[I'd originally posted technical details on testing the Zeo battery, etc., but have deleted them, though correct. Lithium batteries are not as harmless as the batteries we were used to before; they can explode and catch fire. If you know what you're doing, you don't need me to say anything; if not, don't do anything more than replace. Don't try to rejuvenate batteries, or recharge them externally.]
Thanks @tolerant for your information, very useful. I won't quote in full to keep things short. You report that you've opened your non-charging Zeo (mobile phone version) transmitter and are looking to get it working (details on the quantifiedself "Zeo shutting down: export your data!" thread a few posts up).
Batteries are usually soldered into such assemblies, and must be unsoldered for replacement. Be very careful not to heat the battery itself significantly. If you are not accustomed to solder work on miniature equipment you may cause damage to the circuit board.
It appears impossible to get a replacement battery that will fit in the transmitter and supply sufficient energy for a night. Maybe somebody will eventually find one? The external volume of the original battery (20x20x2) is about 80% of that of a rechargeable ML2032 (20mm dia, 3.2mm thick, 65mAh), which suggests ~50mAh for the Zeo battery.
You can replace the battery, either temporarily for testing or permanently, with a larger Li-ion or LiPo one of the same voltage (any single cell will be rated at 3.6-3.7V), being extremely careful not to short-circuit it in the process. If permanent, you'll have to improvise a casing. Capacity needs to be at least 50mAh, maybe a bit more if I've underestimated, but a somewhat larger capacity doesn't seem to be a problem (it might take longer to indicate full charge, but should last a night even if not full).
I'll post a summary of information on the battery below, for reference.
(I wanted this to be a separate message, but it has been appended to my previous contribution.)
This is a summary of information on what is know of the battery used by the transmitter of the Zeo sleep monitor (mobile phone version). The transmitter for the clock version is likely to use the same battery. In case this is copied or posted elsewhere, this was originally posted at
and quotes and uses information supplied by quantifiedself member "tolerant".
There is discussion at quantifiedself of ideas on testing and replacing the battery.
The external battery volume (20x20x2mm) is about 80% of that of a rechargeable ML2032 (20mm dia, 3.2mm thick, 65mAh), which suggests ~50mAh for the Zeo battery; this is an estimate only.
From memory, I think that the battery of the clock version runs for about 16 hours on a full charge, and takes about 1 hour's charging (not 3) until the display indicates full charge (I tested these times). This may vary between units, and my recollection may be wrong!
A comment about lithium technology (Li-ion, LiPo) rechargeable batteries: I believe that lithium cells are very susceptible to damage by either over-charge or under-charge. This is avoided in equipment by circuitry that does not allow these circumstances. Lithium rechargeables self-discharge only very slowly, so that they may be left for long periods; however they may eventually self-discharge below the safe limit and become unusable if left for long enough. It is not safe to try to rejuvenate them in an external charger without protection circuitry. I don't know if this form of damage actually occurs from personal experience; but I have a spare transmitter, and swap the two occasionally to avoid the possibility.
Here is a copy of the picture: