# New guy here with some questions!

Solar Expert Posts: 131 ✭✭✭✭✭
Hello everyone! I have always been interested in powering things with solar power. I am more of a tinkerer more than anything else, but I enjoy it.

I have a couple small about 12v and .100 ma panels that i would like to do something with, though I know I can do alot with them.

I was thinking of making a battery charger for AAA, AA, C and D batteries, whatever type of battery, NiCad, NiMh or whatever would work for this application. I know with these small panels it would take a long time to charge anything, but time I am not worried about, more of something I can learn from and watch and play with.

Any sugguestions on this subject on how to do it, or any other small applications I might be able to do would be great if you could help me out. Thanks

Brandon

Re: New guy here with some questions!

12v*0.1 amps - 1.2 watts... A pretty small panel.

NiMH and NiCad batteries, if charged below C/10 (10% of their Amp*Hour rating) can be charged without using a charge controller...

Typical AA NiMH is ~2,000 mAH (2 AH) and the typical NiCad is round ~0.9 AH...

So, just take one of the panels, an you can put between 1 and ~6-8 maximum NiCad/NiMH batteries in series and just put it out in the sun and let it charge (remember, the panel voltage has to be above the battery charging voltage @ 0.1 amps).

In the summer, you will get about 5 "hours of sun" worth of 0.1 amps of current... So, it will take about 10+ "hours of sun" to charge a NiCad (2 days * 5 hours of sun * 0.1 amps = 1 AH--probably closer to 3-4 four days of sun, hard to guess).

The NiMH will take 2-3x longer to recharge from "dead".

The up side is that you can just leave the cells charging for weeks (or forever). The downside is that at very slow charge rates, it is possible for the NiMH (and probably NiCad) to grow large crystals--which can lead to voltage depression (aka, the NiCad "memory" effect).

Also, check your 12 volt solar panels--if they do not have a blocking diode, they will discharge the batteries at night. You will need to install a series blocking diode to prevent night-time discharge.

Otherwise, there is really no reason to add any electronics, unless you want to look at building a buck mode down converter... Convert the 12 volts at 0.1 amps to 1.2 volts at 1 amp (est.). Then you can charge 1 AA battery in just one day... (otherwise, which is better, charge 2 batteries per day, or 6 batteries every 3 days--your choice).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 131 ✭✭✭✭✭
Re: New guy here with some questions!

Ok great this is a start. I do have some blocking diodes cause I know these panels dont have them built in, but have never wired them in yet. No reason to yet.

I just put my one of the panels in the sun and checked its output. It was about 11.1v and .105ma.

Even with the batteries I have which are rated at 1.2v I dont have to worry about overcharging them?
Re: New guy here with some questions!

Basically, most rechargeable small batteries (NiMH, NiCad) when charged below C/10 rate (AH capacity divided by 10) will not overcharge regardless of the voltage source.

Once fully charged, they have a mechanism inside to reconvert generated gasses back into electrolyte--and the batteries will not overheat at that low of current/energy input (as I recall).

So, if you assume that each battery requires ~1.5 volts to recharge, and you use a standard diode, 0.7 volt drop, then:

(7 batteries * 1.5 v)+0.7v = 11.2 volts

So, seven batteries in series may be a bit much for your panel.

Assuming you got 11.01 volts at 0.105 amps. I assume a typo "0.105 mAmps and 0.100 mA"?--if the panels are really that small, it is not even worth trying to charge AA batteries. It would take weeks to charge a single/set of AA battery with those levels of current.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 131 ✭✭✭✭✭
Re: New guy here with some questions!

What if I wired the panels in series or parallel?

Also if I were to get something like this would I be able to put or add my own panels to it to speed up the process?

http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=3484
Re: New guy here with some questions!

Probably... That charger is 160 MA and can do two batteries in series...

Are your panels 0.1 milliAmps or 0.1 Amps?

Adding your existing panels will make either a good sized improvement or little change (100/160=62% faster, or 0.1/160=0.6% faster). Depending on your panel's rating...

Cost wise, using 12 volt panels on a charger that only needs 3-5 volts (and does not have a buck mode down converter to efficiently drop the 12 volts to 4 volts) will waste 2/3'rd of your 12 volt panel's power.

Play with the equations:

V=I*R
P=I*V=I^2 * R = V^2 / R
Work=Power*Time=Watts*Hours (typical non-scientific units used for batteries and appliances).
Amp*Hours=Power/Voltage (many batteries are rated in Amp*Hours, but you need to know the battery voltage to know the Power involved).

And you will see the waste. As an example:

12 volts charging 3 volt battery:

12v-3v/12v=2/3=66% of energy is wasted (battery sets voltage = 3 volts)

For the example solar charger:

0.160 amps * 3 volts = 0.48 watts

A NiMH battery of 2,000 mA at 1.2 volts:

2.000 amps * 1.2 volts = 2.4 watt*hours

And you need two batteries in series, so you would need 2*2.4 watt*hours = 4.8 Watt*Hours minimum to charge two dead batteries (actually, battery charging is not 100% efficient, so you will need some xx% more energy to really fully charge the batteries):

2.4 watt*hours (battery cap) / 0.48 watts (charger amps) = 5 hours

In reality, full summer sun is around 5 hours per day (really 8-12 hours of sun, but 5 Hours of "full noon sun") and assuming 50% efficient system, two summer days to charge two AA NiMH cells.

And the charger you have linked to, is probably one of the better chargers out there (larger solar panels than most).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 131 ✭✭✭✭✭
Re: New guy here with some questions!

Wow great info thanks a ton! Im not sure if they are 0.1 milliAmps or 0.1 Amps. But I am sure they are rated at 100 milliamps which is .1 amps. When I have them wired together and in full sun they run a computer cooling fan at a pretty high speed.

I know size is not a major factor since some are more efficient, but they are like 7"x12" panels.
• Solar Expert Posts: 131 ✭✭✭✭✭
Re: New guy here with some questions!

If I bought that charger and intergrated my panels into it do you think I have any chance of burning something up?
Re: New guy here with some questions!

I don't know. Your panels are probably 2-3 times the voltage needed to operate that unit. It depends what electronics may be inside that charger that could be damaged (or the voltmeter itself could be damaged when there are no batteries inside).

It also depends on the current of your panels. 0.1 milliamps is not likely to damage the volt meter--but they would also be likely to be useless in charging batteries unless you wired up a 100 of your panels in parallel.

If you have 0.1 amps of 12 volt panels (which it sounds like), it is possible to cause damage (depending on the internals of the charger).

-Bill

PS: If you have 12 volt x 0.1 amp panels--you would spend much less money getting plastic battery holders and a diode from the local electronics store and wiring the batteries (4-6 batteries) in series to your panels directly. Be cheaper and you would be using 4x the amount of power from your panel vs using the 2 cell charger (two cells in series) charger.
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 131 ✭✭✭✭✭
Re: New guy here with some questions!
BB. wrote: »
PS: If you have 12 volt x 0.1 amp panels--you would spend much less money getting plastic battery holders and a diode from the local electronics store and wiring the batteries (4-6 batteries) in series to your panels directly. Be cheaper and you would be using 4x the amount of power from your panel vs using the 2 cell charger (two cells in series) charger.

Doing this I have to use 4-6(1.2v) batteries in series as to not overcharge them correct being they are going to be connected directly?
Re: New guy here with some questions!

Not really... Because you are charging with so little current--you can leave your panels connected for weeks or months and the batteries will not be damaged.

Generally, these small cells, if you charge them at less than C/10, they are safe to leave trickle charging. E.g (2,000 mAH NiMH):

2.00 amp*hours (NiMH) / 10 = 0.20 amps

Your panels are only 0.1 amps, so you are ~C/20 charge rate. You could connect this to a 100 volt @0.1 amp power supply and still not damage the batteries (not that I would recommend that). The limiting factor here is the batteries are able to keep their ~1.2-1.5 volt per cell at that low of current.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 131 ✭✭✭✭✭
Re: New guy here with some questions!

Ok starting to understand this a little better. So its really not the voltage I have to pay attention to as much from my panels, but I do have to watch my amperage?
Re: New guy here with some questions!

Oh boy--If I say yes, I don't want you to get into trouble (no 1,000 volt sources? OK?).

To look at it another way... You are familiar with a car/storage battery... No matter what load you put on it (small LED, 1 HP starting motor for the car, 12 volt Alternator to charge), the 12 volt storage battery will try to keep 12 volts across its terminals--no matter what is connected (within reason). This is the definition of a "Voltage Source/Sink".

A solar panel is just about the exact opposite (in electronic terms). A solar panel is a current source... No matter what load you put on it, the solar panel will output X amps from a dead short to its Vmp (maximum power point voltage). And X amps is proportional to the amount of sun (light energy) hitting the panel.

So, in your case (assuming 11 volt panel at 0.1 amps), if you put 1, or 2, 3, 4 ---6 batteries in series, the panel (in full sun) will still output 0.1 amps and the battery voltage will be whatever the battery wants it to be (voltage source).

If you put 7+ batteries in series, then the output voltage of the solar panel is exceeded, and it cannot output any current to charge the batteries.

This is the classic interaction between a voltage source and a current source.

If you like to look at graphs, this Suntech 210 watt solar panel has probably the best/clearest IV output graphs (2 page PDF file) I have seen.... You could take these graphs, and just change the scales to your voltage and current, it will do a pretty good job of predicting how your panel will work.

And, in this special case (C/10, C/20, or C/XX), these types of AA NiMH/NiCAD batteries are able to withstand continuous charging current without damage (from heat or other causes).

As always, be careful and accept that you may damage/destroy something if you don't know what you are doing... And make sure you understand your limits... It is a lot different working with AA cells and 9 volt radio batteries vs a 12 volt car battery (which is quite dangerous in many different ways).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 131 ✭✭✭✭✭
Re: New guy here with some questions!

Very good information. I am going to have to go over most of what you said a few times to for it to full sink in, let alone I am more of a hands on person so I have to get some experience with it.

But no of course not, I wouldnt be trying something with some crazy voltage. Basically I just have to know that formula for this application.

Thanks again, sure i will have more questions coming.