Voltage drop with acceleration

Sidotian

10 W
Joined
Jun 24, 2017
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85
Location
Buffalo New York
Hello, I'm having some troubles with my winter project, I got my hands on really cheep unused 18650s. I have tested every single cell with a venom duo at 1amp discharge all seemed fine. Each individual cell is around 2000mah with the exception of a few higher ones(2500-3300). I built 2 6s 17ah(ish) packs to be ran in series. My problem is when full throttle on my 500w ebike hub motor the voltage of the cells drops to 3.20v from 4.1v. This can't be normal for li-ion right? So did I just get mostly low discharge 18650s. Is the way soldered them at fualt. I striped the insulation off 22awg servo wire and twisted two together for the battery to buss bar. All of the other wire came from a light duty 110v household 3way extension cord. What could I do to determine the problem I don't want to waste time on cells that arnt capable. Thanks
 

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It does sound bad, even for .5c discharge rate stuff. .5c on 17 ah would be about 8 amps, so you are exceeding that if you rare talking about just one pack. ( If I read right, you are running 12s 17 ah) 12-15 amps for full speed cruise is typical.

More would do better, but there is indeed the possibility that your soldering did in a few of the cells, making performance even worse.

What does it do on half throttle? If sag is ok then, you know what to do, 30 ah or more.
 
If you measure the voltage across each of the series and parallel connections inside the pack (measuring the actual wires, not the cells) and the pack's wires to the controller, while under load, then you can see if the wire itself is causing you problems.

If the 4.1>3.2v drop happens on *every* cell group, measured at teh cells themselves, then then the cells themselves simply aren't capable of the demand you're putting on them.

A "500w" system on 12s "44v" could draw 10-12A continuous, and depending on the controller could peak at up to twice that under load.

It looks like you have 8p, so if the cells were ok under 1A each, then your pack can handle 8A ok. But if you didn't test above that level, then you havent' tested at what your system actually needs from the cells.

The load could be nearly 3x that at peak (you'd have to measure that with a wattmeter or an ammeter between battery and controller), which would be hard on those cells fi they are only ok at 1A each.

If you parallel two of those 8p packs, then they could do up to 16A ok at 1A per cell, which would handle the continous load the controller should put on them. Might still not handle the peak load, but that should be short enough in most cases to not worry much about.



It's also possible soldering to the cells has damaged them; no way to know on that, other than retesting them exactly the same way you did before building the pack, and seeing if they perform exactly the same.


EDIT: I see that here
https://endless-sphere.com/forums/memberlist.php?mode=viewprofile&u=54270
you say your system draws 20A, so that is one definite reason why your pack performs poorly--it isn't enough of a pack to handle even half of that.

You'd have to have at least 20p to deal with that, if each cell can provide 1A.
 
I have built two bikes now. the bike that draws 20 amps I have 32ah 12s lipo with virtually no sag. I haven't measured the amp draw on the bike I'm currently working on. I do know that it is a 500w controller and motor. I have a isdt t8 charger I can power with one of that packs and run some tests. EDIT here's one pack charging the other at 8amps the pack providing the power has a sag of 1.2-1.5v
 

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Considering the effort you put in making these packs and the safety risks, I would first learn how to make a pack properly. You seem to have good DIY skills so here are some guidelines :

- Cells must NEVER touch each other. Use spacers ("battery holder") instead. That's to prevent overheating, thermal runaway, and short circuits
- You are hand soldering. Use insulation gaskets on top of the positive to prevent insulation meltdown. Make sure no solder ball gets trapped under the pole cap.
- Minimize the series connections length and multiply parallel paths. In your pack, the output cable is tied to the end of the pack. It should tap from the center of the cell row, ideally from each cell. Because in your case, the first cell sees more current draw than the last one.
- Don't mix cells ! As they have different internal resistance, cells in parallel will charge/discharge each other when current varies. That's why cells from reputable manufacturers are sorted in batches of similar internal resistance and capacity
- Use cells from reputable manufacturers and reputable sources. A 50-micron metal particle trapped in a cell during manufacturing can cause entire packs to catch fire (= 787 Dreamliner fires)
- Protect your pack from puncture, vibration and internal wiring damage

All this do add to the final cost, but what is the price of troubleshooting an awkward battery pack ? Is saving 100USD on cells worth setting your home on fire ?

A well-built, well-cared pack will last you 10+ years easily. Good luck :)
 
When I get unexpected results, I find doing the reverse maths can bring a sanity check into the situation. Might not help you find the problem, but might tell you whether you're in the ball park for what you're seeing.

4.1v to 3.2v = 0.9v drop

0.9 x 12 = 10.8v drop

You've said 500w on a 49.2v system, so close enough to a 10a draw

r = vDrop / i

r = 10.8 / 10

r = 1.08Ω for the entire pack or 90mOhms per string of cells.

To get a string, you look like you put 8 cells together in parallel. 1/R(total) = 1/R(1) + 1/R(2) ... 1/R(8).

Because I can't do maths until I get my caffeine shot. I'm going to ball park it and say 0.8Ω per cell is close enough (Round up to 100mΩ instead of 90mΩ for the pack)

That's around 800mΩ per cell

According to this thread:

"Typically a "good". 18650 cell would have a discharge IR of between 50 and 100 mohm (0.050 - 0.100 ohm)"

https://endless-sphere.com/forums/viewtopic.php?t=60364

Your cells are out by at least one order or magnitude. To give you another reference, my 16Ah multistars (For comparison to your 17Ah string of cells) are 3mΩ, or two order of magnitudes smaller.

All this maths just to show you what you already know. Nothing gained. (Not much ventured either.)

But maths can still tell us more. You said you had no drop at 1A for each cell individually.

Theoretical loss at 800mOhm internal resistance

800mOhm = vDrop / 1a

vDrop should have been 0.8v at a 1a draw. (Makes sense, you are applying 10 times the load, but 8 times the supply, and seeing 0.9v per cell drop. The difference can be accounted for by the rounding I did earlier)

So if you didn't see that, you can conclude one of three things:

1. There's a problem with your connectors/soldering.
2. You damaged the cells with heat during assembly.
3. Your 500w controller might be drawing a lot more than 500w (Not uncommon).
 
Thank you all for the advice. I think my best bet would be start over from scratch with better quality cells. Got myself a 3d printer so cell holders are no problem now. If definitely the cells I got cells from cloned laptop batteries.
 
800 milliohms resistance would be utterly useless. Those were dead cells to begin with.

Testing cells first, especially recycled ones, is worth the effort.

I bought some A123 round cells from a guy once, said to be good as new, but not factory fresh. Assembled into a pack, they sagged so much they instantly hit 0v, or close enough to it.

I forget what the IR tested to now, but it was not 50 or below. Those "good cells" were total junk. Still have them to this day, and they do hold a full charge. But they can't handle discharge for anything much more than a few small led lights. They work for lighting the tent, and recharging the phone or ipod.
 
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