Battery charged by a solar panel?

[That one guy]

Is it possible for a rechargeable battery to be directly charged by a solar panel?

 

[Arouse1973]

Do you mean connecting without any charge management. If so then I would not recommend it at all.
Adam

 

[Audioguru]

If the output voltage at the charging current is correct and the charging current is safe for the battery then the solar panel can be directly connected to the battery. But a diode in series should be used to prevent the charged battery from discharging into the solar panel at night. The forward voltage of the battery must be included in the voltage calculation of the solar panel.

 

[(*steve*)]

I have a colleague who charges lipo batteries this way (in a sealed outdoor device). However he has a shunt regulator across the battery.

This will work well for lipo's and possibly lead acid batteries, but not with nicad or NIMH batteries.

You need to ensure that the charge current cannot rise too high and that your shunt regulator can dissipate the full power from the solar panel.

In my colleague's case the panel supplies well under 1W to the batteries and full charge is achieved in about 10 minutes of full sun after operating overnight.

Yeah it's a low power thing and may not scale really well.

 

[That one guy]

How can i include charge management?

 

[(*steve*)]

As per @Audioguru, the max current from the panel must not be too high for the battery.

For lithium cells, a simple shunt regulator can be used to limit the cell voltage.

See this thread for the design I've used. You will have to recalculate the voltage divider to get a voltage of about 4.35V. it is quite reasonable to place one of these across each cell when charging in series.

 

[Hicham]

How can i include charge management?

I don't know the capacity of your batteries nor the solar panel output voltage. You can find very cheap modules on eBay for such task. Check this one for 12v battery:
http://www.ebay.com/itm/LED-display...533995?hash=item2caf1de8eb:g:kxYAAOSwbYZXfdMC
If you are charging a 4.2v (nominal 3.7v) battery, here is another option:
http://www.ebay.com/itm/2pc-DC-6-5-...564484?hash=item1eb230d884:g:Z8sAAOSwOVpXU-NJ

 

[Audioguru]

That is a nice charging module. I wonder if it is "prime" or is a reject from a manufacturer. I guess you will get your money back if it doesn't work properly and you pay more than its cost to ship it back to China.

 

[Hicham]

That is a nice charging module. I wonder if it is "prime" or is a reject from a manufacturer. I guess you will get your money back if it doesn't work properly and you pay more than its cost to ship it back to China.

It's a standard module... at the few $ value, i think it's worth exploring.

 

[Charles King]

I have a colleague who charges lipo batteries this way (in a sealed outdoor device). However he has a shunt regulator across the battery.

This will work well for lipo's and possibly lead acid batteries, but not with nicad or NIMH batteries.

You need to ensure that the charge current cannot rise too high and that your shunt regulator can dissipate the full power from the solar panel.

In my colleague's case the panel supplies well under 1W to the batteries and full charge is achieved in about 10 minutes of full sun after operating overnight.

Yeah it's a low power thing and may not scale really well.

Hi,
I'm trying to design/build a similar setup. I googled around and searched this forum and this was the closest thing I could find. If it's not too much to ask, could I get some tips on your colleague's setup? I put all the details of my setup in this post.

Thanks!

 

[(*steve*)]

My colleague has a circuit operating from a single lipo cell. It consumes little power and a 6V 50mA solar panel fully recharges the battery in about 5 minutes after running overnight.

If your voltage or power need are significantly greater there may be additional requirements (thermal, balancing, etc.) that will also need to be met.

See here for discussion of the circuit (it is coming up quite a bit at the moment for some reason)

You simply connect the solar panel up to the battery via an isolating diode and place this circuit across the battery to limit the voltage.

 

[Charles King]

My colleague has a circuit operating from a single lipo cell. It consumes little power and a 6V 50mA solar panel fully recharges the battery in about 5 minutes after running overnight.

If your voltage or power need are significantly greater there may be additional requirements (thermal, balancing, etc.) that will also need to be met.

See here for discussion of the circuit (it is coming up quite a bit at the moment for some reason)

You simply connect the solar panel up to the battery via an isolating diode and place this circuit across the battery to limit the voltage.

Thanks for the reply!

My application will definitely be higher power so I will be looking into heat sinks etc once i have it working.

I've read that thread before- please let me know if i get this right. That circuit basically shunts any voltage over the set amount? Looks like you have a potentiometer in the final version to adjust from 11.6v to 15.4v? So it can be used as a battery charge controller by limiting the voltage seen at the battery?

How does this affect current? My biggest problem right now is the current going from my battery pack into the radio battery.

Sorry for my ignorance. I don't mean to ask so many questions that it seems like I'm not putting in the effort to learn this myself, I'm just hitting a mental road block for some reason and can't seem to get my head around this problem.

 

[(*steve*)]

The version on that page is to similar a lead acid battery. A change in resistor values is required for a different range of voltages.

The circuit works almost exactly like a zener diode. In this application, any excess current passes through the mosfet. This loads down the solar panel so it can't increase the voltage of the battery above a safe level.

If you have a battery where temperature variation will cause a change in voltage, this circuit could also draw significant power from the battery if the voltage rises due to temperature. (This is a bad thing)

The MOSFET will be called on to dissipate the full power from the solar panel once the battery is charged. So... A 100W panel means the MOSFET could be dissipating 100W.

For higher power applications, a series regulator, possibly employing MPPT would be justified.

This circuit should NEVER be used to charge a battery from a voltage source (e.g. battery or power supply). It is designed for use with current sources or at least current limited sources.

 

[Charles King]

The version on that page is to similar a lead acid battery. A change in resistor values is required for a different range of voltages.

The circuit works almost exactly like a zener diode. In this application, any excess current passes through the mosfet. This loads down the solar panel so it can't increase the voltage of the battery above a safe level.

If you have a battery where temperature variation will cause a change in voltage, this circuit could also draw significant power from the battery if the voltage rises due to temperature. (This is a bad thing)

The MOSFET will be called on to dissipate the full power from the solar panel once the battery is charged. So... A 100W panel means the MOSFET could be dissipating 100W.

For higher power applications, a series regulator, possibly employing MPPT would be justified.

This circuit should NEVER be used to charge a battery from a voltage source (e.g. battery or power supply). It is designed for use with current sources or at least current limited sources.

Ok I think that clears up a lot of the questions I had about your circuit, thanks!

My biggest problem right now is the charging rate between the battery and the radio. When the radio is plugged into the original mains charger, it charges at around .74A (half of the 1.5A rating printed on the power supply). That's at a consistent 14v. When I plug the radio into a battery and hold the voltage at 14v, it draws over 3A. What am I missing??

 

[Audioguru]

A battery must be charged at a voltage higher than its average discharging voltage.
A battery can have a discharging current much higher than its maximum allowed charging current.

 

[Charles King]

A battery must be charged at a voltage higher than its average discharging voltage.
A battery can have a discharging current much higher than its maximum allowed charging current.

Ok I'm totally with you so far! My question is: is there a way to regulate the max amperage at a fixed voltage? In other words, can I set the output voltage at 14v (using a voltage regulator), and somehow limit the current to only 1A or lower? Kind of like how a fuse works but instead of shutting down the whole circuit, it just doesn't allow the extra amperage through. That way I don't have to worry about the battery pack discharging too quickly. Or am I fundamentally thinking about this wrong?

 

[(*steve*)]

The mains adapter will have a voltage which drops slightly under load. This limits the current.

This will not happen if your voltage source is stiffer, e.g. a regulated power supply or a battery of sufficient capacity.

You could try placing a low value (say, 1 ohm) resistor in series with the battery. Note that it may be possible for the internal battery to charge the external battery (this wouldn't happen with most power supplies) so a series diode to prevent this would also be a good idea.

It sounds like the battery charging circuit is very simple (perhaps non-existent) so it is your responsibility not to overcharge of incorrectly charge the battery.