I've got Drakino's old Gen1 nest and last night it failed because of a low battery. It was cold in the house this morning.

I can top of its charge by connecting it to a USB charger, and then when I put it back on the wall it works for a while before complaining about a low battery again. Though the indicator screen indicates that there is 37 volts of power coming to it (which their online troubleshooting wizard seems to think is sufficient), it doesn't seem to be successfully keeping the battery above its "low" threshold of 3.6 volts. Since, up until today, the unit worked fine, it looks like it's time to replace the battery.

Looking at the teardown pics, it seems that the battery is a straightforward LiPo unit with a connector plug. I should be able to just replace this puppy by ordering one online somewhere. But google searches aren't coming up with anything.

Anyone know whether I could get one of these things from, like, Digi-key or something, and which one it would be?

Another possibility is trying this guy's hack...

Well, that took all of ten minutes to do. That was super easy! There was already a "C" terminal on the heater's main control board, easy to reach, easy to see, and there was an unused blue wire in the wire bundle, just like the guy said. Strip both ends of the wire and connect to the "C" terminals at either end, and the hack is done. Nest tells me it's got a 24v common wire detected now.

I have no way of knowing whether that's gonna *fix* my problem or not, though. I'd rather still find out what my options are for finding a replacement battery for this thing. Anyone know?

If you successfully recharged it with a USB cable, then the furnace supply will have no problem keeping it fully charged. LiON batteries have a shelf life, so best not to pick up a "spare" until closer to the "need one now" date.

Cheers

And yet, it didn't. The battery, while continuously on the furnace power, in fact dipped below the level at which the thermostat stops functioning and left the system turned off overnight. Their "low" threshold is 3.6 volts, and the thing drops off wifi if it dips below that, and stops working completely at some voltage below that (I don't know what that lower voltage is but it did dip below that too). What I did with the USB cable was to barely push it above 3.6 for long enough to get it to work again for a while.

But just now, I check it, and it's done it again despite the hack I applied yesterday.

Yeah, this thing needs a new battery.

Anyone know where I can get one?

Have you tried contacting Nest ? I got the impression that they worked pretty hard to keep customers happy ? Having a dead battery this soon seems pretty odd.

Agreed. The unit is still covered under their warranty period. I'll e-mail you my proof of purchase.

I didn't realize it would still be in the warranty period. That's cool.

Even so, everything I've seen on the internet indicated that they would be swapping me a whole unit instead of sending me a battery to pop in. I was hoping to avoid that. Don't want to have to wire the old thermostat back in while I wait.

I had hoped that the battery would be something I could just easily source elsewhere.

But if it's under warranty and contacting them is the only way, then so be it. Thanks guys!

That battery should last 10 years due to the very shallow cycling we did on it. It only gets charged to 3.9v max, the usable capacity is less than half the design capacity to extend the life.

Is Voc listed at 37v? That's a bit high to be honest. It's possible you have no issues with your battery but you're hitting overvoltage lockout on the buck preventing any charging from happening. Did anything change with your furnace? I'd expect a lower number from a 24v transformer (I see 21v on mine, which is on the low side, but above 35v is also unusual).

Nest DVT serial number 1 still going strong on my wall (and I only have RH/W wires, no C).

Rectified open circuit voltage from the furnace at my thermostat is around 38VDC. That drops several volts when under a very modest load.

I would not be at all surprised if the real world tolerances on the AC transformers used for HVAC were rather loose.

Even a 25% RMS overvoltage under load would not seem out of the question given the low cost and made in China sourcing I would expect is common. Open circuit voltages could be even farther out, I imagine.

I hope the NEST can cope with 'unusually high' open circuit AC input voltage without causing the battery to paradoxically run itself flat.

Can you measure (using a multi-meter) the actual AC voltage at the Nest terminals? Is it indeed 37 volts?

If yes, does the AC transformer itself (usually located within the heating or cooling system unit) have multiple taps for AC supply voltage?

And recently the battery has indeed been staying at the 3.9v target as expected. It took a while to reach that target though. After topping it off to 3.7v again, and adding the blue common wire hack described in the link at the top of the thread, over a couple of days its voltage slowly crept up to 3.9v and now seems to be staying there again. I have no idea if the blue wire hack had anything to do with the voltage going back up or not.



I think it was, but my memory isn't the best right now. 37 point something, I think, but I don't remember the point something part. I'm in Hawaii for a week now, so I can't check. Too bad that's not one of the pieces of data I can view in the "Technical Info" screen remotely. :-)

The online troubleshooting wizard at Nest.com didn't seem to complain that it was too high when I went through it. The wizard basically said "Voc looks good, you've solved your problem" while of course the unit kept shutting off its wifi.

By the way, it looks like you guys did some kind of a trick with the numbers to the right of the decimal point there. I couldn't just type any random digits past the decimal point into the online troubleshooting wizard, or it would tell me the entry was invalid. So it appears one of those digits wasn't actually a voltage, but rather some kind of checksum or shibboleth. Very clever, guys. :-)




Nothing changed with the furnace that I know of, other than, the weather turned colder so the furnace was running more often.

Maybe the whole thing was just that the wires needed a bit of reseating, and all of my fiddling took care of that?



:-)

Can't check right now since I'm out of the house for a week, but that would be a good check. Not sure which wires to put the circult tester probes on, though, since I don't clearly understand which wires I'm supposed to be testing.

Right now it has connected:
W1 heat
G fan
Rc power
C 24v common

Until I did the hack described at the top of the thread, that C common wire was not there.

What I don't understand is how that can work since none of the above are a ground wire or neutral wire. So I don't understand how thermostat wiring works in general. So I wouldn't know where to put the test probes.



Don't know how to tell? All I know is, that I can open the panel on the furnace and see where the wires go in, and there's a little circuit board in there with screw terminals labeled with the same letter codes as the ones on the inside of the Nest. That's how I could easily add the "C" wire because it was clearly labeled, and there was a blue wire free on the wire bundle.