Battery Level Indicator - del00022
Battery level voltage measurement is best done with
a dummy load. This circuit has four LEDs to show the
charge left. It is important to avoid overcharge or
deep discharge of rechargeable batteries. Never try
recharging normal batteries. Using rechargeable
batteries are good for the environment.
You can also create an electronic load using a "NPN
transistor and Electrolytic cap" with bleeding or
discharge resistor. A 555 Timer can turn on NPN for
a sec and measure battery voltage as soon as NPN is
on. The duty cycle of 555 can be 10% on time. During
off-time cap will discharge.
Description - This circuit uses a LM339,
a quad comparator. LM339 can work on single or
dual supplies, it has a open collector output
that can drive 15mA, low power consumption. The
circuit is an untested design but it should work.
When you measure the open circuit voltage of a
battery with a high impedance DMM (10M), the value
may be a bit misleading. Apply a dummy load to bleed
the battery a bit so that proper readings can be
taken on Load. The load below is a 100 ohms
wire-wound fusible ceramic resistor which will heat
a bit when you test 12V batteries.
Theory of Operation.
R16 a 5W ceramic wire wound bleeder or dummy load.
R15 is a part of an attenuator for obtaining
ranges. D2 is a protection clamp diode. R10-D1 forms
the 5V reference for comparators. Then an attenuator
obtains 1.2, 1.4, 1.6, 1.8 V
steps for each comparator. This circuit is similar
to Audio Level meter or VU meter circuit.
Charger and Rechargeable Batteries
The comparator compares the battery sample voltage
to the fixed reference step. If '+' pin is more
positive than '-', or is '+' is more dominant, then
output goes floating 'open collector', so No LED
light . But if '-' is more dominant the output
transistor of comparator goes low impedance or
saturates or turns 'ON'. But only spec current can
be switched, do not compare with electrical switch
'ON'. Also on a dual supply 0V is more
dominant or positive compared with -12V, even though
it appears -12V is a big number. The direction of
current is what decides, all measurements are
Eagle design schematic sch of above circuit. del00022.zip.