Davis Vantage Vue weather station: stop replacing battery

[Alec_t]

Here's the circuit I used (I think). The J107 replaces the 2N3189 I used. An 18650 could be used instead of the 3 alkaline cells.
The blue and yellow traces are for temperatures of +60C and -40C respectively.

If you do use a 18650, ensure it has a battery-protection circuit to prevent over-discharge.

 

[bushtech]

I really feel that all I need to do is drop 4.2V (if it's really 4.2V- why does an 18650 battery state 3.7V on it?) to 3V.
If I need to drop from 4.2 can I use 2 diodes in series and parallel a resistor over 1 of the diodes thereby reducing the drop in the second diode? (Value of resistor to be calculated)
I don't know much about this stuff so I could be talking absolute nonsense

 

[Alec_t]

if it's really 4.2V- why does an 18650 battery state 3.7V on it?

At full charge the voltage is 4.2V, but this drops non-linearly as the battery discharges.

 

[bushtech]

Thanks Alec_t

non-linearly, heh, been looking at a 18650 discharge graph and it's a rather vicious spiky sawtooth that could make some component rather unhappy. Spikes to about 4.7V, eek! Will have to tiptoe through the tulips rather carefully here.

 

[bushtech]

Just to round this off.

This is the way I went:

Got a design from a friend:
Ooops, cant seem to open it, only attach.
Basically what it does is take 2 x 18650 batteries and get them to output 3V.

Then I also replaced the supercap and added an extra one. Big thanks to Davis agents in South Africa for giving me 2 free caps.

It's a bit of a job getting to the cap:
This is what you're faced with:



You have to cut this potted thing open to get to the supercap. Dremel to the rescue

Once you've got it open



Replaced that cap and added another on the outside (Terrible photo)


And closed up again


Working fine