I use NiMH rechargeables. Built 2 packs with charging ports and use a R/C battery charger. Charges my packs in under 2 hours, and I can charge from my vehicle if needed since it came with a 110 volt inverter built in.
One caveat. If you do not use one particular machine steadily or use the same batterries between them, or are a very casual hunter, consider using Alkaline. Alkalines hold their charge while stored for many months whereas rechargeables self-discharge constantly (up to 1% per day. The newer hybrid NiMH don't have this problem anymore). If it's been a few day since you've used your machine, top up the batteries beforehand. For that reason, do not use rechargeables in any devices that you depend on for long terms of use or storage (smoke detectors, clocks, emergency flashlights, anything that does not get used often or normally runs for several months on an alkaline)
I've had these packs for several years now, and each pack lasts me about 20 hours, but I don't usually wait for them to get that low, so just replaceand recharge pack every 3rd hunt (I avg 3-5 hours per hunt, but could do 2 long 10-12 hour hunts if needed.)
My packs show 11+volts on the MXT meter when fully charged but after a few minutes drop down to 10v and stays there until near the end. That is one feature of rechargeables, they maintain their voltage until the last few minutes, whereas alkalines, although longer lasting, will steadily decrease in voltage throughout their life. The MXT uses a voltage regulator circuit, and I have found absolutely no difference in performance, volume, depth between using alkaline or rechargeables. My original cost on making the 2 packs was about $100.00, and even factoring in the 50% less runtime with the NiMH, I calculate that I would have used $300 worth of alkalines by now. And my packs should have a couple more years of life to them.
Here's a site that has some discharge curves for AA's under different loads. NiMH chart is near the bottom.
AA discharge charts