Thermostat Controller

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Jul 15, 2007
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591
Alright, I'm frustrated.

I have a digital thermostat- runs off 2xAA. Some cells work, some don't. Some power the back light, some don't (backlight is that indiglo EL screen).

I put a fresh pair- right-off-the-charger cells in there and the backlight works great. 10 minutes later - poof- gone.

So... what tricks can I do to get it to work? Should I consider ordering 2/3 AA cells and cramming 3 in there after letting them sit for a little bit ? (3x1.15V - 3.45, 2xAA Alkaline = 3.2).

I've written the manufacturer in hopes of finding out if they have a voltage regulation circuit, but I doubt it.

Should I look at LiFePO4 and it's 3.0V nominal cells? Think one could handle a year of very slow discharge?

Such trouble over a stupid thermostat (actually, one of the smartest thermostats I've every used...)
 

Mr Happy

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Hmmm. Are we in the US or elsewhere?

My digital thermostat is powered from the furnace, so it doesn't take batteries. (Maybe it has a backup battery for power outages, but it has been years since I installed it so I forget. I should probably check in case of leaks.) [Edit: Yes, it takes 2 AA cells, but I haven't touched them in 4 years and it still hasn't given a low battery warning.]

So anyway, if you are handy with electronics you might be able to make a little 3 V regulated power supply that runs off the power wire from the furnace. (I can't quite remember, but I think the standard power wire carries 50 V AC? [Edit: 24 V AC apparently])

Before that, maybe it's worth checking the unit isn't faulty? The back light failing after 10 minutes doesn't sound reasonable with any correctly functioning device.

If you want a recommendation, I have a Hunter model 44550, and it does everything I could want.
 
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Lynx_Arc

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Alright, I'm frustrated.

I have a digital thermostat- runs off 2xAA. Some cells work, some don't. Some power the back light, some don't (backlight is that indiglo EL screen).

I put a fresh pair- right-off-the-charger cells in there and the backlight works great. 10 minutes later - poof- gone.

So... what tricks can I do to get it to work? Should I consider ordering 2/3 AA cells and cramming 3 in there after letting them sit for a little bit ? (3x1.15V - 3.45, 2xAA Alkaline = 3.2).

I've written the manufacturer in hopes of finding out if they have a voltage regulation circuit, but I doubt it.

Should I look at LiFePO4 and it's 3.0V nominal cells? Think one could handle a year of very slow discharge?

Such trouble over a stupid thermostat (actually, one of the smartest thermostats I've every used...)
brand? and you are using nimh in it? do alkalines work fine it is possible your charger isn't working right measure the voltage of the cells.
 
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The brand is Rite-Temp. I believe the battery is used fob backup and to run the backlight (indiglo)- I haven't measured the voltage to the controller but I didn't think it was 50V... however I will check when I get a chance.

The battery chargers are BC-900's- so I'll hedge that they're pretty close to accurate- and I have 2x of them, both confirmed within .01V of each other.

What very low self discharge (I'm trying the Tenergy LSDs I've got) cells can I put in there? Will a Lithium Ion (Mn/Fe) hold a charge at a slow drain over the course of a year?

Yes, I could use the Alkaline AA's- I'd just prefer not to.
 

Flash_Gordon

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You are fighting a losing battle here. Your thermostat is not designed for nor will it run on rechargeable cells.

Even freshly charged cells are at or below the low voltage cutoff point for the thermostat.

Put in two fresh alkaline cells. Change them once a year. Pick a date that is easy to remember such as Dec. 21 (beginning of winter.)

Mark
 

Mr Happy

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More on my thermostat.

It contains a pair of Duracells, expiry date 2011, that I installed in August 2004. The same cells have been running continuously ever since, and currently read 1.38 V each. The Indiglo back light still works fine.

I think you may need to install a better thermostat.
 
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More on my thermostat.

It contains a pair of Duracells, expiry date 2011, that I installed in August 2004. The same cells have been running continuously ever since, and currently read 1.38 V each. The Indiglo back light still works fine.

I think you may need to install a better thermostat.

Grin. It was the best I could afford at the time. Maybe the Batteries were always the problem- but your AA voltage is way above the nimh- so that tells me something there.

Lithium AA it is...
 

jtr1962

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If your thermostat won't work correctly with rechargeables then it's not going to get the full capacity out of alkalines either. Alkaline are considered fully discharged at around 0.9V per cell. Any device properly designed for alkalines will work well with NiMH also. If your thermostat quits after a short time with fully charged NiMH then it's only going to get perhaps one third of the useable capacity of alkalines. Best bet if you're knowledgeable with electronics is to design a regulator to make ~3VDC off the furnace power wire. Either that or get a properly designed thermostat.
 
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If your thermostat won't work correctly with rechargeables then it's not going to get the full capacity out of alkalines either. Alkaline are considered fully discharged at around 0.9V per cell. Any device properly designed for alkalines will work well with NiMH also. If your thermostat quits after a short time with fully charged NiMH then it's only going to get perhaps one third of the useable capacity of alkalines. Best bet if you're knowledgeable with electronics is to design a regulator to make ~3VDC off the furnace power wire. Either that or get a properly designed thermostat.

No, the thermostat works quite properly- just the backlight stops working. When I leave the (old regular nimh) cells in there for over a year it starts to get a little screwy (and I had to do the jump start on the BC900 to get them charging). Those cells are out of service now.
 

Lynx_Arc

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did you measure the voltage of the nimh cells that didn't work with a digital meter and a load? It is possible your nimh cells are bad.
 
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did you measure the voltage of the nimh cells that didn't work with a digital meter and a load? It is possible your nimh cells are bad.

I'll see if I can't sneak a couple of contacts in there and attempt to fire up the light.... whether or not I can capture it remains to be seen.

The manufacturer got back to me- 2.8V minimum to operate the backlight- so not alot of overhead available if the cells drop to 1.3V.

They just recommend alkalines :)
 

Lynx_Arc

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2.8v is nuts IMO..... you should ask them what the maximum it can take is perhaps 3 batteries would work or get a buck circuit for an LED and that could run it at about 3.5v with 3-4 cells. Either that or get energizer lithiums in it.
 
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