Re-celling the Nikon
MN-2 battery pack for the Nikon F3's MD-4 motor drive.
The
MN-2 battery pack is a nominal 16.2 VDC battery used to supply power to a Nikon
F3 equipped with a MD-4 motor drive. The
pack requires the MH-2 charger, which charges the pack in about three hours
with its original cells.
Since
these batteries were made in the 1980’s and 1990’s, they are constructed with
Nickel Cadmium (NiCd) batteries which had the two-fold disadvantages of having
memory and being toxic. After thirty
plus years of service, the NiCd cells in most of these battery packs are now
well past their service life, and likely are not holding a charge and a
probably leaking within the casing and will need repair. Fortunately, they are constructed in such a
way that the cells can be easily accessed and replaced with superior Nickel-Metal
Hydride (NiMH) cells that are readily available.
One of
the changes between the MD-2 for the F2 series and the MD-4 for the F3 cameras
is that the power source of the motor drive will alter the performance
characteristics of the system. In the
F2, both the disposable battery packs and the NiCd packs supplied approximately
15 VDC to the motor drive. This allowed
for a constant frame rate between the two power sources. With the MD-4, the alkaline battery insert
holds 8-cells, giving the motor drive a nominal 12 VDC of power, and a maximum
frame rate of 4 fps with the mirror locked up and 3.8 fps with the mirror
down. This is hardly any better than the
Nikon MD-12 of the same era with 3.5 fps with the same battery supply. Using the MN-2 batteries, with their higher
voltages, the frame rate jumps up to 5.5 frames with the mirror down and 6 fps
with the mirror up. One thing to note is
that the MN-2 as a small plastic nub that pushes in a switch within the battery
chamber of the drive to indicate whether the higher voltage pack or standard AA
cell holder is inserted into the chamber.
Because of this feature, using higher voltage cells (e.g. disposable
lithiums) in the standard holder will not lead to an increase in frame rate.
To
obtain the boost in voltage and still retain the same battery pack form factor,
Nikon had to use more cells, which necessitated that each cell be smaller, and
of less capacity. Since in either case,
the battery packs use each cell in series, the overall capacity of the battery
pack depends on the capacity of each individual cell. So with an AA alkaline cell, each battery
supplies approximately 2,500 mAH of power.
In the MN-2, the cells are only 2/3 AA cells and supply only 250 mAH of
current. The MD-4 manual estimates at
least twice as many film rolls per battery set with Alkalines over the MN-2’s.
In this
re-cell, we will be using NiMH batteries with a 600 mAH capacity, which should
give use more rolls per charge. The
other advantages of using NiMH cells is that they have the same 1.2V per cell
as the NiCds so we can use the OEM charger.
The final advantage is that NiMH lack the memory of NiCds so recharging
mid-discharge will not be an issue.
The
MN-2’s were assembled in Japan from batteries made in the USA (mine was made in
Gainesville, FL). The MN-2’s are assembled with JIS screws which
are different than Philip’s head screws.
Using the wrong tool can easily damage the screw heads, so an investment
in the correct tools is highly recommended.
There
are four screws that attach the top plate of the pack to the plastic body of
the pack. Once these are removed, the
cells are exposed. On top of the cell
bank are two wires stretching between one side of the pack to the other.
The
first task is to carefully desolder the two long wires. These are the leads that connect the charging
socket on the underside of the pack to the pack leads. The power leads are lead from the bottom of
the pack to the contact plates of the pack.
Each cell has a tab that connects it to the next cell in series. Each cell is inserted in the opposite direction
as its neighbor to allow for connecting them in series. Seven batteries are connected in a row and
then the row is shunted to a second row that is soldered in a similar way.
After
the leads are de-soldered, the next step is to unscrew and remove two small
brass tabs that hold the batteries down.
Be sure not to lose either the tabs or the screws. Next, work each row of cells out of the
plastic holder and dispose of them per local regulations. NiCd batteries cannot be disposed of in
regular household waste. If the
batteries have leaked, there will be a whitish-blue powder that may be green on
the wires if they came in contact with copper.
This powder is toxic and should not be handled. Once the cells are removed, any residual
powder can be neutralized by vinegar and then the case is rinsed with water and
thoroughly dried. Any metal contacts
displaying corrosion should also be treated the same way with vinegar.
Using
solder, connect each cell in series in the same configuration as the original
cells. I did have one tab be a bit short
on the last cell in the line and used a bit of tinned desoldering braid to make
a jumper. I tested each row of cells
with a voltmeter. Each row should be
approximately 8.5-9.0 volts if the cells are charged.
Before
inserting each row of cells, solder the lead from the battery pack to the
leading cell in the first row, with the red wire to the positive terminal, and
the black wire to the negative terminal of the second row. Carefully insert each row of cells into the
plastic holder, taking care not to damage the lead wires. Once each row is inserted, you can then
solder the final shunt between the two rows.
At this point, test the voltage on the battery pack. The voltage on my pack read 17.7 V, although
if the cells are not charged as much, it may be as low as 16.8 volts. Zero voltage indicates an open in the circuit
somewhere and a lower voltage may indicate a cell that was installed the wrong
way. Any issue must be corrected before
the next step.
Once
the voltage is determined to be correct, re-insert the tabs and their
respective screws to keep the cells in place.
You can then resolder the charging leads to their terminals. These wires were initially taped down and
small squares of electrical tape can be used to keep the charging wires in
place.
Before
re-installing the top plate, make a label with the date of the re-cell and
install it on the bottom of the plate. Fit
the plate on top of the cells, If there is a bit of play, a small bit of foam
can be installed on top of the cells.
Just be sure no to cover any of the four screw holes. Carefully install
the screws, retest the voltage at the terminals for a final time and then you
should be ready to test the camera with the refurbished battery pack.
The
best way to test the battery pack is to first insert it and hit the battery
test button on the back of the drive.
Two red LED’s should illuminate.
Next set the shutter speed to 1/500th , lock up the mirror
and set the MD-4 to “C.” Fire off a
series of frames. You should be able to
hear a noticeable increase in the speed over the AA-cell battery insert. If you care to, you can also set the frame
limiter to a set number and measure the time it takes to run through that
number of frames.
The
MH-2 charger is designed for NiCd cells but has no auto switch off when the
charge cycle is complete. The manual
states it takes 3 hours to charge NiCd MN-2’s at 250 mAh cells. Since we are using 600 mAH cells, it may take
up to 7 hours 15 minutes to charge the new pack. Lesser discharging will require less time on
the charger, but knowing how much time will be difficult to determine. A bit of experimentation will be in order to
dial in the process.
Re-celling
these battery packs is well worth the effort since it is relatively easy if you
have some rudimentary soldering skills.
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