[TriLUG] OT: battery charger voltages - using 18V charger on 16V system

[Rob Lockhart]

Quoting Joseph Mack <mack.joseph at epa.gov>:

> I have an IBM thinkpad with a 9 cell (nominal 10.8V) Li-ion battery.
> The charger is nominally 16.8V charger (measured 16.0V). 
> 
> (Anyone know why you charge a 10.8V battery with a 16V charger?
> Surely the internal impedance of the charger can't be that high.)
> 
> I also have available a Toshiba charger with the same plugs, except that
> it is a 18V charger (nominal and measured).
> 
> Does anyone know if you can use an 18V charger in place of a 16V charger?

As others have said, I wouldn't trust the higher voltage would NOT cause damage.
 In fact, if the supply is nothing more than a buck converter, if you can
probably find the feedback voltage loop, you can change the resistors to obtain
the voltage you need (i.e., change the voltage divider).  I've done this before,
to use some thrown-away 21V Dell laptop AC/DC adapters for use with my Compaq
laptop (needing 19.6V).  You may have to measure some voltages to determine the
location of these resistors.

Regards to the higher voltage, as someone else said, you need to have a higher
voltage for the charging circuit.  It is my guess that, if it's a NiMH or LiIon,
you need to carefully monitor the voltage charging.  Not sure about the LiIon,
but for NiMH you have to do some cycling of charge cycles.  To make it simple,
the charger would current-limit charge for about 80% of the time, wait some
settling time (charging not applied), apply a load for a short period of time,
then at the end of the load time, measure the voltage.  You keep doing this, say
once a minute, until you reach negative delta-V or zero delta-V (how the voltage
is changing with respect to previous voltage "points").  When you reach this
point, you know the battery is nearly fully charged and is at capacity.  After
this point, the charging usually slows to trickle rates (i.e., 10mA current) to
top it off from 96% charged to nearly 100%.

Regards,
  -Rob