These slides introduce the design of lead-acid standby battery charger. Which would be used for 12 V batteries. including the schematic and layout designed using KiCad.

1. A 12V lead acid standby
battery charger

2. Problem
A 12V battery is used as a backup power source
for an alarming system in a farm. A charger is
required to be continuously connected to
ensure that the battery is charged as ready to
be used in cases of emergency.
Lead-acid batteries support two types of
charging, standby-charging and cycle-use
charging. Both of them will be discussed in the
following slides besides the requirements for
them and which one to choose.

3. Battery specifications.
Any battery has the following specifications:
- Nominal voltage: in this case = 12V.
- Ampere-hour: represents the amount of energy a
battery can deliver. It represent the period of
operation for a fully charged battery before a
recharging is needed
𝑡𝑖𝑚𝑒 𝑢𝑛𝑡𝑖𝑙 𝑟𝑒𝑐ℎ𝑎𝑟𝑔𝑒(ℎ𝑜𝑢𝑟𝑠) = (
𝐴ℎ 𝑟𝑎𝑡𝑖𝑛𝑔
𝑙𝑜𝑎𝑑 𝑐𝑢𝑟𝑟𝑒𝑛𝑡
)
in this case a 7Ah means that this battery can
drive a load of 7 amperes for an hour, a load of 3.5
Amperes for two hours, or a load of 14 Amperes for
half an hour.

4. Charging methods:
• Cycle-use charging voltage: the
voltage level a charger should supply in
case of charging a discharged battery.
In this case the charger should be able
to supply at least the initial charging
current mentioned on the battery. In
this case:
 cycle-use charging voltage: 14.5-14.9V.
 Initial current: 2.1A. (remember: at room
temperature).
The battery should be disconnected
once it is fully charged or it would be
damaged.

5. Charging methods:
• Standby charging: in case of standby
charging, a charger is always connected to
the battery to maintain it charged and
ready to be used whenever needed.
• The charger should deliver a voltage
between 13.6-13.8V.
• It should be able to withstand an initial
current of 2.1A. (remember: at room temperature)
The battery can be connected to the
charger for a long period without
problems. Standby charging could be
used in cases of storing a battery for a
long time or when it’s used as emergency
power source.

6. Which charging method to use?
• As mentioned previously, the battery is required to be always fully charged and
ready to be used when the line voltage drops.
• The best method to keep it charged is to use a standby charger with the following
specifications:
• Voltage: 13.513.8 volts.
• Current capacity: ~2.1A.
Let’s get to work 

7. Component:
A 220V/12V
transformer: I found one
that can deliver 2.1 A
with 12-0-12 v outputs.
Rectifier KBU 808.
Capacitors, resistors,
1N4001 diodes and
potentiometers.
LM317T adjustable
regulator.

9. • Two LM317T regulators are used in
parallel to deliver the required 2.1A
initial charging current since the
current rating of a single LM317T is
about 1.5A.
• Diodes are used to isolate the
regulators from each other.
• The output of each regulator should
be at the same voltage, more than
the output voltage by ~0.7V ( diode
forward bias voltage).

10. PCB layout using KiCad 4.0.7

12. PCB
making
and
testing
The circuit
was tested
vs two 6V
lead-acid
batteries in
series and
was
confirmed
to work.

13. The charger is now
backed and ready to
work.