12V thermostat with Battery Backup

[Aqati]

Hello all!

I am looking for a thermostat. It needs to run off 12V when 12V is available, and when Power is lost, switch over to battery backup. When power is restored, after a 3-5 minute delay, it should start cooling again. This is how pretty much every 24VAC thermostat works, but after searching and searching, I'm unable to find a thermostat that pretty much does the same thing as simple home thermostats. I don't want a home thermostat as I need to be able to use both fan speeds. I don't want just a battery operated thermostat. I don't want an analog thermostat as it doesn't have a time delay built in to prevent short-cycling. Does this mythical unicorn really exist?

[tourdejour]

Can you more simply describe what you are trying to do?

I.e., I see you have a 1996 30FT Bounder. Are you trying to replace your RV thermostat with an aftermarket thermostat? ...And you want a battery backup?

...If so why? I.e., if you your house battery bank drops below 12V then you will not have any power to run your AC or heat pump blower fans anyway... so, I don't get the need for a 12V battery backup in your thermostat?

[hamm2018]

Originally Posted by Aqati

Hello all!

I am looking for a thermostat. It needs to run off 12V when 12V is available, and when Power is lost, switch over to battery backup. When power is restored, after a 3-5 minute delay, it should start cooling again. This is how pretty much every 24VAC thermostat works, but after searching and searching, I'm unable to find a thermostat that pretty much does the same thing as simple home thermostats. I don't want a home thermostat as I need to be able to use both fan speeds. I don't want just a battery operated thermostat. I don't want an analog thermostat as it doesn't have a time delay built in to prevent short-cycling. Does this mythical unicorn really exist?

https://www.rvupgradestore.com/shop-...-air+easytouch

[twinboat]

The battery powered thermostats run for a year on the few AA batteries in them. They have the delay built in so your compressor can unload.

Lots of Utube videos on adding a high/low fan switch.

[zrock2010]

i could be wrong but that is not how home thermostats operate once the onboard battery's re dead they no longer pull power through the furnace/ac at least any of the ones i have installed.
As said above if your batteries are dead your heat/ac is not going to work anyway. You must have a large battery bank to run your AC off 12v

Also as said above use a battery operated thermostat the battery's last for well over a year.

[Gary RVRoamer]

I guess I'm confused about your ask, cause all my RV thermostats have worked like that anyway, simply because the RV 12v system switches from 12v battery to 12v converter as needed. Meaning the thermostat gets constant 12v power as long as the RV battery bank is live. If I lost 120v power while the a/c was running, the thermostat was still operating and the a/c came back on when power was restored or if I started the generator.


Seems to me that any modern RV thermostat from Atwood/Dometic or Coleman-Mach meets your requirements. Am I missing something?

[Skip426]

Originally Posted by Gary RVRoamer

Seems to me that any modern RV thermostat from Atwood/Dometic or Coleman-Mach meets your requirements. Am I missing something?

About the only thing a standard RV t'stat doesn't have is the 3>5 min. delay at restoration of 120 volt power .

My surge protector , will wait 3+ mins when shore power is restored , to insure it's in range ( 108V >132V ) and stable; to avoid roof A/C compressor damage due to a brown out start up; or overload due to compressor being mid cycle .

[bigb56]

Originally Posted by Skip426

About the only thing a standard RV t'stat doesn't have is the 3>5 min. delay at restoration of 120 volt power .

My surge protector , will wait 3+ mins when shore power is restored , to insure it's in range ( 108V >132V ) and stable; to avoid roof A/C compressor damage due to a brown out start up; or overload due to compressor being mid cycle .

Yep, this was my issue, a momentary line voltage dropout would cause the AC to try and re-start immediately against the high head pressure. At first I solved it with my EMS like you have but later I installed a micro air which has the delay built in, so I put the EMS back to the 15 second delay.

[chuckftboy]

The Micro Air ASY-359-X02 thermostat will do exactly what the OP wants to accomplish.

[Gary RVRoamer]

About the only thing a standard RV t'stat doesn't have is the 3>5 min. delay at restoration of 120 volt power .

None of my RVs with an auto-transfer-switch would allow the a/c to restart immediately under those circumstances. It wasn't a thermostat function - it was basic power management. Those were Fleetwood designs, with a built-in 2 minute delay to restart power.


I think some newer a/c units do that on their own now - the fan comes back on but the compressor won't try to restart until its been off for 2m or so. Does anybody know yea or nay?

[hohenwald48]

Every RV air conditioner I've had has a delay to start built in. Not sure if the delay is in the thermostat or in the RV air conditioner itself. When power is restored the fan will immediately begin operation but the compressor is on a delay of a few minutes. Never timed it so not sure how long the delay is.

[Captain8]

They make 12v UPS devices like below, but I think you could just use a diode and a 12v battery

https://a.co/d/1BT42D6

[Mudfrog]

OP, why are you losing 12 volt power in the first place? That power should not go away unless there's a problem somewhere.

Do you have a good coach battery? Converter working ok? Are you trying for some reason, to keep it powered while the coach battery disconnect is off?