I have an Accrite 5-in-1 weather station. It uses 4AA batteries in the outdoor sensor. Inside the unit each pair is connected in series, producing 3v. Each pair is then connected to the same terminals in parallel. This is supposed to be so the batteries can be replaced one pair at a time without interrupting power. So I believe the unit runs on 3v, or whatever 2AA batteries produce.
The batteries don't last very long, so I would like to construct an inexpensive solar power supply for it.
What sort of device would be best for this application, and what kind of rechargeable batteries should I use?
I have seen some solar battery chargers that have a holder for 2 batteries attached, but they say the batteries are 1.2v, so I'm not sure if that would power the unit. Why is the rechargeable battery voltage less that the 1.5v of normal batteries?
I'm open to purchasing from AliExpress, since I'm in no big hurry.

 

Interesting questions! Here is the answer about battery voltages: Standard "flashlight" batteries have one chemistry, (Zinc based) that produces nominally 1.5 volts per cell for much of the battery life The rechargeable batteries use a different chemistry, (Nickle Cadmium) that produces about 1.2 volts per cell. So it is the different chemistries that deliver the different voltages. The complete, detailed, explanation is rather tedious and complicated .

The result would be that for the weather station you could have a string of THREE rechargeable cells that would provide about 3.6 volts when they were new. Quite probably the weather station would work with that slightly higher voltage quite well. And the solar cell array could recharge it every day.
You might also use the solar cell array to recharge the non-rechargeable batteries every day, and they would last a much longer time.
The challenge is that recharging batteries must be done from a higher voltage than what they are rated at. so with the solar cell array it will make sense to charge a battery pack of at least three cells and then have a regulator provide the desired 3.0 volts to the weather station. So it gets more complicated .

 

Thanks for the quick response, and great information.
Yes, I think 3.6v would probably be okay. I've seen solar cells that are rated for 6v, but that would probably wreck the batteries? What sort of regulator would I need?

 

(-)small about SIX cells 3 to 4V 1W solar panel(+)----------(+)2 AAA NiMh in series------------(-)

Connect in paraller to the AA pack. If needed, each Si diode in series will drop ~0.7V to match the AA pack.

 

(-)small about SIX cells 3 to 4V 1W solar panel(+)----------(+)2 AAA NiMh in series------------(-)

Connect in paraller to the AA pack. If needed, each Si diode in series will drop ~0.7V to match the AA pack.


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I was looking at something like that, only with AA batteries. I didn't think the 1.2v rechargeable batteries would work. Are you saying, leave the non rechargeable batteries in and connect the others in parallel? I don't think I need the original parallel arrangement, since I've had no issues removing and replacing all 4 batteries at the same time. I would probably just run with 2 if I can keep them charged.

 

Are you saying, leave the non rechargeable batteries in and connect the others in parallel?

Connect the tiny panel to any pair, rechargeable or not, tailoring the voltage to match.

 

Connect the tiny panel to any pair, rechargeable or not, tailoring the voltage to match.

Won't 2 rechargeable batteries in series only give me 2.4v? And with the solar panel rated for 4v, would I get 4v out of the battery terminals if the panel was in full sun?
Trying to understand how to get 3ish volts consistently.

 

(-)-----------------your panel(+)----------|>|-----------------(+)-------------yourbattery-------------(-)

Every diode -----|>|------in series you add, reduces the panel voltage by ~0.7V. Match your voltages that simple way.

Example: 3V battery and two diodes in series can use a 4.5V panel.

 

I understand, but MisterBill2 said the panel voltage needs to be more than the battery voltage to charge the battery. So how does that work?

 

Rechargeable or not; make it slightly less than full charged voltage. (0.1V ~ 0.3V less) at full sun.
If 3V, will work happy set at 2.8V.
If recharge to 1.4V each; will work happy with 2.5V

 

So if I want 3-3.6v I can use a 4v panel & add diodes until I get output down to 3.3v in full sun and the batteries will still charge okay? Also, will I want 3 batteries since the rechargeable ones are only 1.2v?
What type of rechargeable batteries should I be looking for? Ni-cad or nimh, or...?

 

A single LiFePO4 cell would be about the right voltage. There should be a charger module out there which will work from a solar source. You should also have a protection module for the cell, if it doesn't have one built-in.

 

A single LiFePO4 cell would be about the right voltage. There should be a charger module out there which will work from a solar source. You should also have a protection module for the cell, if it doesn't have one built-in.

Thanks for the info. Would something like this work?
https://a.aliexpress.com/_mOkJU14
It says it has protection.
Paired with a 6v solar panel.

 

That module looks like it should work, although the review is discouraging. It's cheap enough to buy a couple from different sources and test them out. There's also this TP5000 module, which could be more efficient, since it has an inductor. https://www.aliexpress.com/item/1005001419622897.html
If you're using unprotected LiFePO4 cells, it would be wise to add a protection module.
https://www.aliexpress.com/item/32807056032.html

 

That module looks like it should work, although the review is discouraging. It's cheap enough to buy a couple from different sources and test them out. There's also this TP5000 module, which could be more efficient, since it has an inductor. https://www.aliexpress.com/item/1005001419622897.html
If you're using unprotected LiFePO4 cells, it would be wise to add a protection module.
https://www.aliexpress.com/item/32807056032.html

That's great. Which panel & battery would you recommend?
Would be great if I could get this all from 1 seller to save on shipping.

 

For cells, I prefer to salvage them from brand name packs from the recycle bins, but LiFePO4 aren't common. You don't need much capacity, so unless you're doing this on an absolute shoestring, you should be able to buy them from somewhere that deals in real cells. As for panels, I guess whatever you can find a deal on that has real feedback; 6 volts nominal, likely 10 volts no-load. I wanted to suggest yard sale 1 to 5 watt battery maintainer panels, but those are 12V nominal, usually nearly 20V no-load. And they're usually too well sealed to easily modify for lower voltage.

As for battery capacity and panel power... AA cells are a couple of amp-hours and run for days or months, so I'd expect a 400 to 2000 mAh cell and a 100 mA panel could be ample.

 

Great. Thanks for the tips. I assume the connections for everything are obvious. I'll get on AliExpress and see what I can find.
What do you mean by real cells? I shouldn't get them from China?

 

Having a difficult time finding a deal on AliExpress. The shipping is making the parts I find unreasonable. Looking at eBay I've found what I think are the parts you described.
Panel
https://www.ebay.com/itm/355662586181
Charger
https://www.ebay.com/itm/145683704409
Protection board
https://www.ebay.com/itm/145683691719
Do these parts look correct?
All of the LifePo4 cells I see are 3.2v. The board you selected shows a 3.7v cell. Will 3.2v cells work?
I'd order from AliExpress if it weren't for the shipping, and that I can't find a single seller with everything I need.

 

Received all of the parts & put everything together. When I tested the voltage going to the unit, it was almost 4v, so I installed some diodes to get it down closer to 3v. When I hooked it up, the low battery light was on & it wouldn't update.
I brought everything inside today and tested the voltage with the weather station powered up and I had under 2v. I shorted around the diodes and the voltage went up to 3.6, so I removed them. I also shorted the terminals on the charger board for the higher voltage setting for the lipo battery. Put everything back outside and no more low battery light and the readings are updating now. Hopefully I'm good to go for several years. Thanks everyone for the help.