Electronic Engineering Schematics
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 relative.
Eagle design schematic sch of above circuit. del00022.zip
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20th Mar 2020