thats really the most efficient way to do it if im not mistaken. What you need to do is get yourself a battery pack with a lot of Ah(Amp Hours).
I read in another one of your posts that your only using a few NiCd cells, am I right?
Not sure how much you know about batteries so Ill start with the basics:
One AA battery has 1.5v and x amount of mAh(milliamp hours).
If you put 2 AA batteries in series with each other, you get 3v and x mAh
If you put 2 AA batteries in parallel with each other, you get 1.5v and 2x mAh
see what I mean?
for a better example, I have a 12 volt battery that supplies 18Ah or 18 amps for an hour(18000mAh). That means, If im running a 12 volt circuit and that circuit is using a constant 18 amps, the battery will only hold up for one hour until it needs to be recharged. So if I had a second battery just like it and I put the 2 in series, I would get 24 volts and 18Ah. If I put the two in parallel, I would get 12 volts and 36Ah which means that now if I was running this 12v, 36Ah battery setup, I could run that 12v, 18A circuit for 2 hours instead of one because I doubled up on my Amp Hourage.
Hopefully you didnt already know that so I didnt just waste all this typing and I was actually helpful for once.
basically the more batteries you put in series, the more voltage you get. The more batteries you put in parallel, the more Ah you get