One of the most asked questions that we get is "When should I recharge the battery in my cordless drill?" often followed by the statement " I run my batteries all the way out and I still don't get a full charge. What gives?"
I don't know where the practice of running the batteries all the way down started, but most people tout the dreaded "memory syndrome" as their reason. They heard " somewhere" from those ubiquitous "THEY SAY" that if they don't run the batteries "all the way down" then they will not take a full charge.
Nothing could be further from the truth. In fact, the practice of running your batteries into the ground before charging them is most certainly doing permanent harm!
This occurs for a variety of reasons. First of all, the memory effect in NiCAD batteries was a phenomena first observed in the power systems on board satellites in orbit. What scientists discovered was that when a satellite's batteries, charged through its solar cells, underwent precise and repeated charge & discharge cycles as it orbited into and out of the sunlight, the capacities of its batteries declined. The batteries in effect, would not discharge beyond this point, diminishing cell capacity . They named this phenomena the "Memory Effect" They found however, that this was not a permanent effect, and in fact could be erased by a few "deep" charge/recharge cycles. Then the batteries would return to normal. It is important to note that this occurred only in early type "wet cell" NiCAD batteries. Memory effect with today's batteries can only be demonstrated in the lab when subjected to very precise cycles. In fact, memory effect in today's batteries is a non-issue as it does not exist in normal practice.
So then, what's going on?
What is most often mistaken for memory effect is usually something called voltage depression, or cell reversal. To understand this, it is necessary to explain a little about the NiCAD battery.
A NiCAD cell is essentially a chemical factory. It consists of two plates called the cathode usually made of cadmium, and the anode made of nickel-oxide separated by the electrolyte, potassium hydroxide. Each cell in a battery has a nominal voltage of 1.25 volts. Tool batteries are made up of multiple cells, usually some multiple of 1.25. Therefore, a 12 volt battery has 10 cells in it. One of the more useful qualities of the NiCAD cell is its very low internal resistance. This allows very large currents to flow through it. This explains the large torque loads that these tools can produce.
Another characteristic is that the discharge curve of the batteries is pretty
much flat until they reach about 1.1 volts per cell, then they drop off rapidly.
This sudden drop in voltage on the discharge curve is called the "knee".
Batteries should never be discharged beyond this point. 
Figure 1 shows the discharge characteristics of a typical NiCAD cell. Note the sudden drop at the knee. The second graph shows the internal resistance of a cell. Note that as the voltage falls below about 1 volt, the internal resistance rapidly falls toward zero. Here is where the trouble is. You can see that in this region, a very small change in voltage of the cell can mean a large change in internal resistance. Most of the major manufacturers try to "match" the characteristics of the cells in a battery, i.e. match the internal resistance curve and their capacities . What can happen in a battery pack if it is discharged beyond the knee is that one cell might be at a slightly lower voltage than its neighbor, and therefore may have a substantially different internal resistance. Remember Ohms law! Now when charging is started, the cell with the lower resistance is the current hog and will start to recharge sooner than its neighbor. Taken over a couple of cycles, this cell may overcharge and its neighbor will never fully charge. Again, this happens because of the large differences in resistances below the knee. This effect is not as serious when recharging above the knee since changes in voltage above the knee result in only small changes in internal resistance.
One of the results of charging when the pack is below the knee is a condition called "cell reversal". Most failures of NiCAD packs can be traced to this condition. What happens is that in packs that have not been matched properly, especially the cell's capacity rating, is that when the battery pack discharges, one or more cells may discharge more that its neighbor. A point is then reached when the lower capacity cell becomes the recipient of a charge from the remaining cells in the pack. At some point its voltage becomes zero and is then charged in the reverse direction, causing the cell to reverse polarity.( In a discharge mode, the cell is being charged in the reverse direction.) When a cell is charged in a reverse direction, hydrogen gas is produced which is not reabsorbed ( unlike oxygen, which, produced during normal charging, is), but is lost to the chemical cycle through the cells overpressure vents. The cell then is permanently lost to the pack, causing decreased performance.
Matching cells properly is an extra expense to the manufacturers. Cells that are matched well perform their job for a long time. One example of this is the black battery system made by Makita. We have not seen a failure of a black Makita battery in a long time. Other manufacturers who will remain nameless, have not had the same results.
Remember this the next time you are tempted to buy one of those "$69.00 12 Volt " specials offered by some people on off- shore brand tools. That is the reason that they can sell a battery drill so cheaply, but the major manufacturers can't ( or won't).
There is more to charging a battery pack then just applying a charging voltage. NiCADS have to be recharged in a special way. There are usually two ways to recharge a battery, fast charge and regular charge. Fast charge is the predominant mode in use today in most cordless tools. These are batteries that will recharge in one hour or less.
Batteries have to be engineered specifically to take a fast charge. You should never try to fast charge a battery not made to fast charge.
I once tried to recharge a lithium battery not made for recharging. I had set the battery on top of the steel cage of the power supply and applied a current of about 100 ma.
Later that night, I was awakened by the sound of a substantial explosion in the cellar. I thought the furnace blew, it was so loud. Turns out, it was the battery. The explosion was so violent that it put a 1 inch depression into the heavy steel top of the power supply and embedded shrapnel in the walls of the room! This was only one "D" sized cell. Lesson: When recharging batteries, follow the manufacturers directions!
Most of the tool chargers in use today use a simple temperature sensor in the pack to tell when the battery is charged. In theory, when a battery becomes fully charged, the temperature of the pack rises suddenly. The sensor, which is connected to the third terminal of the pack, opens and turns off the charger. One of the drawbacks of this type of charging is that the sensor is in contact with only one cell. Who's to say that when this cell is charged, the rest of the pack is?
Manufactures get around this by providing a "trickle charge" mode to their chargers. The expectation is that once the charger drops out of fast charge, the trickle charge will take over and "top off" the rest of the batteries. This normally works well, but think of how you use your tools.
Most guys when working will grab a battery right out of the charger when the light goes out. Bad idea. The battery may only be 90 percent charged or so. Trickle charging can take a couple of hours. The answer to this is obvious. Leave the battery in the charger longer. Buy extra batteries if you have to.
The other problem with temperature sensors is that ,again, when your battery dies in the middle of a job, you grab the other one and drop the dead one in the charger. What's wrong with this? Well, the battery you just removed from the tool is hot because you were just using it. Two problems. Batteries don't recharge well when they are hot and also there is the chance you may trigger the heat sensor in the pack prematurely, since it's starting out at an already elevated temperature. Again, use extra batteries. Let them cool to room temperature before charging. We have seen a case where the customer complained that their battery will not charge at all, and we determine that he had left his tool in a closed car during a hot summer day. When put into the charger, the thermal sensor was already so hot it would not let the charger turn on.
Newer chargers are coming out now that eliminate the temperature sensor. They use a technique called "Delta V".
These chargers look for the slight drop in voltage that occurs at the end of a charging cycle in NiCAD batteries. When this occurs, the charger shuts off and usually goes into a trickle mode.
This newer technology, combined with other tricks like pulse charging, advanced temperature sensing, and built in diagnostics, have made a lot of the problems of earlier systems go away.
So then some tips on NiCAD charging:
When buying a new tool, ask questions about the charger and battery. If the seller can't answer them, go somewhere else.
If you have any specific questions on battery systems you can Email me here;
Notes@sawcenter.com and I will try to answer them for you.
CUL and happy charging!
Click here for more info on Charging NiCd and NiMH Batteries...
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