# How do I charge a 6V, Lead Acid, 4.5 Ah battery using a 3W solar panel?

I have a solar panel with ratings: Vmp=8.8V and Imp=0.3A

Using this i have to charge a 6V Lead Acid battery whose capacity is 4500mAh.

I have searched the internet a lot and tried many circuits but haven't got any proper solution. Can you people help me out with this?

• You haven't indicated what you want help with. BTW, what's the "mp" in "Vmp" and "Imp"? Commented Dec 28, 2015 at 10:57
• @EMFi: "mp" stands for "maximum power". Commented Dec 28, 2015 at 12:04
• You have not told us all you can. Elsewhere you say "my application is a solar lantern. a have got 7.2V using PWM & Mosfet. but the current m getting to charge the bateery is too low (20mA-50mA)." -> This is meaningless without a circuit and context. What is the battery state of charge? Under what conditions is this true. || TEST: In full sun at midday with panel optimally oriet=nted to the sun, short the panel with an ammeter (eg DMM on 10A range) and report the current. This is ~= Isc actual. | In same sun conditions load the panel with 27 Ohms. What voltage do you get? SHOULD be ~= 8.5V. Commented Dec 29, 2015 at 10:54
• IF panel specs are as you report a partially charged battery (say about 50% SOC) you should get the full 300 mA in full sun using just a series diode from panel to battery. Commented Dec 29, 2015 at 10:55
• What is the FET & PWM abou? Why do you think you need them? How are you connecting them? Circuit diagram essential. Commented Dec 29, 2015 at 10:56

Because the solar panel puts so little power compared to the size of the battery, and lead-acid batteries are rather forgiving, all you need is a Schottky diode.

Connect the diode in series with the solar panel, cathode to battery +, anode to panel + output. Tie together the panel - and the battery -. That's all you need. The float charge level of the battery will be around 6.8 V, but the solar panel is too wussy to deliver enough current at that voltage to cause any harm. If you're really worried about overcharging, add a shunt regulator across the battery set to 7 V or so.

What Olin says. There are quite a few answers on this on stack exchange already. Such a charger will not treat the battery as well as you'd like but will work OK. What is your application? How often will you discharge the battery and by how much? - Those matter.

If you are in eg Amritsar then GAISMA tells you that you typically get about 7 SH (sunshine hours) per day in May-June and 3 sunshine hours/day in December-January in Winter.

To fully charge the battery in full sun you need 4500 mAh/ 300 mA = 15 hours at 100% efficiency so maybe 16 to 20 hours in reality.

In December-January you would need 16 hours/ 3 SSH =~ 6 full days. In May-June you'd need 16 hours / 7 SSH = about 2.5 days.

For long battery life you should not use more than 50% of capacity so charging from 50% to 100% capacity would take about 3 days in Winter and 1+ days in Summer.

• my application is a solar lantern. a have got 7.2V using PWM & Mosfet. but the current m getting to charge the bateery is too low (20mA-50mA). Commented Dec 29, 2015 at 7:20