What's up with this batt pack?

[cgandersen]

So I built up a battery pack (6cell, 3800 mAh, NiMH) and at first, it would charge, so I had to "force" a charge into it and it worked fine up until recently... I tried Novak's SE Discharger and now the pack wont charge again! It's saying that there's an "open circuit". So I checked the pack, it's running about 1.2V per cell, but when I do check the resistance, it is greater than my meter's 20MOhm range... odd, no? So I tried an ICE charger from a guy at the field his charger was saying it was short circuited! Well, this clearly wasn't the case... I'm at a loss. I'm gonna measure each cell's resistance at this point, maybe one of them has an inordinately high resistance... this happen to you?

[rccardude04]

Is it your first built pack? Do you have the cells shrinkwrapped together? Shoe-Goo'd?
I bet a bar popped loose. Most of us, when we build our first packs, have REALLY CRAPPY SOLDER JOINTS. What usually happens is that you'll get a cold joint, and the solder will pop loose from the cell. Then nothing works properly. A matching tray will still see it because it contacts the cell directly (almost). If yours are ugly, I would probably find someone with more experience to solder them for you so that you don't overheat and destroy the cells.
-Eric

[cgandersen]

It's my second pack... so close ><

Still, I shouldn't get a voltage differential across the pack and have it work periodically? Although you do bring up an interesting point... it was shrink wrapped together and to get it to work with the discharger, I cut holes around the perimeter to make contact with the battery bars... it's possible I did have a bad joint and by opening up the ends, I released what little connection I did have... it should be pretty easy to isolate: check the resistance across each joint! Thanks for the tip... I'll let you know how it goes ;)

[GSMnow]

You may need to test the voltage drops under current. I like to use a moderate current light bulb discharger (mine is just 4 tail lights, pulls less than 8 amps) and then measure the voltage of each cell while under load. They should obviously be about 1.2 volts and all very close to each other. Then measure from the terminal of each cell to the battery bar on it. On the end cells, measure from the wire to the end of the cell. If your solder joint is good, the voltage will be very low on these tests, in the very low millivolt range. If the pack is so dead that it won't light the lisghts, then you have a bad problem somewhere. With just a little luck, you will find the cell to bar connection that is dropping 7 volts. This test will also show up a bad cell very nicely. Just be sure to watch the polarity. I had a dead cell that went to negative 1.3 volts. When I first scanned the pack, I missed it because I didn't bother to switch my leads so I just ignored the "-" sign as I knew 1/2 should have been anyways. I found it when I saw 1.25 to 1.35 on every cell, but the total voltage was just 5.0 volts under load.

I never had a set of 3800's, but my IB4200's really don't like going below 1.0 volts per cell. If you discharge them too low, the cell willlose a lot of performance, both current and run time. You almost have to treat them like a LiPo. Never over charge or discharge them or you greatly shorten the life.

In my XXX-T chassis, I had a battery bar break loose, and all I noticed during the race was what looked like radio glitching. It would hesitate and stutter every once in a while, and even the steering would freak out a little under power. At the end of the race though, one of the cells was hot enough that the shrink wrap was tearing. That cell never took a good charge again. The battery well is tight enough in the XXX that it held the bar on but the resistance was high causing the heat and glitching. Without load on the pack, it all measured good, but when I took it out of the truck and put the bulbs on it, they would not even light.

[cgandersen]

very good info... I'll carefully check it out tonite. Thx!

[rccardude04]

I'd take the whole pack apart and check the solder joints. See if you can pry the bars away from the cells.
-Eric

[cgandersen]

LOL! I pulled the shrink wrap off and the thing fell apart in 2 pieces :p Mystery solved!

[zueslilbuddy]

Tip get clear shrink wrap, Loose cells leave signs from where they got hot before letting go.

[GSMnow]

The best results I have seen for soldering cells is to use a soldering iron with a very fat tip with alot of thermal mass. I see peoplle rocomending 40 watt irons. I feel 40 watts is a bit too small for good joints. Use a 75 watt or so, temp controlled would be good so it does not cook while sitting between joints. You want to get on the cell, get it hot enough for the solder to flow, and get back off as quick as possible. The high power and fat tip do that the best. I have used a gin as well, but it just does not have the thermal mass, so the tip cools too much when you touch the cell. Even at 180 watts, it is not as good as a 75 watt iron.