Inexpensive Gell Cell battery charger.

Hi all. I thought I would start this thread to see if anyone knows of an inexpensive, but good quality, regulated gell cell charger that would be good at charging say a 12v 7ah gell cell without frying it. I'm kind of new to gell cells (haven't played around with them in years) and I don't want to get a charger that will fry the new one that I'm thinking about purchasing.

It seems to me that a 12v gell cell would be an ideal power option for a MR16 headlamp. Not too overly heavy but a lot of power per pound/square inch and since it's sealed, you don't have to worry about getting it wet or spilling anything out of it. Just seems like a great power source to experiment with.

Also, are there any good web sites on the proper care and charging of gell cells out there? Are gell cells charged the same as lead acid batteries or differently? Will a lead acid charger work well with a gell cell? I kind of want to generally learn more about their characteristics as well as find the best charger for the money.

Thanks for any info/advice. /ubbthreads/images/graemlins/smile.gif

Has anyone here had any experience with the chargers that this site sells? Would the $10.95 charger likely fry a gell cell? http://www.alzatex.com/cgi-bin/webshop.cgi?command=link_list--00kits&setuid=yes

I've been using a couple of the A & A Engineering 1 amp kits for about 10 years to charge 2 amp to 17 amp gel cells. They have taken very good care of all my gel cell batteries in that time. /ubbthreads/images/graemlins/thumbsup.gif
Don't know if you want to pay this much or build a very easy kit. /ubbthreads/images/graemlins/icon3.gif

snuffy: Thanks for the link. I'm checking it out now.

Anyone else out there playing around with Gell Cells? The more advice and suggestions, the merrier. /ubbthreads/images/graemlins/smile.gif

snuffy: I checked it out and the price is more than a bit steep, but perhaps worth it, given the quality and all. I am wondering though, how hard is the kit to assemble? Are there many heat sensitive components to worry about frying in there? My soldering skills are rusty, and my equipment is primitive, so I don't want something that is going to be easy to fry like really heat sensitive micro chips or anything. Thanks for any info on this. /ubbthreads/images/graemlins/smile.gif

Also, I noticed that you could get a 14 volt version. Does this mean that you can get a 14 volt gell cell? That seems like it would be excellent for overdriving a 12 volt bulb. I've overdriven my MR16 at 14.4 volts with 12 NiMh AAs with no ill effects so far so a 14 volt gell cell might be just the ticket for maximum brightness + best runtime. What do you guys think?

X-CalBR8:
This ( 12v ) kit is a snap to build. About as easy as the old Heath-Kits. Anyone who can solder should have no trouble at all. It's a complete kit case and all. It has one socketed UC3906 IC, one pass transistor mounted to the back of the case for heat sinking, 12 resistors, 3 capacitors, 2 diodes and a transformer. Plus the usual nuts, screws and misc. bits & pieces.
Oh yes , also a red LED and a green LED. /ubbthreads/images/graemlins/grin.gif
You just hook it up to your battery and it's completely automatic. I leave mine hooked up 24/7 except when I use the batteries to run something.

Edit: The circuit board is the thru-hole type. No surface mount components. /ubbthreads/images/graemlins/grin.gif

If it's that simple...might someone be able to make their own from Radio shack parts? Seems like that would be quite a bit cheaper...

i use a big wallwart 12v@800ma which open circut gives 22v.
i feed it to a 7815 with a 1n400x diode in series with the output.floats around 14.2v
about as cheap and simple as they come.

snuffy: That does sound like an easy kit to build kit. Thanks for recommending it. Now if only I could get around that steep price tag. No more components than that unit has, I have to wonder if they are ripping people off on the components. It also seems that for that amount of money, it should have an adjustable voltage to, but from the way it sounds, you have to build a separate unit for each voltage battery that you want to charge. If I spend that much on a charger, I would like it to at least do 6v and 12v and it would be icing on the cake if it would also do one of those 14v to, assuming I can even find a 14v Gell Cell, that is.

I still think a 14v gell cell would be great for overdriving a 12v bulb. After all, a 12v bulb is usually rated for thousands and thousands of hours so even when you overdrive one of the suckers, it is still going to last much much longer than a standard flashlight bulb would.

I just had an idea. I don't know if anyone has tried this before, but I thought that I would throw it out there and see what you guys think.

What would be the result of buying one of those cheapo Wal-Mart wall warts that has an adjustable voltage at 1000ma current and connecting it directly to a gell cell to charge it. Let's see, 1000ma is 1 amp, IIRC. Most of these gell cells can be charged at 1 amp no problem, right? So, why would this not work? If you have a 7ah gell cell and you are charging it at 1 amp, would you just not let it charge for 7 hours? This would be a 1C charge rate, right? Isn't 1 amp also safe as a float amperage?

Any ideas as to why this would not work? I mean, it must not work for some reason or else everyone would be doing it instead of buying really expensive chargers, but I can't see why it would not work. I sure would rather pay like $8.00-12.00 for one of these at Wal-Mart than to pay a fortune for a kit that I have to spend hours building and it would have an adjustable voltage as well. Well, guys, let me have it. I know this logic must be flawed somewhere, but I just can't find the flaw, so help me out here please. /ubbthreads/images/graemlins/smile.gif

The only problem is that, neglecting charging efficiency, you shouldn't put more charge into the Gel Cell than you took out. If you know you took 3A-hours out you can hook the battery up to 1A for 3 hours and be OK.

You'd be better off setting the wall wart output (if it's very well regulated) to 14V and monitoring the charge current. When it fell to a small level - say 100mA or so - set the voltage on the supply to 13.2V and leave your battery on the charger till you need it. If you can't monitor the charge progress - set the wall wart to ~13.5V and leave the battery on it for a few days. It will charge slowly and safely at this voltage - but maybe not completely - you might reach 90% pretty quick.

Having said all this - my kid's grandparents gave them a powerwheels jeep that runs on two Gel Cells. The charger outputs ~14V. They say to charge for no more than 24hours. Sometimes we remember and sometimes we forget to disconnect them. The batteries have lasted probably 40-50 cycles so far without a lot of capacity loss.

OK, here is the scoop on gel cell batteries.
Gel cell batteries are great sealed batteries because of energy density and the fact that they don't spill acid. However, when considering a charger you have to keep a few things in mind.

ONE: gel cell batteries require special charging voltage. Normal deep cycle, automotive, or marine batteries can tolerate up to 14.8 or even 15 volts. Gel cells on the other hand cannot be charged over 14.1 volts, or permanent damage will occur. This automatically eliminates most chargers. The charger has to specifically say "can charge gel cell batteries" or chances are it won't work. Typical wall warts won't work because the boost the volage too high when the battery is nearly charged. Even if they are rated 14 volts it won't work, it'll damage the battery.

TWO: You do not want to damage the battery. This seems obvious, but if it is charged to too high a voltage too quickly, it will start releasing hydrogen gas (you can hear it hissing out). While most other batteries can tolerate this somewhat (sealed batteries less) gel batteries will immediately lose capacity, and there is no way to get it back. If you severely overcharge a small battery like the one you have, chances are good it will be totally destroyed, while other sealed batteries with liquid electrolytes can tolerate overcharge a little bit better ( a little).

Three: at open voltage, the charger cannot be more than 14.1, and 1 amp sounds good for a 7 amp battery. 13.5 is good for float, but you won't get a full charge. Call me paranoid, but I've had alot of trouble getting a charger for my gel cell because I wanted to be sure I wans't damaging it. I finally settled with a microprocesser controlled charger from Sears that has an option specifically for gel batteries. My battery is 51 amp hours.

By the way, the typical voltage for a rated 12 volt battery is between 12.75 and 13 volts. If the voltage reads 12 volts for the battery then it is fully discharged. If a 14 volt battery does exist, it's typical voltage would be about 14.8 volts. I know it sounds weird, but that's how it works.

At work we use this:
http://www.accumate.co.uk/

Works really well for charging and reconditioning a variety of sizes.

Snakebite's solution is similar to what those cheap spotlights use. A friend's was dead so I of course opened it to play. It uses a 12v (~18v no load) dc supply fed to an internal circuitboard with a bunch of (I assume) zeners in series to charge the 6v gel-cell battery. I guess current was limited by what the little transformer could put out, but it still killed the battery.

~ned

php_44: I used to have one of those Power Wheels chargers like you mentioned, but that was years ago and I guess somebody around here must have thrown it away, not knowing what it was. I never really trusted that charger anyway because it depended on you to disconnect it within a certain number of hours. The way I figure is that if a charger is designed even half way as good as it should be, it should know when to float the battery without any human intervention at all.

LEDlightman: You say that you got a charger at Sears that was specifically for a gel cell? How much did it run you and how large is it?

[ QUOTE ]
X-CalBR8 said:

I still think a 14v gell cell would be great for overdriving a 12v bulb. After all, a 12v bulb is usually rated for thousands and thousands of hours so even when you overdrive one of the suckers, it is still going to last much much longer than a standard flashlight bulb would.

[/ QUOTE ]

You might want to check that out. I think you'll find such high output bulbs are rated in *dozens* of hours, not hundreds let alone 'thousands and thousands' of hours.

They are subject to the same rules of physics as flashlight bulbs are, right?

Otherwise, my money in the 'poor man's high tech charger' contest is on a wall wart and LM317 (variable voltage regulator IC). You'll need a trimpot a resistor and two caps. Check out <http://www.jameco.com/jameco/Products/ProdDS/23579.pdf>, the spec sheet for LM317, and note the first 'typical application'. Adjust it to the desired voltage. You get automatic current limiting (just over one and a half amps), automatic graceful shutdown if it tries to overheat (like trying to charge a shorted battery) and voltage stability of a very small fraction of a percent (probably more accurate than the DMM you'll use to 'calibrate' it). A few bucks with Radio Shack parts.

Doug Owen

Doug: No mistake on the life ratings. When you get to the 12 volt range, you are dealing with much much larger filaments. That's why you don't have to constantly change the headlights on your car is because they have gigantic filaments in them. The 12volt MR16 that I'm using right now is rated at a 4000-hour lamp life! I just double checked it. So, even if you add an extra 2 or 3 volts for super brightness, you are still talking about a run-time that is much better than your average flashlight bulb. This is one of the big advantages that attracted me to the MR16 format to begin with. Now if only I could add a long life gell cell battery to it, I would be set. /ubbthreads/images/graemlins/smile.gif

I also use a 7815 with a diode in the output. My last lot I bought tested @ 14.6v (30ma LED load) without the diode, so I only used 1 on the output. I haven't tested my new lot - but I would add the diode on the input if I need two. I like 13.6-14.0v for float on 4 - 24AH gel cells.
I bought several 4amp power supplies from MPJA for about $25 ea. The first one I got uses a 317 (adjustable version of LM7815) and a transistor for 13.9v output. The next 3 used an older discrete transistor regulator, otherwise they were the same. This is probably the cheapest way to go if you do not build your own. I'm modifying one of these for 10 amps (LM317, 2x 3055 transistors) The only part that may not work @10 amps is the transformer, although I did add fan cooling. We'll see.

the matsushita(panasonic)7815s i just got are right on at 14.95-15.02v
with a 1n4002 right at 14.2.
great for cyclic use but if i need long term float i just add another diode in series with the first.

There's a problem with the simple 'add a series diode' idea to reduce Vout, the forward drop from the diode goes down as current does. This means as the battery starts to 'float', the voltage goes up.....

"Load regulation" (the ability to maintian correct output voltage as the load (battery) demands change) is a most important parameter.

A much better solution is to use a lower voltage regulator (say a 5, 9 or 12 Volt unit) or the 317 (which is in reality a 1.25 Volt 3 terminal regulator). Two resistors can set the output voltage accurately, making one variable makes it easy to adjust to just the right voltage.

Doug Owen