kwf@ecersg.ncsu.edu (Ken Fernald) (09/13/90)
I'm interested in designing and building a fast-rate nicad charger probably using a "dump-and-time" technique where you discharge the cells to place them in a known state, then charge at a high rate for a timed period. I've been told that you must be careful about how you discharge cells in series because if some cells are weaker than others, they may be forced into a reverse current state and be damaged. Given this is true, is there a proven technique for the fast and safe discharging of nicad cells? Should you discharge them individually? Discharging in series would certainly be simpler. thanks, -- +----------------------------+------------------------------------------------+ | Kenneth W. Fernald | "To recognize that nature has neither a | | kwf@ecersg.ncsu.edu | preference for our species nor a bias against | | North Carolina State Univ. | it takes only a little courage" - James Randi |
phil@ux1.cso.uiuc.edu (09/14/90)
IMHO, the slower you discharge the cells, the less likely you are to reverse the weak one given a particular starting point. --Phil Howard, KA9WGN-- | Individual CHOICE is fundamental to a free society <phil@ux1.cso.uiuc.edu> | no matter what the particular issue is all about.
larson@snmp.sri.com (Alan Larson) (09/26/90)
In article <1990Sep13.123227.24228@ncsuvx.ncsu.edu> kwf@ecersg.ncsu.edu (Ken Fernald) writes: > >I'm interested in designing and building a fast-rate nicad charger >probably using a "dump-and-time" technique where you discharge >the cells to place them in a known state, then charge at a high rate >for a timed period. I've been told that you must be careful about >how you discharge cells in series because if some cells are weaker than >others, they may be forced into a reverse current state and be damaged. > >Given this is true, is there a proven technique for the fast and safe >discharging of nicad cells? Should you discharge them individually? >Discharging in series would certainly be simpler. You should discharge them individually. This came from a lecture I heard given by a battery company rep a few years back. Otherwise, the lowest cell will be reverse charged if you go to low. I would question the wisidom of discharging the cells first (especially doing so quickly). This is likely to increase their temperature, and to cause shorter life (in number of cycles of service) from the greater amount of chemical changes done. Alan
phil@ux1.cso.uiuc.edu (09/29/90)
> You should discharge them individually. This came from a lecture I > heard given by a battery company rep a few years back. Otherwise, the > lowest cell will be reverse charged if you go to low. ^^^^ Use "may" instead of "will". If the different between the lowest and the highest cell is small enough, this reversal won't happen or won't go far enough that routine charging cannot correct. Reversal certainly WILL happen in extreme cases, such as a very weak cell among strong ones and a high discharge current. If you want to discharge, and especially if you are going to deep discharge, do it with a low current. > I would question the wisidom of discharging the cells first (especially > doing so quickly). This is likely to increase their temperature, and to > cause shorter life (in number of cycles of service) from the greater amount > of chemical changes done. On the converse, by doing the full discharge regularly, and of course slowly, then following that up with a slow charge, you are more likely to keep each of the cells in a battery at a relatively equal level of charge. It is said that periodic full discharging can prevent the so called "memory effect". I have 4 battery packs I use for my ham radio HT's. They each go through (at 4 different phases) a 4 day cycle: 1. slow charging for 24 hours 2. more slow charging for 24 hours 3. regular use for 24 hours 4. slow discharge for 24 hours At the end of the 48 hours of slow charging, the battery packs are just barely at the verge of being warm. Occaisionally I cannot tell if it is warm or not, and get worried that the charge did not make electrical contact or some other problem prevented a charge. But such a problem has never happened. Occaisionally, a battery pack still has a charge left on on after step 4. This is usually when I accidently left the power saver ON on the radio during step 3. These battery packs have NEVER been rapid charged. 3 of them have been working for over a year and a half at this and the 4th was added several months ago. Even though the packs are only 8 cells, they are exhibiting typically over 10 volts even under high power transmit load (1.8 A on the 1.2 Ah rated cells). No cells have reversed or shorted yet. --Phil Howard, KA9WGN-- | Individual CHOICE is fundamental to a free society <phil@ux1.cso.uiuc.edu> | no matter what the particular issue is all about.