My volt meter tells me the current voltage of my lipo, but is there a way to tell how charged or many Mah have been used after a flight.

You need to use a charger that records how many ma it used to charge the pack. There really isn't any other way.
John

ditto I use a balance charger from FMA direct it can charge 4Cell lipo at 4amp without damage although not reccomended. It has whats callled a fuel guage but better yet it tells you how much it puts back in which is a much better reference than some calculated discharge percentage. Its a DC only charger but I modified an old PC power supply to power it off AC and the charger can be bought around $60 with discounts. I have had mine for a few years now And it has been used thousands of times LITERALLY!!! I charge my 4 packs 2 or 3 times a day when I am flying. for about 2 1/2 years on the charger.

I have a lipo charger and it's great. It tells me how many Mah it put in the battery while charging. But what I wanted to know was. Once the battery is fully charged. If I use it... let's say for 20 minutes, or 10 or whatever, how do I know how many Mahs were used. without having to charge it again.

Simply put unless you do a graph of mAh used over time verses voltage you can't. And that graph will be inherently inaacurate because you draw current while flying in spikes not steady state.

You'll simply need to use the charger. Sorry but it is the easiest solution.
John

Thinking more on this - perhaps one of the in air data recorders like Eagle Tree will record voltage and current drawn. That relates to power used over time, which is another way to say mAh used. You could use this to get a reading of how much power is left.

Still it is easier to know you have a 1500 mAh pack and took 725 mAh out for a 7.5 minutes flight to figure out you have about 7.5 minutes of flight left. Though I would only fly an additional 5 minutes. LiPos do not like to be fully discharged! Pretty easy to do the math and a whole lot cheaper.
John

Do you take your charger with you?

I dont charge my pack but I will just plug it in and see what the fuel gauge says without charging takes maybe 5 seconds. It will give me a calculated percentage of whats left in the pack.

Yes the eagle tree system will record how many Mah's were used. if your willing to keep it hooked up and monitoring in flight.

http://www.fromeco.org/Products/05FRCWEASD/Default.aspx

or

http://www.fromeco.org/Products/04FR...0/Default.aspx