I have a device that has a bad battery and I am trying to find a suitable replacement. There are no replacement batteries for the device so I am trying to put something together myself. The original battery has written on it 3.7 V, 1.41 W. I have found a battery (CR123A) that is 3.7 volts and 700 mAh. Can I safely use it?
Also, the original battery has 3 wires - red, white, and black. How would I connect the new regular battery?
** It's for my Nest thermostat.
The new battery (700 mAh) comes up with 3.7 * 0.7 = 2.6 Wh, so it should be capable to power your device.
Your original battery (1.41 Wh) comes down to 1.41/3.7 = 380 mAh, or it is smaller than CR123A. It should be expected that your built-in charger uses no more than 200 - 380 mA charging current, which should be fine for CR123, it will just take a longer time to re-charge.
More challenging would be the handling of "white" wire. I assume it is a plain thermistor, although it could be more complicated. You can measure the resistance between thw white wire to ground (black wire). If it comes up as, say, 10k +- 25%, then it is a thermistor. You would need to fool the white wire with the same value, plus-minus. Without the termistor the charger would think that the battery is overheated (or broken), and would refuse to charge it. The negative side is that the thermistor was there for a reason of extra protection, and your new setup will be lacking it.
However, the Nest thermostat is a fairly popular product, so the replacement batteries should be freely available. It is advisable to get a direct replacement.
The mAh rating of a battery is two-fold generally.
First it tells you something about its capacity (at full state of charge). The product of Current and Time is definitively Charge (as in Coulombs). Using a lower rated mAh battery pack will most likely just mean its battery life is shorter.
Second, for rechargeable batteries, it tells you something about the safe rate at which the battery can be charged, often stated in C's. So a 1400 mAh battery might ideally be charged at 1.4A maximum. This is just a rule of thumb type of thing, certainly refer to your battery manufacturer's data sheet for definitive information on this. But if you are replacing a 1400mAh battery with a 700mAh battery, I would be really careful about using the same charging system with a lower mAh battery pack.
An Amazon ad says Duracell CR123A batteries but their photo shows CR123 that is very different. Somebody else's ad says Energizer CR123A but they don't make it, theirs is also a very different CR123.
If you try to charge a CR123 then it might explode.
