Steve's Gel Cell Charger Page

A Practical Gel Cell Battery Charger

by Steve Pituch, W2MY/5 , (N2MNN - July 4, 1996)

Introduction

Gel cell batteries are ideal for portable QRP operations.  The gel cell batteries that I own are 1.2 and 2.3 AH.  The 1.2 AH battery was intended for use in the Adventure Radio Society Spartan Sprints where contestants are judged by QSOs per pound of radio station.  These are 2 hour contests.  The 2.3 AH battery was intended for Field Day.  As it turns out, the 1.2 AH battery powered my Sierra for my entire 1996 Field Day effort (10 hours), and had plenty of juice left over.  Gel cell batteries are the most economical batteries available, but ready made chargers can be quite expensive.  You have probably noticed that there are chargers offered for sale in the 75 to 100 dollar range.  Although I don’t doubt the usefulness of these devices, it is easy to build an inexpensive automatic charger yourself.  The charger in this article will charge 12 Volt gel cell batteries from 1 to 3 AH capacity, and higher capacities if modifications are made, and will automatically shut itself off when the battery is 100% charged.  In addition, since most of the components of a good power supply are included in this design, a separate voltage regulator has been added to allow you to use this project to power your QRP rig when at home and using the mains.

 Approach

From documentation that I have gathered from a major gel cell manufacturer[1] I learned that the recommended way to charge a 12 Volt gel cell battery is to provide a constant voltage of between 14.4 and 14.7 Volts to it, and limit the maximum possible current to approximately 20% of its AH capacity.  This current value may seem high to those who use a constant “C10” (10% of AH capacity) charge to charge their batteries, but the 20% value only occurs at the beginning of the charge cycle since this is a taper charge.  As the battery voltage rises and approaches the limiting 14.7 V, the DV across the limiting resistor decreases, therefore reducing the current.  When the charging current decreases to a C100 (1% of AH capacity) charge, the battery is fully charged and must either be switched to float charge, or charging must stop.  A simple OP amp circuit acts as a comparator and samples the voltage on either side of the charge limiting resistor.  When the DV is such that it represents a C100 charge current, the OP amp comparator turns the circuit off.

 Design

Table 1 contains my design data, and enough information to customize this charger for other battery AH capacities.  I read on QRP-L that gel cell batteries discharged to a no load voltage less than 11.8 Volts can experience temporary damage (reduced capacity) that takes many charge cycles to undo.  I therefore do not discharge my batteries to less than 11.9 Volts (no load).  Column 1 lists the various battery AH capacities.  The rows containing the 1.2 and 2.4 AH values are the actual design basis for this charger.  The row containing the 3.0 AH value shows that the charger will still adequately charge this size battery.  The row containing the 6.0 AH value shows that the charger will charge this size battery if some of the components are modified.  Column 2 lists the ideal maximum charge current (20% of AH capacity). Column 3 lists the ideal limiting resistor value in Ohms.  This limits the charge current at the lowest battery voltage (11.8 Volts) to the value listed in column 2. This resistor value is rounded off to the nearest available size, and is shown in column 4.  For the 1.2 AH battery scenario four 47W one-quarter Watt resistors were used in parallel, and for the 2.4 AH battery scenario two 10W one Watt resistors were used in parallel.  Since the limiting resistor value has been rounded, the actual maximum charging current is shown in column 5.  The actual charge rate as a percent of AH capacity is shown in column 6.  Remember that the ideal is 20%.  The actual power dissipated by the limiting resistors is shown in column 7, and the power capacities of these resistors is shown in column 8.  The C100 cutoff current (1% of AH capacity) is shown in column 9.  Since the OP amp comparator will be measuring voltage and not current, the voltage drop across the limiting resistor corresponding to the cutoff current is calculated in column 10.  The end of the resistor connected to the voltage regulator U1 will see 14.7 Volts.  The other end of the limiting resistor will see the voltage shown in column 11 when the battery is fully charged, which is simply 14.7 Volts minus the DV across the resistor shown in column 10.  This voltage is too close to the upper voltage sweep of the OP amp so a voltage divider is used to cut it in half, and this value in shown in column 12.  Note that the values shown in column 12 are very similar.  It was decided to use 7.28 Volts for both the 1.2 and 2.4 AH capacity scenarios, as the cutoff current values are still close to the manufacturer’s recommendations, and this eliminated an extra voltage divider and switching arrangement.

 For a 3.0 AH battery, the 5 W limiting resistor will still deliver an 18.6% charge.  This is the practical AH capacity charging limit for this charger without making some additional modifications.  For a 6.0 AH battery the limiting resistor should be changed to one that can dissipate more heat, and I suggest heatsinking the regulator.

 Description of Schematic

The schematic is shown to Figure 1.  In the upper left corner is P1, a power outlet conditioner salvaged from a PC power supply.  Not only does it help reduce noise, but I like it because it provides an easy way to connect AC power to the charger box.  This is not mandatory, and any safe power cabling arrangement can be used.  Next is a fuse F1 followed by the main power switch S1, and then T1, a small 120V to 12.6V transformer.  Rectification is by way of a full wave bridge consisting of D1-D4.  C1 is the filter capacitor, and R24 is its bleeder resistor to discharge the capacitor when the unit is turned off.  U1 is the voltage regulator for the charger. R1-R3 determine the regulated voltage going to the limiting resistors.  They are configured to give high resolution in the 14.4 to 14.9 Volt range (R2 is a 15 turn pot).  Two sets of limiting resistors are shown next ( R4-R9).  They are selected by switch S2.  R13-R15 sample the regulated (14.7 V) voltage before the limiting resistors.  They set the fixed voltage that  the comparator’s (U3) non inverting input sees to 7.28 Volts.  The R11-R12 voltage divider samples the voltage after the limiting resistors.  This voltage is the actual battery voltage during charging, divided by two.  Thus the inverting input of the comparator will see 5.9 Volts (11.8/2) when the battery is depleted and 7.28 Volts when the battery is fully charged (14.56/2).  When U1’s inverting input voltage reaches 7.28 Volts the comparator output goes low, opening the relay and stopping the charge cycle.  The voltage at the inverting input will then rise to 7.35 Volts (14.7/2) since there will be no current flow in the limiting resistor, and the OP amp output will stay low.

When a depleted battery is connected to the charger the relay is normally open.  To activate the charger the output of the OP amp must be made temporarily high to trip the relay closed.  Once the current starts following into the depleted battery, the voltage of the inverting input of U3 will drop to less than 7.28 Volts, keeping the relay closed.  S3 is a momentary switch used to start the charging cycle.  Transistor Q1 is used to switch the relay since the OP amp can not supply the current needed.  Q2 simply drives an LED to indicate that charging is taking place.

 U2 and its associated components provide for a nice QRP 13.8 Volt power supply.  It can also be used as a float charger for a fully charged battery.  If it is to be used as a float charger, limiting resistor R23 must be used in place of the shorting wire to limit the current to the charged battery.  The manufacturer recommends that to float charge a fully charged battery, it be connected to a constant 13.5-13.8 voltage source.  As the battery voltage reaches the regulated voltage the current will drop to the necessary charge replacement current.  A battery can be left indefinitely on float charge with this configuration.  The problem is that a battery that has been correctly charged to 14.56 Volts will drop quickly in voltage as soon as the charging stops.  Putting a 13 Volt battery across a 13.8 Volt regulated supply will result in a brief but very high current if no limiting resistor is used.  R23 can be calculated by dividing the 13.8-13.0 DV = .8  by the maximum allowable current (about 20% of the AH capacity).  For a 1.2 AH cell, a .8/(1.2*.2) = 3.3 W resistor should be used.  I would rather use this circuit as a radio power supply and not a float charger.  I check on my batteries once every one or two months.  It they fall below 12.5 Volts I recharge them using the regular charger circuit.

Construction

Nothing unusual here.  Since you are working with the mains be sure to pay special attention to fusing and grounding.  If you use a larger filter capacitor value, make sure the bleeder resistor discharges the filter capacitor within about 20 seconds.  You don’t want to get zapped by a charged capacitor.  Make sure that R11 and R12 are closely matched.  This is because you want to ensure that pin 6 of U3 sees 7.28 Volts when the voltage at S2 is exactly twice that (14.56 Volts).  I used a Radio Shack prototype circuit board (276-168).  Although I have used these boards for many projects, it got a little cramped on this one.  I had some trouble seeing all the traces ( I just got bifocals).  A 7X eye loupe helped to check for any solder bridges.  I would use a larger board or split the project onto 2 separate boards.  An old RJ-11 A-B switch box provided a nice enclosure.

 Alignment and Testing

Turn on the charger.  Adjust the voltage coming out of U1 to 14.70 volts.  Adjust R14 so pin 5 of U3 sees 7.28 Volts.  Set S2 to the 1.2 AH position.  Hook up a 1 to 3 AH 12 Volt battery that reads at least 12.00 Volts at rest to the normal charging output.  The charging light should be off.  Hook up a voltmeter across the battery.  Press S3 momentarily.  The LED should stay on.  Watch the battery voltage go up.  The rate of voltage increase should gradually slow down.  Toggle S2 to the 2.4 AH position for about 10 seconds.  The voltage should increase more quickly, and then go down when toggled back to 1.2 AH.  Monitor the initial charging every hour.  When the voltage approaches 14.55 Volts start monitoring every 5 minutes.  You want to verify that the charger shuts off correctly.  While the charger is running check pin 5 of U3 occasionally to be sure it is stable at 7.28 Volts.  Once you successfully charge a battery with no problems you can charge by the “set and forget” method.  I let the charger run overnight and it is off when I check it in the morning.  This is a real convenience.

 Conclusion

The most expensive part of a battery power system can be the charger, and this is not necessary.  With a good junk box you should not have to buy any parts for this project, as all the parts are common types.  This was my first project where I was able to scrounge up all of the needed parts from my junk boxes and discarded projects except for the circuit board.  Your best strategy is to put your money into buying new gel cell batteries (not the used ones from standby devices), and then to properly maintain them with a good home made charger that is designed to fully charge them without the danger of damaging them from overcharging.

 Please let me know how you do with this project.  

TABLE 1

Design Data

*              use four 47 W resistors in parallel, 1/4 Watt
**           use two 10 W resistors in parallel, 1 Watt, RS 271-151
***         use three 1W resistors in series, 10 Watt, RS 271-131 (overkill)

FIGURE 1 - Schematic

[1] Power-Sonic Web Page: WWW.Power-Sonic.COM

*     all resistors 1/4 Watt unless otherwise noted.

**   any general purpose OP amp should work as a comparator.  Be sure to get the pinouts correct if using a different OP amp.