By John Andersen
All good things come to an end, usually at the most inappropriate time. Bike light
batteries are no exception.
After three seasons of service my bike light battery was getting really wimpy even
after a full charge. It seemed dim, and the run time was poor. Even after an
overnight charge, the best it could muster was just under 9 volts.
It would cost over a hundred dollars to replace the battery with a new one from the
manufacturer.
This was the second time this had happened. I still had some salvaged NiCad cells
from the first battery that failed, several years earlier, also after delivering years of
service. I decided to see if I could come up with another season of service by
combining the cells from these two dead batteries into one serviceable battery. I
had nothing to lose, in that the newer battery was no longer serviceable in its current
(or lack there of) condition.
In my case the batteries were both of the same brand. NightSun in particular.
These are water bottle batteries, composed of 10 cells in a bicycle water bottle designed
to fit in your water bottle cage.
Before doing anything to the the newer battery, I
gathered the older cells and set out to determine which were salvageable I separated
each cell from the old battery pack. Using power from the charger supplied with the
batteries, I charged each cell individually for 30 seconds. (Plus wire to Plus terminal,
Minus to Minus).
I was charging with 12 volts into a 1.2 volt cell. This may seem excessive but
the charge was for only 30 seconds per cell. I didn't want to fully charge the
cells, just to give them all the exact same charge opportunity, hence the 12 volts for 30
seconds routine. NiCads can take a lot of juice when they are dead, and these were
quite dead. Besides, I was carefully timing 30 seconds each.
After the charge, I let them sit for 5 minutes (NiCads exhibit a self-discharge just
after charging - I wanted to avoid measuring any misleading voltage during this period.)
I then arranged the cells on the bench in order of those having the highest
voltage, and kept notes.
I let them sit over night, and measured the voltage again. Some were completely
flat. Others were still near the voltage I had measured just after charging.
From these I selected the 3 or 4 best. I expected to need two or three because my
NightSun was measuring only 8.75 volts after a full charge. I guessed that either
all cells were in moderate decline, or there were two or three that were really
bad. I was hoping for the latter.
So with resignation, I decided to murder another
NightSun battery. I got my tin-snips out (any scissors would have done as well) and
split the water bottle open from stem to stern.
As expected (from prior experience) the bottle was filled with foam, and the batteries
were incased in such a way as to prevent them from jiggling around. The newer foam
was far more uniform than that found in the older battery and easier to peel away.
But here I received a surprise. The newer battery which carried a higher rating
than the old one, had smaller cells, a bit taller, but smaller in diameter. I
worried about compatibility, but not for very long. I had two dead batteries, and
nothing to lose.
Carefully I peeled this foam stuff off by hand, so as not to damage any of the wiring.
NightSun uses flat wires that snuggle right up to the battery and take no room.
These are covered with a plastic sheath to prevent shorting.
Without separating any of the batteries, I laid them
out on the floor and measured each cell individually for voltage and then again for
resistance. Note that they came out of the bottle in sets of two, stacked one on top
of the other and shrink-wrapped together. I measured each set looking for weak sets,
then further separated the weak sets into individual cells to find the bad ones. Two
adjacent cells were completely dead, no voltage at all, and no resistance either.
These two cells, which were shrink-wrapped together, had simply shorted out, and taken
themselves out of the picture, neither helping or hindering the rest of the cells.
Probably the result of over-charging, I blame no one but myself.
I figured these two had caused most of the loss of power, but that there might be
another weak cell in there as well. But careful metering of all the remaining cells
showed no other contender for worst of the lot. I decided to use the rest, and a
couple of my salvaged cells from my prior battery.
Note: when obtaining cells of any kind for repairing your batteries, always try to buy
the kind with solder tabs. Its generally not advisable to solder on batteries unless
you are careful to do it quickly. The key to getting a quick solder job is a fine
(thin - very thin) layer of soldering flux on the to-be soldered surfaces. This way
you will get the job done in one go without heating the battery for more than a few
seconds.
Picking the two best of my previously salvaged
batteries, I soldered their tabs together (Plus to Minus). Then I cut the flat wires
of the newer battery as close to the dead cells as possible and soldered these to the tabs
of the replacement cells. Again, always watch your polarity.
I found a suitable water bottle from my collection. I was looking for a wide
mouth to make it easier to fit the chain of cells into the bottle. You have to
carefully jiggle the batteries down into the bottle, so that they stager themselves side
by side. You don't want any to be sitting on top of each other where they might
short out. Careful work with a pencil can generally fit them in almost like they
were arranged in the original battery.
I had the added problem of fitting the two fat batteries into the bottle. I ended
up with two layers of batteries instead of the one layer that NightSun and produced.
But my second layer was sitting on top of the cell they were connected to (plus to
minus) so shorting out would not be a problem, as that is where the wire ran anyway.
At hardware stores or building supply centers you can buy spray cans of foam used for
insulating around window casements during construction. One can in way more than you
will need. Spray this stuff into the bottle around the cells. Do a little at a
time, trying to spray to the bottom of the bottle first ('cuz you can't get there once the
top fills up). This stuff expands like crazy. Spray a little, watch, see where
holes are, and spray a little more.
Finally I strung the wires out of the bottle spout, and hot glued the opening to
prevent water intrusion.
Charging the battery overnight brought it up to 12 volts. After an hour of
sitting around it was down to 11.9, but that was acceptable for old cells. I could
have put another one in there, and still been within the limits of what the bulbs could
handle.
I've used this battery for several weeks now and it's run time is sufficient for a
couple days without a recharge and its brightness is restored.
NiCad Cells for repair jobs can be found through a number of sources on the internet, a
quick web search will find dozens of such places. It is always better to obtain
something as close to the original cell as possible. Sometimes you can tell by the
markings what ratings are needed. Other times the original manufacturer will sell
you replacement cells, but by and large they only want to sell you complete batteries - at
high prices.
Never throw "dead" NiCad batteries away. In the first place, they must
be recycled as the Cadmium is quite toxic. In the second place there are usually
just a few bad cells in the battery that can be replaced, or, conversely, several that can
be used to replace cells in other batteries.
Update: I got another season of use out of this repaired battery,
but the next season, I replaced it as additional cells began to fail.
Home of the Winter Cyclist