Charging batteries in the scanner as it is operating will degrade the ability of the battery to properly hold a solid charge and battery life. I suggest getting a few sets of batteries for your scanner. I have 4, so when one dies, I can swap out to a fresh set and charge externally the dead set.
Get a decent charger, I bought my first few sets of RayOVac NiMH batteries and charger 10 years ago, but the batteries have all become worn out, you would after several hundred charge cycles. My hand held scanners are powered up nearly all my waking hours. I have newer sets that I have picked up over time and they are still going strong.
As for choice of brands, that really doesn't matter, what does matter is the current rating. If you can find 2650 miliamp/hour rate (or higher) batteries they will have longer battery life. They also require a slightly longer charge time, but that's to be expected.