which time the current automatically reduces to a trickle
charge. CV/CI power supplies do not have temperature
monitoring to prevent battery explosions, so do your
homework on the type of battery you’ll be charging to
determine the maximum voltage and current. When
charging batteries, do not leave them since the charge
termination is not always automatic.
You must first set the final charged voltage based on
the number of cells in series for the type of cells you are
charging. Then, set the maximum current to charge the cells.
When determining the charge time, a reference “C” is
used for the rate of charge current. A “C/1” would be the
Ah (amp hour) value to fully charge or discharge a battery
in one hour. We must supply more energy to the battery
than its actual Ah capacity to account for energy loss
during charging. A trickle charge rate might be C/10. One
might think that it would take 10 hours, but 14 hours is
necessary because of the energy lost charging the battery
— usually by heat.
If we had a 100 mAh battery, then C/10 would be
100/10 or 10 mA for 14 hours. To rapid-charge this battery
(C/1), we would have to charge 100 mA for about one
hour and 15 minutes. If we try to increase the charge rate
faster than C/1, we run the risk of overcharging and
damage to the battery that results from heat generated in
the battery. Caution must be taken when charging C and D
batteries as many manufacturers will disguise an AA battery
by packaging it in a C or D battery case. This affects the
C/1 value.
To wrap things up, here are some tips for specific
batteries:
Nickel–Metal Hydride (NiMH) and Nickel–Cadmium
(Ni–Cd) batteries: Charge to 1.41 volts per cell and use a
timer to prevent overcharging. Do not continue past 13 to
15 hours (with C/10). A moderate charge rate might be
C/2 for 2. 5 to 3 hours. Refer to Table 1.
Sealed Lead–Acid Batteries: The volts per cell vary
with temperature and manufacturer. For example, 2.6V at
0°C, 2.45V at 25°C, and 2.3V at 50°C. The C/1 or Ah is all
over the place with each type of battery and manufacturer.
You have to do your homework on this one, too.
Charging lithium-ion, lithium-polymer, and lithium-iron phosphate batteries can be dangerous and requires
close supervision. Be sure you research this thoroughly
before proceeding. NV
32 May 2014
; FIGURE 14.
Wiring for the
voltage pot
and current
pot.
These numbers can vary from
manufacturer to manufacturer
NiMH to
1.41V mAh
NiCd to
1.41V mAh
AA 1,300 700
AA 2,450 1,000
AAA 850 300
AAA 1,000
C 4,500 3,000
D 10,000 5,000
Table 1.
; FIGURE 13.
Overall power
supply wiring
complete.
