# Low-voltage charge control for solar

Discussion in 'The Projects Forum' started by Kefka666, Nov 9, 2008.

1. ### Kefka666 Thread Starter Active Member

Mar 4, 2008
38
1
I'm trying to create a solar power battery charger. I don't want to over-charge the batteries, so some type of charge controller is probably necessary. Here's what I have available right now:

* Four 1.2v rechargeable NiMH batteries (2650mAh), pretty standard.
* Four 4v/200mA solar cells

From what I've learned online I would need a low-voltage charge controller so I don't run the batteries down to v=0 or overcharge them. However, I have found surprisingly little information about where to find and how to use charge controllers. Is it possible to build a simple charge controller for this low-voltage application? Otherwise, do you know where I could find one?

The ultimate goal of this project is to use solar cells to charge batteries during the day, which will power LEDs at night. The light-deactuated part of the circuit is no problem, but the solar-battery charging component has perplexed me. Thanks in advance.

2. ### Audioguru Expert

Dec 20, 2007
10,797
1,217
If you live near the equator of earth then the solar cells produce 200mA only at noon.
Anywhere else and the max current is lower.
The average current for 10 hours is about only 60mA. The solar cells are too small.
2650mAh cells overcharge when the current is 66mA and more so use a battery charger IC to prevent overcharging.

3. ### Kefka666 Thread Starter Active Member

Mar 4, 2008
38
1
I don't understand the last part; if 2650mAh cells overcharge with a current >66mA, why is 60mA too small a current?

4. ### Audioguru Expert

Dec 20, 2007
10,797
1,217
If the 2650mAh battery is discharged overnight then it will take 1.4 times more mAh to recharge it. It needs 3710mah to recharge it.
If the solar cell provides an average of only 60mA for 10 hours each day then the battery will need 6.2 sunny days to recharge.

You need a much bigger solar cell that can charge the battery in one day or less.
And you need a battery charger IC to prevent overcharging.

5. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
8
Solar cells have come a long way. Many are now marketed with stats reflecting output in the temperate regions instead of maximum possible output. What's more, the "equator at noon" bit is not fully accurate. A solar cell generates more power in Eastern Oregon in January than an identical panel in Southern Texas in July. They work better in the cold, you see.

Without seeing the full stat's on Kefka's panels, we can't really predict their performance one way or another.

We also need to know how much power his LEDs require.

6. ### Kefka666 Thread Starter Active Member

Mar 4, 2008
38
1
I'm using 20mA LEDs. So assuming I can find the right solar panels, what current would be necessary to charge those four 2650mAh batteries most effectively? Audioguru recommends a battery charger integrated circuit for charge control--would this by itself be effective and practical for someone who has never built a charge controller, or is there a good commercial alternative?

Also, as I said earlier this will be a light-deactuated circuit; it will only run at night, for a maximum of 12 hours at a time in the dead of winter. I was confident that those batteries would easily carry a few LEDs through the night.

7. ### Audioguru Expert

Dec 20, 2007
10,797
1,217
2650mAh will power four 20mA LEDs for 2650/80= 33 hours.
If the LEDs are powered for only 8 hours each night then 80 x 8= 640mAh is used and a charge of 1.4 x 640mAh= 896mAh is needed each day.

The mAh from your solar cells depends on how high is the sun in the sky and for how long each day. If you are in North America then the sun is low in the sky for only a few hours each day in winter. Then your LEDs will perform like my solar garden lights: all night long in summer but for only 1 hour in winter following a sunny day.

8. ### raybo Member

Oct 18, 2008
22
0
audio guru disregard the fact that battery do not stand there and dicharge at one set rate. the rate is set per one hour not 33 hours. the discharge is always exponential never linear.