Li-ion to NiMh charger conversion

[tony18m2001]

Flecc mentions in his pages on chargers - chargers that the eZee Li-ion charger can be used to charge NiMh batteries, but the cut-off voltage means that the battery wont be fully charged. I recently had my Li-ion charger apart to repair the output cable, and noticed that there are 3 potentiometers inside. I had previously been advised by 50Cycles not to change settings because of internal damage limiting electronics in the Li-ion battery, but I wonder if anyone has tried tweaking them to increase o/p voltage. I'll do some investigation myself, if no-one has the answers already on this forum. I would probably add a switch which would change the settings between the two batttery types. I realise I have to be careful of overcharging NiMh since the Li-ion charger doesnt monitor the internal thermistor in the battery, so I am also proposing to make a little unit which will interpose between the battery connector and charger and monitor the thermistor to cutoff the charger at a suitable safe temperature - probably powered directly off the battery, so will not need any other power supply. The system will save money over buying an additional charger for a NiMh battery.
Thoughts welcome
Tony

 

[Tiberius]

Hi Tony,

Sounds like you understand the principles. With NiMH all the intelligence is in the charger. It has to spot when the battery is full and switch to a low trickle current. If it gets it wrong the batteries can explode (there's a thread about this on the forum). A good NiMH charger looks for a number of indicators of full charge (temp, rate of temp rise, voltage, change of voltage, overall time). It might be easier to get a new charger.

Nick

 

[Ian]

I tried a bit of tweaking and found that of the pots does change the output voltage, but not to much higher than the nominal setting. The NiMh charger is a totally different design, not simply a tweaked Li-ion charger and it's primary method of detecting a full charge is by looking for the rapid increase in voltage that occurs at full charge, and not for the voltage reaching an absolute value, likewise the thermistor monitoring circuit is looking for a rapid change rather than an absolute value although it will undoubtably have a high temp cut out as well.

 

[flecc]

Yes, as Nick and Ian post Tony, a new charger is the only safe way. The Li-ion charger even when tweaked won't have the headroom NiMh needs, and I only used it on NiMh for the reason stated, to give a very safe low level preliminary charge to an NiMh pack.
.

 

[tony18m2001]

Thank you all for your useful replies. I guess I'm going to have to bite the bullet and buy a new charger. I still want to do a few experiments with a separate power supply (1A) and monitor the voltage/temperature characteristics with a data logger. Before I do that I'll look for the exploding NiMh thread! I also keen to try looking at the discharge all cells procedure recommended by flecc as an occasional requirent to keep the cells 'synched'
I bought an XLR plug and socket from Maplins yesterday for this experiment.

 

[flecc]

But not completely discharge of course Tony, best done with a charger having the refresh/discharge facility or on the bike by riding it.

Discharging to zero by other means, lamps or resistors, will destroy the NiMh cells.
.