MrAl
Flashlight Enthusiast
Hello again,
Recently i revived a set of sub C NiCd's i havent used
for maybe a year or so, and decided to recharge them and
take a look at how well they performed as well as to
look for their minus delta V. As some of you already
know, the minus delta V is sometimes used to trigger
the end of charge time for a NiCd cell.
There are various debates about using the minus delta V
method as an end of charge indicator which involve
whether or not the battery is already charged too far
ahead of the indication and so waiting for the minus
delta V condition is not good for the cell.
Im presenting the results of the test i did myself to
try to understand the pro's and con's a bit better.
The test was performed using two sub C NiCd's wired
in series.
TEST #1
Starting conditions:
The cells were sitting around for a year or so discharged
to about 0.8v each. Pumping them up with 300ma for seven
hours to make sure they were fully charged. The discharge
consisted of one 5 ohm resistor across the two cells.
From the discharge time and the cells terminal voltage
the ampere hour rating of the two cells was estimated
at 1.24 ampere hour.
TEST #2
The cells were totally discharged from the first test so
now they were connected to a constant current source for
charging. The constant current source puts out 1.045 amps
regardless of voltage and maintains that level within about
+/- 0.1 percent or less.
Right after the cells were connected their series voltage
went up to 2.75v, then gradually rose up to 3.00v, 3.10v,
etc., until some 2 hours later the voltage was up to
3.30v exactly. Funny thing is, NO MINUS DELTA!!
Since the cells were rated at 1.2 to 1.3 and they had been
charging for at least 2 hours they were getting hot. I had
to stop the charging after about 2 hours and 10 minutes.
TEST #3
This turned out much more interesting results.
The cells were discharged at about 500ma for
3/4 of an hour (45 mins). This means they need
about 25 mins of charging at 1 amp constant to get
the charge back in, so connecting the constant current
again i monitored the cell voltage of each cell
individually with two digital voltmeters set on their
2.000v scales. AMAZINGLY, this time the minus delta V
showed up! The cell voltages continued to decrease
until it was decided they were overcharged slightly.
Even more interestingly, just before the voltage of each
cell topped off, the voltage shot up very quickly.
To get a better idea of this phenomenon take a look
here:
http://hometown.aol.com/xaxo/page6.html
This pic clearly shows the quick rise as well as
the minus delta.
TEST #4
I havent done this test yet. What i'd like to do is
find out why the delta V didnt show up in Test #2.
Any ideas?
Take care,
Al
Recently i revived a set of sub C NiCd's i havent used
for maybe a year or so, and decided to recharge them and
take a look at how well they performed as well as to
look for their minus delta V. As some of you already
know, the minus delta V is sometimes used to trigger
the end of charge time for a NiCd cell.
There are various debates about using the minus delta V
method as an end of charge indicator which involve
whether or not the battery is already charged too far
ahead of the indication and so waiting for the minus
delta V condition is not good for the cell.
Im presenting the results of the test i did myself to
try to understand the pro's and con's a bit better.
The test was performed using two sub C NiCd's wired
in series.
TEST #1
Starting conditions:
The cells were sitting around for a year or so discharged
to about 0.8v each. Pumping them up with 300ma for seven
hours to make sure they were fully charged. The discharge
consisted of one 5 ohm resistor across the two cells.
From the discharge time and the cells terminal voltage
the ampere hour rating of the two cells was estimated
at 1.24 ampere hour.
TEST #2
The cells were totally discharged from the first test so
now they were connected to a constant current source for
charging. The constant current source puts out 1.045 amps
regardless of voltage and maintains that level within about
+/- 0.1 percent or less.
Right after the cells were connected their series voltage
went up to 2.75v, then gradually rose up to 3.00v, 3.10v,
etc., until some 2 hours later the voltage was up to
3.30v exactly. Funny thing is, NO MINUS DELTA!!
Since the cells were rated at 1.2 to 1.3 and they had been
charging for at least 2 hours they were getting hot. I had
to stop the charging after about 2 hours and 10 minutes.
TEST #3
This turned out much more interesting results.
The cells were discharged at about 500ma for
3/4 of an hour (45 mins). This means they need
about 25 mins of charging at 1 amp constant to get
the charge back in, so connecting the constant current
again i monitored the cell voltage of each cell
individually with two digital voltmeters set on their
2.000v scales. AMAZINGLY, this time the minus delta V
showed up! The cell voltages continued to decrease
until it was decided they were overcharged slightly.
Even more interestingly, just before the voltage of each
cell topped off, the voltage shot up very quickly.
To get a better idea of this phenomenon take a look
here:
http://hometown.aol.com/xaxo/page6.html
This pic clearly shows the quick rise as well as
the minus delta.
TEST #4
I havent done this test yet. What i'd like to do is
find out why the delta V didnt show up in Test #2.
Any ideas?
Take care,
Al