Things to keep in mind:
The rechargeable battery discharge rate when not in use. Alkaline batteries loose energy at a very slow rate (years) when not in use but rechargeable batteries don't hold a charge for long so it is important to recharge the battery before a performance if the battery hasn't been used for a while even if it was fully charged up after the last time it was used. |
While a rechargeable NiMH battery puts out a lower voltage, it maintains a steadier voltage than an alkaline battery as it runs down. The difference in the average voltage of a rechargeable battery is closer to that of an alkaline than their maximum voltage rating suggests.
Non-Rechargeable Lithium: 9.6 Volts, 1200 mAh (milliamps per hour)
Non-Rechargeable Alkaline: 9.0 Volt 560 mAh
Rechargeable Lithium-ion: 8.4 Volts 520 mAh
Rechargeable NiMH: 7.2 to 8.4 Volts 175-300 mAh
As you can see from the mAh rating, a non-rechargeable alkaline battery lasts considerably longer than a rechargeable NiMH battery in most cases.
Lithium-ion batteries (or lithium-ion polymer ones) are best but at around $20 (the last time I checked which was quite a while ago), they are expensive. They retain their charge longer, have a higher initial voltage and don't wear out as quickly as other types.
Batteries are being improved all the time so it is possible that some newly released battery will have better specifications than those I provided.
Voltage and amperage ratings are nominal.