This might sound like obvious advice but if you can, buy a charger that charges a range of different types of battery and then try and design any future robots to operate on the same voltage or type as your current batteries, youll save yourself a lot of hassle and time in the long run.

For example, I have one robot that runs on two 6V sealed lead-acid batteries (because they were cheap when I started robotics) and I have a charger specifically for them which I bought at the same time as the batts and that I use to charge the bot up.
I have another bot that uses four 7.2V 1800mAh nicad race packs (when I chose to upgrade to better quality components) that I link in series and parallel to get 14.4V and 3600mAh and bought two chargers that only charge 7.2V race packs to charge those batts (since my SLA charger wouldnt work on them and the nicad chargers were cheap).
And my most recent robot runs on 9.6V 3700mAh nickel metal-hydride batteries (that I needed to use to meet the capacity requirements of that robot) that I then had to buy chargers for because neither my SLA charger or nicad race charger were suitable for them.

Ive finally got myself a couple of chargers that will charge both nimh and nicad packs but over the long run it has cost me much more than if I had saved a bit more money to start off with and invested in widespread compatability battery chargers and specific battery types.