Digital electronics need a consistent and stable voltage (5 volts, 3.3 volts), but battery voltage drops, as it discharges. So, a voltage regulator is built into the circuitry, to provide the required voltage, and requires a higher voltage to work with. We tend to add about a 20% tolerance into circuits, when selecting components, as most tend to get hot, sometimes fail, when pushed to capacity. A few volts over, shouldn't be a problem, a basic linear 5 volt regulator (LM7805) can safely handle 18 volts. There are more efficient choices these days.
Rechargeable batteries are a good investment, but you needs to change some usage habits, if you want them to last a long time. Eneloop are low-self-discharge, which simply means they are more expensive, and hold their charge longer, when not used. If you use your detector, and charger your batteries, least once a month, no real advantage to paying the premium price. Depends on your hunting habits, if you go months between hunts, probably good choice though.
My hunts only last an hour or two, and get several between charges, which is where the detector tells me the batteries are about half discharged. It's not that I worry about the batteries dying in the middle of a hunt, it's that rechargeable batteries have a longer life, when kept charged. When you completely discharge them, they start to lose a little capacity, adds up over time. If you don't expect to use them for a while, make sure you fully charge them.
Battery chargers... Fast chargers are battery cookers and killers. The heat from the rapid charge, destroys the chemicals in most batteries. There are a few, which are suppose to handle the heat, but expensive, and not sure if it's true. Batteries are consumables, companies don't make much money, if you seldom have to buy replacements... You want to get a slow, smart charger. These test each battery individually, and monitor the charge, terminate when completed. Most of the chargers bundled with batteries, are simple, timer based, and will allow you to overcharge (heat), and shorten the lifespan of the batteries. You still get reasonable use, most people still think they are getting a good deal, new batteries every year, not know that they can get 7-8 years, or more, if they took good care of them. Yep, lot's of people buy batteries weekly, just to throw away, convenience. Changing batteries, charging batteries, lower capacity, takes a little time, but saves a ton of money in the long run.
