Chargers

To discuss chargers, it is probably best to first discuss how to charge the different chemical compositions of batteries.  To charge NIMH packs, the charger will apply a voltage to the battery to get a certain current into the pack.  For a 7.2V pack the voltage will increase to somewhere around 9V, and when it is fully charged, the voltage will begin to drop.  This drop is called the delta V and is normally fixed on cheaper chargers or configurable on higher end chargers.  If configurable, I would suggest 3mV per cell delta V and so for a 7.2V pack (6 x 1.2V cells), this would be 18mV.  So say the voltage peaks at 9V, the charger will cut out when the battery voltage drops to 9-(18/1000) = 8.982V. 

 

A LIPO charger works a different way.  If you are charging a 2S pack, the charger will charge at a certain current (you should be using 1C as explained in the LIPO battery section) until the voltage hits 8.4V .  The charger will then reduce the current to maintain 8.4V until the current has dropped so low (0.1 amps), it stops charging the battery. 

 

Due to the different ways the chargers work, you must ensure you have the correct charger, set to the correct charge mode to match the battery you are charging.  Never try to charge a LIPO pack on a NIMH charger it will never end in a good way.

 

When looking at a charger, make sure you keep the power supply in mind.  Some chargers are 12V input only so you will need a separate power supply to run the charger, some are 240V so can run off the mains and some are both.

 

For LIPO, I would always recommend getting a LIPO charger with balance function built in.  Balance charging is where the voltage of each of the 2 lipo cells is monitored during charging and the voltage carefully maintained so that each cell charges equally.  As a pack is used, one cell may be slightly better than the other so the weaker cells voltage will drop quicker than the good cell.  If you don’t balance charge then you risk having one cell go over the 4.2V limit.  For example you could have one cell at 4.2V and one cell at 3.5V, if you charge normally (i.e. not balance charge) the voltage would be 7.7V at the start but the charger will not stop until 8.4V.  You can see one cell is already at the limit at the start of the charge and so further charging is dangerous.  If you balanced charged then this situation is detected by the charger which then prevents one cell being overcharged.