Chris, What Don said is correct about the cells needing to be equalized. This is a bad side effect of most quick-charge chargers, they will shut off when the "peak" is detected, but one or more cells in the pack may not be fully charged and then, when the pack is discharged, the cells that are not fully charged will take a nose-dive before the rest of the pack is anywhere near being discharged. Some fast chargers will switch to a slow or trickle charge once the peak is detected, but the user may pull the battery out before the cells can equalize. The best way to equalize the pack is to use a C/10 charge rate for 16 hours. In some applications (like _competition_ R/C cars and planes) the operator will buy more cells than needed and will *match* the cells in the pack so all the cells are very close to the same capacity. Out of a production batch some cells may have 1850 mah capacity and some may have 1750. Matching the cells will give some assurance that they all will reach the knee in the discharge curve at the same time, and help avoid any of the cells going into reverse-charge mode which will severely shorten the life of the pack. Also, all chargers are not created equal. It is hard to beat a standard C/10 charge rate to completely top-off a pack. Bob Richards, NCT Square D.
Chris Maxwell <[email protected]> wrote: Hi all, I have a question for you not specifically related to EMC; but I hope that I can “hijack” the group’s attention for another sort of problem. Recently, I’ve been allowed to do some non-compliance design work. Specifically, I’m working on a battery charger/power supply design. My question relates to battery discharge. I’m charging a NiMH battery pack. The pack consists of four AA-sized cells in series. At full charge, the pack is starting out at about 5.7Volts. I then load the pack down with our system and take measurements in order to see our discharge characteristic. Well, the load is about 360mA, and the pack is rated at 1800mAH. I was assuming that I could get about 4 hours out of the pack. I’m not getting four hours; and the discharge characteristic is strange to me. It takes about 3 hours for the pack to get down to 4.9V. At that point, the voltage takes a “dive” down to about 3V. This dive sometimes takes only seconds. If I disconnect the battery pack, it still measures 4.9V; but the voltage dives again as soon as I plug it back in. This bothers me for a few reasons: Firstly, I know that the NiMH battery discharge characteristic has a “knee” in it where the voltage dives; but most of the literature that I’ve read puts this knee at a lower voltage. Most literature suggests that I could use the batteries down to a cell voltage of 1.0 to 1.1V (which would put my pack at 4.0 to 4.4V). I’m not getting down there. Secondly, the unloaded “recovery” bothers me, it’s suggesting that my 1800mAH pack is having trouble with a 360mA load. (This would put me at about a C/5 rate) Most literature that I’ve read suggests that NiMH batteries should work just fine at C/5. I was expecting the pack to get down to about 4.8 V and run for a while (the flat portion of the discharge curve), then I was expecting to kick our system into auto-shutdown when the battery voltage got to about 4.0V. The way things are going now, our users would have to save data when the battery gets to about 5.0V just to make sure the batteries don’t dive and put the processor into oblivion. Could this be because the battery packs are new? I’ve heard some say that NiMH batteries take a few charge/discharge cycles to reach full potential. Any other suggestions? Thanks, Chris Maxwell Nettest ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Sociiety emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Ron Pickard: [email protected] Scott Douglas [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Bob Richards [email protected] http://www.toprudder.com -------------- ------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Ron Pickard: [email protected] Scott Douglas [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
