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Discharging Cell
by Sanj Varah of
http://www.schumacher.clara.net/index.htm
Introduction
To achieve the best performance from RC car batteries, they have to
be discharged. This article guides you why and how you should go
about discharging your batteries.
NiMH Batteries
If you own Nickel Metal Hydride batteries, you don't need to
read the rest of this article. They do not need to be flattened off
before recharging as they do not suffer from the dreaded memory
effect.
Simply let them cool before recharging. If storing them for more
than a week, only charge them up to 50% of their rated capacity (in
most cases 1500mAh).
Memory Effect
Memory effect is a fundamental problem with nickel cadmium
batteries. Memory is where after a number of charge, discharge
cycles the capacity or voltage output of the battery starts to fall.
Its very hard to create memory in a lab. So I have a graph to show
you what it leads to:
The above graph is by no means exhaustive. Battery memory comes
in a variety of forms but has two common characteristics, those
being reduced voltage output and reduced capacity.
Preventing Memory
Research by General Electric has shown that eventually all batteries
will become bad but there are a few things that can be done to
reduce the chances of memory.
The first method is to discharge the batteries. General Electric
state that if each cell is taken to below 1.0v then memory effect
cannot exist. The batteries can still go bad but not through memory.
The other methods are to give the pack full overcharge and use
discharge cycles that are non uniform. These two do not apply to RC
(or are hard to recreate)- so I'm going to ignore them!
Discharging
The majority of dischargers flatten off at around 20A, or you can
make one out of light bulbs which could do about 10A (6 bulbs).
There is an article on the Orion website that states that batteries
actually remember what rate they were discharged and you should
always discharge at a high rate.
I had a battery friend of mine have a look at it and he verified
it was bollocks. The discharge rate doesn't really matter but there
are two distinct routes to follow, High discharge rate for a short
period of time or Low discharge rate for longer period of time. The
aim here is to keep the cells as cool as possible.
The average discharge rate in a car using a stock motor is about
12A, so you could work from here.
Deep Discharging and cell Reversal
From the above, you can see that discharging the batteries to below
1v per cell (6V per pack) should prevent the onset of memory. Pretty
easy you might think? The problem is if you discharge the pack until
it is completely flat (Deep discharge), you run the risk of cell
reversal.
Not all cells are made equal and some go flat before the others.
Once a particular cell has gone flat it continues to receive a
charging current (from the other cells) but the current is reversed,
so the +ve terminal receives a -ve charge and vice versa. The flat
cell starts to vent and its life is shortened. This process is
called cell reversal.
Cell reversal is preventable by stopping the discharge when it
reaches 6v (most dischargers stop at 5.4V), however this leads to
the problem of some cells being more charged than others, Time to
equalize!
Equalizing (conditioning)
Battery equalizing (conditioning) is a process by which a load is
placed across each cell individually rather than the whole pack. The
advantage is that all the cells are taken down to exactly the same
voltage level. When charged they receive a uniform charge. Equalized
packs are noticeably punchier than non-equalized packs, they also
last longer.
Equalizing cannot be performed on stick packs as they are sealed,
it can only be carried out on saddle or side by side packs.
Cycling
Cycling is a process where batteries are charged and discharged
several times in an effort to bring back their life. This is all
well and good but if you do not Equalize the cells after every
discharge then you run the risk of cell reversal. So be warned if
you have an automated charger/discharger/cycler.
Storing
If you are going to store NiCad's, they should ideally be stored
flat. If they are not, this isn't usually a problem because they go
flat eventually through self discharge (about 3% every day)
If you are storing NiMH, store them at 40-50% full.
Conclusion
Discharging and Equalizing are absolutely imperative towards keeping
your battery packs in top condition.
Discharging until a battery is flat is not a good idea. An auto
cut off is absolutely essential. Dischargers from manufacturers such
as M-Troniks and Orion do the job but they are priced quite highly.
Dischargers are relatively simple pieces of electronics, and thus
pretty easy to make one.
Equalizing is an advert for changing your stick packs to saddles
if ever there was one! I always Equalize my cells as I can tell the
difference. The voltage output and run time are longer as well as
reducing the chances of memory and thus trashing your battery packs!
sanj.varah |
