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Two Panasonic lithium cells, a circuit board
and ribbon wiring are inside of the 830 mAh packs from Mark Levitt. The Goldmine packs
are similar, but the cells are larger. |
Background
Recently there have been discussions on the eZone about
converting
surplus Qualcomm cellphone battery packs for use in small electric R/C planes.
There appear to be two batteries available on the surplus market at the moment. Both
are two cell Li-Ion batteries encased in a plastic shell. One is rated at 830 mAh capacity
and is available from Mark Levitt. The other is
rated at 1100 mAh and is available from Goldmine
Electronics. Also available from Mark is a Qualcomm charger that can be used to charge
the battery packs.
These batteries work very well for eFlight within their limitations. They cannot be abused
the way eFlight practitioners do with NiCd's and NiMH's. The manufacturer characterizes
them for up to 2C discharge and they should not be use much beyond that discharge rate.
The cells in the 830 pack are officially rated to 1.66 amps. I've found them an excellent
battery for any GWS IPS-"A" powered plane (LiteStik power system). I've flown them
in both stock and heavily modified LiteStiks and a GWS Tiger Moth with flight durations in
excess of 30 minutes. The Tiger Moth cruises on 1/3-1/2 throttle with a fresh pack.
The 830 packs contain two Panasonic CGR17500 cells wired in series. For reference,
you can check out the Panasonic Li-ion Catalog.
As delivered, the batteries are encased in sealed plastic cases that have finger contacts
designed to mate with, what else, Qualcomm cell phones. Fortunately, it is simple to
break open the cases and remove the battery assemblies. Once you remove the battery from
the case, you'll discover a circuit board connected to one end of the battery pack and a
couple of other components. My goal was to use the batteries, not to understand the
workings of the pack; however, I think it is safe to assume that the circuit board serves
three functions. One is to act as a current limiter to keep the cells from being over
stressed (one item in the middle of the board appears to be a pigtail pico fuse). The second
function is a low voltage cutoff to shut down tghe pack when the voltage falls to the
"discharged" level. The third function is in charging the batteries - I suspect
there is a balancing circuit in there to allow the two cells to each reach full charge.
With respect to the first function, current limiting, there are two opinion camps. The first is
that the circuit can be used as a circuit breaker in the event of the motor stalling (from an
obstructed prop) - the circuit will kill power to the motor if this happens, there by saving
the motor, batery and ESC. Unfortunately, when the circuit breaker function trips, ALL power
is killed - this means that the receiver and servos are dead in addition to the drive motor.
The only way to reset the circuit breaker function is to disconnect the battery and reconnect
it. This can be a problem if a glitch causes an momentary over current condition - all
control is lost. The second camp connects the load directly to the battery contacts,
instead of the protection circuit output. This allows uninterrupted power. Since I've
had planes both crash and fly away due to dead battery packs in the past, I opted to
connect directly to the battery terminals. Although I chose to remove the circuit board,
it is possible to connect directly to the battery contacts with the board intact. If
you choose to remove the circuit board for flight use, it needs to be attached during
charging to provide the charge balancing function.
Much of the eZone discussion has been in regards to different ways to accomodate the protection
circuit board and hooking the battery and board up to the charger. Some have refitted the
battery packs back into the plastic cases to charge them. Others have simply opend the pack
enough to attach directly to the battery for output, but the pack is essentially intact and
can be dropped back into the charger. Since I intended to use these packs to power LiteStik's
and other small, low current draw (under 2 amp) applications. I don't think the protection
functions of the circuit board are useful in this application (for one thing, long before the
batteries are below their safe discharge level, the plane's performance is degraded to the
point where the plane will be on the ground), so I chose to remove the
board fom the pack. However, I see the recharge function as useful, so I devised my own
solution to the problem. I use homemade connectors made from DB25 computer connector pins
for power systems up to speed 280/300, so I was looking for some way to interface the
batteries into my existing power systems and at the same time allow them to connect to
the charger. I accomplished this by adding a third wire to the battery lead connected
to the center tap of the battery pack (shades of 1970!). The planes use battery plus
and minus, ignoring the third wire. The charger uses all three leads.
Modifying the batteries
Warning - Before you consider cutting these cell phone batteries up and repackaging the cells, you
should be aware that these batteries have potentially violent failure modes if the cells
are over charged or are discharged at too high a rate - here is a reference to a site that
knows far more about Li-Ion cells than I plan to know NEC Moli Energy.
You should evaluate your use and not exceed the recommended current draws. You should
also not over discharge the packs - it appears that 3v/cell is the minimum discharge level.
(I use the packs in conjunction with GWS IPS-A drives. On this drive, I get in excess of
30 minutes flight duration. I've never discharged the packs below 6.5 volts. At 6.5 volts,
the flight performance is pretty poor.)
Let's start by taking the battery apart. This will get us some of
the parts for the charger modification.
Carefully remove the circuit board from the battery.
Begin by carefully removing the circuit board from the battery pack. Start
by unfolding the ribbon board from the side of the battery. Then clip the
centertap strap near the bottom of the battery. Using a solder sucker or
solder wick, unsolder the positive and negative battery terminals from the
circuit board and then pop the board off and set it a side.
Next, make up the connectors and leads. If you use the same connector
system as I do, you'll need about 6" of three conductor servo wire, a male
DB25 pin, two female DB25 sockets some 3/32-1/8" heatshrink (in black,
red and white) and some 1/4" heatshrink.
Parts to make the battery and connector.
Strip and tin the servo wire and then crimp the male pin onto the black wire,
and one female connector each onto the red and white wires. Next cut short
pieces of the small heatshrink tubing and shrink it over the connector pins.
Finally line up the pins and slip the piece of large diameter tubing over the
three pins and shrink it in place.
Make up the connector first.
Now strip and tin the other end of the cable. Trim off the little nibs on
the end of the battery and while you're at it, tin the battery contacts.
Solder the red and black wires to the positive and negative terminals of
the battery. Solder the white wire to the center tap at the other end of
the pack.
Connect the leads to the battery.
Cut some narrow strips of fiber reinforced tape and wrap the ends of the
packs to keep the cells together. Be sure to trap the power leads under
the tape otherwise, it'll easily rip through the packing tape covering.
I cover the ends of the cells with masking tape and then wrap the entire pack
with clear or colored packing tape - you can substitute heat shrink tubing if
you like. You can shrink the packing tape tight with a heat gun and if you
fold the ends over when hot they seal really well.
Add reinforcing tape and covering to the pack.
CONTINUED...
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