As I understand it, the rechargeables have slightly less voltage. For instance, 8 AA alkaline give 12 volts give or take when fresh and 8 rechargeables give arppox 9.6. This is enough voltage to still run most detectors okay as they have a regulator that cuts off the voltage below that. The manual for my MXT says 8 volts for instance. The main practical difference is the rechargeables will maintain a working level for less total hours, then drop below the acceptable level very quickly. I"ve seen where some have rigged up battery paks to take 1 or 2 extra rechargeables to get near the 12 volt level, but I don't know if this is really necessary. Also, the extra battery or two in an improvised pak, while raising the voltage probably won't actually give much more in total operating time as the whole pak will drop quickly as they near the discharge level. If one detects a great deal of the time, alternating 2 rechargeable sets can offer a long term savings, but good quality alkalines are the better choice for those of us who detect just occasionally.
HH
BB