Shunting damaged battery cells

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Magendanz
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Shunting damaged battery cells

Faced with a frustrating inability to purchase replacement battery cells, I've been considering short-term solutions to getting my Vectrix back on the road. Right now I can only ride a few miles before completely losing power, and I suspect it's due to only a single damaged cell.

Considering that there are 102 cells that each only provide about 1.3V, I'm thinking that a simple shunt across the damaged cell might do the trick. Overall range would certainly be less with 101 cells, but it should be considerably better than what I'm seeing now.

I should be able to create a bus bar shorting across the cell that could be easily bolted on top of the existing inter-cell bus connectors. My concern is that if the cell isn't totally depleted when I connect this shunt that I could cause heat (or an explosion) that would damage neighboring cells. Probably starting with a resistive shunt first makes sense, but what would be an appropriate resistor to use?

X Vectrix
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Re: Shunting damaged battery cells

This method is a good solution. If the cell is below .5 volts just short it with the bus bar. You wont create much heat. If you think its too high put a small light light bulb or 50 ohm resistor across it overnite then put on you bus bar. If its just the one cell that is bad you should see a significant improvement in performance after this fix.

siai47
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Re: Shunting damaged battery cells

I hope you don't have something else going on or more than one shorted cell. Usually, when a NiMH cell discharges completely, it acts like a short and shouldn't affect your range as much as you have seen. If it were my Vectrix, I would remove the cell and install a spacer to replace it. If you don't want to remove it, I would jumper around the cell and just leave it there----open circuited. In either event, the battery cells don't like to see more then 1.45 VPC during charge. Removing just one cell brings you to the limit, more and you will overcharge. You will need to time you charges so you don't exceed the voltage limits if you remove more than one cell.

X Vectrix
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Re: Shunting damaged battery cells

Actually it can affect your range because affectively it becomes a resistor. On discharge it drops the overall voltage and on charge it increases the overall voltage such that cutoff is reached way too soon. I have removed 2 cells before w/o any problems with the charging routine. Again, as long as it is just that cell that is bad.

Jumping over the cell is a better idea. That way you dont have to monkey around with discharging it first.

antiscab
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Re: Shunting damaged battery cells

it actually does more than become a resistor, it becomes a voltage controlled current dump.

the voltage of a reversed nimh cell for the first 20% of rated Ah of reversal is around -0.2v
after this, it becomes -1.2v.

and it does this at full traction current.

+1 jumping the cell.

Matt

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah
Vectrix 60Ah Lithium Tyres Fuel Registration Insurance cycle analyst 2 x TC Charger & MC
conversion

AndY1
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Re: Shunting damaged battery cells

But, won't this method cause cells' overcharging, when charging?
If the current charging cut-off is at, let's say,
152V / 102 cells = 1.49V / cell
152V / 101 cells = 1.505V/ cell

and it may happen, that, after a complete discharge, a red battery telltale appears, next charge goes up to the cut-off Voltage (+ to measure battery's capacity).
That may overcharge the cells or even in the worst case scenario, cells may never reach 152V and CP overcharging will go on until... I don't know... battery cooks?

amberwolf
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Re: Shunting damaged battery cells

For every cell you take out of the loop, get two or three power diodes capable of handling the full current of charging, times at least two or three, and place them in series with the charger to the pack. Under load they'll drop about 2 to 3 volts total, enough to compensate for the cell that's missing during charge. Since it's only on the charger circuit, it won't affect operation on the road.

If the charge current is more than a few amps, then parallel several diodes (with plenty of space between them for heat dissipation) for every voltage drop you need to make. If you need 3 diodes in series to get the drop necessary for a cell compensation, then you might put 4 or 5 diodes in parallel, then 3 of those 4-5 diode groups in series with each other.

If you have to for heat dissipation, you can get TO220 or TO223 cased diode packs (usually with two in each pack, common cathode center pin) and bolt those to individual heatsinks that don't touch each other (so you don't have to electrically isolate the diode from the heatsink, which makes heat transfer less efficient).

--
Michael Elliott
Cybernetic Necromancers, Discorporated
Phoenix, AZ
Watch me build an electric-assisted recumbent bike from recycled junk:
http://electricle.blogspot.com

Mik
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Re: Shunting damaged battery cells

For every cell you take out of the loop, get two or three power diodes capable of handling the full current of charging, times at least two or three, and place them in series with the charger to the pack. ...
...
...

Great idea, but only if you have a charger separate from the scooter. It will not work on a Vectrix because the on-board charger is highly integrated and disconnecting the battery and reconnecting it with diodes in series before each charge is very impractical.

This information may be used entirely at your own risk.

There is always a way if there is no other way!

Magendanz
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Re: Shunting damaged battery cells

Well, I discovered that it was only one cell and it was in the top layer of modules. Moreover, the voltage across the cell was 0.0V, so that really set to rest any fear I had of shorting it. All-in-all, it took about 20 minutes to fashion the shunt from a cheap piece of stock aluminum bus bar.

Damaged Cell Shunt

Since I'd depleted the battery earlier in order to minimize the residual charge in the damaged cell, I really haven't had a chance to measure the new capacity of the battery. All I've done after reassembly was ride it down until the battery light was illuminated. This should give it a full charge and I'll test my range in the morning.

AndY1
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Re: Shunting damaged battery cells

Please, watch the end of CP charging, since there will be more Voltage per cell at the cut-off, as I mentioned above!

I wish you success and please, report back :-)

X Vectrix
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Re: Shunting damaged battery cells

I hope they are big diodes! Plus you wont be able to charge as current wont flow in the charge direction. The charge routine is flexible enough to charge the pack minus a cell or two so dont worry about it. (nevermind...i read the post too fast, i see what you are saying now..)

Mag...nice shunt. should work well

R
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Re: Shunting damaged battery cells

Don't forget that the "regen" function uses the charger to insert these receovered electrons inside the battery....

Magendanz
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Re: Shunting damaged battery cells

Okay, power and range definitely seem to be restored. I just went 30 miles with lots of hills, acceleration, and freeways trying to see if I could cause an overtemp, "buSULt" warning, or early battery light. The bike performed well, just as it had before the blown cell.

When I finished the run, however, I still saw elevated temps on the affected module (~34C vs. average 30C, ambient at 9C) which has me concerned that neighboring cells might have been damaged when squeezed by the swollen cell. There's a lot less airflow through those pancaked spacers, so I'll have to watch them carefully.

Clearly, the better solution is to replace with a new cell, but until I can find a distributor for the GP battery module (or Vectrix starts honoring its warantee again) this seems an effective stopgap.

R
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Re: Shunting damaged battery cells

that's a really useful way to temporally overcome a damaged cell. Thanks for keeping us posted.
Does it charge well? I suggest you avoid stressing the battery until you get a replacement...

AndY1
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Re: Shunting damaged battery cells

How about taking out the damaged cell for better airflow??

Mik
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Re: Shunting damaged battery cells

How about taking out the damaged cell for better airflow??

You need some sturdy dummy of the same size, because the metal strap that holds the battery modules together needs to be tight to prevent the remaining cells from expanding and rattling around.

This information may be used entirely at your own risk.

There is always a way if there is no other way!

Magendanz
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Re: Shunting damaged battery cells

Does it charge well? I suggest you avoid stressing the battery until you get a replacement...

I haven't noticed any difference in the charging cycle so far, but I understand that each cell will now be getting 1% more than they were before. Honestly, with the limited airflow through those flattened spacers I'm more concerned about overheating than overcharging.

My goal with the shakedown ride was to really stress the battery, so I drove it pretty aggressively to see if I could generate a fault. (Ideally, I would have also ridden on a hot day, but that wasn't an option.) From now on I'll ride more conservatively, but I'll have some confidence that it can handle a little abuse if needed. I don't think I could handle the embarrassment of blowing another cell when crossing a floating bridge.

Mik
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Re: Shunting damaged battery cells

This thread has been added to the Vectrix Collaborative Handbook, please stay on topic!

This information may be used entirely at your own risk.

There is always a way if there is no other way!

R
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Re: Shunting damaged battery cells

I have a friend with a death (very short range) battery.
Do you still have the battery with the cell shunted? Is it still operating ok?
anybody else tried this method? Thanks.

Magendanz
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Re: Shunting damaged battery cells

My bike has been out of commission since early April because of a motor controller issue, but the battery repair had been serving me well right up until that point. I'd put about 4000 more miles on it commuting daily to work since shunting the cell. While I'd seen some decreased range, I believe that would have occurred regardless.

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