporting the one bad cell in a bad pack.

To me it makes sense. If your pack really needs that support from the paralelled one, it's in need of junking it. After awhile, you have a stash of remaining good cells handy for doing something kooky with.

My attitude would be different if you could just buy 2p, 3p or 4p packs. I'd definitely be interested in buying 2p3s packs! Or even just 4p packs. 4p 20 ah lipos in hardcase for 25 bucks would be great for long haul bikes.

neptronix said:

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Ohh man... how many things wrong can i point out with that picture..

- controller inside canvas bag ( weather proof yes, heat sinking no )
- lipo packs with no support from side impacts, nothing seperating each pack from rubbing against each other.

see, just because someone gets away with a certain method, does not make it right for everyone ! :wink:

1) controller never gets hot, running it below it's rating . In the summer, i crack the top of the bag open a little just to be safe.
2) seat and cranks prevent the triangle from being damaged and i do not ride offroad. I've dumped the bike twice. The bag is really narrow and has never touched the ground even if it theoretically could ( like if i removed my cranks or seat )
3) packs are well insulated, do not vibrate much, and there are no markings/wear indicating that they rub on each other.

I have gone through a half dozen lipo mounting and managing techniques to find the perfect one, and this one has never left me wanting.

Given a discharge rate so low you can smother your controller, it would be a BIG suprise if your packs didn't stay balanced.

Personally I'd be inclined to throw a layer of coroplast in the bag just for the murphys law crash I seem good at finding a way into. But you are correct, that package doesn't stick out far enough to contact the road in a laydown. Pedals tend to interfere enough to prevent a rash on that bag.

I think it's a great example of how easy lipo can be when you have one bike, don't race, don't flip it regularly on the rocky trails. Ride like me, and you carry the lithium in hard metal boxes. Every one of my metal boxes has a bash mark.

The 75% of the magic is in throughout testing and not using the duds hobbyking sends, and the other 25% is cutting off around 3.5v/cell resting, go much farther than that and the RI starts to differ greatly, to the point where some cells are putting out more current than the others in series. The C rate used doesn't matter all too much if you have a good set of cells.

I'd protect my packs a bit better if i had your kind of terrain, definitely . For me, it's mostly streets with some minor potholes but nothing major. I really don't like permanently sealed packs though, since if a pack starts puffing, you can't catch it if it's permanently sealed. That's why i always liked rear racks and center triangle mounting.

My cells will probably never puff or do anything nasty, but i have 2.5kWh worth of lipo in an old upstairs victorian apartment and i don't take any risks with lipo. That's why i'm borderline OCD about getting it dailed in 100% perfect, to the point where i can mostly ignore it and use it like it was a ping pack.

If you have a 100A load, and the balance wires are only transferring 20A (if they even could do that), you've got the other lipo pack putting out 80A and taking all the stress. You've got one pack sagging and making heat, losing watt hours, then you've got another pack making heat as it tries to push more juice through 20-24awg wires

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I don't think it'd work like that. The current flowing through the balance harness would boost the voltage of the weak pack enough to make up for it's higher Ri. In practice I think very little current would actually flow through the balance harness. But I'm not sure of the math needed to figure that out.


Nep doesn't wear a helmet so I guess he figures if he crashes, the batteries will be the least of his worries. Live dangerous

Try it out, auraslip, and you will see for yourself how the current sharing works when there is a choke point. The more amps being drawn from a cell, the more voltage sag. The only way to lower the voltage sag is to lower the current draw, or say, parallel more cells up and share the current between them evenly by using a properly sized linkage..

At 10 amps, you probably won't run into a problem since the balance wires can carry around that much. At 100 amps, you will see another story.

Do the test in real life with the proper equipment and prove me wrong.

Oh, and i do wear a helmet when riding above 25mph if the trip is over a mile or the route is not bike friendly.

If i crash my bike so hard that the pedals / seat / handlebars break or fall off, i have more problems than just a scuffed up lipo pack, so i don't worry about it. That's how i see it. Batteries in a rear or front rack could be damaged just as easily. Strangely enough, i have not heard anyone criticized for a rear or front rack, i guess my mid-mount is just super unsafe, right?

Why would the load be only traveling through the balance wires? Wouldn't they be also paralelled at pack level with the discharge wires? Any big imbalances would then be adjusted at that connection. Only smaller differences in individual cells voltage would be getting adjusted by the tiny balance harness wiring.

It's part of why I don't get it, why it would be mandatory to paralell at the balance wires. Damn right it works better for you, since all your wiring spagetti gets tucked into a tidy bag, and you only use a tested pack in the package.

But not mandatory. 99% of any voltage equalizing is of course happening on the big wires, at pack level.

Isn't the OP's question regarding what happens when 2 cells that are parralled the pack level and the balance taps have a large difference in voltage under load? I would be interested to know more as well, seems to me the balance wires would not be able to handle it and would vaporize like they do when you connect packs with unbalanced cells via the balance taps.

In most, normal situations, this works fine...

If you were to hook up an empty and a fully charged pack in parallel, expect problems... sure..

But, a slightly out of wack but usable cell will be kept in check without any melting ballance wires.. i know this from plenty of personal experience.

Path of least resistance, by paralleling power AND ballance leads of multiple packs, the current is shared among the pcells and the voltage will equalize, to melt ballance wires you would need a severe imballance and an abnormally high amp load that most of us simply do not see on ebikes ( unless you are liveforphysics.. )