How much voltage do I need from solar panels? Help please

System

Hi, I am new at using solar panels, but I thought I might give it a try. But I needed some questions answered before I began buying anything.

I have 10 AA Rechargable batteries, which are 1.2v's each, making them all together 12v's, and I want to charge them using solar panels.

Now my question is: How much voltage do I need to give those batteries to charge them? Do they need 12v from a solar panel to charge 12v's of batteries? Also, How long will the charging take, if slow, how can I make them charge faster? For example, some wall chargers these days can charge batteries in about 15 minutes only!

BB.

Re: How much voltage do I need from solar panels? Help please

Oh boy... That is a tough question to answer simply.

Here is a multi-page discussion over at the Candle Power Forum (family friendly place) about solar chargers for AA/AAA batteries.

The short(er) answers... NiCAD and NiMH batteries if you charge them with a 1/20 to 1/10 * Capacity (i.e., 10-20 hours to charge a battery), then you can do almost anything (resistors, simple 3-T regulators, etc.)...

However if you want 1-4 x C (1 hour to 15 minutes to charge), you need a pretty sophisticated type of charger (monitor temperature, current, voltage) to detect when to end the charge... Plus, if I recall correctly, the NiMH batteries that can take 4xC charge rates (internal protection) to prevent damage (i.e., rapid energetic disassembly).

And, we can also come back to what is it that you wish to accomplish. A solar panel that can collect ~5 hours of sun per day every day to charge your batteries (typically summertime) and only use if for 15 minutes (once in a while?) means that the solar panel would need to be 20-40 times the size (and cost) of a similar 1-2 day solar charger system.

You can mitigate some of the solar panel issues if you a solar panel to charge a local battery that then is used to quick charge your AA/AAA cells--but that is again going to cost you more money, size, and weight than the more simpler chargers...

The small "solar chargers" work, but are like 0.150 AmpHours... Just one NiMH AA battery can be 2.7 AmpHours--so a "portable" solar unit could take 4 days of 5 hour per day sun to recharge 1 AA battery...

Lastly, it also depends on the environment that you want to use the batteries...

Rechargeable:
NiCAD--low(er) capacity, pretty rugged and forgiving. High self discharge
NiMH--very high capacity, less tolerant of over/under charging. High self discharge.
Hybrid NiMH--high capacity, less tolerant of over/under charging. Low self discharge. (Sanyo Enloop type cells, and other's called "hybrid" NiMH)

Non-Rechargeable (Primary Cells):
Alkaline--Long shelf life, good capacity at 1/10*C discharge rates. At high rates (like digital cameras), very poor performance.
Lithium--Long shelf life, very good capacity at high discharge rates. Expensive. Can explode if miss-handled.

I will stop here for now... Questions (that I can answer)?

-Bill

PS: Forgot to add that Lithium cells (Primary) are about the only ones you can use in cold weather (near freezing or below)... The rest of the batteries pretty much die once they get cold.

niel

Re: How much voltage do I need from solar panels? Help please

viruz5 wrote: »

Hi, I am new at using solar panels, but I thought I might give it a try. But I needed some questions answered before I began buying anything.

I have 10 AA Rechargable batteries, which are 1.2v's each, making them all together 12v's, and I want to charge them using solar panels.

Now my question is: How much voltage do I need to give those batteries to charge them? Do they need 12v from a solar panel to charge 12v's of batteries? Also, How long will the charging take, if slow, how can I make them charge faster? For example, some wall chargers these days can charge batteries in about 15 minutes only!

i would say you'd need a steady 18v+ source capable of about 3 amps. seeing as how most 12v pvs have a vmp around 17v in sunny conditions, it may be advisable to go to either an odd pv rating like those with vmps in the mid to low 20v ranges or get a 24v pv. the 24v pv will have far more than enough voltage to do what you wish and it could even be 2 smaller identical 12v pvs of the proper or higher current than you'd need placed into series.
a fast charge of up to 4hrs may be to your advantage as solar does not afford the luxury of charging reliably beyond 5hrs everyday as many other charger types can do from utility power. now i should mention that many nicds and nimh types charge better with longer times and affords the greatest overall charge cycles and that decreases somewhat the faster the charge takes place. a small float type charge could help top off the batteries when not being used right away from their full charge and can be safely done with 1/500 to 1/1000 of the battery ah ratings. this can extend battery storage and battery cycles under some conditions and can be accomplished fairly easy, but i won't go into it unless you later indicate you wish to do this.
i'd recommend getting a commercial fast charger as most can't fabricate the chargers available. of the few that take a dc input few also accomodate all 10 batteries, but this one does that i've linked to here.
http://www.mrnicd-ehyostco.com/universal_charger_plus!.htm
editted to add:
this too may be an option for you as a similar charger:
http://www.mahaenergy.com/store/viewItem.asp?idProduct=185&cur=specs#mid
to go with a 12v pv (> or equal to 6 amps) you would need multiple small chargers that can accomodate the dc input. some only take 4 batteries at a time, while others take up to 8. in exploring the link websites i gave you may come up with a differing or more suitable system for yourself. to add to your field of choice and price here's the link to thomas distributing:
http://www.thomas-distributing.com/nimh_battery_chargers.htm
let us know what configuration you pick and how well it works for you.

mike95490

Re: How much voltage do I need from solar panels? Help please

One way I solved the "5 hours of sun" is I use a charger that has a FAST and SLOW charge settings. reading the manual, [play ominous tones here] I discovered that a half hour at FAST, then switching to SLOW rate, lets me get a full charge, without cooking the batterys. I do have to babysit them more while charging.

System2

Re: How much voltage do I need from solar panels? Help please

I am trying to absorbe all this information, but my head is about to explode..lol because remember i am a newb at this stuff. But I was wondering if there was a way, if it is this simple, to just connect a bunch of little solar panels, like ones i can take off an outside garden solar light ( http://www.eco-lights.com/solar-lighting.php )... My understanding is by connecting 4 of these small solar panels, I can get up to 4.5v - 5v's or maybe more, as I seen in some video.

So is it easy enough just to connect a bunch of those together to get the required voltage? Or is there more to it than just to connect them all together and connect then to the batteries to charge it.

BB.

Re: How much voltage do I need from solar panels? Help please

Hmmm.... You want to do solar powered lighting using LEDs?

For very simple lighting, like solar yard lights, they typically use one LED and a pair of NiCAD batteries and a little solar cell, blocking diode, and photo transistor to turn off during the day.

The reason this works is several... 1) NiCADs are pretty tolerent of overcharging and running flat. The batteries can last a couple of years in daily use.

However, there is one mode that will kill NiCAD and any other rechargeable battery. That is "reverse charging" one. For example you put ~8-10 cells in series for a 12 volt light. With the "run flat" method with one or two cells, the battery get discharged to 0.0 volts and all is well.

But in a string of 10 batteries, if the "string is run flat", what usually happens is that one (or more) cell has less capacity than the other 9. And what happens is it goes to zero volts while the others are still at 1.1 volts. And then, the battery gets "reverse voltage" and the other batteries begin to charge the one weak cell... Once the battery gets more than a few ten'ths of a volt of reverse voltage--the cell is destroyed.

So, long around explanation is that the larger and higher voltage your system becomes, the more things need to be accounted for.

There are 12 volt solar charge controllers that are specifically made to support low voltage lighting like this.

So--right now, you are still a little unclear (to me) what it is you are trying to do. Some folks just want to experiment with solar around the home, others want to equip a weekend cabin with lights, and others are trying to bring electric lights to the third world.

Each of those problems probably has a different solution.

I guess you are in the US... You can go by a Home Depot or Lows and get a pretty nice LED (older ones used filament bulbs) solar powered yard/security light for $35 or so... Take one of these apart and play with the pieces and add your own LEDs strings (plus resistors), use the motion detector, etc. and see what you like.

-Bill

niel

Re: How much voltage do I need from solar panels? Help please

as bill said the nicd batteries take some abuse, but they are lower in amphours than nimh types. you did not say which type you have or the application that the batteries will be used in as this may shed some light on a better answer. in general i do like chargers that allow for individual batteries to be charged, but most need at least 2 in series with some out there in the links i gave capable of individual charging. the voltage from the solar cells is low (about .5v each) and charges, fully only if you're lucky, 1 battery with 4 cells every day. my nicd solar lights have the nicd at 700mah and charging a nimh with a capacity at 2500mah would require 3-4x as much of the cells paralleled to get the current up to charge it. given the costs of those garden lights it may be advisable to do a larger panel with a commercial charger. for fast charging as you require, the commercial chargers are the only way for you to go without any real solar or electronics knowledge and that same fast charging requirement forces a certain amount of power to be needed in short term so a pv with a higher current rating would be needed. remember the garden lights take all day for a low capacity rechargeable and you requested a fast charge ability.