I am using the motors from Dewalt 24v Hammer Drills. Due to the problems some
have had in over-volting these motors, I will power them at the rated 24v. Their
are four motors, and a match lasts 3 minutes (a rumble goes for 5 minutes, but
I'm not going to worry about that).
I looked into the different types of batteries. Sealed Lead Acid (SLA) batteries
would be cheapest, but NiCAD (Nickel Cadmium) or NiMH (Nickel Metal Hydride) would be more
effective (lighter, and retain more even level of voltage over charged life). I initially wanted
to use 3AH NiMH (AH stands for amp-hour, a measure of how long voltage can be
supplied at a given amperage), but found them to be more costly than NiCAD,
and not used as much in this field (less data on how they perform). I
decided to go with 2.4AH NiCAD batteries that are packaged in Dewalt 9096 18v XR+
batteries. Each pack contains 15 1.2v 2.4AH cells. The packs retail for $86, but
I was able to get them for $48-$52 on e-Bay. With postage, that's less than $60
each, or less than $4 for each sub-c cell. I will need two battery packs so one
can be charging while the other is fighting I've got to assume I'll win, and
their is a chance I could need to fight again without much turnaround time. If
not, at least I've got a backup in case something happens to one.
Deciding how much battery you need is difficult. A Dewalt engineer says that
the motors I'm using pull 19amps at MWO (maximum watts out), and 34amps at
thermal run away. If I assume 4 motors at 19amps, I get 76amps. I'll need this
for 3 minutes, or 1/20th of an hour. Multiplying 76amps by 1/20hours gives
3.8AH. 40 2.4AH cells could provide 24 volts at 4.8AH, which would more than suffice.
Is the base number, 19amps, the correct number to use? Using 34amps, I get the
following: 34amps x 4 motors=136amps; 136amps x 1/20hours=6.8AH. 60 2.4AH cells
could provide 24 volts at 7.2AH, again, more than enough.
The last thing I need is to lose because I don't have enough power. Since I'm
a rammer/pusher, I'm going to assume the higher amp rating, and go with 60
cells. I've seen some talk on the BattleBot Forum that suggests that 66amps
might be a more accurate number for one motor at peak usage. I hope this is far
from true, or I'm going to be left sadly short of power in the box.
I bought 9 Dewalt 9096 18v XR+ packs. Only would have needed 8, but I screwed
up a few cells building the custom packs. Got most of them one auction at a time
on e-bay. Never bid over $52, ended up spending a total of approximately
$500.
I separated the packs to better construct the 24v, 7.2AH setup that I need.
Creating two 12v, 7.2AH packs would allow use of two AstroFlight 112D chargers
(each charger can charge up to 40 cells). The schematic below shows how I
created 12, 6v, 2.4AH packs by wiring 5 1.2v cells in series. I connected the 12
packs into 6 12v, 2.4AH pairs wired in series (series means connecting the
positive of one battery to the negative of another battery, resulting in the
voltage being added, while AH becomes equal to that of the lowest AH battery). Three of these pairs are then
wired in parallel to provide 12v at 7.2AH (parallel means connecting positive to
positive, negative to negative, resulting in the voltage remaining the same, but
the AH of the cells being added). Those two packs need to be kept
separate for charging, since the 112D can only handle 40 cells at most. They
will need to be connected in series to run the bot at 24v at 7.2AH.
Caution: Diagram below is not correct. Messed
up a few lines. Below diagram would give 12v at 14.4AH. I will be redoing the
diagram as soon as I can...
Here
is the process I used to construct the packs:
Click
images for
larger version |
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Photo from the Dewalt web
site. |
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Another photo from the Dewalt
web site. |
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Here are the eight Dewalt
batteries. |
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You'll need a #10 Torx
screwdriver to open the case. |
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Their are six screws accessible
from the top of the battery. |
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Some of the batteries opened
easier by pulling the bottom off first, while for some, the top slid right
off. |
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Some required using an object
to push against the terminals to push the batteries out the bottom of the
top. I'd recommend using a non-conductive implement to push, since the
batteries will likely have some charge, and could cause injury or fire. |
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Here is the battery pack
removed from it's case. |
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The first thing to do, which
I learned the hard way, is to cut the wire shown in the photo. This wire
was white on most of the batteries, but a few were red. Failure to do this
could result in some of the batteries contacting each other in a bad way,
resulting in orange hot metal, which tends to be somewhat hot. |
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Next, cut the metal tab
connecting the top battery to the lower batteries. |
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A temperature probe extends
from the top battery into a void in the lower batteries. It simply pulls
out, leaving the top battery now fully separated. |
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Cutting the sides of the
plastic strip that runs around the top battery makes it easier to release
the terminals connected to the top battery. |
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Cut the tabs connecting the
battery to the plastic connector. DO NOT try to peel the tabs off the
battery, you will rip the weld out of the battery case, leaving behind a
hole, and a dead battery. |
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Cut the tab connected to the
side of the battery. You can peel this one off without harm to the battery
case. |
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Peel the tape and the foam
rubber off the battery. |
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Remove the cardboard sheets
from the top and bottom of the battery pack. |
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Some of the batteries are hot
glued together. You'll need to break them apart. |
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Here's what we've got so
far... |
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Cut the tabs between all 14
batteries. |
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Here's what 60 separated
cells look like. |
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Use a utility knife to
remove any hot glue from the cells. |
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Next, I used an old chisel to
aid in the removal of the tabs from the battery cases. |
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I gently clamped the cells in
a vise, and used a hammer and chisel to knock the tabs off. Hold the
chisel about 10 degrees off horizontal and give a quick tap with the
hammer. Most of the tabs pop off pretty cleanly. |
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Here's the copper tabs from
the top battery. I was able to use the chisel to get these off as well.
Again, use caution with these as they are firmly mounted to the case and
improperly removing them could result in damage to the case (OK, I wrecked
two batteries this way). |
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Here's the 60 cells
without tabs in a more organized fashion. |
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Remove the cardboard sleeves
from the batteries. |
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Use sandpaper to rough up
both ends of the battery so the solder can grip better. |
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Apply flux to both ends of
the battery. This cleans the metal, and helps the solder flow evenly. |
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Apply solder to both ends of
the battery. I used a 35 watt soldering iron. Apply as little heat as
possible as to much heat could damage the insides of the battery. |
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Replace the sleeves on the
batteries. |
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I originally used 18 gauge
wire to connect the cells. After getting all the packs together, I
realized this wasn't going to be sufficient. Not only was the wire to
small, it was also stiff, which would transmit any movement in the cells
to the solder joints. I desoldered everything and started over. |
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For connecting the batteries,
I used some 10 gauge speaker wire. The wire is comprised of many strands,
and is very flexible. With this, any movement in the cells will not be
transmitted to the solder joints. |
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Strip about 1 inch of
the insulation from the wire. |
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Apply flux to the wire. |
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'Tin' (apply a thin layer of
solder) the wire with solder. |
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Solder the wire to the
battery by applying the soldering iron to the top of the wire and apply
pressure until the solder on the battery liquefies and flows into the
wire. I used my 40 watt desoldering iron for this, as it provided more
surface area than my normal soldering iron. |
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Trim the wire to a length,
strip the insulation, apply flux and tin the wire. Solder the wire to the
next cell. Note that because we want to increase the voltage, we must
connect the cells in series, which means the positive terminal should
connect to the negative terminal. |
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Here's one 5 cell, 6
volt pack. |
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Connect the negative terminal
of one pack to the positive terminal of another with the same wire, but
only strip the ends of the wire (leave insulation on). Solder them so the
packs sit side-by-side. This provides a 12 volt pack. We need eight of
these. |
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Do the same for four more
packs, but use a stripped piece of wire and keep the batteries as close as
possible. The two packs involved will end up with their butt ends
together. |
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Here's all of the 12 volt
packs together. |
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Hard to see here, but this is
the heat shrink tubing from McMaster-Carr, cut into 24 6" lengths. |
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A 5 cell pack gets inserted
into the heat shrink tube. Cut a 3/4" slit in one end to slide over
the wire connecting the two packs together. Heat the tubing with a heat
gun on medium heat. |
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Here's the pack after
shrinking the tubing. |
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Use a utility knife to cut
off the excess heat shrink tubing. |
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Here's the packs stacked
roughly the way they will be arranged when together in the robot. |
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Use electrical tape to strap
two of the 12 volt packs together with the four unconnected terminals at
one end, with the odd and even terminals alternated. |
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Use a utility knife to cut
openings over the unconnected terminals. |
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Use some of the same wire to
connect the positive terminals together, with excess wire allowing for a
third pack to sit on top of these two. |
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The two negatives connected
in the same fashion. |
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Strip some insulation to
allow the wires to connect to the third 12 volt pack which will be stacked
on top. |
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Here's the third pack in
place with the wires connected. |
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Use electrical tape to
tightly wrap the entire pack, preventing movement within the pack. |
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Here's the four 12 volt
packs, each at 7.2AH. Two of them will be connected in series to give 24
volts at 7.2AH. Their are four so one set can be charging while the other
is fighting. |
