Solar charging.

[cps13]

Hi,

I am just starting to look into turning a product I have into wireless version which solar charging. I was hoping someone could help me with the basics of charging which I haven't looked into much before.

My circuit requires 205mA max at 12VDC. I have a 12VDC SLA battery with a 14Ah capacity.

What I would like to know is how to calculate charge times.

For example I have found a panel which will charge at 150mA @ 7.5V. Can I use this or is the 7.5V output simply no good for 12V battery?

In my head if you a 12V 1Ah battery and a 6V 1Ah charger then it would take two hours to charge the battery? Same as a 12V 0.5Ah charger would.

What I need to be able to do is keep the battery constantly charged. When the device is powered on it is likely to be left on for a couple of weeks. So I need a way of calculating how much power I would need from a solar panel to keep the battery topped up.

Any help appreciated, thanks.

 

[hevans1944]

Your circuit requires 205 mA (max) and 12 VDC for a "couple of weeks," so the power required is 2.46 watts (max) continuously during that time. The panel you found provides 150 mA @ 7.5 VDC or 1.125 watts, but only when fully illuminated by the sun. So this panel (alone) will not provide enough power for your product, much less any power to keep a SLA battery charged. You need more solar panels with higher voltage and/or higher current ratings. In other words, more solar power.

A 12 VDC SLA battery requires about 14 VDC to charge, so you must step up the solar panel output to at least that voltage, by either connecting two 7.5 V panels in series or by using a boost regulator to go from 7.5 VDC to the required SLA charging voltage. You will also need more solar panels to obtain more power (when the sun is shining) than is consumed by your product, plus a little more to "top off" the battery before the sun goes down, and a lot more to re-charge the SLA battery the next day.

You will need a lot more solar power because during the night (say for at least 18 hours) your 14 A-Hr battery will be delivering all the current to the product... about 3.69 A-Hr worth of energy at 12 VDC or about 42.28 watt-hours. This energy will have to be replaced by the solar panels during the next six-hour day. So, if the solar power output were constant (it never is), and you want to recharge over a six-hour effective daylight period, you would need 7.38 watts to recharge the battery and another 2.46 watts to provide power to your product, or roughly ten watts of solar power for six hours per day.

However, there are inefficiencies in charging and discharging SLA batteries, inefficiencies in controlling the power to re-charge the SLA battery, and some days are cloudy and the sun provides virtually no energy from the solar panel. To be on the "safe" side, I would look for a solar panel with at least 100 watts capacity and a terminal voltage of at least 12 VDC. Purchase or build a quality SLA constant-voltage charger with 0.1 C current limiting and "float" the SLA battery across its output along with your product. With any luck on the sunshine, this might work for "a couple of weeks," maybe even longer. Be sure to mount this rig way up high, or in an inaccessible location, to discourage vandalism and theft.

 

[ADRT]

Here's a site that has some great information from a DIY guy. His name is Michael Davis and he is an astronomy professor in Florida. Anyway, he built a windmill, solar panel, and charge controller. You will find in great detail his work, failures and achievements in charging a 12 volt Lead Acid Battery which he uses on his remote property in Arizona. I have built both of the charge controller designs myself and was able to get them to work well in the shop with a bench top PSU. The 555 design is the better one in my opinion and is easier to find components for.

http://mdpub.com/

Good luck!

 

[Colin Mitchell]

A 12v panel will NEVER charge a 12v battery.
The panel must output at least 18v (max 18v is sufficient) because a charged 12v battery will create a "floating charge" of up to 15v, and some are even higher, before producing a gas.
You need a diode and some form of control to prevent over charging and all these have voltage-drops across them.

Some of the calculations above are too complex as you only need to introduce current into the equation as you can assume the voltage is 12v.

You really need to know the AVERAGE current required by the circuit. It may only need 205mA for 5 minutes a day.
If you need 42.28 watt-hours per day, you don't need a 100 watt panel that delivers 600 watt-hours per day.

 

[hevans1944]

A 12v panel will NEVER charge a 12v battery.

Sure it will. You just need a boost converter/charger.

The panel must output at least 18v (max 18v is sufficient) because a charged 12v battery will create a "floating charge" of up to 15v, and some are even higher, before producing a gas.

Nonsense. Even a 1.5 V panel providing sufficient current to a boost converter will work.

You need a diode and some form of control to prevent over charging and all these have voltage-drops across them.

Yeah, you need a diode, maybe a choke, some more diodes, possibly some Schottky diodes, some MOSFETs, and an Integrated Circuit Charge Controller, TBD. And, yes, everything has voltage-drops. Live with it.

Some of the calculations above are too complex as you only need to introduce current into the equation as you can assume the voltage is 12v.

The math is what it is.

You really need to know the AVERAGE current required by the circuit. It may only need 205mA for 5 minutes a day.
If you need 42.28 watt-hours per day, you don't need a 100 watt panel that delivers 600 watt-hours per day.

I think 600 watt-hours per day is very optimistic for a 100 watt solar panel. The sun doesn't shine in a clear blue sky every day. Unless the solar panels are mounted to track the apparent motion of the sun across the sky, they will not produce their rated output power for six hours, even on a clear day. The OP wants the battery to stay charged for weeks at a time, so best to be prepared for some cloudy/rainy days.

 

[Colin Mitchell]

"I think 600 watt-hours per day is very optimistic for a 100 watt solar panel. "
You have no idea where he lives.
Even if it is 400 watt-hours, it is far in excess of what he needs.

"The OP wants the battery to stay charged for weeks at a time,"
Your understanding of English is different to mine.
Where does he say he say he "wants the battery to stay charged for weeks at a time."

"A 12 VDC SLA battery requires about 14 VDC to charge, so you must step up the solar panel output to at least that voltage, "

14v DC will NEVER charge a 12v battery.
You keep making the same mistakes. Where is your headroom?

 

[hevans1944]

"I think 600 watt-hours per day is very optimistic for a 100 watt solar panel. "
You have no idea where he lives.
Even if it is 400 watt-hours, it is far in excess of what he needs.

That is absolutely true, I don't know where he (or she) lives. Feel free to jump in with an estimate of your own. Maybe a 100 W panel is a wee bit more than is required if the energy requirement really is less than 50 watt-hours per day... but it will work.

"The OP wants the battery to stay charged for weeks at a time,"
Your understanding of English is different to mine.
Where does he say he say he "wants the battery to stay charged for weeks at a time."

Read near the bottom of the original post where it says: "What I need to be able to do is keep the battery constantly charged. When the device is powered on it is likely to be left on for a couple of weeks."

"A 12 VDC SLA battery requires about 14 VDC to charge, so you must step up the solar panel output to at least that voltage, "

14v DC will NEVER charge a 12v battery.
You keep making the same mistakes. Where is your headroom?

That's not a mistake. I used a 12 V SLA battery to power an isolated high-voltage secondary-electron suppression power supply. Next to this Lexan-encased and Teflon insulated contraption was a voltage and current adjustable B&K laboratory power supply used to periodically recharge the SLA battery. I set it for constant current output with 14 VDC compliance voltage. Worked just fine. Plenty of headroom.

 

[Colin Mitchell]

A power supply set to constant current is COMPLETELY DIFFERENT to a solar panel.