Battery charging Li + NiMH temp

Yesterday I charged one Li and two NimH batteries after a long cycle. I left them all for just over 4 hours and when I returned they all had the green light on the charger (originally orange and all chargers were working).

I noted that the NimH batteries were warm to the touch, but the Li battery was not. Is this correct for some technical reason to do with Li batteries. I am as confident as I can be, without actually riding the bike, that the Li battery is charged as I noted that the charger was on orange when I passed by my bike shed a few times during the evening. I always double check the lead is firmly in the battery when charging.

Thanks for any replies

Jeanette

That's normal Jeanette. It's a function of NiMh batteries that they heat up as the charge nears completion, and in fact it's that temperature rise that's used to determine when to switch off the charge. The middle connector in the plug connects to a thermistor in the battery, that being a type of resistor which detects the temperature rise and allows through the switch-off signal when the temperature is right.

On the Li-ion charger, the middle connector on the plug isn't used as there is no corresponding temperature rise to detect. Inside the Li-ion battery there's complex electronics which constantly monitor all the cells both individually and as a set, and those in an eZee battery I'm showing in two photos below:

http://users.tinyworld.co.uk/flecc/images/battop2.jpg

http://users.tinyworld.co.uk/flecc/images/closemon.jpg

Yes that is normal.

Nimh are always 'overcharged' to get them fully charged up and this overcharge ends up as heat. In fact it is this rapid increase in temperature that the charger looks for.

Lithium chargers terminate when the battery no longer takes charge, based on their voltage and the current drawn. This leads to little wasted energy and consequently little heat.