Chris-
What you have going on is the classic "weak link in the chain" battery
problem. One cell has less capacity than the others, and once it is
exhausted (at 360 mA load), the voltage across it collapses and then
reverses (reverse charge). Thus the drop to about 3 volts.
Charging can be thought of as a plating process. The plating has area,
which determines current capacity. The plating also has depth, which
determines the amp-hour capacity. It is possible to have some small areas
with extra depth, which can provide small amounts of current after the
apparent depletion of charge. This is why the voltage can spring back to
normal when the load is removed. On the other hand, if there are large
areas of uniform plating, the output will drop suddenly once those large
areas are exhausted.
A load of 360 mA on a 1800 mH-h battery is not excessive, so you should be
able to get very close to 5 hours of operation. Three hours is too little.
However, you should do at least 3 charge-discharge cycles before making a
capacity determination. But you need to make sure that you are getting a
full charge on all the cells. Your charger may be shutting down the charge
cycle prematurely when the three other cells reach full charge, but the 4th
cell is still not fully charged. If you can fully discharge each cell
individually, you can start them all from the same place (zero charge).
Also, be careful to disconnect the load as soon as the voltage plunge is
detected. Reverse charging is very destructive to that weak link cell.
Voltage is not a really good indicator of state of charge of NiCad and NiMH
cells. The voltage under load at the same state of charge tends to drop as
the cell accumulates cycles.
Don Borowski
Schweitzer Engineering Labs
Pullman, WA
"Chris Maxwell"
<chris.maxwell@Ne
tTest.com> To
Sent by: <[email protected]>
owner-emc-pstc@li cc
stserv.ieee.org
Subject
NiMH battery discharging
10/11/04 01:27 PM
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 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
