Author Topic: Battery temp sensors failing  (Read 1960 times)

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domeguy

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Battery temp sensors failing
« on: October 21, 2016, 09:32:51 AM »
I've been living off-grid for the past 8 years on a pair of SW4024 inverters with a Dan-built 12' mill and various solar charge controllers. Just this year BOTH of the battery temp sensors going to the inverters have quit working. I'm not sure how these sensors actually work. They have a 4 wire interface and are intended to stick to the side of a battery.

I can't find a place to buy replacement sensors. Anybody know a way to replace these wierd things?

Thanks for your help.

-Lee

Eraser3000

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Re: Battery temp sensors failing
« Reply #1 on: October 24, 2016, 09:21:24 AM »
Using an ohm meter test between wires and you should have two loops around 100 ohms each. Assuming each loop is still good.   If so these are RTDS.  And are very common.
Tell us what you find.

richhagen

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Re: Battery temp sensors failing
« Reply #2 on: October 24, 2016, 01:57:46 PM »
I recall this from a few years back if it helps. Rich

http://www.fieldlines.com/index.php/topic,141617.msg947517.html#msg947517
A Joule saved is a Joule made!

richhagen

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Re: Battery temp sensors failing
« Reply #3 on: October 24, 2016, 02:06:25 PM »
In case you don't get to the meat of it, the trace ones were 100k thermisters centered at 25 degrees celsius wired to a modular RJ11 connector per both the tests and per boB who helped design them. 
A Joule saved is a Joule made!

domeguy

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Re: Battery temp sensors failing
« Reply #4 on: October 27, 2016, 03:55:22 PM »
Thanks for the info, folks.  I did actually do a search before I posted; if it had been a good search I would not have needed to post, of course.

So I see that I need a 100K thermistor wired to the RJ11 connector. It seems that doubling up the connections (2 instead of 4) to the RJ11 connector might be simple redundancy - that way if one of the wires in the phone line cable is severed by a little rodent (they LOVE these low voltage wires) the sensor still works.

domeguy

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Re: Battery temp sensors failing
« Reply #5 on: October 27, 2016, 04:55:54 PM »
OK, one more complication.  Thermistor data sheets put great weight on the B-value (beta) of a thermistor.  For example, I can get from Amazon 10 each of ntc mf58 3950 B 100K thermistors (5% tol).  This part has a B value of 3950.  Is the beta very important for the range of measurement here - about 45°F to 100°F?  At different B-values the resistance at temperatures other than 25°C are different.

At this point I'm inclined to blow off worrying about the B value.  Without any sensor my error maxes out at 0.9 V at the minimum temperature, which is huge.  I'm guessing that 90% of this error goes away no matter what the B value is.

Reasonable?

Thanks again!

Lee