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Old 15-05-2009, 05:26   #1
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Battery Temp Sensor Vital?

I'm in the process of buying an Outback Flexmax 80 charge controller for my solar array, and was wondering if the optional battery temp sensor is neccessary.

I'll most likely be charging a bank of AGM's around 600 amp/hours capacity, from 420 Watts of solar panels, possibly upgrading to around 600 Watts later.

Thanks in advance for any advice.
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Old 15-05-2009, 06:16   #2
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Assuming you have no drain on the batteries and maxiumum sunlight and (miraculously) no line or conversion losses and that your panels put out their rated wattage, you will be putting 35amps @ 12V into your bank.

AGM batteries are often listed as 5/C, lead-acid at 8/C; so 600Ah/5 would be 120A maximum charge rate, you are at under 30% of that so not even close to the limit of warming up the batteries.

On the other hand, the temperature sensor is probably only a few bucks and if you ever run solar concurrently with a genset or shore power or the engine then you might reach a rate of charge that might cause the AGMs to gas out. I'd put one in.
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Old 15-05-2009, 06:28   #3
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My MaxCharge 612 had bat sensors. I removed them. There is hardly a reason for them to go outside normal parameters.

Even when using the alternator they are rarely receiving oodles of amps as the program ramps it down inevitably.

My guess is it's a bit of overkill and something to go faulty.
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Old 15-05-2009, 18:21   #4
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Thanks for the answers. Sounds like unless I seriously increase my charging capacity, the batteries will be perfectly fine without the temp sensor.
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Old 15-05-2009, 20:13   #5
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Not so fast...AGM's are not sensitive to high charging current...some of the TPPL AGMs can accept a charging rate as high as C; e.g., a 210 amp-hr battery can accept a charge rate up to 210 amps.

Howerver, they are very, very sensitive to high charging voltage and charging voltage is a function of, among other things, temperature. The TPPL AGMs that I am most familiar with specify +/- 4 mV/degree C/cell from 25C. This becomes 24 mV/degree C for a 12 volt battery.

As an example: At 25C (77F) nominal float would be 13.62 VDC and nominal absorption voltage would be 14.70 VDC. At 40C (104F, not uncommon), the nominal float voltage is 13.26 VDC and the nominal absorption voltage is 14.34 VDC.

For your batteries' sake, spend the $40 for a temperature sensor for each and every one of your charging sources.

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Old 18-05-2009, 18:46   #6
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My vote is with CharlieJ as well. Why not use it the way it is intended to be? Believe it that if the engineers thought is was not of use they would have left it off.
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Old 18-05-2009, 19:37   #7
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Well they do leave it off, kind of. The unit comes without the temp sensor as standard, it's an optional extra you have to ask for and pay extra for.

Having said that it is now sounding like it's worthwhile having one, so I probably will.

Thanks everyone for the advice.
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