i. Warning

The author makes no warranties that this document is free of errors.

Be careful to use correct charging voltages and currents to the lead
accumulators, avoid short circuit and use high quality components. It is very
important to connect the charger and discharger with correct polarity to the
lead accumulator and power supply otherwise the circuits will be immediately
destroyed. This can be dangerous because lead accumulators produce very
high currents when short circuited.

1. Introduction

Small sealed lead accumulators should be carefully charged with correct
voltage and current. Here are described an automatic charger regulator and
an automatic discharger for 12 or 24 V accumulators. An extra power supply
(19 or 31 V DC) is also required. Both the voltage and current are automatic
regulated to desired level and can be used for continuous charging.

First the battery is charged at maximum current (for example 0.1·C A). When
the adjusted voltage is reached (for example 13.80 V for a 12 V accumulator)
the current is gradually reduced but the voltage is kept constant.

The discharger is used to discharge an accumulator without going below
a voltage that can damage the accumulator. When the voltage drops to the
adjusted voltage (for example 10.50 V for a 12 V accumulator) the current
gradually goes down to zero.

It should be pointed out that lead accumulators should always be charged again
after being used and discharged. They should always be stored fully charged.

2. Data for the charger regulator

Note, it seems as if this charger regulator is very well adapted also to charge
Li-ion batteries Under the assumption that the charging voltage is not higher
than 4.10 V (4.20 V at maximum) per cell. See this very good article:
Charging lithium-ion batteries.

Note, the charger regulator can be used also for 6 or 18 V lead accumulators
if the regulator charge voltage is adjusted to 2.30 V per cell at maximum
(i.e. 6.90 or 20.70 V respectively). As usual the power supply voltage should
be at least 7.0 V above this.

Data for the charger regulator (lead accumulators):

Charge voltage (12 V acc): 13.00 - 14.00 V; recommended 13.50 - 13.80 V

Charge voltage (24 V acc): 26.00 - 28.00 V; recommended 27.00 - 27.60 V

Charge current (12 or 24 V acc): 0.3 - 2.0 A; recommended 0.1·C - 0.2·C A (*)

Approximate charging time at 0.1·C A current: 15 h

Input voltage from power supply (12 V acc): 19.00 V DC

Input voltage from power supply (24 V acc): 31.00 V DC

(*) Here C is the charge capacity of the accumulator in Ah.

3. Data for the discharger

Data for the discharger:

Lowest discharge voltage (12 V acc): 9.00 - 12.00 V; recommended 10.50 V

Lowest discharge voltage (24 V acc): 18.00 - 24.00 V; recommended 21.00 V

Maximum discharge current: 2.00 A; One or two car lamps 12 or 24 V, 10 W recommended

4. Advantages and disadvantages with the charger regulator

Advantages:

• The charger is very careful to the accumulator - regulates both the voltage and current.
• Can be used for continuous charging.
• Works for all (sealed) 12 and 24 V, 5 to 20 Ah lead accumulators.
• Quite cheap and easy to build (if you already have the power supply).

Disadvantages:

• The voltage must be carefully calibrated and monitored the first time the accumulator is charged.
• The maximum charge current is only about 2.0 A making it suitable only for small accumulators.
• An extra power supply (19 or 31 V DC max 2 A stabilized) is required.

5. Advantages and disadvantages with the discharger

Advantages:

• Discharges the accumulator carefully down to a certain pre-set voltage that doesn't cause damage.
• Can be used for long time discharging.
• Works for all (sealed) 12 and 24 V, 5 to 20 Ah lead accumulators.
• One or two ordinary car lamps (12 V, 10 W) can be used as load.
• Very cheap and easy to build.

Disadvantages:

• The maximum discharge current is only about 2.0 A making it suitable only for small accumulators.

6. Calibration of the charger regulator

Read this section through carefully before starting to use the charger regulator.
It is recommended to have one (or better two) digital multimeter(s) at hand during this process.

1. Adjust the current trim potentiometer to the middle position (about 5 revolutions from the end).

2. Adjust the voltage trim potentiometer to the middle position (about 5 revolutions from the end).

3. Adjust the power supply to 19.00 V for 12 V accumulators or 31.00 V for 24 V accumulators respectively.

4. Connect the charger regulator input to the power supply. Note polarity. One LED should light.

5. Connect a load (about 200 mA) to the output of the charger regulator. Three Christmas candle
lamps (34 V, 3 W each) connected in parallel should work fine.

6. Adjust the charger regulator output voltage to 13.00 V for 12 V accumulators or 26.00 V for 24 V
accumulators respectively with the lamps as load.

7. Connect the charger regulator output to the lead accumulator (note polarity) and immediately adjust
the current to not more than 0.1·C A (or 0.2·C A at maximum). Perhaps the switch on the 1 Ohm
resistor must be changed to get desired current. Perhaps this step can not be completed before also
the voltage is adjusted.

8. During the whole first charge process the voltage must be carefully monitored and adjusted to not raise
above to 13.50 V (13.80 V at maximum) for 12 V accumulators and 27.00 V (27.60 V at maximum)
for 24 V accumulators. This step is very important but can be somewhat tedious.

9. Fine adjust also the current to desired level during the first hour of charging.

10. The charge is finished after about 15 hours at 0.1·C A charging current or when the charge
current goes down below 50 mA (somewhat dependent of the charge capacity of the accumulator).

The voltage and current potentiometers must only be adjusted the first charge time for a certain set of
accumulators. When the desired voltage is reached the current starts to go down and the first LED goes
dark and the second is light. The heatsinks of the transistors become quite hot during the charge
process.

7. Calibration of the discharger

Read this section through carefully before starting to use the discharger.
It is recommended to have one (or better two) digital multimeter(s) at hand during this process.

1. Connect one car lamp (12 V, 10 W or max 20 W) for 12 V accumulators or two car lamps in series
(12 V, 10 W or max 20 W) for 24 V accumulators to the output of the discharger.

2. Adjust the power supply to not lower than 10.50 V for 12 V accumulators or not lower than
21.00 V for 24 V accumulators.

3. Connect the power supply to the input of the discharger.

4. Adjust the potentiometer on the discharger so the lamps goes from light to completely dark at
the pre-adjusted voltage.

5. Connect the input of the discharger to the lead accumulator (note polarity). The lamp(s) should light.

6. The current of the lamp(s) (10 W) is about 0.8 A. So a fully charged accumulator is discharged
after about C / 0.8 hours. Here C is the charge capacity in Ah of the accumulator.

When the lamps goes dark the current drops to about 20 mA (after a while). Then the accumulator is
almost not further discharged and can be connected to the discharger for several days. The heatsink
of the transistor on the discharger becomes quite hot just when the lamp(s) goes dark.

8. Building instructions

It is quite straight forward to build the charger regulator and discharger on
standard laboration cards. The power transistors (BDX 34 C and BDX 33 C) must
have quite large heatsinks. To avoid short circuit and wrong polarity power
diodes (1N5404, 3 A) can be connected to the input and output cables. But
then the voltage from the power supply must be raised with about 1.4 V. It
should be noted that the maximum voltage for the LM 324 IC is 32 V (absolute
maximum ratings).

The charger regulator can be used for both 12 V and 24 V accumulators by
having a switch (SW1) for each accumulator type voltage. Both voltages must
be calibrated once by the potentiometers. Don't forget to change the power
supply voltage as well. The current adjustment can be the same under the
assumption that the charge capacity for both 12 V and 24 V accumulators
are approximately the same. It can be wise to have two power resistors
(1 Ohm, 4 W) connected in series in the charger regulator circuit. One of the
resistors can then be short circuited by a switch (SW2) to make it possible
to double or half the charge current.

9. Lead accumulator charger regulator

The lead accumulator charger regulator. Circuit layout:

10. Lead accumulator discharger

The lead accumulator discharger. Circuit layout:

11. Power supply 2 - 32 V DC max 2 A stabilized

A power supply 2 - 32 V DC max 2 A stabilized that can be used with the
charger regulator. Note, this circuit has not been built and tested but should
work fine anyway. The LM 350 voltage regulator must have a quite large
heatsink. Circuit layout:

12. Some images (photos) of the project

1. The lead accumulator charger regulator. Note the switches for voltage
and current selection. The piece of wood stabilizes the heatsinks.

2. The lead accumulator discharger with two car lamps (12 V, 10 W) connected
in series as load.

3. The power supply adjusted to 31.0 V.

4. A lead accumulator 12 V, 7 Ah.

13. References

Some useful data sheets (mostly from ELFA) to download:

• lm324n.pdf (138 kbyte), "LM 324 N, Low power quad operational amplifiers",
  by STMicroelectronics, December 2001
• bdx34c.pdf (41 kbyte), "BDX 34 C, Complementary Silicon power Darlington transistors",
  by STMicroelectronics, October 1999

Some useful Internet links:

• ELFA (A nice electronics component shop in Sweden)
• Biltema (A nice shop in Sweden where cheap small lead accumulators can be bought)
• BatteriGrossisten AB (in Swedish)
• Yuasa (Batteries etc)
• Charging lithium-ion batteries