I have a new metal Taigen Tiger 1, 15 Lbs. I run it in short grass up and down 15% inclines. I have four 5000mah 7.2V NiMh batteries that are tired 4 year old batteries- I have an old beloved HiTec charger which I used to try and recover the old batteries. I can both monitor the charge going in total and how fast it is trying to recharge. I've used this before to discover batteries that just are not taking a charge and will last only a minute or so in a tank vs good batteries. These chargers are almost generic, I've found many companies selling them, like the cheaper IMax B6:

(If you don't have a 12V power supply, you may want to get the version with that built in)
Three of the four batteries just are gone, but I was able to screen out one battery that was still able to hold a charge. For this 5000mah which can hold a full charge, the tank lasted 50 minutes, with a couple minutes left over to spin the tracks to dislodge grass. I also have a new 3000mah "Imex" battery, which lasted 30 minutes. I would say it's important to have a charger which displays all the detail and allows parameter adjustment- not just a simple single red light on/off. The Tamiya/Kyosho white plugs can also come loose after awhile and stop the tank. I sometimes use Deans plugs, XT60 plugs are popular in general also.
Batteries: In USA I use MaxxPacks, Batteriesamerica, but a lot of brands are good.