Temperature Limits

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Mark
Site Admin
Posts: 411
Joined: Sun Jul 28, 2013 6:47 am
Location: Brisbane, Australia

Temperature Limits

Postby Mark » Sun Oct 27, 2013 12:29 pm

Since things are starting to get a bit warmer again down under, I've run into another charge being aborted due to the temperature limit being set to 35 degrees Celsius.

I've made a change for the next firmware version: The temperature limit is now set dynamically at the start of the charge. I've currently got the defaults set to a maximum limit of 50 degrees Celsius, or 20 degrees above the starting temperature of the cell - whichever is the lowest.

My reasoning behind this is that just checking the data from charging a good Eneloop, I saw a temperature rise of about 5 degrees C. A cell in poor condition with high internal resistance will obviously heat up more - with the maximum limit of 300 milliohms, the temperature rise due to the internal resistance will be about 6 times higher than on a good Eneloop. Some of the temperature rise is however due to the fact that charging a NiMh cell is an exothermic reaction, and the temperature rise due to that won't change with internal resistance, hence why I haven't set the limit to 30 degrees higher than the starting temperature.

The upper limit of 50 degrees I've taken from a Panasonic data sheet which specifies an upper limit of 55 degrees Celsius for AA cells - I've just dropped the limit down a little to be on the cautious side. 55 degrees C is a bit toasty for the health of the cell, but should still be quite safe - dropping the upper limit a little just allows for a bit more margin of error.

If anyone has any suggestions on changes to these values or another approach, I'm all ears!

DeweyOxberger
Posts: 5
Joined: Sat Sep 14, 2013 12:37 am

Re: Temperature Limits

Postby DeweyOxberger » Sun Oct 27, 2013 5:29 pm

How noisy is the temperature reading? Is this just a "threshold is too close" problem or a "spikes on the temp reading" problem?

Mark
Site Admin
Posts: 411
Joined: Sun Jul 28, 2013 6:47 am
Location: Brisbane, Australia

Re: Temperature Limits

Postby Mark » Sun Oct 27, 2013 9:44 pm

It's a threshold too close problem.

The temperature readings have very little noise in them - a bit of noise would actually be useful so that we could do oversampling to get a bit more resolution!

One exception to this is that it appears that if the input voltage starts to sag, that causes the measured temperatures to drift (higher) and that has led to over temperature failures as well.

Tcepsa
Posts: 46
Joined: Tue Aug 06, 2013 5:26 pm

Re: Temperature Limits

Postby Tcepsa » Fri Nov 22, 2013 2:30 am

I think that's a great feature! I was running into this pretty frequently in my house over the summer--now that things are cooling down on this side of the planet it hasn't been an issue, but that's only a temporary solution. I like yours better ^_^


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