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    This circuit will safely charge a GellCell or other lead-acid type battery. The circuit uses a 3 terminal regulator with current limiting circuit to charge the battery. The charger starts by charging the battery at a maximum current limit and a lower voltage and progresses until the charging current is small and the output voltage approaches the set maximum.

    The GellCells that I use are 6 volt 7.5 and 8.2 amp/hr batteries. The manufacturer's recommendation for charging the battery is to charge starting at the maximum current until the battery voltage reaches the maximum voltage and the current has dropped to between C/5O to C/100 (where C = the battery's amp/hr capacity, or 8.2 for the 8.2 amp/hr battery). The manufacturer's recommendation for charging limits are:

Max current = Battery capacity (C) / 5

Max charged voltage = 2.4 volts X # of Cells

Charge until current = C / 50 to C / 100

# of cells (lead-acid) = Rated voltage / 2

    For the 8.2 amp/hr battery these would work out to: 1.64 amps max current, 7.2 volts max volt, and charge until 0.164 to 0.082 amps. Some manufactures recommend a maximum current of C / 10, so if you are not sure use this value, though most manufacturers say C/5 is ok..

    The voltage regulator in the parts list is limited to a maximum output of 1.5 amps. The regulator listed was therefor just a little under powered, but charges only a little slower. If you feel you need more than 1.5 amps use a LM350 which is rated at 3 amps. The transformer listed is good for any 6 volt or less battery that needs less than 3 amps. If you need to charge a higher voltage battery or need more current the transformer you choose should have a voltage rating of the (battery voltage X 1.5) + 3v.

    If you use a battery with a voltage higher than 6 volts you will also need to use a higher resistance pot than the one listed for R3. For batteries up to 12 volts (14.4 v max output) use a pot bigger than 2.6 K ohms.

    Most of the parts are available from Radio Shack, including the case I used. The only parts that I had to get elsewhere were the sense resistor (R1) and an amp-meter to monitor the charging. I bought a separate case for the ammeter and connect it into the circuit externally to the charger case with alligator clips. This is so I can use the meter to measure other circuits, very handy.

    The only critical part value needed is the value of the sense resistor R1. The voltage across this resistor is used by the current limiting circuit to control the output of the regulator. The current limiting circuit turns "ON" when the voltage across this resistor reaches 0.6 volts. To choose the value for this resistor use the formula below:

    The value you get is guarantied to not match any available resistor value. Choose the next higher value available. Do not however go from say a calculated value of 0.456 ohms to 1 ohm, this is too big a difference, try looking for say 0.5 ohms. If you can only find 1 ohm resistor connect 2 of them in parallel, this will give you 0.5 ohms.

    If you want to dial-in the current even closer put a larger value resistor in parallel with R1 to decrease its total value even more. The formula to figure out what values to use to decrease the total value is:

R2 = Rt/(1 - Rt/Rl)

Rt = Total resistance - final desired value of sense resistor.

R1 = Value of main sense resistor

R2 = Value of "tweaking" resistor

    The main sense resistor should have a power dissipation capacity of 2 watts minimum (for up to 1.4 amp charge current). The power dissipation needed for the tweaking resistor will be

Power = 0.6 X0.6 / R2

    When building the circuit keep the parts a reasonable distance apart, as it will dissipate a fair amount of heat when running. Also be sure to use a heat sink and heat conducting grease on the voltage regulator. If you use a different case be sure that there are plenty of holes for air circulation.

    To adjust the circuit after you have finished building you will need a volt meter (digital is best), ammeter, and a load resistor (a few 10 watt 10 ohm resistors). First plug in the power and look/smell for smoke. Assuming no smoke, next with no load on the output adjust the output voltage to the value for your battery (2.4 volts per cell). This is where the digital meter is better, due to the easier and more accurate reading. Now hook the ammeter in series with one of the load resistors across the output. There should be a current reading on the ammeter and the output voltage should drop some. Keep adding resistors in parallel to the first one until the current stops increasing. The value on the meter is now the value of max current for the circuit with that value sense resistor. If it is not correct by a large value there is something wrong with the circuit. Otherwise, if it is close, you can leave it as is or try to tweak it some more. Be sure that it is equal to or less than your desired value, you do not want to fry your battery.

    For my setup with a 6 v 8.2 amp/hr battery, I have my charger setup for 7.2 volts max and 1.25 amps. I can recharge a battery that has gone 2 sorties in about 2.5 to 3.5 hrs. I stop the charging when the meter reads 0.2 amps. With this circuit a battery can be left on overnight, without harm. I also, every now and then, charge the batteries overnight to top them up (generally before each major event and once or twice during the winter).

    The transformer listed is capable of driving two of these circuits at the above current level and you might want to consider building your board this way, especially if you have a number of batteries to charge. This is what I did for my second charger. Now I can charge three at a time between the two chargers. I also installed the two ammeters in the second charger. If you have different capacity batteries set the current for the smaller set. You could also tie in an additional tweaking resistor, via a switch, to up the current for the larger batteries. The resistor in series with the pot is to up the effective resistance, due to the fact that Radio Shack does not carry the 1.5K or 2.6-3K pots. If you can find the correct value at another store eliminate this resistor.

Parts available at Radio Shack

Part Description                                         Radio Shack Part #            Price (1990)
Case                                                                270-253                             $6.79
Circuit Card (IC Spacing) perfboard            276-1395                           $2.99
Fuse Holder Chassis Mount                          270-739                             $0.99
Fuse ¼ Amp                                                    270-1270                           $0.79
LM317 Voltage Regulator                             276-1778                           $1.99
Full Wave Bridge 4 Amp 50V                       276-1146                           $1.39
Heat Sink TO-220                                          276-1363                           $0.79
Mounting Hardware (TO-220)                       276-1373                           $0.99
Heat Sink Grease                                          276-1372                           $1.59
1K pot 15 turn                                                 271-342                             $1.49
(2)Resistor 470 ohm ¼ watt                         271-1317                           $0.39
(2)Resistor 1K ohm ¼ watt                                                                      $0.39
Capacitor 1000 uf 35v                                  272-1032                            $1.59
Transformer 12v 3 Amp                                273-1511                            $8.99
AC Line Cord 6ft                                            278-1255                           $1.99
Strain Relief for power cord                          278-1636                           $0.79

Other Parts

Resistor 240 ohm 1/8 watt
Resistor Sense (see text)
Ammeter 0 - 5 amp (or to suite)
Leads for ammeter (if separate from case)

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