12 Volt Gel Cell Charger


12 VOLT GEL CELL
CHARGER

BY

Recently, a fellow amateur was
looking for a gel cell charger which would first charge at a
fixed rate and then later switch to a trickle charge when the
cell was fully charged. After reviewing several catalogs and web
sites, the MAX712 IC was discovered. This IC meets all the
requirements for almost any type of battery charging system. The
circuit in Figure 1 was designed specifically for 12 volt gel
cells. 

When a discharged gel cell is
connected, the charger goes into a fast charge mode at a fixed
rate of 400 ma. After the chip detects the voltage leveling off
or when 4 1/2 hours has elapsed. (which ever happens first.) the
fast charge will stop. After the fast charge has ended, the IC
goes into a trickle charge rate of about 50 ma. This trickle
charge continues until 13.8 volts is reached which will stop all
charging current since the cell is now fully charged. If the cell
voltage should drop for any reason, either a fast charge or
trickle charge (IC will detect what is needed) will start
again. 

When constructing this circuit, be
sure to attach a small heat sink to Q1. Apply a DC (partially
filtered) voltage of at least 15.3 volts. The
voltage must never go below this level even under load
conditions. Many of the DC wall transformers available will work
just fine as long as they meet the minimum voltage requirement.
The input voltage can be as high as 24 volts. If the input
voltage must be in the 30 volt range, increase R1 to about 820
ohms. 

The output voltage must be aligned
prior to use. Disconnect the battery from the circuit and apply
power. Connect a digital volt meter or other accurate volt meter
to pin 2 (positive lead) and to pin 12 (negative lead). Adjust R7
until exactly 13.8 volts is read. 

Because this circuit will not
overcharge a gel cell, the battery can be connected indefinitely.
This circuit is designed primarily as a 12 backup system and can
be connected to the load provided the device to be powered only
draws current during power line interruptions. Use a diode from
the battery to load if needed. This circuit makes an excellent
battery backup to an amateur transceiver.

The MAX712 IC and the .62 ohm
resistor are available from Digi-Key, 701 Brooks Ave, Thief River
Falls, MN 56701 (1-800-344-4539). Order part numbers MAX712CPE-ND
and 0.62W-1-ND respectively. All other parts are available at
Radio Shack. 

DE  

PARTS LIST 

C1
MAX712 Battery
Fast-Charge Controller IC (Cost is $6.27 from Digi-Key)

R1
680 ohm 1/2 watt resistor
(Blue Gray Brown)

R2
150 ohm resistor (Brown
Green Brown)

R3
68K resistor (Blue Gray
Orange)

R4
22K resistor (Red Red
Orange)

R5
.62 ohm 1 watt resistor
(Blue Red Silver) (Cost is 27 cents from Digi-Key)

R6
1.8K resistor (Brown Gray
Red)

R7
10K PCB trimmer resistor
(103)

R8
470 ohm resistor (Yellow
Violet Brown)

C1
1 microfarad tantalum
capacitor (observe polarity)

C2,C4
.01 microfarad capacitor
(103)

C3,C5
10 microfarad
electrolytic capacitor (observe polarity)

Q1
TIP42 PNP transistor or
similar (attach heat sink)

D1
1N4001 Diode (observe
polarity)

LED1,LED2
2 volt standard LED
(observe polarity)

12 Volt Gel Cell Charger
12 Volt Gel Cell Charger #2

12 Volt Gel Cell Charger #3


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