Batteries
NiCad and NiMH Care
NiCad Care
You can discharge NiCad packs with a bulb discharger.
However you must never allow the pack to fully discharge. If you do, you
are taking a risk of reversing a cell. Disconnect the discharger when
the lights begin to go dim. A better idea would be to use a cut-out
switch to prevent the pack from discharging below a safe point. For a
6-cell pack a 5.4 volt switch would work, and for a 7-cell pack you
would want a 6.3 volt switch. The idea here is to bring each cell down
to .9 volts.
Another method of discharging your NiCad's is to run
them in your truck until the point where it slows down dramatically on
pavement, and just about stops in the grass. You will notice that when
you pick the truck up, it still has enough power to turn the wheels
easily. Just like with the bulb discharger, you don't want to run the
battery all of the way down. Otherwise you'll take the chance of
reversing a cell. At this point you should be around .9 volts a cell.
It's not exact and perfect, but this method will work fine for the
average backyard basher.
One of the more common mistakes I see is to take the
pack at this point and slap it back on the charger. This is wrong, and
will shorten the life of your pack. Make sure your pack cools completely
before charging it again. You can use a fan to help move cooler air
across the pack. However I discourage you from using the freezer,
fridge, or any other extreme cooling procedure. While many people do
this and they may not have any problems, it's not the best procedure for
ensuring long life from your packs.
I also often see many people warn against using NiCad
packs more than once or twice a day. I think this stems from the fact
that you must ensure that your packs are completely cool before
recharging them. I generally disregard this rule and feel that if you
ensure your packs are cool, you should have no problems with running a
pack several times in one day. I've got several packs that have been
used in this manner for over a year and a half, several times a week,
and have suffered no ill effects. In my opinion it's heat that causes
battery failure, not how many times you run it in one day.
When charging NiCad batteries, you'll want the charger
to see a higher voltage drop-off than NiMH would, when the pack is full.
This is what the threshold setting on most chargers addresses. If you
are using a charger that allows you to vary the threshold setting,
you'll want it around 10-15mV for each cell. Therefore, for a 6-cell
pack, you'll want to set it in the range of 60-90mV. The exact setting
may vary between packs, and require some experimentation. Always start
out on the low side and adjust upwards if you find you packs are not
getting fully charged. When charging your NiCad pack, you should
generally charge at a 4-5 amp rate.
Many high-end chargers will also provide you the
capability of flex/pulse charging. This method of charging starts out at
a fixed rate but at several points during the charge, it's interrupted
with short bursts of discharging. This is often used to revive older
batteries and restore much of their usefulness. This is used on NiCad
packs, and isn't recommended for NiMH packs.
You should always make sure that your NiCad packs are
discharged when they are stored. If you store a pack for a few days
that's fine. However don't store the packs for a week, month, or any
other prolonged period while they are charged. Doing so can affect the
discharge rate and their capacity.
NiMH Care
The discharge procedure for NiMH packs is the same as
for NiCad packs. Again, you want to discharge until you reach .9 volts a
cell, but no lower than that so that you don't reverse a cell. As with
the NiCad packs, you can also just run the truck until it slows.
Heat will kill a NiMH much easier than a NiCad battery.
So you need to be especially careful that the pack is cool before
recharging it. Again a fan is fine, but extreme measures can damage the
pack. Also, you should always use a NiMH compatible peak charger when
charging NiMH batteries. A NiMH battery pack has a much lower voltage
drop-off when the pack is full. If you are using a charger that allows
you to vary the threshold setting, you'll want it around 3-5mV for each
cell. Therefore, for a 6-cell pack, you'll want to set it in the range
of 18-30mV. The exact setting may vary between packs, and require some
experimentation. Always start out on the low side and adjust upwards if
you find you packs are not getting fully charged. When charging your
NiMH pack, you should generally charge around a 4 amp rate. It is
possible to charge at a 5 amp rate, but some NiMH packs may be less
tolerant of the higher voltage. Flex/pulse charging isn't recommended on
NiMH packs.
Probably one of the most significant differences in
NiCad and NiMH packs is how to store them. When storing your NiMH packs,
you should always charge them back up to around a third of their normal
charge. Which, to say the least can be inconvenient. Who wants to sit
around and charge their packs partially back up after they're done
running. However it is necessary. NiMH packs can be damaged if you store
them fully discharged. Contrary to popular belief NiMH can still suffer
the same memory condition that NiCad's can. So when bringing your NiMH
packs out of storage, it's a good idea to discharge them before you
charge them. This will help them maintain their optimum level of
performance. Running them down in your truck will suffice for this.
For more information on caring for your NiMH packs, see
RCCA's article entitled
NiMH Battery Care and Maintenance.
Repairing a Pack
There may come a time when a pack just doesn't seem to
offer what it used to. One of the signs of a good pack is that it's
seems to lose most of it's power all at once instead of throughout it's
run. However if the pack seems flat soon after you start using it, or
even from the beginning, it's possible you have a bad cell.
Just because a cell is bad doesn't mean you have to
trash the whole pack. It's possible to replace just the problem cell and
continue using the pack. Try to determine which cell is giving the
problems. After a run, or after charging the pack, feel the individual
cells and see if there's a drastic difference in temperature between any
of them. If so, then that cell is a suspect. You can verify this when
you break the pack open by checking the voltage on that cell to see if
it is extremely lower than the others in the pack.
After determining the bad cell(s) you can replace them
and rebuild the pack using the procedure found in the RCCA article
entitled
Build a Battery Pack. This article will walk you through the
procedure of how to reassemble the pack once you have broke it apart. In
addition you may also find the article entitled
Rebuilding a Stick Pack of help as well. |
