NiMH Heat Problem

[nickB722]

Hey Guys,

Well this past week I took my Impulse 31 out to the family cottage and suffered a small mishap. Despite running 3-4 times a day in the 100+ degree heat last week with no issues, my 2nd run there resulted in one of my new Dynamite 5100 6 cell packs splitting the shrink wrap over both the outer wrap and the inner covering the batteries. The batteries are usually pretty warm after a run (no temp gun to verify), but hadn't split any shrink before. The very next run (after ensuring there was no shaft binding) both of my 4500 packs also split both layers of shrink. Again, none of the batteries seemed much warmer than normal, though they were in the boat slightly longer due to the distance from my pit area to my running location.

Any idea what could have caused this? The boat was running with the M445 on it when the 5100 split and the 4500s split with the stock prop. I had cooling flow to the motor and ESC the whole time (neither were warm to the touch and the connectors weren't even warm compared to a normal run). The biggest difference in running at this location compared to others is that I had to do a bit more starting and stopping to clear weeds from the rudder and turn fins, but the prop never got fouled and the electronics all stayed cool. Runs were under 10 minutes (probably closer to the 5-6 minute range) and all batteries were checking out at about 7.24-7.28 volts on my Onyx 245 after the splits. None of the cells were shorted out (verified by an uncle who is an electrician and was puzzled by the splitting/battery heat but impressed with the boat nevertheless). Is this normal with certain packs or could it just be a bad batch? In a test run afterwords (with high grade electrical tape repairing the battery), the boat ran as normal and the heat stayed down using the stock prop. All packs were less than 2 weeks old.

I know most everyone here uses LiPOs, but if anyone can think back to the good ole NiMH days and help me out I'd appreciate it!

[BHChieftain]

While you can technically run NIMHs all the way down (unlike lipos), they can get very hot when eeking out the last bit of power. I used to run NIMHs in my Fastech and once I "blew" a pack when I ran to too hard for too long on a larger prop. Try either backing off on the prop choice, or back off on the time.

Chief

[sanyijr]

Nick,

While i am not 100% sure, I am beting your packs are not rated for 20C (90 amp) use. The high output NIMHs are usually maxed at a much lower discharge rating... I am sure the first few runs may not have pushed them quite so hard, but as they get cycled, a little more resistance is added and then comes the heat.

Note that even guys with 20C lipos have battery and esc heat issues because of not having enough discharge capacity in the batts.

And you say you are pushing a M445? If you ahve another pack, prop down a size and retest. But I would be cautious about running MH packs that have already shown signs of fatigue....

I used to have that happen in my Hornet and Fox cars. geared up too much and blew packs and escs.... Now we have more expensive problems and solutions to contend with. :doh:

But I really think you are working with packs (NIMH technology) that cannot deliver the power draw needed for that setup.

And to note, the smaller battery with the stock prop has the same basic issue... The connectors would not heat up. Those would only heat up IF you are pushing more amperage through them than the connectors can handle. With the packs, the power source itself is the limitting factor.

The other thought I had was that due to ambiant air temps, the packs were already warmer than usual due to air temps. And that you are running at the packs limits as is, but this additional factor pushed them over the edge. Either way, I think you are at teh limit of the packs.

How do you feel about lipos?

Sean

[sanyijr]

Hey Nick,

Another thought came to me....

What packs are they? You say the wrapping split. But were the cells actually HOT? Could you hold them in your hand for 10 seconds without discomfort? You may have just popped the wrapping, but teh reason may not be heat at all if the cells were just warm... Cheap wrap can pop and split. They don't used thermal insulation on many NIMH packs. Just cheap plastic that can rip easy... Just another thought....

[nickB722]

Sean and Chief

Thanks for the help/ideas/suggestions!

Well my LHS replaced them on warranty, but I'm going to monitor these ones a bit closer to make sure it doesn't happen again.

The batteries were Dynamite Speedpack 7.2 volt 5100 mah and 4500 mah. The cells themselves were definitely hot (couldn't hold for more than a second or two before discomfort set in), but nothing else in the boat was warm which is why I was puzzled.

I was not running them in the high heat as I had the week prior (it was a solid week of runs in the 90s/100s with no issues then a run in the low 80s is when everything started to split)

Everyone at the LHS was surprised by the condition of the wrapping on the cells but seemed convinced it was a combination of bad/cheap shrinkwrap and probably running a bit too long. I'm going to keep runs a bit shorter and bring the boat in as soon as it starts to show ANY sign of slowing.

As far as pushing too large of a prop, I know the M445 is probably a bit big for NiMH packs, and I'll have to monitor it more in the future, but the batteries got warm even on the stock prop in the past, so I think it may be more of a how long I was running the boat and the starts/stops than anything.

Again, thanks for the ideas/suggestions/thoughts, it's much appreciated!

[sanyijr]

Nick,

I am telling you. If the cells were HOT, you are really at the packs limits. So I would find a way to prop down, raise strut height a bit or something to prevent that amount of power being pulled from the packs. AND if the cells were hot, you are luck they did not pop. That could take out the whole boat! Definitely keep and eye on it.

My cousin was running a Impulse 26 (brushed) and he has a high discharge pack and it gets hot just pulling the 30 amps or so the motor uses.... I am sure you are pulling more.... so without revisiting what you are doing and managing amp draw, I would expect this to happen again.

Though it says it is NIMH campatible, I don't think it was really intended for NIMH packs.

I cannot find anythign about these pack's discharge rating.... They say anything on them about max dischage current?

They come with Tiyama connectors as well, so not very high.... in any case.

[nickB722]

Sean,

The thing with the heat is that the cells only get HOT when I run them down to nothing (or at least really low). I've had similar heat with similar cells (not splitting shrink though) in my Evader ST (with a Castle 4600 system) when running down to nothing (they don't get HOT when running until the end, warm, but not NOT). I'm probably going to bump the strut up a hair anyways to try and adjust the ride of the boat a bit. I will definitely be keeping an eye on it! I had been checking heat on everything (batteries, motor, esc, connectors, discharge water) between 2 and 4 times every run prior to the runs on Monday and everything stayed within comfortable limits.

As for discharge current, there is nothing about that on the packs, box or the data sheet included (which seems to be a LiPO sheet with a different title at the top), and I can't seem to find anything about it online either.

I've melted Tamiya connectors and even a motor connector (between the Sprint ESC and 20T motor that came on my truck originally) with the same batteries before (though not the same specific pack), granted that's much lower current draw and lower end materials, but the pack wasn't even warm.

I will be trying a few different things with it this weekend and I'll report my results.

As for the LiPo thoughts, I'm not exactly a fan. I love the speed, power, and consistency, but I really don't have a practical means of safely charging them and my family refuses to allow them in the house under any circumstances due to a friend's bad experience.

[sanyijr]

I can understand the Lipo hesitance....

As for the heat, as long as you are aware and comfortable with when to cut the run short, you are good. Good luck and enjoy!

[nickB722]

Sean,

Just ran the boat twice tonight, stock prop kept the 5100s cool after a full run, and the 4500s were only a bit warm (10 second touch test was passed with flying colors) after a 5ish minute run with the m445. I'll be continuing to keep an eye on it and check the batteries mid-run, but so far so good!