NiMH tripping low voltage cut-off early

[D-Mon]

Hi everyone,

 

I've noticed some odd behaviour with my NiMH battery and ESC. I have a theory and a possible work-around that might improve things but wanted to run it by the forum and see if anyone has had similar issues. ESC is a hobbywing quicrun 1060, battery is an overlander 3300mah 7.2v pack. Buggy is a Tamiya DT-03 and motor is Etronix 17t brushed modified.

 

Battery and buggy run fine for about 15 -20 minutes off-road (grass or sand), before ESC slows the car and steering becomes intermittent before both cut out. Do the usual checks of ESC and motor temp and both seem to be ok, not too hot to the touch. However, the battery at this stage is hot, not dangerously so but I think its a lot hotter than it should be under normal operation. I don't think its a faulty ESC or battery although I don't have another battery to test yet.  Turning the car off then on again allows a brief (<1 minute) of run time before the same thing happens.

 

The reason I think its related to battery temps is that by the time I've walked home, the car functions normally when switched back on. At this point the battery is still warm but the temperature is significantly lower than at the point of cut out. Using my multimeter, the battery voltage reads about 7.2v after getting in. When left to cool for another 30 minutes to room temperature, a further reading on the multimeter shows the battery sitting at 7.8v. In both cases I can run the car indoors for a further 20-30ish minutes, albeit not at full throttle due to space constraints before the battery runs flat. For reference, fully charged the battery registers about ~9.3v.

 

I know that the internal resistance of batteries and other electronics increases with temperature and that an increase in resistance lowers voltage so I'm reasonably confident that hot batteries are the issue, especially as it works properly for a while after cooling down. I know switching to LiPo would be the best option but there are a couple of reasons I can't do that at the moment, so have to persevere with NiMH for the time being. With that in mind, would swapping the connectors from Tamiya style to something like a Deans or EC3 help to reduce the heat build up in the battery? I've read that the Tamiya style connectors are quite inefficient and considering the max discharge rate of NiMH is already fairly low, I can't imagine leaving these on is helping. Motor is only a brushed 17t and gearing is 17/55 so nothing crazy and I think in-spec for off-road use so I doubt the issue is with the ESC, but respect that such conditions would probably cause the motor to draw a lot of current and I think this is the root cause of the battery heating up. I should also note that I didn't have this issue when using the TBLE-02s and Tamiya torque-tuned motor that the kit came with.

 

Any thoughts or ideas are appreciated. I guess I could just get a second battery and swap them out as they get hot to allow each one to cool and then hopefully run them flat instead of ending my bash when it cooks but would prefer to reduce heat build up to begin with.

[Stormbringer]

15-20 mins on a 3300 isnt bad mate and nihms do get warm when being run especially when starting to go flat/tired

when it slows down like that basically battery is flat and needs recharging only way round it is more batteries really hth

 

[D-Mon]

Yeah I did wonder if it was just the max life of the battery, it was getting 25ish minutes on the kit motor but that was a much weaker motor. Was just a bit put out by the how hot it was getting and thought that might be affecting things, especially with the voltage climb after being given time to cool off and if there was anything odd about that.

 

Will probably get a couple of 5000mah in the near future for a bit longer per battery and so I'm not hammering the same one all the time 😁

 

Might still swap connectors though and see if that reduces temperatures. At the very least would future proof it for eventual LiPo I guess, when that does become an option for me.

 

Edited April 4, 2021 by D-Mon