Troubleshooting a Bad NiMh Pack

[Casadetrevino]

I had a Duratrax 8 cell Nimh pack put together for me at the LHS. It did good for several runs, but the last two runs it has gotten REAL hot. The heat it generated melted the heat shrink around one certain cell and even the duratrax label melted on one of the cells. When I shot it with my temp gun it was over 170 degrees and that was few minutes after I had quit running.

I am wondering if I have a bad cell that is causing excess resistance or bad solder point somewhere.

What is the process on testing these cells individually to find out which one(s) are bad?

Thanks in advance for your help!

[kaceywells]

I would take it back to the LHS that put it together for you. Let them know what its doing and maybe they will make you another one for free....or at least fix that one for no cost. Don't spend your bills on a problem you didn't create.

[Grouchman]

I bought a 7 cell Durtrax to test and the same thing happen to me... 4 runs for me. It got really hot and melted the plastic. Now the battery doesn't hold a charge.

[DrFizz2k1]

One would think, after ALL these battery threads, that people would stop buying low quality NiMH cells..

I'm not trying to be rude.. but if you're going to have a pack built, you should have it built by the best, using the best cell... Promatch Racing is the builder.. EnerG 4600s are teh cells... buy a high quality charger as an investment in ALL of your batteries present and future. biggest problem most people have is poor charging habits... NiMH are often more sensitive to bad charging habits than Lipo.

Good luck.

[Casadetrevino]

One would think, after ALL these battery threads, that people would stop buying low quality NiMH cells..

I'm not trying to be rude.. but if you're going to have a pack built, you should have it built by the best, using the best cell...

When you've been in the hobby for two weeks you generally don't know what good cells are. That's about how long I had our cars when making the purchase. You go in and trust that what your LHS recommends and has on the shelf is good.

I would take it back to the LHS that put it together for you.

I would do that but it has been a at least a couple of months since I bought it. The batt hasn't seen a whole lot of use because we went LiPo soon after buying it, so I'm not sure they would want to mess with it. On top of that hobby shop I got it from is about an hour away.

Also, I have new Weller soldering iron, that I just got today, begging to be used on such a project as this one.

We still do run these NiMhs though and I would like to fix this one if possible, even if making it into a 7 cell is necessary.

I'm just looking for some pointers from those of you who may have tackled a project like this for yourselves.

Doing little things like this is half the fun of this hobby, right?

[Vinny_Sabatini]

Doing little things like this is half the fun of this hobby, right?

So true. So true.

I would bring it back to the LHS and talk with them about it. If they blame you for it, and won't help (for free), you should tell them to shove it and find a new LHS to hang at. Don't act angry or accusative though, because then they will get defensive and blame you.

If they can't help you out, I would recommend going with some good promatch nimhs. They are really good. Of course, that is if you want to stay with nimh.

Well, whatever you do, good luck!

[rupert]

This happens very often and can be cell imbalance and/or cell mismatch. Generally individual cells are tested (for capacity is most important) and arranged so that they all get full and run out at same time. Even the cheapo cells can be matched very well if there's a big enough batch.

Without the proper equipment to test one can find the hot cell and 90% of time this is the weak/small/bad cell. One can also get the best cell balance by simply charging at c/10 for 15 hrs (no testing or dissassembly)

[SteveC2375]

I'm surprised the cells didn't vent at 170 degrees. I am assuming this is a side by side pack?

To determine if you have bad cells, you'll need a temp gun and a voltmeter. Fast charge the pack as you normally would, and as the cells start to heat up, check the temp each individual cell. It is normal for cells to heat up during a fast charge, but they should not go over 125 to 130 degrees. Any cells that are more than a few degrees lower then the rest of the cells is probably not charging correctly. You can also use a voltmeter to check the voltage on each cell. A bad cell will show a lower voltage or no voltage at all.

Even Pro-Match packs have problems sometime. I have several Pro-Match 6-cell 4600mah matched packs. I had two cells in two different packs vent on me. Still not sure why, because I take very good care of them. Possibly just a bad batch of cells. The outstanding customer service at Pro-Match replaced the cells, tested the packs, and shipped them back free of charge. I only paid for shipping them to Pro-Match.

[SteveC2375]

This happens very often and can be cell imbalance and/or cell mismatch. Generally individual cells are tested (for capacity is most important) and arranged so that they all get full and run out at same time. Even the cheapo cells can be matched very well if there's a big enough batch.

I agree, but this is only true for more expensive matched packs. These packs are usually matched for voltage, resistance, and capacity. The cells are matched and the pack is built by the battery manufacturer. I highly doubt the LHS has the test equipment to match cells, especially if they are using low quality cells like Duratrax.

Matching cells takes more time and expensive equipment, so your low cost, mass produced battery packs are not matched cells.

[rupert]

Let's rule out cell's that don't have 1 volt since they are junk. 90% time the coolest cell may be the best cell or at least the biggest or least charged cell. For this app the cell's IR and voltage are irrelevant since even if 1 cell had high IR they are all in series and the IR and voltage affects the whole circuit.

Testing cell temp with knowing the SOD of each cell can be problematic , so it is advisable to put all cells at the same SOD and then test with desired method.

When I said "Generally individual cells are tested (for capacity is most important) and arranged so that they all get full and run out at same time" I was referring to the op's dtx packs and "what is generally done" not what a battery matcher does with new cells when assembling "matched" packs.

[SteveC2375]

Casadetrevino,

So the cells with the melted shrink wrap are more than likely your bad cells. Test them with a voltmeter, and if they show a low or no voltage, they need to be removed. Get your new Weller soldering iron and rebult it as a 7 cell pack.

[Casadetrevino]

Rupert and SteveC, Thank you very much for the informative posts. This was the info I was looking for.

The pack was an 8 cell hump pack. Two cells on top made the hump. It is one of these two cells that melted the heat wrap, I believe.

As for testing them. What voltage should these cells read after quick charging the pack? And just for info sake, what do these cells read after they are discharged?

Thanks.

[Casadetrevino]

I found out what the capacity charge rate was for NiMhs. Looks like it is anywhere from 1.35 to 1.5 per cell.

What I need to know now is how to wire the 7 cell in series. The 6 cells that make up the most of the pack are self explanatory. It is the one cell on top that I am not sure how to wire.

Any tips?

[rupert]

Ultimately packs need to be wired so that all cells are in series , that is +-+-+-+-+-+-+-

A NI cell is very hard to tell it's state of charge by checking voltage , but generally a resting voltage of 1.2 is discharged and 1.4 is charged. The voltage is very springy so right after the quick charge they can read up to 1.6 volts and will quickly bounce back down to the resting voltage of 1.4-1.45. At the other end they can read from .5-1.2 , and will quickly spring back up to the resting voltage of 1.2.

Testing cell voltage is not always a good idea when trying to decide cell match or whether or not a cell is acceptable for a certain application.

The process that (I think) you seek is as follows:for each cell

Charge cell up
Test capacity
Sort a batch of cells according to capacity for best match.

Without a large batch of cells it is a waste of time. I always got a kick out of users and companies that offered to "re-match" a pack. By and large there is no such thing as "re-matching" a single pack. You'll get the exact same pack as you started with.

[traxxasbrothers]

Duratrax cells do not run good when assembled,

The best quality cells are pro match racing.
http://www.promatchracing.com/prodde...p?prod=5000-7M

[Casadetrevino]

Here is how I would arrange the bottom six cells in series. What do I do with the 7th cell? It will be the hump cell.

|-{===]--{===]--{===]-|
|-{===]--{===]--{===]-|

?---{===]---?

[jcjhl69]

Turn them sideways and alternate which side has the + and build two 3 cell packs and then put the last one in the middle of the two packs and you should see it come togther.

That would be for six you would flip ove one of the three in order to properble attach the seventh cell in the middle

[Casadetrevino]

Thanks jc, that helps.