The alternator tensioner keeps the drive belt at the correct tension around the pulley that is fixed on the drive shaft of the alternator. The alternator has two pivotal fixings and a sliding tensioner bracket or a sprung wheel type bracket.
If the drive belt is not tensioned it will slip on the pulley and the alternator will not give a full charge to the battery, and so may not provide the required output to run the electrical systems as required.
The tensioner may be a simple bar with an end fixing and a slotted opening for adjusting, or a more complex sprung loaded wheel on a bracket.
If the drive belt slips and the alternator does not fully deliver the required power to the vehicle's electrical systems and keep the battery charged, this can cause damage to the battery and also to the alternator. The alternator tensioner may need to be replaced if the sprung loaded wheel is not functioning and is not putting pressure on the belt.