Solar battery charger

ruriimasu · Jan 27, 2009

Hi,
I am making a solar universal battery charger (mainly eneloop AAs and 18650s) and I need tips. The solar panel I bought off DX claims 5V and 130mA. I will use a diode to prevent the charge from flowing back (so I heard). Do I need any resistor from the wiring to the battery to prevent shorting the battery or something as the panel is 5V and my batteries are 1.2V and 3.7V? I plan to use a double AA battery holder and charge 2 AA batteries at same time, or charge a single 18650 at any time each. Any help is appreciated!

linterno · Jan 27, 2009

I guess you are far from making a reliable and safe battery charger.

It is not just to connect a power source to a battery to charge it. Batteries have a well defined method to charge them.

NiMh batteries should be charger until a voltage drop of ~5-10mv occurs (dV). But, that is not it. You should also control temperature and time.

Li-Ion batteries are, in theory, easier to charge. But, easier doesn't mean safer. Li-Ion batteries need a constant current applied to them until 4.2 volts a reached. After that point you must provide 4.2 constant volts until the current to the battery is between 0.05C and 0.10C. 1C = the battery capacity, 0.05 means 5% of the battery capacity and 0.1C means 10% of the battery capacity.

In conclusion, using the solar panel you bought is not a good idea to charge any of the batteries you want. Additionally, a 130ma solar panel is far from the required current to charge batteries with actual capacities in the market (2000+ mAh). It will take more than 3 days at full sun. Additionally, NiMh batteries should be charged at 0.1C minimum to work properly; Some one could say something different here, but that is what I have learnt after many readings and tests.

Sub_Umbra · Jan 27, 2009

ruriimasu · Jan 28, 2009

Sub_Umbra said:

serious?

ruriimasu · Jan 28, 2009

linterno said:
I guess you are far from making a reliable and safe battery charger.

It is not just to connect a power source to a battery to charge it. Batteries have a well defined method to charge them.

NiMh batteries should be charger until a voltage drop of ~5-10mv occurs (dV). But, that is not it. You should also control temperature and time.

Li-Ion batteries are, in theory, easier to charge. But, easier doesn't mean safer. Li-Ion batteries need a constant current applied to them until 4.2 volts a reached. After that point you must provide 4.2 constant volts until the current to the battery is between 0.05C and 0.10C. 1C = the battery capacity, 0.05 means 5% of the battery capacity and 0.1C means 10% of the battery capacity.

In conclusion, using the solar panel you bought is not a good idea to charge any of the batteries you want. Additionally, a 130ma solar panel is far from the required current to charge batteries with actual capacities in the market (2000+ mAh). It will take more than 3 days at full sun. Additionally, NiMh batteries should be charged at 0.1C minimum to work properly; Some one could say something different here, but that is what I have learnt after many readings and tests.

how about i store the charge from the solar panel to a 4.2V li-ion or a set of 4 1.2V NiMh batteries, then use the batteries to charge up other batteries?
i have plenty of sun here in singapore, so i really think it could charge up the battery.

a silly qn here, if i use a lens to focus the sunlight onto the solar panel, will it poof the panel or actually give it more intense sun?

linterno · Jan 28, 2009

ruriimasu said:
how about i store the charge from the solar panel to a 4.2V li-ion or a set of 4 1.2V NiMh batteries, then use the batteries to charge up other batteries?

If you have a multimeter you could be measuring the voltage in your Li-Ion battery. When the voltage is 4.2V then you disconnect the panel from the battery.

Charging NiMh in series, even if they do, it is not advisable. Additionally, 130ma going to 4 batteries would be to 32.5 ma to each battery. Your batteries will never charge.

ruriimasu said:
i have plenty of sun here in singapore, so i really think it could charge up the battery.

Yes, you could, but it will take a long time to do that and you need to babysit your battery while charging. It is no worth.

ruriimasu said:
a silly qn here, if i use a lens to focus the sunlight onto the solar panel, will it poof the panel or actually give it more intense sun?

You could do it. But, as the temperature increase in the solar cell the current from the solar cell/panel decrease.

Mr Happy · Jan 28, 2009

linterno said:
ICharging NiMh in series, even if they do, it is not advisable. Additionally, 130ma going to 4 batteries would be to 32.5 ma to each battery. Your batteries will never charge.

No, no, no, quite wrong. Charging NiMH in series is fine, it is done all the time. Furthermore the current will be the same through each battery, it will not be divided by four.

Mr Happy · Jan 28, 2009

ruriimasu said:
Hi,
I am making a solar universal battery charger (mainly eneloop AAs and 18650s) and I need tips. The solar panel I bought off DX claims 5V and 130mA. I will use a diode to prevent the charge from flowing back (so I heard). Do I need any resistor from the wiring to the battery to prevent shorting the battery or something as the panel is 5V and my batteries are 1.2V and 3.7V? I plan to use a double AA battery holder and charge 2 AA batteries at same time, or charge a single 18650 at any time each. Any help is appreciated!

OK, let's try to give you some good answers here.

Firstly you can charge your Eneloops by connecting them to a solar panel, but you should not try it with lithium ion cells. You must use a properly designed charger for those.

When your solar panel says it can produce 5 V and 130 mA, this will be the maximum in bright sunlight. It will be lower in weaker light. If you connect the panel to an Eneloop with the right polarity, and using a diode for reverse current protection, the panel will charge the Eneloop. A common recommendation is to have the panel voltage at least twice the battery voltage, so a 5 V panel could probably be used to charge one Eneloop, or two Eneloops in series, but not four in series.

If the panel is in bright sunlight and producing 130 mA, then it will take up to 20 hours to charge Eneloops (the calculation is 2000 mAh / 130 mA * 140%). If you have 8 hours of bright sunlight per day, that will be nearly three days for a full charge. Is it worth it?

linterno · Jan 28, 2009

Mr Happy said:
No, no, no, quite wrong. Charging NiMH in series is fine, it is done all the time.

Not that wrong. Batteries usually have different internal resistance and tend to charge/discharge ones more than others. If you charge in series you are going to finish with batteries with more charge than others and that is not good. Or, Am i wrong?

Mr Happy said:
Furthermore the current will be the same through each battery, it will not be divided by four.

You are right here. I was wrong. Excuse me.

linterno · Jan 28, 2009

Mr Happy said:
OK, let's try to give you some good answers here.

Thank you for coming to the rescue.

Mr Happy · Jan 28, 2009

linterno said:
Not that wrong. Batteries usually have different internal resistance and tend to charge/discharge ones more than others. If you charge in series you are going to finish with batteries with more charge than others and that is not good. Or, Am i wrong?

It is true to say that NiMH batteries for series charging should be of the same type and as closely matched as possible. However, if you charge a series pack at a very low current for a long time as we are talking about here, the pack will tend to balance itself as all the cells will eventually come up to the same state of charge.

Sub_Umbra · Jan 28, 2009

Sub_Umbra said:

ruriimasu said:
serious?

There used to be a POPCORN emoticon (graemlin) that people would put into posts to show that they were reading along and also interested in the poster's question.

This search shows lots of "popcorn code" in CPF posts, although the emoticon doesn"t show up any more. I don't know where it went...so I found my own and put it up from time to time. It's smaller than 4k which is much smaller than many avitars currently in use here.

I am also interested in your question.

ruriimasu · Jan 28, 2009

Sub_Umbra said:
There used to be a POPCORN emoticon (graemlin) that people would put into posts to show that they were reading along and also interested in the poster's question.

This search shows lots of "popcorn code" in CPF posts, although the emoticon doesn"t show up any more. I don't know where it went...so I found my own and put it up from time to time. It's smaller than 4k which is much smaller than many avitars currently in use here.

I am also interested in your question.

i thought u meant :poof:

ruriimasu · Jan 28, 2009

Mr Happy said:
OK, let's try to give you some good answers here.

Firstly you can charge your Eneloops by connecting them to a solar panel, but you should not try it with lithium ion cells. You must use a properly designed charger for those.

When your solar panel says it can produce 5 V and 130 mA, this will be the maximum in bright sunlight. It will be lower in weaker light. If you connect the panel to an Eneloop with the right polarity, and using a diode for reverse current protection, the panel will charge the Eneloop. A common recommendation is to have the panel voltage at least twice the battery voltage, so a 5 V panel could probably be used to charge one Eneloop, or two Eneloops in series, but not four in series.

If the panel is in bright sunlight and producing 130 mA, then it will take up to 20 hours to charge Eneloops (the calculation is 2000 mAh / 130 mA * 140%). If you have 8 hours of bright sunlight per day, that will be nearly three days for a full charge. Is it worth it?

i was thinking of using it during hikes when there is a possibility of running out of batteries or doomsday when there is nothing left to charge up the batteries except the sun (yeah.. i am going overboard here :crazy:

)

if i needed to depend on solar charging, i intend to do it everyday even if the battery is about 80% full. so the charging should not take too long right? no?

Woods Walker · Jan 28, 2009

Mr Happy said:
OK, let's try to give you some good answers here.

Firstly you can charge your Eneloops by connecting them to a solar panel, but you should not try it with lithium ion cells. You must use a properly designed charger for those.

When your solar panel says it can produce 5 V and 130 mA, this will be the maximum in bright sunlight. It will be lower in weaker light. If you connect the panel to an Eneloop with the right polarity, and using a diode for reverse current protection, the panel will charge the Eneloop. A common recommendation is to have the panel voltage at least twice the battery voltage, so a 5 V panel could probably be used to charge one Eneloop, or two Eneloops in series, but not four in series.

If the panel is in bright sunlight and producing 130 mA, then it will take up to 20 hours to charge Eneloops (the calculation is 2000 mAh / 130 mA * 140%). If you have 8 hours of bright sunlight per day, that will be nearly three days for a full charge. Is it worth it?

Mine seems to charge in 10-15 hours.

Tested it with AA Duraloops and Energizer NiMH. Don't know if the charge was full however ran the L2D Q5 on high for about the right amount of time.

Un folds to fit on my pack. Onced used the thing to recharge my GPS.

65535 · Jan 28, 2009

The only decent way to use solar panels to charge is with a 12 volt panel putting out at least 500mA preferably 2A+ and a gel cell to stabilize the power, using the smallest hobby type charger you can find.

Mr Happy · Jan 28, 2009

Woods Walker said:
Mine seems to charge in 10-15 hours.

Tested it with AA Duraloops and Energizer NiMH. Don't know if the charge was full however ran the L2D Q5 on high for about the right amount of time.

That's good. It all depends on how much current you can actually get out of the particular solar panel being used.

Now of course if you are walking in the woods you won't get so much direct sunlight to work with :grin2:

ruriimasu · Jan 28, 2009

Woods Walker said:
Mine seems to charge in 10-15 hours.

Tested it with AA Duraloops and Energizer NiMH. Don't know if the charge was full however ran the L2D Q5 on high for about the right amount of time.

Un folds to fit on my pack. Onced used the thing to recharge my GPS.

may i know how many Vs and current are your solar panels?

Woods Walker · Jan 28, 2009

Mr Happy said:
That's good. It all depends on how much current you can actually get out of the particular solar panel being used.

Now of course if you are walking in the woods you won't get so much direct sunlight to work with

It works ok with indirect sunlight too. I put it in the sun when breaking camp. So got maybe 3 hours of good sun when packing my gear and eating. Add in another 4 hours of mixed and it was enough to push the GPS up a few bars. This was all I needed to find the trail head before a nasty T-storm with marbel sized ice. I do pack extra batteries but was using my AAA gear (headlamp) so forgot all about AA. Packed the charger mostly for fun but didn't know that I would need the silly thing.

"Charging capacity is 100% when perpendicular to full sun, 70% when at a 45 degree angle to full sun, 60-90% in light overcast, and even 20-30% in heavy overcast."

Woods Walker · Jan 28, 2009

ruriimasu said:
may i know how many Vs and current are your solar panels?

I don't know here is a link to my Silva charger. It does work. Still kinda suprised by that.

http://www.thesolarpowerstore.com/silvacharger2.html

Solar battery charger

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