Reprinted courtesy of Vencon technologies

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By Marc Venis B.A.Sc.,  M.A.Sc., P.Eng. - President Vencon  Technologies Inc.


Standard Charging

In this article I discuss the two methods of NiCd and  NiMH charging -- standard and trickle. The "overnight" charger that comes with  most rechargeable powered products charges at a rate of C/10 (the C rate is the  hour capacity of the battery, i.e. a typical AA NiCd battery of 600mAh capacity  has a C rate of 600mA, and a C/10 rate of 60mA). There is a very good reason why  the manufacturer chose this rate. If the charger uses a higher rate, it would  have to detect when the batteries are fully charged and shut off, or risk  damaging them. This would make the charger more complex, and hence more  expensive. Lower charge rates than C/10 unnecessarily extend the charging time,  and in fact at very low rates (below C/50) the batteries never fully charge no  matter how long you wait.

Thus the C/10 charging rate is a compromise between keeping the charger  simple, yet charging the batteries in an acceptable amount of time. At the C/10  rate the battery will reach a full charge after approximately 14 to 16 hours. If  the actual battery capacity was the same as its rated value, and its charging  efficiency was 100% then only 10 hours would be necessary to fully charge a  battery. But, actual capacity is usually greater than rated and charging  efficiency is always less than 100%, thus 14 to 16 hours of C/10 charging  ensures a fully charged battery. At this point any further charging only results  in an increase in temperature and internal cell pressure. This does not damage  the battery, although it accelerates their deterioration thus reduces their  reliability.

Now that we know what the standard charger is, how do we use it? Most  important, follow the manufacturer's instructions. A typical manufacturer  recommends a 15 hour charge to fully charge a discharged battery. If your  battery is only partially discharged, your can prorate the charge time. For  instance, a battery that is one third discharged will fully charge in only 5  hours. If you don't know the condition of your battery, then you should charge  it for the full 15 hours. What happens if you forget to disconnect the charger  and end up charging for more than the required amount? If you overcharge by only  a few hours, don't worry. If you leave your charger connected for a couple of  days, then you're unduly stressing your batteries. If you're the forgetful or  worrying type, then you might want to use a timer. I like to use the standard 24  hour security timer, used to switch lights on and off when you're away. It is  sold almost anywhere, usually less than $10 on sale. To use a timer with  removable ON/OFF pins for standard charging, first rotate the time dial until 12  midnight. Now insert an OFF pin at the 3pm position (15 hours) and remove any  remaining pins (figure 1). Now plug your charger into the timer and the timer  into a wall socket. Rotate the on/off switch on the timer until your charger  turns on, and leave it alone. After 15 hours your charger will shut off and  remain so. If you have a timer without removable pins set it up as above, but  instead of inserting an OFF pin at 3pm, set an ON/OFF pin (or if separate pins,  the ON and OFF pins together) at 2:30pm such that the timer will turn on at  2:30pm and off at 3pm (figure 2). It will operate similar to a single cycle  timer, except on subsequent days the charger will turn on for one half hour each  day (actually an advantage as we'll see later).

art2fig1 Figure 1.

art2fig2 Figure 2.

Trickle Charging

I'm calling this section "Trickle Charging" because  that's the term we are all familiar with. This section is really about keeping  your batteries fully charged. Trickle charging is one way to do that, charge  replacement the other. To avoid confusion, I'll use the term "trickle charging"  to refer to any method of keeping your batteries fully charged.

Now that we know how to fully charge our batteries, when do we trickle  charge? If you usually charge your batteries the day before you intend to use  them, you don't need to trickle charge. If, on the other hand, you're like me  and want to have all your batteries charged and ready to go, you need a trickle  charging system.

NiCds and NiMH cells, like all batteries, self-discharge. NiCds and NiMH  actually have a relatively high self-discharge rate of about 1% per day at room  temperature (i.e. a 600mAh cell loses about 6mAh each day). The purpose of  trickle charging is to replace the charge that is continually draining off.

Trickle charging is similar to standard charging (i.e. it uses a continuous  charging current), only less current is used, between C/50 and C/20 (i.e.  between 12mA and 30mA for a 600mAh cell). This rate is high enough to maintain a  charged battery fully charged, yet low enough to permit continuous charging  while keeping cell temperature and internal cell pressure at a safe level.

Since I doubt a trickle charger came with your battery, we will need to be a  bit more inventive if we want to trickle charge our batteries (or you can go out  and buy a commercial trickle charger). There are three basic methods used in our  hobby to trickle charge.

Method 1. Continuous Current.

The simplest method of trickle charging  (at least for a manufacturer) is to just reduce the charging current to about  C/40. If the charger already charges at the standard C/10 rate, then all the  manufacturer need do is add a resistor (and possibly a switch for  trickle/standard mode and a charge indicator LED). This is the method used by  most trickle chargers.

Method 2. Pulsed Current.

If we switch a C/10 standard charger on and  off such that it is on for only 10% of the time, we would be continuously  replacing any lost charge. We could do this by switching a C/10 charger on for  one second off for nine seconds. This is similar to the method used by the Ultimate Battery Analyzer.  This is more complicated for the manufacturer than the continuous current  method, but easier for the user. If we switch the AC side of our standard  chargers, we can simultaneously trickle charge all our batteries. Unfortunately  the 24 hour security timer can't switch on and off fast enough for this method,  but it can do something else, as shown below.

Method 3. Daily Charge Replacement.

This method allows the battery to  self-discharge during the day then it replaces the lost charge once per day. It  is a simple and inexpensive method. Set the timer to turn on for at least one  half hour each day. Plug your chargers into the timer (a power bar comes in  handy here), and the timer into the AC socket. Everyday your batteries will get  a top-up charge and will be ready for use.

What would happen if you accidentally connect a fully or partially  discharged battery to your trickle charger? Method 1 has the advantage that it  will charge the battery in about 3 days (although this isn't recommended because  the battery might not reach its maximum capacity, you should always charge at  the C/10 rate).
Methods 2 and 3 with their slower charging could take over a  month. Increasing the duty cycle 'on' time of methods 2 and 3 will reduce the  charging time, but at the expense of slightly greater heating. You can increase  method 2's duty cycle up to 25% (1 second on, 3 seconds off) with little adverse  effects. At 25% duty cycle, full charge will take four times as long as at the  C/10 rate (about 3 days). Increasing method 3's duty cycle to 25% will warm the  pack for 6 hours every day.

An important consideration of which method you choose is verifying that the  packs are on trickle charge. Method 2 has the advantage that you can see the  flashing charge lights and know that the system is working. Method 1 does not  draw enough current from the charger to light its LEDs, thus unless you have  installed additional charge LEDs it is impossible to tell if the battery is  actually charging. Since method 3 only turns on the charger for one half hour  each day it is difficult to verify its operation. Method 1 also has the  disadvantage in that being a continuous charging system, it promotes cadmium  migration. Cadmium migration is something we want to avoid, and pulse charging  (methods 2 and 3) reduce the chances of it happening.

Which trickle charging system is best for you? Unless you have a convenient  method of pulse charging, I recommend method 3 for its simplicity and economy  (and that's the method I use).

A Personal Charging System

Now let's combine a standard and trickle  charger to assemble your own charging system. You can use your security timer to  charge your batteries, then automatically trickle charge them. Remember where a  timer without removable pins was used to charge up you batteries. During the  first day the batteries charge for 15 hours. On subsequent days the batteries  charge an additional one half hour. This will keep them fully charged and ready  for use. You can do the same using a single cycle timer. Just insert the ON pin  one half hour before the OFF pin. You can now leave and forget about it.

If you know that you're not going to be using your battery for more than a  couple of months, take your batteries off your trickle charger and just remember  to fully charge them before use. Also, even though you're batteries are on  trickle charge, you still need to do a full discharge test every month or so to  ensure their condition. One final note of caution. The first couple of times  using a new trickle charger system insure that the batteries are really fully  charged before you use them. Ideally, you would measure their capacity at least  a day before using them by a discharge test and then fully charge them.

I hope that this information instills in you a bit more knowledge about  charging your batteries.



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