Interesting issue. What happened to me is probably the best reason to go with dual batteries (as setup explained by Overbored and Bruce) and after this incident, I added voltwatch to the setup which worked out very well.

What happened is this. My Venus II (pattern plane) has dual NiMH batteries, 6 volt packs, both 1200 mah. That's lots of available amps for this plane to fly just about all day... or so I thought.

After the third flight, the plane seemed to be acting somewhat sluggish, so I just decided to check both batteries to see how they were doing. The first battery read 6.3 vdc which was just fine, and the second battery read.... hmmmm.... didn't read anything. The display on my Dubro voltmeter showed a flashing display then nothing. Hmmm what's wrong here.

So, I borrowed someone else's voltmeter and checked it thinking that perhaps my voltmeter was going bad or something. Same reading. Flashed the LED's then nothing.

I started to get a tad concerned so I called the "club pro" over.. he just happened to be around that day.. and he checked it both in the plane then we removed the wings and removed the batteries (both of them) and checked them both again. One read 6.3 under load the other.. well, flashing LED's and then nothing. WOW! A bad battery!! And it was only a few weeks old.

Fortunately, I had a spare, so I put that in, put both on the field charger and let them recharge.

The charger jack is disconnected when the switch is turned on so the VM only works with the switch off. But, we decided that the bad battery (which I checked after I charged it on the way to the field that morning) had failed during flight and the good battery had taken over the load.. and was also trying to charge the bad battery. So, had I just checked one battery thinking that both would be the same, I would have added some fuel to that fire and had a serious problem.

Moral to this story is, yes, dual batteries are a great idea and frequent checking of batteries is even a better idea.. or a great way to insure that your setup, either single or dual, is working properly.

I just have to think about this a bit and consider what would have happened if I had a single battery setup in that plane, and that battery failed in flight. Ah my nice Venus would be history now.

Oh, by the way, the voltwatch is always connected to a vacant channel (I had one more left..ha..) and monitors both batteries. You just have to learn the trends of discharge during operation to realize what is going on. Of course, you cannot see the voltwatch during flight, but before and after, you certainly can. I usually start the plane, make engine adjustments, check controls (make sure the ailerons are not reversed for instance), then I turn off one battery switch and watch the voltwatch... if ok, I turn it back on and turn the other one off and check the voltwatch. If all is ok, I turn it back on so both are on, then play with the controls and watch the voltwatch. You can detect binds in controls, end stops for throttle, and so on. Cheap insurance.

Sorry for going on and on, but I feel this is an important issue and thought it worthwhile to add my 2 cents worth.
DS.