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Demystifying NiCd's
Part 3
Copyright, ElectroDynamics, Inc.
This page may be printed and distributed freely, only if it includes the ElectroDynamics
Logo, this copyright notice, Website link and an acknowledement of its
source.
http://www.electrodynam.com
Hi! This is the third part of our "put-it-in-layman's terms" articles on NiCd batteries.
Charging NiCd Batteries
Of all things in the modeling world, nearly nothing generates more controversy than the care and feeding of NiCd batteries! Let's try to clear the air up...
As we discussed in our last installment, our NiCd battery is an energy store, containing a "tank" of electrons ("fuel"), and an "electron pump".
In order to use our battery, it is necessary to first fill up the fuel tank. Simply put, "charging the battery" is really "fueling up the tank". But, there are a number of restrictions with regard to how we can fill up this tank.
Restriction 1 Not Too Quick: The "filling lines" and "internal plumbing" of the tank are not as big as we would like, so we can't pump in electrons at any rate, it'll only take so much. If we try connecting our "electron tank" to a high pressure fueling source, in an attempt to force electrons in faster, in all likelihood, we will either overheat or rupture the "internal plumbing".
Restriction 2 The Perils of Overcharging: (Obviously!) if we keep trying to force-fill the tank after it is full, it may burst!
Restriction 3 Not Enough Oomph: If we fuel the tank too slowly, insufficient fuel inflow will fail to properly fill all the "internal spaces", and the tank won't fully fill up.
What's this stuff mean in practice?
Restriction 1 says we cannot charge the battery faster than a set limit, or it will overheat and possibly be damaged. Most modern NiCd's may be charged up to the maximum of the 4C rate, i.e., with a charging current up to 4x the "rated capacity" number...
For example, a 1200mAH cell may be charged with up to 4800mA, or 4.8Amps.
BUT!
And a B-I-G BUT ... due to Restriction 2: The Perils of Overcharging, charging at a high currents (aka fast or rapid charging) must be carefully controlled and monitored to avoid catastrophic and possibly dangerous consequences! (Read: EXPLOSION RISK!)
We have learned in Part 1 of this series, sealed NiCd cells have a built-in mechanism to handle overcharging at a moderate (C/10) rate. But, when fast charging, the NiCd's overcharge tolerance mechanism does not work fast enough to keep up with heat and gas generation, so overcharging must be carefully avoided when fast charging.
To avoid overcharging when faswt charging, we can either time the charge (assuming we know how "empty" the cell is beforehand, so we can calculate how much time to charge the cell), or we will have to find some method to signal when the cell has reached full charge.
Fortunately, NiCd's have a nifty characteristic when charging...
As the cell reaches full charge, the terminal voltage rises slowly, then peaks in a "hump" when 100% charge is reached. If charging is continued, the terminal voltage then falls off.
A properly designed fast charger can monitor the charge voltage and stop charging when it detects this peak. These types of chargers are known as "delta-peak" or "peak detecting" chargers.
Timed chargers are simple and cheap, but they may abuse your batteries. To help with this, NiCd manufacturers make special "fast charge" cells, designed to have a larger physical size than absolutely necessary, to help tolerate and dissipate the heat generated when overcharging. Sanyo, for instance, have their "R" series cells, e.g., the N-1300SCR and KR-2000CR cells. Timed chargers are usually easy to recognize by their prominent timer dial on the front face, and are usually designed for use with electric motor power packs, since in most cases these are nearly exhausted before the user desires to recharge them. so their intial charge state is somewhat "known".
Delta-peak chargers are more sophisticated, and obviously, cost a bit more. But, they offer the advantage of automatic charge management, and a properly-designed unit will protect the battery from abuse from overcharge. They can be used with a battery in any state of charge, since they automatically detect when the battery is fully charged. Many of them are have, in addition to the delta-peak detection circuit, a backup maximum timeout circuit to shut off the charging current in the event that the 100% charge peak was missed.
When choosing and using a fast charger, two important factors must be borne in mind:
- What type of batteries are you planning to charge - electric motor power packs or receiver (Rx) and Transmitter (Tx) batteries? This will determine the maximum charge current and output voltage specification for the charger. Match these with the specifications of your battery.
- Although the maximum charge current of the NiCd is 4C, in the case of Tx and Rx batteries, the battery cables and connectors are rarely capable of supporting more than 2 Amps continuous current. There is no point investing in a 20 Amp charger for charging Tx and Rx batteries, you will have to turn the output w-a-a-a-y down to avoid melting the wires and connectors!
Having talked about fast charging (and everybody in in a hurry these days), it must be repeated that the best way to charge NiCd's is the overnight, C/10, rate, for 14 to 16 hours! Not c-o-o-o-l, not snazzy, but your batteries would really appreciate it very much! If you like to fast charge, at least treat your batteries to a full 16 hour overnighter at the C/10 rate every 4 fast charges or so.
Why??
Remember the nifty NiCd charging curve? A battery pack is composed of a number of these NiCd cells in series...
...and these cells are likely at different levels of charge. There will always be at least one cell lagging behind the other ones. Since the batteries are in series, the charger can only "see" the average characteristics of the sum of all the cells, so it "sees" the "fuller" cells peak close to each other, and interprets this as the end-of-fast charge signal. The lagging cell doesn't get a full charge, and, when the battery is fast charged again, it's going to fall further behind ...
For example:
First charge, cell #4 is 90% charged when charging ends.
Next charge, cell #4 gets 90% of that, i.e., 81% (0.9 x 0.9 = 0.81)
Next charge, cell #4 gets 90% again, and now's only at 73%...
You get the idea...
But, when we put the whole pack on a C/10 overnight charge, the "fuller" cells will get all filled up, and go into C/10 overcharge (not harmful, see Part 1 of this series) and the "emptier" cell gets to catch up to at leat 100%. This procedure is called "equalizing".
Restriction 3 is not so obvious - if we try to charge the battery with too low a current, we will not be able to fully charge it! A NiCd needs a minimum charge current to fully saturate the internal plates and fully charge it... charging for longer periods at less than the minimum current achieves nothing! The higher the capacity of the cell, the higher this minimum current is. The minimum recommended charging current to fully charge a NiCd is the C/10 rate, +/-10%.
Yeah, bubba, charging a 1200mAH battery at 60mA for 48 hours just won't cut it!
One of the most common "non-problems" we get here at ElectroDynamics is customers returning batteries 'cos "they didn't test out to the specified mAH rating". We only supply premium SANYO cells, which we think are the best NiCd's available on the market. These "faulty" units almost invariably turn out to be high capacity batteries charged "for a couple days" on the standard 50mA wall charger that came with the radio system!
If you charge with 50mA, you can't expect to get more than 800mAH out of the battery no matter how long you left it on charge!
What about trickle charging? As mentioned in Part 1, trickling is not really necessary for NiCd's, since the C/10 rate is OK for virtually indefinite charging without fear of overcharging. However, some chargers on the market do offer an automatic switch-over to trickle charging after completing the fast or overnight charge. A trickle charge is technically the C/20 rate or lower, i.e., the current number obtained by halving the C/10 overnight rate.
NiCd's do have one bad characteristic, they are electrically "leaky". Not that they leak actual "stuff", but they do lose charge over time due to self-discharge. Most manufacturers quote their cells' self discharge rate to be 30% of the remaining charge per month. Trickle charging is used to make up for that small self-discharge, to keep the cell topped up and ready for use in short notice.
Note that while the trickle charge rate is OK for long term keeping of the battery topped-off, it is not enough to fully charge it. To charge a NiCd, you MUST charge at the minimum of the C/10 rate.
In a nutshell...
- Charge your NiCd's at the C/10 rate whenever possible
- If you fast charge, do an overnight C/10 charge at least once every 4 fast charges
- When fast charging, remember to consider the maximum current that your system cables and connectors can handle.
- Trickle charge only to keep batteries topped up AFTER overnighting or fast charging. Charging at lower than the C/10 rate won't fully charge the batteries.
On that note, we'll adjourn to the next part in this series...
May all your landings be soft ones...
Third Order Intermodulation (3IM)
De-Mystiflying NiCd's Part 1
De-Mystiflying NiCd's Part 2
De-Mystiflying NiCd's Part 3
Installing RC Equipment in Larger Aircraft pdf file (250kB)
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