What charger do you have? For field charging it depends on a number of factors. Do you have electricity  available? How many batteries do you have or want to have? I have 7 my son, Kevin has 6 so we have no need to charge at the field. If you are only looking for an extra 3-5 charges a car battery should be fine. The actual number of field recharges will depend on the size of the batteries. The speed that you can charge is also a factor. I can charge 4 batteries in 11-13 minutes providing I have enough power available to run my chargers at full speed using 2 chargers. At my field I would need a  portable generator to run my power supply to produce that kind of power. I could never go that fast off a car battery.
By the way your biggest limiting factor with most chargers is the max # of watts they put out, NOT the # of amps the manufacture claims they can put out!

Thanks guys for this input.

My current power supply is 1.75amps 40w

It take almost 2hours to charge one 4s battery!

I think i need a bigger power supply.

It's a Turnigy

Here is what's on the front.

Thanks. Can you post a link?

Paul

Here is another link that handles different types of pwr supplies: http://www.buddyrc.com/power-supply.html                                                    Now you have lots of options, including building your own.  

This is my favorite one. http://www.rcgroups.com/forums/showthread.php?t=1581061&highlight=dps+600pb These are generally available all over E bay or from multiple places on the net if you don't like e-bay. One of these will power at least  4 of your chargers at full power.

No offense to the builder but that does look a little OTT. I think if i was going to to go that far I'd probably just go for the montorous 1700w power supply from Revolectrix, and get the case while I'm at it so it, and my powerlab can look all proffesional like.

That setup looks like it needs a flux capacitor, and gets plugged into the DeLorean!

I'm in the market for a couple of little ones for travel. 

I see the ESP114 is now $15 at Server Supply. 

http://web2.murraystate.edu/andy.batts/ps/POWERSUPPLY.HTM

I followed these instructions.  I put in all the lead outs like the instructions state because I was needing the supply for some proto-board work.  You can omit quite a few of the lead outs and have a much cleaner setup if you only intend to use the +12V line. (However if your hobbies have some variety you may want to have the added options. )

I put some self-adhesive rubber feet on the bottom so it won't scratch any surfaces.  It also helps with cooling as my PSU has vents on the bottom too.

Phil