Several points raised here, but let's focus on the 9 V batteries.
The first thing to be said is that 9 V NiMH batteries are inherently a bad idea. There are limits to what you can expect from them. If you want a better rechargeable solution you should consider Li-ion batteries instead. Something like the ones on this page:
http://www.batteryjunction.com/9v-lithium.htmlSo, 9 V NiMH--what's the problem with them?
The basic problem is that such batteries are made from 7 or 8 cells in series. Anyone who has worked with NiMH technology will recognize such a configuration as a "battery pack", and will know that managing an 8 cell battery pack needs particular care. You must keep the cells balanced during charging, and you must not over-discharge the pack.
A 2 hr charger like the Maha C490F is not good for long term use. A 2 hr charger has minimal balancing capability. For balancing you need to do a slow timed charge overnight for 12 to 16 hours at a low charging current. Ironically some of the cheaper "overnight" chargers you will find in retail stores are just about perfect for this.
The second problem with 8 cell packs is over-discharging. If you run the pack until the voltage fades, then it is almost certain that one of the cells inside the pack has become completely empty and has been subjected to voltage reversal. Over discharge to the point of voltage reversal is a sure fire way to kill an NiMH cell. And once one cell inside the battery has gone bad the whole battery will need replacing.
So, tip one for looking after such batteries is to recharge them long before you think they are empty. If you think the battery can run for three days before fading, then recharge it after two days while it is still going strong. Avoid running the battery down to fading power at all costs.
Tip two is to avoid fancy expensive chargers. Get an overnight trickle charger and charge the battery for a calculated amount of time. If the battery is 250 mAh, then charge it with a current not more than 25 mA and time it to 120%. E.g. 250/25 + 20% = 12 hours.
You can use a fast two hour charger two or three times in between slow charges if you are in a hurry, but always do the slow overnight charge at regular intervals.