Davis Vantage Vue weather station: stop replacing battery

[bushtech]

This weather station has a non rechargeable CR123 that goes flat about every 18 months. Now for a system that has a solar panel I find it unacceptable that every 18 months or so you either have to drop the mast to get to the station or grab a ladder and scale the mast (9m high in my case) For somebody in his sixties this is no joke and as I get older and more crotchety my language is not going to improve when I have to replace the battery.

Now how this thing works is the solar panel runs the station during the day whilst simultaneously charging a supercap that is meant to run the station during the night. Any shortfall by the cap is made up by the non rechargeable CR123.

For more info read here: https://www.manula.com/manuals/pws/davis-kb/1/en/topic/supercap-faults.

Let's just be clear, the station is operating as it should, I just feel Davis's design sucks.

As I see it options are:
1. double up on the supercap
2. increase the size of the solar panel. ( Davis do a bolt on solar panel but not for the Vue)
3. add in a charging circuit to facilitate the installation of a rechargeable battery

I will probably need to do all three.

I have considered feeding it mains power via a transformer but it means laying quite a long underground cable which also increases lightning risk getting back into the house.

I would appreciate some input here, am I going about it the wrong way?, are there any alternative solutions I haven't thought about, any bright ideas will be welcomed. I want to knock this problem on the head once and for all.

 

[kellys_eye]

A battery box mounted at the base of the mast seems a logical solution. CR123 rechargeable fitted.

https://www.amazon.co.uk/Universal-CR123A-CR-123-rechargeable-battery/dp/B005D7UYR4

 

[bushtech]

Thanks kellys_eye. So box at base with 2 wires snaking up the pole and soldered to the 2 poles of the now empty battery spot in the station?

 

[davenn]

Thanks kellys_eye. So box at base with 2 wires snaking up the pole and soldered to the 2 poles of the now empty battery spot in the station?

sounds like a good plan

 

[kellys_eye]

Thanks kellys_eye. So box at base with 2 wires snaking up the pole and soldered to the 2 poles of the now empty battery spot in the station?

'zaktly

 

[bushtech]

Thanks for a brilliant solution kellys_eye.

Now those 3V CR123's are not mainstream. I have lot's of 3.7V 18650's and chargers. What are the chances that the weather station will be happy being fed 3.7V instead of 3V. Or will I have to go with step down transformer or buck convertor?

 

[kellys_eye]

A simple silicon diode in series with the battery output will suffice (drops 0.6V). The current draw seems to be of the order μA, certainly no more than a milliamp or so.

 

[Alec_t]

A silicon diode will drop less than 0.6V if the current is only a few uA. If you're really concerned that 3.7V is too high then an alternative would be to use a couple of alkaline cells (nominally 1.5V x 2, but when fresh could be ~ 1.65V x 2).

 

[(*steve*)]

What about an LiFePO_r?

A nominal cell voltage of 3.2V would likely be safe.

The only thing I don't like about using this type of battery is the risk of damage when they are over discharged. They could easily become single use :-(.

A pair of high quality D cells would probably last their shelf life.

 

[bushtech]

Thank you kellys_eye, Alec_t and *steve* for all the great advice.

I am still leaning towards using 18650's as they are batteries that I use a lot around the house. I have however remembered that a fully charged 18650 sits around 4.2V. Hmmm. Isn't this a problem?

Just did an inventory of my diodes:
1N4001
1N4004
1N4005
1N4007
1N4148
1N5819
And some humongous 1N5405's that could probably rectify mains voltage
Also some 5.6V and 10V zeners
Could any of these work?

 

[Alec_t]

For current in the 1-50uA range, a power diode (e.g. 1N4007) drops about 50mV more voltage than a signal diode (e.g. 1N4148), if that's of help.

 

[(*steve*)]

TheThere some very low power regulators available. When I had to replace a 1.35V mercury cell in a piece of equipment (that was used as a voltage reference) I used a pair of AA cells and one of these regulators. The quiescent current is so low I decided a power switch wasnt required.

Something like that may be an option if you want and accurate 3V.

I was repairing a Keithkey 600B, so searching for that here may give you more information.

Edit: Mercury cell and 600B

 

[(*steve*)]

Oops, 600B. The thread is here. And the regulator is an AP7365. I used the adjustable one. It is a low drop out device with a maximum input voltage of 6V. So a lithium cell or three 1.5V batteries would work fine for a 3V output.

 

[bushtech]

Thanks *steve*

That's getting a bit complicated for me to understand.
Been looking at datasheets of the diodes I got but non comprehendo.
But I see you do get 3V Zeners, only trouble is our esteemed postal service is on strike again, so making something work with what I got is currently first prize

 

[bushtech]

I have no idea how much leeway I have around the 3V CR123

 

[Alec_t]

I have no idea how much leeway I have around the 3V CR123

Do you know how low the cell voltage can go before the cell is regarded as dead?

 

[bushtech]

Alec_t: On the Davis forum's the consensus seems to be 2.7V at which you get a low battery warning.

 

[Alec_t]

That's useful to know.
Just remembered: I replaced a 3V memory back-up cell in my O'scope with a stack of 3 or 4 partly-used AA alkaline cells and a crude regulator made using a JFET (2N3189, now obsolescent) from my junk-box. Perhaps something similar would suit your application?

 

[(*steve*)]

A zener side is not ideal. I would recommend 2 X 1.5V batteries as a temporary measure.

I know I bought more than 1 on those regulators because I have 2 of those units to repair. If I bought more than 2 (and I can locate them), I may be able to knock up a 3V regulator for you. Of course, that will also rely on your postal service working

The good news is that I bought quite a few of these (in going to use them as regulators for the heaters of some very low power tubes) and since I've already used some for other projects, I'm going to have to reorder them if I make my valve circuit in quantity (it will be a kit for my local hackerspace).

I'll let you know if they're where they should be, and if so, I can make you a low quiescent current 3V regulator. Powered from 3 1.5V cells, they'll be close to dead by the time the low battery warning comes on.

 

[bushtech]

Thanks *steve* but the 18650's would be ideal. Low battery warning comes on, I grab a charged 18650, walk outside to the pole and swap out flat for charged in battery box at bottom of pole