MrAl
Flashlight Enthusiast
Hello,
Note the pics were introduced in post #10 of this thread, and here
is the link:
http://mral.peu.net/index.php?page=EnergizerChargerMod
The Energizer 15 minute charger has been successfully fooled
into thinking it was charging two AAA cells when really it was
charging two AA cells. This trick allows AA cells with higher than
usual internal resistance to be accepted by the charger and
charged effectively because AAA cells normally have higher
internal resistance anyway.
Here's the story...........
The Energizer 15 minute charger has a slight problem
where if an AA NiMH cell has a somewhat higher than
usual internal resistance the charger rejects it and
does not attempt to charge it at all. Apparently the
philosophy adapted by Energizer is that if the internal
resistance is beyond a certain limit there will be too
much power heating in the cell and could cause a melt
down of the cell and/or charger.
Measuring the approximate internal resistance of several
cells that work and several that dont work in this charger
i have determined that the threshold is approximately
0.100 ohms. That is, a cell that had an internal resistance
less than 0.100 ohms will be accepted in the charger and
will be charged in about 15 minutes, while a cell that has
anything over 0.100 ohms will be rejected and will not be
charged at all (note the 0.100 ohms is approximate).
Calculating the power for each cells internal resistance
at 10 amps charge current and 0.100 ohms Ri leads to
10 squared times 0.1 which equals 10 watts. Since the AA
cell has a surface area of about 3 square inches, the
temperature rise due to the internal resistance alone would
be enormus, should the cell be left to run in this condition for
an hour or so. The plastic of the charger would eventually
melt. Of course with half the resistance we get half the
power heating and this too would get hot if left long enough.
Luckily, it's only allowed to run for 15 or 20 minutes, and
there is a fan to help cool the cells down.
In any case the main issue here is that once the cell reaches
a point where it's internal resistance prevents it from being
charged in the Energizer charger we have to look for another
way to charge this cell. I had four RS cells, two of which
would not charge in this charger and now just recently one
other cell (now a total of three) wont charge in it either.
Interestingly, since the Energizer charger also charges AAA
cells and AAA cells have inherently higher internal resistance
we could fool the charger into thinking it was charging AAA
cells when really it was charging AA cells if there was a way
to stop it from sensing the longer AA cell once it was inserted
into the charger. Since it accepts the higher resistance of
the AAA cell it should accept the higher resistance of the AA
cell once it's fooled. The drawback is that when it senses
an AAA cell it also cuts back the current to one-half the normal
AA cell charging current, so it's going to take twice as long
to charge an AA cell. The good news is at least it will charge
it, and also 30 minutes isnt too bad either.
Ok, so the only thing left to do is find out how to fool the
charger into thinking it has an AAA cell in it rather than
an AA cell. Lucky for us, the charger has a switch mechanism
inside (one for each cell position) that closes when a cell
that is longer than an AAA cell is inserted into it. The switch
closing tells the charger to charge at a high current and also
to look for low internal resistance and reject any cells with
an internal resistance that is too high. Peventing this switch
from closing means the charger thinks it's got an AAA cell in it
(the terminal contacts do not change) and it starts detecting
the internal resistance that would be typical for an AAA cell
and switches the current to about half max output.
The only problem then is getting to these switches. They are
all inside and not accessable from the outside of the case.
This means the case has to be opened up, and the circuit board
removed. The four switches become clearly visible as they are
each about 1 inch long and look more like relay contacts.
The only thing necessary to do at this point is to slide a thin
card (like an index card) under the switch contacts so that
they can not close, even when an AA cell is inserted.
The drawback here is that a slot would have to be cut in the
side of the charger case to allow sliding the card under the
switch contacts when we want to charge AA cells using the AAA
cell charge algorithm (ie fool the charger).
Alternatives to using the card trick:
1. Remove the switch contacts completely and mount four SPST
switches on the case and manually switch between AA and AAA cells.
2. Solder a new four position
AA cell holder externally. Note there are no common terminals
for any of the cells, so this means 8 wires would have to be run
from the AA cell holder to the charger circuit board.
3. Plug 'dummy' AAA cells into the charger and have their terminals
connected to a four cell AA holder. Each cell *MUST* be wired
independently however, so this requires a minimum of 8 wires. The
wires should be very short and heavy gauge too. The wires internal
to the charger look like #12 flat. The idea is to keep lead
resistance to a minimum, or else the charger will still reject
the cells. The nice thing about this idea is that it does not
require modification to the charger itself.
Actual Test
-----------
An actual test was done to see just how good the card trick
would work out. The case was taken apart via three small
screws and the circuit boards removed (connecting wires folded a little).
After the circuit boards are removed the wires are still attached
and the unit still operates, the only difference is the switches
can not be activated via a cell insertion, so the charger always
thinks it's charging AAA cells. This turned out to be the best
way to test this, so leaving the two circuit boards out of the
case (and inserting another insulating card between circuit boards
to prevent shorting) and inserting two AA cells the unit was ready
for the test.
After plugging the unit in, the LED lit red and stayed red!
This was a first for these two cells. This meant they were
actually charging in this charger when before they would not.
As noted previously, i expected a longer charge time, and that's
exactly what i got. After 20 minutes the charger LED turned
green, indicating that the cells where charged. I suspected that
this was not long enough, so i unplugged the charger and plugged
it back in after about five seconds. The LED turned red, and
the cells were being charged a second time. I expected to see
another 10 minutes charge time, but it actually took closer
to about 20 more minutes, after which the charger LED turned
green again. To make sure they were really charged this time,
i unplugged the charger again and waited another five seconds,
then plugged back in. Within about 30 seconds the LED turned
green, indicating that the cells were really charged this time.
I would guess that the charge algorithm has a cut off time
period of 20 minutes, even if they are not done. Unplugging
and plugging back in allows them to get the needed charge time.
It took a little longer than expected (40 minutes vs 30) but
hey at least they charged this time!
The heat output of the cells during charging was minimal, maybe
warm, but no where near hot, and this was also expected because
the AA cells have about 50 percent more surface area than the
AAA cells have, and this keeps them cooler than AAA cells would
be powered by the same charge current.
I will probably go with the card trick, and slot in the side
of the case for a long term solution. I like this better than
having to solder to the circuit board or trying to provide
a second four cell AA holder. I also cant see mounting four
switches on the case, even small ones.
If anyone is interested i'll try to get some pics of the dissassembly and
where the card is to be placed to prevent switch closures.
Note the pics were introduced in post #10 of this thread, and here
is the link:
http://mral.peu.net/index.php?page=EnergizerChargerMod
The Energizer 15 minute charger has been successfully fooled
into thinking it was charging two AAA cells when really it was
charging two AA cells. This trick allows AA cells with higher than
usual internal resistance to be accepted by the charger and
charged effectively because AAA cells normally have higher
internal resistance anyway.
Here's the story...........
The Energizer 15 minute charger has a slight problem
where if an AA NiMH cell has a somewhat higher than
usual internal resistance the charger rejects it and
does not attempt to charge it at all. Apparently the
philosophy adapted by Energizer is that if the internal
resistance is beyond a certain limit there will be too
much power heating in the cell and could cause a melt
down of the cell and/or charger.
Measuring the approximate internal resistance of several
cells that work and several that dont work in this charger
i have determined that the threshold is approximately
0.100 ohms. That is, a cell that had an internal resistance
less than 0.100 ohms will be accepted in the charger and
will be charged in about 15 minutes, while a cell that has
anything over 0.100 ohms will be rejected and will not be
charged at all (note the 0.100 ohms is approximate).
Calculating the power for each cells internal resistance
at 10 amps charge current and 0.100 ohms Ri leads to
10 squared times 0.1 which equals 10 watts. Since the AA
cell has a surface area of about 3 square inches, the
temperature rise due to the internal resistance alone would
be enormus, should the cell be left to run in this condition for
an hour or so. The plastic of the charger would eventually
melt. Of course with half the resistance we get half the
power heating and this too would get hot if left long enough.
Luckily, it's only allowed to run for 15 or 20 minutes, and
there is a fan to help cool the cells down.
In any case the main issue here is that once the cell reaches
a point where it's internal resistance prevents it from being
charged in the Energizer charger we have to look for another
way to charge this cell. I had four RS cells, two of which
would not charge in this charger and now just recently one
other cell (now a total of three) wont charge in it either.
Interestingly, since the Energizer charger also charges AAA
cells and AAA cells have inherently higher internal resistance
we could fool the charger into thinking it was charging AAA
cells when really it was charging AA cells if there was a way
to stop it from sensing the longer AA cell once it was inserted
into the charger. Since it accepts the higher resistance of
the AAA cell it should accept the higher resistance of the AA
cell once it's fooled. The drawback is that when it senses
an AAA cell it also cuts back the current to one-half the normal
AA cell charging current, so it's going to take twice as long
to charge an AA cell. The good news is at least it will charge
it, and also 30 minutes isnt too bad either.
Ok, so the only thing left to do is find out how to fool the
charger into thinking it has an AAA cell in it rather than
an AA cell. Lucky for us, the charger has a switch mechanism
inside (one for each cell position) that closes when a cell
that is longer than an AAA cell is inserted into it. The switch
closing tells the charger to charge at a high current and also
to look for low internal resistance and reject any cells with
an internal resistance that is too high. Peventing this switch
from closing means the charger thinks it's got an AAA cell in it
(the terminal contacts do not change) and it starts detecting
the internal resistance that would be typical for an AAA cell
and switches the current to about half max output.
The only problem then is getting to these switches. They are
all inside and not accessable from the outside of the case.
This means the case has to be opened up, and the circuit board
removed. The four switches become clearly visible as they are
each about 1 inch long and look more like relay contacts.
The only thing necessary to do at this point is to slide a thin
card (like an index card) under the switch contacts so that
they can not close, even when an AA cell is inserted.
The drawback here is that a slot would have to be cut in the
side of the charger case to allow sliding the card under the
switch contacts when we want to charge AA cells using the AAA
cell charge algorithm (ie fool the charger).
Alternatives to using the card trick:
1. Remove the switch contacts completely and mount four SPST
switches on the case and manually switch between AA and AAA cells.
2. Solder a new four position
AA cell holder externally. Note there are no common terminals
for any of the cells, so this means 8 wires would have to be run
from the AA cell holder to the charger circuit board.
3. Plug 'dummy' AAA cells into the charger and have their terminals
connected to a four cell AA holder. Each cell *MUST* be wired
independently however, so this requires a minimum of 8 wires. The
wires should be very short and heavy gauge too. The wires internal
to the charger look like #12 flat. The idea is to keep lead
resistance to a minimum, or else the charger will still reject
the cells. The nice thing about this idea is that it does not
require modification to the charger itself.
Actual Test
-----------
An actual test was done to see just how good the card trick
would work out. The case was taken apart via three small
screws and the circuit boards removed (connecting wires folded a little).
After the circuit boards are removed the wires are still attached
and the unit still operates, the only difference is the switches
can not be activated via a cell insertion, so the charger always
thinks it's charging AAA cells. This turned out to be the best
way to test this, so leaving the two circuit boards out of the
case (and inserting another insulating card between circuit boards
to prevent shorting) and inserting two AA cells the unit was ready
for the test.
After plugging the unit in, the LED lit red and stayed red!
This was a first for these two cells. This meant they were
actually charging in this charger when before they would not.
As noted previously, i expected a longer charge time, and that's
exactly what i got. After 20 minutes the charger LED turned
green, indicating that the cells where charged. I suspected that
this was not long enough, so i unplugged the charger and plugged
it back in after about five seconds. The LED turned red, and
the cells were being charged a second time. I expected to see
another 10 minutes charge time, but it actually took closer
to about 20 more minutes, after which the charger LED turned
green again. To make sure they were really charged this time,
i unplugged the charger again and waited another five seconds,
then plugged back in. Within about 30 seconds the LED turned
green, indicating that the cells were really charged this time.
I would guess that the charge algorithm has a cut off time
period of 20 minutes, even if they are not done. Unplugging
and plugging back in allows them to get the needed charge time.
It took a little longer than expected (40 minutes vs 30) but
hey at least they charged this time!
The heat output of the cells during charging was minimal, maybe
warm, but no where near hot, and this was also expected because
the AA cells have about 50 percent more surface area than the
AAA cells have, and this keeps them cooler than AAA cells would
be powered by the same charge current.
I will probably go with the card trick, and slot in the side
of the case for a long term solution. I like this better than
having to solder to the circuit board or trying to provide
a second four cell AA holder. I also cant see mounting four
switches on the case, even small ones.
If anyone is interested i'll try to get some pics of the dissassembly and
where the card is to be placed to prevent switch closures.
Last edited:
)- would be interested in photos if possible.
