Charging C battries with small solar panels

hi all.
i have an outdoor weather station which uses 2 c bateries, im over changing them ever couple of months and have a set of old garden led solar lights that i would like to use if possible.
I was looking at some circuits around for a charger but they only seem to use the reverse blocking diode and i thought it might cook the batteries if they are over charged with our Australian sun.
Is there a simple circuit i can use to regulate the charge.
The solar panel full sun is reading 3.3v and the batteries ill be using are
**broken link removed**

any help would be appreciated. thanks

stuee said:

... station which uses 2 c bateries,...

Click to expand...

Are you talking NiCads, NiMH or Alkaline? You cant recharge Alkalines, but you could supply power to the Wx station during daylight hours from the panel, and draw power from the batteries only at night.

The link to the batteries i was going to buy are nimh. thanks

A solar garden light usually uses only one very low capacity Chinese AAA cell or AA cell and it does not get over-charged. It will not fully charge one C cell unless you leave it charging for a couple of weeks or more without any discharging.

The ebay no-name-brand, probably Chinese battery cells are a joke and might not even work (fakes?). The four in parallel might produce a capacity of 10,000mAh but each cell is probably only 2500mAh or less.
Why don't you buy name-brand high quality batteries locally?

Hi. I can get the more expensive ones if needs be, The sensor only uses 2x C batteries. I have 3 x solar cells which i can use in parallel. How can i test what the amp output is from the cells?

The solar cells on my garden lights produce about 3V with no load, about 2V/30mA in summer noon sunlight with a series Schottky diode feeding a an AAA or AA Ni-MH cell and about 40mA in summer noon sunlight into a short (my DVM set to measure current).
Then to charge a 2500mAh C cell it would take a few weeks if the cell is dead because it needs about 250mA or more all day long to charge in one day.

The solar garden lights are cheap and made in China so some solar cells are much worse than others. The solar cells are so cheap that the plastic ones get sunburned (the plastic is destroyed by UV radiation), the glass ones do not.

I'm pretty certain the last time I purchased some from Amazon it was less than $20 for a five 6v 200ma glass panels. It would appear the solution is more solar output and a charge controller. I typically use a linear voltage regulator I learned about from Afrotechmods YouTube channel.

Ok on my investigation the solar panels are
IN FULL SUN.
18.45ma (no load test direct to cell)
1.98ma (tested inline current while connected to the charging circuit and battery)
3.2v full sun.

The weather hydrometer only uses 2 C cell and i have to replace every 3 months but hoping to modify to free energy instead of dumping batteries in the bin and using the spare bits i have around.
Im thinking that surely 2 or 3 of these cells parallel will produce enough power / charge to top up the batteries in the day. they batteries will never run flat but more just just keep them topped up.

Does that make sense?

My solar garden lights light an LED from its voltage stepup circuit and the battery discharge current is about 8mA for about 9 hours at night. Then the battery must be charged with a little more than 72mAh.
The solar panel has a current into the battery of about 30mA at noon in summer when facing directly into the sun but it averages maybe 7mA for 11 hours pointing upwards all day long which is 77mAh and it works all summer.

You need much less current so maybe you can use a smaller battery cell and a single solar panel from a garden light.
But don't you want it to work in winter when the sun is low in the sky, frequently covered with clouds and when daylight is short and night darkness is long? Some of my solar garden lights light for only part of an hour in winter following a sunny day and do not light following a cloudy day.

I have 18 solar garden lights and looked at them just now. Today was partly cloudy and the garden lights turned on exactly one hour ago. Two stopped working and some are dim. In the morning only a few will be lit. They were bright every night all night long in summer.

well i guess the only way to find out is to try lol.
How would be the best way to hook up 3 or even 4 panels to the 2x c batteries so it does top up while on but not overcharge. I have an old set of lights which have the glass panels so i can use 10 if needs be
thanks for you help thus far.

Energizer and a Japanese battery manufacturer say to limit the over-charging current to 1/40th its mAh rating or less. But you will not know the actual mAh rating of cheap Chinese battery cells from ebay.

i will pay the extra and get this pack from Jaycar electronics
https://www.jaycar.com.au/Power-Pro...,500mA-Ni-MH-C-Batteries---Pack-of-2/p/SB1733

Those Powertech batteries are extremely expensive. Powertech is in South Africa and make lead-acid batteries. Maybe they buy then re-sell Ni-MH batteries. Aren't you in Australia, MATE?
Isn't there a battery manufacturer in Australia?

yes 'Mate' Im in Australia. I don't think Australia as advanced to modern technology like producing their own Batteries. We have just upgraded from a 33k modem to v56 so hopefully batteries will be next for technology

My first pc was bought in 1994 (21 years ago) and used an Intel 486 processor and a dial-up 33k modem. A year later I was excited to get a 56k dial-up modem that rarely operated at 56k. Now I have cable internet (on my cable TV cable) and it is extremely fast.
Most people have fast internet on their cell phones today.