How can I charge a 12V Car Battery?

Just tryin to think outside the box. I didn't know if the car was anywhere nearby or if you were hiking somewhere remote and wanted a more self sufficient solution. It might not take that many pumps to charge a battery, lessee if I can take a quick guess:

Lets say your setup can make two pounds of thrust for 600 seconds (10 minutes), or 1200 lb. seconds.

Lets also say you weigh 200lbs.

that means for you to make the same output, you would have to put all your weight on the pedal for 1200/200 seconds or 6 SECONDS!

Of course there are losses all over the place in this analogy, and the lipo battery will complain if you charge it in 6 seconds, but it is just a quick starting point for a guess.

It'll work just fine, only very slow. Go to Walmart and get a cheap auto charger. Prolly not more than $20.00.

I use a Die Hard lawn tractor battery for my electric flying. I charge it using one of those Battery Tender charger/minders. Much smaller than a standard car batt with a high capacity. Cool thing with the tender is you can leave it hooked up all week and it keeps the battery up without overcharging. The lawn tractor battery also fits in the bottom of a rolling plastic tool box I have keeping me self sufficient at the field.

but I only get about 3 charges before the 12V has dropped too far. You need to use a power supply. Using a battery to supply voltage to a battery charger is a no win situation. It works with field chargers only because the supply battery is MUCH larger than the battery being charged, and can be kept peaked by the car's engine.

(about 70Ah). Good luck finding a car battery that small. They usually come in ratings of 600Ah or more.

The question is; will I be able to charge the 12V car battery with the same charger that I use to charge the 12V sealed lead acid? Yup but you may be there for days.

Dr.1

I think on this point your are way of the mark a 600Ah battery is massive fork lift truck sized, the maximun sized battery fitted to my Vauxhall Astra is 70Ah

Charging from my car is an option I had thought about since it has got a 12V output in the back. The thing that put me off was that the LiPos are best charged outside of the car (since in certain circumstances they can catch fire) and I didnt really want to leave 2 or three LiPos (at £50 each) laying around on the floor where someone might walk off with them.

Not too sure if thats a serious suggestion? :-)

At £50 each I have bought 3 batts and 2 chargers as this allows for almost continuous flying.

Thanks

David Bevan

If you're worried about a LiPo burning up your vehicle, make yourself an extension cord so your charger and LiPo are away from it. If you're flying from a grassy area, it would be a good idea to bring something like a bucket of sand or some other nonflammable substance to put your pack on or in while it's charging. You could also fix up a lockable top on the bucket and chain it to your vehicle's frame if you're worried about theft! (just kidding)

Morris

FWIW - car batteries are supposed to be used to start engines and nothing else. The most important number therefore is the maximum current that can be drawn.. Logically - thats the number placed in the big letters on the side of batts:)

Biggest 'car' batts I have personally seen are 100AH capacity - these delivered 700A - at least thats what it said on the side;) Methinks if you were to drain it in 8 mins it would be a bit warm!

SC

I bought a charger for 69 dollars from Sears. It charges anything that is

12V lead acid under 200 AH, including small gel cells. You can connect the battery up backwards with no harm to charger or battery. It can also provide a 100A burst to help start an engine (car engine, that is).

I have been charging 7AH gel cells with no problem after every use. It normally takes less than a minute. I suspect most of them die from overcharge or being left in a discharged state. I have also been charging

30 AH deep-cycle batteries for field use.

I know that it is a lot of bucks for a charger, but I wanted an automotive charger that worked better than the one I had. It is quite an amazing gadget.

-- Mike Norton

Something is very wrong. A battery that charges in less than a minute either did not need to be charged, was charged *WAY* too fast, or wasn't really charged, due to the charger shutting off when it shouldn't.

Check the instructions for your charger - is there something different you need to do when charging gel cells vs car batteries?

Ed

DR1 , the 600 rating is cold cranking amps. Thats how much your starter can suck out of it for a short burst. A 70 AH battery is a big one for a car.

Ken

| >>3. How about some sort of foot pump like generator that you can pump the | >>small lead battery back up, or possibly a lipo directly to save a few | >>strokes, perhaps even while you are flying. | | >Not too sure if thats a serious suggestion? :-) | | Just tryin to think outside the box. I didn't know if the car was anywhere | nearby or if you were hiking somewhere remote and wanted a more self | sufficient solution. It might not take that many pumps to charge a battery, | lessee if I can take a quick guess: | | Lets say your setup can make two pounds of thrust for 600 seconds (10 | minutes), or 1200 lb. seconds. | | Lets also say you weigh 200lbs. | | that means for you to make the same output, you would have to put all your | weight on the pedal for 1200/200 seconds or 6 SECONDS!

Your understanding of the idea of work (in the physics sense) is very flawed.

Mere thrust (force) does not do work. Force * distance is what gives you work (energy) -- you'd need to not only push on that pedal, but actually move it. A long distance.

Assuming that you could push down on the pedal with 200 lbs of force, if you moved it one foot that would produce 271 joules -- which would give you one watt for 271 seconds or some combination thereof.

Assuming you had a 7Ah 12 volt battery (a popular size for field boxes) and it was completely discharged, and you wanted to completely charge it by pushing on your pedal with 200 lbs of force, you'd have to push it 1116 feet. (This is assuming that everything is 100% efficient, and that the voltage is exactly 12 volts and stays there, of course -- assumptions that are not accurate. In the real world, you'd have to push the pedal more to compensate for the losses and the increase in voltage.)

That also means that this battery has enough power, fully charged, to raise your body (if it weighs 200 lbs) 1116 feet in the air, assuming all is 100% efficient.

It certainly is possible to charge batteries via muscle power, but a lot more muscle is required than one might think.

I believe that a top athlete can produce about 1/2 horsepower (380 watts) for several minutes. Making the same (incorrect) assumptions as before, this would mean that the athlete could charge that battery in about 13 minutes, if the athlete could keep up that rate of work.

And this is just for a pretty small battery ...

To make this R/C related, your LiPo battery in your plane is probably smaller than this field box battery. Perhaps 1/4th the size, So your athlete could charge it in about three minutes, which is more realistic, except that the battery couldn't tolerate that :)

[...]

Yup. In the real world, you probably can't get much more than

50% efficiency in the generator, and the voltage is closer to 14V.

That translates into pushing with 200lb of force through a distance of about 2600 feet (let's just say a half a mile).

If I remember correctly from articles on human powered flight,

1/2HP (373W) was about the best you can get from a trained cyclist for any decent period of time. IIRC, an average joe probably can't produce more than 50-100W for more than a few minutes. 7AH @ 14V == 352,800 Joules.

Assuming a generator efficiency of about 50% and somebody pedalling at 100W, it would take about 7000 seconds (352800/(100*0.5)). Roughly two hours.

I guess you missed the relationship between thrust and time. Hint: pretend the airplane is just a fancy rocket. In this example it is putting out 2 lbs. of thrust, for 600 seconds, we are disregarding the weight of the rocket.

Yes, you actually have to be moving the pedal with your entire weight for those 6 seconds, but with a spring and proper gearing or leverage, you can reproduce the same two pound push for 600 seconds.

Reaching for the work formula will not help you see this relationship without a bit of algebra, and that woud be contrary to my stated goal of a quick guess ;).

Wrong.

Gears and levers do not provide a ratio between thrust/time and thrust/time. The ratio provided by gearing/leverage provides ratios of thrust/distance -- which is what was explained to you using the formula for work.

Because there is no such relationship.

Quick, but not founded in real physics.

You haven't discovered the 'Cyclon' range then. I've been using a pair of small 2.5AH Cyclons in a flashlight since about 1982 with no problems until just last week when one of them developed an internal short. That's longevity for you! You can get 2v cells in 25AH, so you'd need 6 of them. An advantage of building using single cells is that if one goes bad you don't have to replace the whole lot--just the one cell. Search: hawker cyclon lead

I hope you can forgive me. This way off topic. I don't know what happened, but I have been unable to log onto this usenet for the longest time and am just trying to see if I could do it. Strangely, now I can whereas I could not for almost a year.

However I do know people have been using car batteries for charging Li Polies sucessfully. They just use a 12 volt charger to recharge the batery. No problems.

Wan

Andrew

I have been away from this discussion group for a long. Now I am on a roll. Good to be back. I thought everyone knows what a Lipo (Lithium Polymer) is by now. This is the type of battery that is all the rage now. Beats NiMh, Nicad. Lipos are lighter, smaller, longer flight times.

I am by far not an expert, been using them for about 3 years. Loved them.

Wan

Andrew

I have been away from this discussion group for a long. Now I am on a roll. Good to be back. I thought everyone knows what a Lipo (Lithium Polymer) is by now. This is the type of battery that is all the rage now. Beats NiMh, Nicad. Lipos are lighter, smaller, longer flight times.

I am by far from being an expert, been using them for about 3 years. Loved them.

Wan