I believe there was an issue with the charging circuits of some 350's such that batteries were not being charged properly. I don't take any chances and recommend using a good quality external charger. Old/low capacity batteries will cause problems. I use 2700mAh batteries and normally get 2-3 days use. If you can I'd also recommend recharging/replacing the batteries before the tester actually shuts off, as this will sometimes reset the unit loosing all the accounts/settings. Yes be aware battery voltage is temperature sensitive, so keep a spare set close to you ie in your pocket and don't store your tester/batteries anywhere cold.