-dV and high cap C & D cells

sstmax

Newly Enlightened
Joined
Feb 7, 2008
Messages
31
Location
UK
The consensus for NimH seems to be to charge between 0.5 and 1C in order to achieve proper termination.

A lot of the cheap "smart" chargers seem to charge at far lower rates than would be required to achieve proper termination, such as this one:

http://www.component-shop.co.uk/html/mw6278.html

Even the Maha C808M has a max charge rate of 2000mA which would mean that proper termination would only be guaranteed on a cell of <= 4000mAh capacity??
 

SilverFox

Flashaholic
Joined
Jan 19, 2003
Messages
12,449
Location
Bellingham WA
Hello Sstmax,

Welcome to CPF.

Most chargers have a back up to the primary termination method. Often, they have several back ups.

One back up method is to set a timer and terminate the charge based on time. Another method is to set a maximum voltage for the cell, or pairs of cells, and terminate when that maximum voltage is reached. Another method involves monitoring the change in cell temperature, or setting a limit for maximum cell temperature.

It is possible to generate a -dV termination signal at lower charge rates, but it is not reliable throughout the life of the cell. It may work at first, but as the cell ages it becomes unreliable.

Another problem that I have run across from time to time is that C and D cells don't move as fast as AAA and AA cells do. The battery manufacturers may only do a run of C and D cells every 3 - 5 years. This means that the probability of getting old cells is higher. Old cells that have been sitting in storage develop higher internal resistance, and then they get hot when charging at 0.5 - 1.0C. Most people don't bother with a forming charge, so it is probably better that the consumer chargers limit the charge rate and rely on secondary charge termination.

Tom
 

sstmax

Newly Enlightened
Joined
Feb 7, 2008
Messages
31
Location
UK
Thanks for the insight Tom.

I guess it still pays to go for a better charger than the one I linked to in order to make sure the unit actually has several "backup" termination methods.

I'm currently using a cheapo timer-based charger to charge high capacity C and D cells and am using a wall timer to ensure no overcharge.

A C-9000 takes care of the AA/AAAs though.
 
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