Good type of battery for solar cells?

Ravenkallen

I have looked at a bunch of different chemistry's(Lithium ion, lithium polymer, nickel cadmium, nickel metal hydride, even sealed lead acid). I wonder. I know they say that sealed lead acid types are good for trickle charging, but they are kinda weird... I like lithium polymer's, but i watched a few documentary videos that showed them exploding and leaking toxic gas when over charged. I think if i am careful, lithium polymers will be my best bet? A lot of tutorials also use lithium polymer's to!

Kevin Brown

like anything it depends what you're trying to do. What voltage you need, how much it draws. I've used all the different chemistries over the years but when it comes to low-cost simple to implement I like the little 6 V sealed lead acid you can get them in for six or eight amp hours sizes. You can get them at the hardware store there used for emergency lighting as long as you do not exceed the maximum charge current they're very forgiving about the voltage. Even in hard use you can typically get a couple years out of them. Depending what your current draw is a couple of the 9v solar cells running through a diode make a very nice charger for them without needing all the hassle of a charge controller. the battery is designed to be continuously charged so they're pretty rugged compared to most of the other chemistries. Regardless of the chemistry.comes down to how deeply they are discharged in a cycle. Discharging lead acids below 50% can permanently damage them same for the lipos for 12 V 50% is 10.5 V for 6 V to believe is around 4 V so if you discharge a battery to 50% every night it has a life of about 360 cycles where is if you only discharge a battery 10 percent it will have 12 to 1500 cycles so often times it's better to double up if you need it to last a long time.

I have a GSM project that needs 3.3 V the module that I'm using also has a lipo charging circuit built in so a 6 V solar panel with a diode will give me 5 V which is what the charging circuit needs if you need 5 V at a lipo's then you also need to step up circuit and there are some boards out there to make that easy.

My favorite is still the little 6 V they are very rugged little beast

the general rule of thumb I use on solar powered systems when calculating battery and panel sizes, I like to have 3 to 5 days reserved capacity and I use five hours per day to calculate my charging. I built some solar powered LED lights that the city uses on signs coming into town that have been running for over six years without a single issue.

Ravenkallen

I guess i should have put up some more details... I am using this solar cell,
http://www.sparkfun.com/products/7845

I need to find a small battery and one that is light weight(Sealed lead acid types are pretty hefty)... Right now i am only starting to experiment with solar. If i like it(and i am sure i will), then i will move up to higher powered cells and beefier storage systems. The battery i am thinking of purchasing is this one..
http://www.sparkfun.com/products/341
I will have to lower the voltage a little, but that shouldn't be to hard!!

LoopyByteloose

Lead Acid (gel cell or not) is about the heaviest you can get - heavier than Alkaline on a per amp=hour in the the packaged. But they are 'storage cells' - which means that they hold are charger longer than justs about any other technology. That may be a big advantage in some solar cell charger situations.

At the opposite end is Lithium technology that pack the most power per ounce of weight,and don't hold charges as well as lead acid. But they do tend to recharge very quickly, I suspect faster than any other chemistry when handled with the right charger (thus their use in cellular phones and laptop computers is not all about the light weight or length of charge - they tend to do well in a deep cycle of charge and discharge situaition)

Finally you have NiCd or NiMh that are in between, easier to build a charger for than Lithium and also less costly. I've never actually built a Lithium battery charger as they seem to demand rather complex charging (pre-charge trickel, then regular charge, then top off WITH temperature sensing and often with balanced charging.

I guess this is for your Propeller PDA project and single cell 3.7 Volt Lithium flat packs are likely to be a first choice. But it also helps with solar to 'think big'. It is difficult to get a successful solar charger in a small package for DIY - much easier to build a cost-effective big one.

SparkFun's disclaimer "Do not attempt to charge these with anything but a specialized LiPo charger." points out the problems with all Lithium technology - What do I do for a charger and what exactly is going to happen if I make a poor design choice? The powers that be don't really want you to DIY Lithium chargers.

From what I can figure out, nearly all batteries work better and last longer if they are kept above 50% of their rated voltage, so some illusions about how much increased capacity from higher voltage batteries need to be considered. There is a gain, but roughly 50% or less of what most people expect. Sometimes it is much better to have a larger capacity nearest your desired use instead of regulation burning up power resources.

And so.... add this item.

http://www.sparkfun.com/products/726

bsnut

A Deep-Cycle battery is the best way to go for your type of application. Kevin and Loopy have covered the other battery types.

Here is a LiPo charger that I found for the type of battery that you posted. www.sparkfun.com/products/10217
or search LiPo charger on the Sparkfun.com and will get 16 results for that type of charger.

Ravenkallen

@Loopy... you are almost as good at reading minds as Mike Yeah, i want to use the battery for my Prop PDA! I saw those chargers on Sparkfun and i was already planning on throwing one in for good measure. I also really like solar cells and i want to experiment more with them. I believe that solar cells are the future of power generation(Coupled with solid state lighting and more efficient appliances). We will need a few other types of energy to get us through extended cloudy period, but i think solar is the best alternative energy out there, even better than wind, geothermal or hydroelectric. They are literally working on a solar "paint" that can be easily applied to any surface. Imagine if we could cover everything in solar cells!! But yeah, back to the point...

I just want to try something to get my feet wet a little. I thought that you could charge lithium polymers from any power source as long as you didn't go over the voltage rating and made sure to not charge it with more current than its capacity in mAh's? You would also need some way to shut off the power once the battery was charged, right?

bsnut

@Raven you have part of it right. The voltage only shuts off when you remove it. Most chargers that are on the market today cut back on the amps, not the voltage.

I have 2 solar panels and a 2 solar chargers for the USS Zuni/USGC Tamaora, which is a ship that I am helping to restore. This system can be switched between 12v and 24v to charge lead-acid batteries at 1 amp hour. These chargers cut back in the amps only. You may think, that the voltage will be higher. Yes, that is correct when the amps are low and only when the voltage doesn't on the panels.

I will be using this system for another project on our ship as well. You can see this system on EFX-TEK.com forums under completed projects.

LoopyByteloose

A few years ago, I wanted to build my own DIY Lithium cell charger, but to begin with there are 3 different chemistries with 3 different voltages - 3.0v; 3.6v, and 3.7v. Added to that is a problem that absolutely everyone wants to sell you their 'charger chip' and not explain the proprietary algorithm by which they charge and monitor the safety of the cell. And of course all these chips are surface mount and many are only documented and not sold to hobbyists in small quantities. You may find yourself quickly frustrated by trying to locate on line material that is truly useful about charging Lithium -- too many trade secrets.

And so, matching up a set of Sparkfun products that they have managed to make work well is an easy way to go.

I have no trouble with building a Lead Acid or NiCad or NiMh charger, but Lithium sees a bigger challenge. For a short period extreme high rates are often employed and thermal sensing is quite important in such context in order to avoid explosive failure. If good information evenually leaks out, fine. But until then, I personally feel it isn't worth the trouble. Buyer beware - commercial chargers sold by battery makers actually may be built to quick charge the battery and NOT to optimize the batteries useful life. After all they are in business to sell MORE batteries, not less. That is certainly true of NiCad and NiMh chargers.

Ravenkallen

Good points. I wondered why i couldn't find any useful diy LIPO chargers on line.

localroger

They are about the opposite of light weight but if you can get away with it I'm a big fan of EnerSys Cyclon sealed spiral lead acid batteries -- the ones in the metal cans. I once built a robot a bit bigger than Stingray using surplus windshield wiper motors powered by 5 Cyclon X cells -- I swear that thing could drive up a vertical wall if it got the traction. They're almost impossible to destroy, can be permanently hooked to a voltage source that is a bit higher than the cell voltage (I used a regulated 12VDC supply to keep my 10VDC pack charged). You can recharge them as fast as you discharge them, they don't deteriorate the way normal lead-acid batteries do because the can keeps gases sealed in so they recombine into the original chemistry a lot more effectively. And those dinky little D and X cells can deliver several hundred amps (be careful not to short 'em out, I vaporized a 12 gauge jumper wire that way). They're about half as energy dense as NiCD by volume and even worse by weight, but if you have the room and load bearing capacity for them there's nothing easier to work with.

LoopyByteloose

Lead Acid batteries have a very valid use in providing robots with a low center of gravity. And the ones that are NOT gel cell can tolerate aggressive charge rates that would swell a gel cell. Slowly I have been drawn towards motorcycle batteries being more flexible and rugged than gel cells. But of course, you have to deal with fumes, spillage, and leaks.