did you mean 4000 Mah batteries?
if so, then in round numbers,, ( remembering you will only take about 80% of the battery capacity in use) that's about 4 Ah per charge taking into account the loss inherent in the charger. so charging 4 or5  batteries will take somewhere around 20 amps. Were it me, I would look at a small deep cycle lead acid if you cant justify a Gel cell type. and to insure you had enough capacity, I would not look at a motorcycle battery, but I think there is small tractor type batteries that are around 100 to 150 amp capacity that might be decent,, although, in retrospect, a standard car battery might prove more cost effective, larger capacity, and typically cheaper than the smaller specialty batteries

Dunno if it's necessary, but I'd check the costs on a deep-discharge battery -- car batteries are optimized for starting, and don't live long and happy lives if you run them significantly down on charge very often.  Deep discharge batteries (for RV, boat, or golf carts) are designed for it.

Or get a solar panel and regulator, and spend more on the charger than you did on the planes.

Hi Wynn,

If cost is not too important then I fully agree with Peter. He did a great job of laying out the many advantages of 12V AGM batteries. Another good source for these, and probably close by in most towns down under, is your local marine store. Most larger pleasure boats, both sail and power, use these. There smallest ones will work for our ECL needs.

With the above said, it is hard to beat the tried and true method used by one of our ECL pioneers, Mike Palko. Mike uses a 12V auto battery. This has great price/performance and ease of charging with your standard inexpensive auto battery charger. It does require Mike's youthful strength, OR a good small wheeled luggage cart like you get at an airport, or wheeled tool cart, with the battery in some form of FG or wood box for protection. I have seen nice DIY boxes with input and output jacks for all charging. 

One of the best compromises is buying two WHEEL CHAIR 12V 20 amp batteries. This is what I use for the rare times that I am flying at a field that does not have 120V power, or solar power. I run one charger off each battery and I can get two full charges on each of my 5S 4000 mAh flight batteries from each 12V battery, or 3 on my 4S. I keep these in one of those wheeled tool boxes from the Home Depot (HW store down under? ;-). Two of these are inexpensive and are much easier to carry than one large 12V. I ordered them on line from one of the big battery suppliers.

One of the guys that flys at our E only RC field uses an excellent Honda generator. As quiet as they are, they are still very LOUD at a quiet ERC field. They smell too. They are really annoying.  No one wants to pit near them and we are all glad when the guy leaves. I almost bought one until I saw it in action. It kind of spoils one of ECL/ERC many benefits: quiet flying. ;-)

The BEST system is the SOLAR system we have at the government parks dept. RC/CL field in Van Nuys CA. This is close to perfect for the ONE ECL flyer (me ;-) and the many ERC flyers at the field. Thanks to the local RC club and the local government we have about 15 12V plug ins for our chargers and a very nice CL/RC field. :-)

As others have mentioned, you can't just divide the 12V batteries capacity by the amps in your battery. A 17 amp 12V will NOT give you 3 full charges of your 4S with 3 amps used, even though it looks like it should. Once your 12V gets a little low your charger will stop and give you a "low V" warning. It takes more extra amps in the 12 V than you think to get the last charge.

Hopefully we will all soon have standard Solar PV units on our cars/vans for recharging our batteries and we can then also use them for our ECL planes. This will be the ideal solution for this issue.   

I just use my car battery and a HobbyKing quatro charger. If the car battery voltage drops too low the charger cuts out with a low input voltage error.  So then I just start the car and let it idle while charging until complete.

Beats carrying around another battery that I have to recharge.

Is it practical to leave the car idling to charge the battery and to have no battery at all on the field?

I dont know about you guys, but when I go flying the car is usually only about 10 or 20 metres/paces away.  I take about 6 batteries with me anyway so I usually only have to recharge them once. The car battery has plenty of power to do this with needing to start the engine.

The only time I need to start it is if I am camping and I have already been using the battery for other things.