________________________________
From: Chris Maxwell [mailto:[email protected]]
Sent: Monday, October 11, 2004 1:28 PM
To: [email protected]
Subject: NiMH battery discharging
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?
Chris:
I thought the manufacturer specs the Amp-Hours at a C/10 discharge rate.
Discharging at a faster rate, you get less capacity out of the battery.
Ed
Ed Price
[email protected] WB6WSN
NARTE Certified EMC Engineer & Technician
Electromagnetic Compatibility Lab
Cubic Defense Applications
San Diego, CA USA
858-505-2780 (Voice)
858-505-1583 (Fax)
Military & Avionics EMC Is Our Specialty
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