battery solutions
NiMH / NiCd cell
A cell is defined as a single vessel containing electrodes and electrolyte for generating current. A battery consists of two or more cells. NiCd/NiMH cells are rated at 1.2V for design purposes although they normally develop about 1.25V. Under full charge they require about 1.5V to 1.6V. They can supply very large amounts of current and display a remarkably flat discharge characteristic, maintaining a consistent 1.2V throughout discharge. The voltage then drops quite suddenly, and they are almost completely flat at 0.8V.
NiCd / NiMH Charging
Rechargeable battery capacity is rated in mAH (milliampere-hours). The total capacity of a battery is defined as "C", that is it can supply C mA for 1 hour, or 2C for 30 minutes and so on. Charge rates can vary from trickle charges to keep the battery 'topped up' of 3.3% of C to 5% of C, a slow current charge of 10% of C to 20% of C or a fast charge of 50% of C to 100% of C. Slow charges normally last about 8-15 hours. Fast charges such as 100% of C should be terminated after about 1.5 hours, providing the battery is flat to begin with. Once a battery is fully charged, the battery produces gas creating a high internal pressure, and a sudden rise in temperature. The charge should be switched to a trickle charge at this point or the battery will begin to vent and release its electrolyte. NiCd/NiMH batteries are about 66% efficient. These cells should be good for 500 to 1000 cycles of charging if properly treated!
Example and circuit
Battery : NiMH 3000 mAH 1.2V x 10 cells.
LM317 Constant current battery charger. Calculation for resistance R1 are done as R1 = (final battery voltage) / C. So if we are slow charging at 300 mA we need to put R1 = 1.25 / 0.3 = 4.16 ohms.
For higher currents use a lower value resistor using the formula in the diagram R1 = E / I . i.e. for 1000mA (1/3 C) the resistor would need to be R1 = 1.25 (V) / 1 (A) which equals 1.25 Ohms.
Li-Ion Battery
Lithium-Ion (Li-Ion) batteries are gaining popularity for portable systems due to their increased capacity at the same size and weight as the older NiCad and NiMH chemistries. Li-Ion batteries commonly require a constant current, constant voltage (CCCV) type of charging algorithm. In other words, a Li-Ion battery should be charged at a set current level ( should be less than the capacity C ) until it reaches its final voltage. At this point, the charger circuitry should switch over to constant voltage mode, and provide the current necessary to hold the battery at this final voltage (typically 4.2 V per cell). Thus, the charger must be capable of providing stable control loops for maintaining either current or voltage at a constant value, depending on the state of the battery.
