For 45 years, nickel-cadmium (NiCd; also Ni-Cad™, see Editor's Note below) batteries have plagued me. That's because I like to spend my Sunday afternoons relaxing with radio-controlled aircraft, such as my Century Predator helicopter. This beauty costs well over a $1,000. Unfortunately, if the NiCd battery powering her unexpectedly dies before landing, let's just say that my whole day is shot.
NiCd batteries are notorious for losing their capacity over time and it's not just model plane enthusiasts who suffer. With their high current capability, NiCd batteries are used in everything from computers to power tools. I was thinking about this as I read Batteries in a Portable World (see Reference). Looking at Chapter 10, ("Getting the most from your Batteries--Exercise and Recondition"), I realized that a solution was finally at hand.
The Problem with NiCds
Depending on how they are handled, as NiCds age, the nickel crystals inside them will enlarge. This decreases the total crystal surface area, which in turn effectively reduces the cell capacity and increases its internal resistance. Fortunately, nickel crystals in a battery can be re-formed by very slowly discharging each NiCd cell from 1 V down to 0.4 V.
As explained in the reference, this helps to restore the cell's capacity and decreases its internal cell resistance. You won't recover 100% capacity, but you can prolong the useful life of a battery by up to 40% with a once-per-month cycle. How much is recovered will depend on many factors, such as the condition of the battery when you start, battery age, and how many charge cycles it has gone through, among others. I built a simple circuit, based on reconditioning a 1900 mA-hr, 4-cell battery pack, which handles this task.
This circuit, Figure 1, uses Linear Technology's LT6700-1 dual comparator to control a constant-current discharge circuit.
Figure 1: NiCd battery discharge and conditioning circuit, in this example, for a four-cell battery pack.
(Click to Enlarge Image)
A battery connected to this circuit will be discharged at a rate of 1900 mA until its voltage reaches 1 V per cell (4 V for the 4-cell battery pack in this example). The discharge current is then reduced to 38 mA until the battery voltage reaches 0.4 V per cell (1.6 V for the 4-cell battery pack).
To begin, the start switch, SW1, is closed and the comparator U6 senses if a battery is connected. If the battery voltage is above 4.4 V, the output of comparator U6 will be high, the output of comparator A of U7 will be low and the output of comparator B of U7 will be high. These comparators control switches U2and U4 of the current-set circuit (highlighted on the schematic).
The current-set circuit generates one of three output-voltage states which, in turn, establishes the amount of current drawn from the battery by the current-sink circuit (pin 3 of U5), see Table.
Reference U1 supplies a stable 2.5 V across a resistor-divider string. When the battery voltage is above 1 V per cell, the current-set circuit will output 190 mV to the current-sink circuit, commanding it to discharge the battery pack at a rate of 1900 mA.
When the 4-cell battery pack voltage goes below 4 V, analog switch U4 is closed. The current-set output control voltage drops down to 3.8 mV, and the current-sink circuit discharges the battery at a rate of 38 mA. When the battery voltage reaches 1.6 V (0.4 V per cell), the output of comparator B goes low, switch U2 opens, the current-control voltage goes to zero, and discharge current is set to zero.
MOSFETs Q3 and Q4 are used in place of positive feedback resistors for the 1 V and 0.4 V comparators, to generate a large amount of hysteresis. This is needed because when a NiCd's discharge current is very low, the battery voltage can float up above the higher threshold. Without sufficient hysteresis to hold off the comparator circuits, the circuit could continuously flip between high- and low-current discharge. In this version of the circuit, the lower-current (38 mA) reconditioning cycle is maintained even as the battery voltage floats up above 1.6 V.
Conclusion
No battery is without problems. But even with all of its shortcomings, the NiCd battery is still the number-one choice when high discharge currents are required, because the NiCd cell has the lowest internal resistance. The tradeoff is that the user must take care to keep them in top shape. The circuit presented here is a tool to keep NiCd batteries in the best possible condition and reduce the uncertainties of their use.
A final note of caution: the circuit presented is not a "plug-and-forget"
circuit. Care needs to be taken, as should be done with any battery circuit,
to monitor the cell voltages as the conditioning cycle progresses.
In the example give, with four NiCd cells in series, the most likely case would be
to have three good cells and one cell that does to zero volts quickly. In
this case, the 4.0V threshold will be reached and the circuit will switch to
the low-conditioning-current mode. It is at this point where it is important that you check to see if the cells are discharging evenly. If you have questions, email me at the address below.
Reference
Isidor Buchmann, "Batteries in a Portable World— A handbook on rechargeable batteries for non-engineers", available at http://www.buchmann.ca/ and also via Cadex Electronics Inc.
Editor's Note
NiCad is a trademarked name for a Nickel-Cadmium battery, owned by SAFT Corp.
Author's biography
Jim Mahoney is an associate applications engineer at Linear Technology Corp, Milpitas, CA, and is primarily responsible for op amps, comparators, and other specialized signal-conditioning products. He started career in the US Army, followed by the US Air Force, Electra Physics Labs, Hewlett-Packard, H-P Labs, and Yokogawa-Hewlett Packard, prior to joining LTC in 1994. Jim studied anthropology, mathematics, and physics in various schools over the years. In addition to enjoying time with his wife Aline, Jim builds and flies radio-controlled helicopters and travels to Japan and France to visit his daughter and grandchildren. He can be reached at jmahoney@linear.com.
