Hi guys,

Do any of you use a 12V Battery to charge your 4,200 mAh batteries at the flying field? If yes, which one do you use?

How many charges do you get from it before you need to recharge the 12V?

I would like to charge each of my four 4,200 mAh batteries at my local ballfield. This would give me 8 flights instead of 4.

My AXI, turning a Bolly CF 11x4 3B (from Randy Smith) @ 11,200 uses 3,300 mAh per flight. This means I will need at least 13.2 Amps to charge the four batteries. I know it takes much more power than this to operate the MAGIC BOX (AKA charger) to get all the pixie dust back into our flight batteries. ......... The 12V should = ?? AMPs (min?) (max?)   

Since this 12V will be portable, it would be nice if it were as small and light as possible, and still have enough power to get the job done.

I was going to buy 4 more 4,200 mAh batteries, but at $200 each, I thought a portable 12V charging battery might be a better way to get in my 8 flights?

Ideally I would like to use two of my chargers to charge 2 batteries at the same time. At our club field, with AC power available, I do this all the time. Is this possible from one 12V battery?

I looked at some Marine Deep cycle 12V batteries between $100 and $185, but they weighed around 45 lbs. !!!! I was hoping for something <25 lbs. if possible?

TIA for any help on this.   

Hi Gang,
I use the car battery and never run it down more than 30% without running the engine for the first 20 minutes of the next charge. Most chargers are perfectly happy with the runningh voltage, just be sure to have the car engine running already before you connect the charger. The load dump when the starter current goes away suddenly can produce high voltage spikes that will harm the charger electronics. Shutting the engine off while charging has never been an issue. The sort of surge suppression that would be necessary at the charger input is minimal but has some cost. Nobody does ... probably for that reason alone.
Dean

Linheart, Dean, and Mike,

Thanks Linheart,
I am glad to hear that we can get 3 charges from a light weight (<16 lbs) MC battery. This looks like a good option for my small park flying. Your generator idea sounds like the ultimate solution for the park without power. It may be a little overkill for one, but will be great when there are several elec. CL flyers at the same site. Perfect for the electric CL contest of the future! The new ones are as quiet as our planes. :-)

Thank you Dean.
The car battery option is attractive because it is automatically charged, and most of the time it travels with us. In my case I walk to one park, and the other park requires the car to be parked out of my view, farther away than I trust my batteries and chargers to be safe. ... BTW: How many charges do you get before you reach your 30% cut off point?

Thanks Mike,
I don't think I will ever be as dedicated as you guys. If I can get in 8 CL flights in one session I will be VERY happy. I usually fly my R/C 3D elec. plane 3 or 4 times between CL flights, so it makes for a full morning. If I go flying all day with my larger R/C planes I go to our club CL/RC field which has AC power. There I can fly elec. all day.   
     Linhart was right, he said someone would come up with the #s! Thanks :-)  For my 4 charge needs, I get:  55.44 x 4 = 222 watt hrs used for 4 flights,    222/.6 = 370 total watt hours needed, with reserves,        370/12V =  31 AMP/hr battery required.

The ideal would be an AGM 12V battery. They charge faster, can be used in any position, and are very safe. Serendipitously I just read a long article on batteries in one of my sailing magazines. Below is an excerpt from one of the tables they had. This really shows what you were talking about Re discharging the 12V less than 100%. Like you, they said to keep the battery fully charged, and when stored for the winter, keep it on trickle if possible. They also said DON'T use a regular auto charger. I use my Trinitron charger, and it seems to do fine.   

Discharge      No of Cycles
5%                  10,000
20%                  2,800
50%                  1,050
80%                     550
100%                   360

If anyone out there reading this wants a battery, you may find what you need at this excellent web site:

http://ebatteriestogo.com/merchant2/merchant.mvc?Screen=PROD&Store_Code=EBTG&Product_Code=D5745-AB

If you have any questions, just call the Ph number in the upper left on their site. The guy is a battery expert who was very helpful. They have a sale on now with free shipping. I ordered two of the batteries shown on the page address above. Using Mike's #s these should meet my needs and give me flexibility, and at 15 lbs, not break my arm carrying it when I walk to the park. :-) At $33.95 ea. with free shipping, for an AGM battery it should work for our needs.

Thanks again for all your complete answers. It has helped me, and I hope it helps others getting into elec. CL flying.   

Well if you use a generator, at least pick a quiet one. Otherwise I am sure you will be teased to death about your "quiet" plane.