Two of my hobbies are hiking and camping. Usually I take my QRP gear with me, including a low capacity 12V gel cell battery, which must be properly maintained or its life is short. Some time ago, I bought two gel cell batteries at the same time. I used one of them frequently, and put the other one aside as a backup. When the time came that I wanted to use the backup battery, I found it was no longer useful as the battery voltage had dropped to a few volts and couldn't be charged. A brand new battery went dead simply from disuse.
There is a simple and effective way to maintain the gel cell battery properly, and that is by charging the battery under a constant voltage. If a fresh battery is not being charged from time to time, it will eventually die due to the internal leakage. I use the constant voltage charger that is shown in Fig.1 to maintain my gel cell batteries.
The operating principle of this charger is quite simple. The heart of the charger is a 3-terminal LM317 adjustable regulator. The output voltage is set at 13.5V for a gel cell, but other batteries may require a slightly different voltage.
Diodes D1, D2, D3 and D4 form a full-wave bridge rectifier. The rectified current is smoothed by capacitor C1 and applied to the input of LM317. The output voltage of the LM317 is set by the resistor R1 and potentiometer Pot1. There is an internal voltage 1.25V between the output and adjust terminals of the regulator. As shown in the circuit, R1 is placed in between these terminals. Therefore, the current flow through R1 is 1.25 volts/270 ohms = 0.00463 amp or 4.63 mA. This current also flows through Pot1 and establishes a voltage drop across it. What voltage drop is needed? As mentioned above, the output voltage of the charger is set at 13.5 V, and we get this output voltage after diode D5. Since D5 has a voltage drop of 0.6 V in the forward direction, the output voltage of the regulator must be 13.5V + 0.6V = 14.1V. Hence the voltage drop across Pot1 should be 14.1V – 1.25V = 12.85V. Because there is 4.63mA on Pot1, the resistance of the pot should be 12.85V/4.63mA = 2775 ohms. Since there is no standard value resistor of 2775 ohms, instead a 5K potentiometer is used.
Transistor Q1, resistors R2 and R3 form a current limiter. To complete the circuit, the charging current must go through R2 and R3. When this happen, a voltage is developed between the emitter and base of Q1. If this voltage is higher than 0.6V, Q1 turns on and it's collector draws down the voltage across Pot1. Hence the output voltage of the charger is reduced and so is the charging current. What should the charging current be to charge a low capacity 12V gel battery? As the rule of thumb, the charging current is one twentieth of the capacity of that battery. For example, if you have a 12V, 7.3A gel cell battery, 1/20 of 7.3A is 365mA. When 365mA goes through R2 and R3, it establishes 0.73V to the base of Q1. This voltage turns on Q1, therefore the charging current is always limited to 365mA.
Measurement of the voltage between the test points T1 and T2 will tell you the charging current. If the reading is 0.27 V, the charging current is 270 mA; if 76 mV, the current is 76 mA, and so on; 1mV corresponds 1mA.
In the circuit, D5 serves two purposes: first, it prevents damage to the charger and the battery if the output polarity is incorrect; second, it prevents the battery discharging itself through the charger while there is low or no power to the charger.
Under normal operation, there is a 3V voltage drop between the input and output terminals of the LM317, as we have calculated, the output voltage of the LM317 is 14.1V, therefore, the input voltage to the LM317 should be 14.1V + 3V = 17.1V or higher.
If you use DC only to power the charger, you can omit D1, D2, D3, D4 and C1 to simplify the circuit. You can simplify the circuit even more by removing Q1, R2, and R3 if you don't need the current limiter. Fig.2 shows the much simplified circuit.
When a solar panel is used to power the charger, apply the output of the panel directly to the input of the LM317 rather than through the bridge rectifier, because the bridge rectifier will cause a 1.2V drop. Even a loss of 1.2V represents a significant power loss under a cloudy sky.
Since the bridge rectifier takes away 1.2V from the power source, then why use it? Well, the reason is that with it, either an AC or DC power source can be used. When only DC power is used, it helps to prevent incorrect polarity hook up. With the rectifier being used, the input voltage of the charger should be 17.1V + 1.2V = 18.3V or higher. 18V is acceptable.
After the charger is built, a simple adjustment is needed. Adjust Pot1 to the mid-range before power is applied to the charger, then use a voltmeter to monitor the output voltage. Adjust Pot1 until the voltmeter reads 13.5V. This is it, all done. Since this charger provides a low charging current, a heat-sink is not needed for the regulator LM317, but no harm in using one. Fig.3 and Fig.4 show the construction and physical size of this charger.
Because this is a constant voltage charger, there is no over-voltage to the battery to harm it. Also, because the charging current is limited to 365 mA, not much heat will be generated inside the battery. The charger can be connected to the battery for a long period of time and will keep the battery fully charged all the time even there is internal leakage of the battery itself. This charger may be suitable if you want to have emergency power handy to operate your QRP rigs. All the parts used to build this charger are from my junk box. Happy home brewing!
~ Hiu VE7YXG
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