Gel Cell Charging

Discussion in 'General Electronics Chat' started by Metalfan1185, Jul 17, 2009.

  1. Metalfan1185

    Thread Starter Senior Member

    Sep 12, 2008
    ok, I have some Lead Gel cell batteries that are used in solar power systems. I have 4 at 12V and 7.0 Ahours a piece.

    the part number is


    I am looking for a suitable circuit to charge them. It would be nice to charge them in pairs (24V) I know that EXIT signs use a charging circuit for a Gel cell battery but the amp hour rating is different since they use a smaller battery.

    does anyone have a circuit that will allow me to charge these? via solar or by the grid?
  2. lightingman

    Senior Member

    Apr 19, 2007

    Jelly acid / Gell batteries are generaly charged with a constant voltage. This is usualy at around 13.8 volts for a 12 volt battery. There should be plenty of curcuits on the web.
    And yes, you can charge these in series at 27.6 volts for two in series (24 volts).

    The constant voltage supply should be capable of a few amps, as the batteries on recharge will pull a large current at first, falling of to around 50 to 100mA charged.

    Hope this helps.

  3. k7elp60

    AAC Fanatic!

    Nov 4, 2008
    Here is one that I built using a LM317T.
    LM317T 12V Lead-acid charger​

    The AC/DC adapter in use is capable of 600mA @ 18V. (I used an Magnetek WDU18-600 adapter) Using parts on hand I chose to limit the charge current to about 450 mA. True the LM317T has built in current limiting, but it exceeds the rating of the AC/DC adapter. This adapter is a Class II transformer and has internal circuit protection, and using this internal protection without external current limiting could cause premature failure of it. For this reason I chose to use current limit in the LM317T circuit. The minimum input voltage for 450mA charging is very close to 17V.
    LM317T circuit. R1 is selected to limit the current to a maximum amount and is determined by 0.7/ILimit. In the case of the shown circuit the current limit = 0.7V/1.5 ohms or 0.466A or 466 mA. To change the current limit value, just calculate the value of R1=0.7V/ILimit.
    The charge voltage can be adjusted with no load on the charger. With a calibrated DVM connected + lead to point A and - lead connected to point B adjust the 2K potentiometer so the DVM reads 14.0 VDC. This sets the charger to the float or standby method of charging, or 13.7V.
    The circuit boards are Radio Shack 276-159B, they come as one unit and can be snapped apart. Looking at the top of the board. The two larger red and black wires go the battery to be charged. The smaller red and black wires to the 2.1mm power jack. The various colored wire jumpers are the same type used on solderless breadboards. The color indicates their length in 0.1's of inches. For example yellow is 0.4" long. The long black one was made out of #22 solid black wire. I used a thermopad when fastening the LM317 to the heatsink, but regular heatsink grease could be used.

    Charge Indicator
    The charge indicator monitors the charge current and when this charge current decreases to the set level the LED is turned on. The colored wires are as follows: Red connected to point A, Yellow connected to point B, and Black connected to point C. All connections are to the charger schematic
    IC1 is a voltage comparator that is using the reference voltage from the TL431AIZ precision regulator. The reference voltage is 2.5V @ 1%. This reference voltage is applied to the non inverting input (pin5). The sample voltage from the current limit resistor (R1) is amplified by U2, and is applied to the inverting input (pin 6). When this input voltage decreases to and is = too or less than the reference voltage, the comparator output goes high and turns on the LED thru the current limiting resistor connected to pin 7. This voltage amplifier U2 has a voltage gain = 1+20K/(R2 + R3).
    The circuit as shown, when adjusted for a 7AH battery, the LED turns on when the charge current drops to
    70mA. (7AH/100). With R1 of 1.5 ohms this voltage is 0.126V. From ohms law E=IR. So E=.070 x 1.5 =0.105V. The gain of U2 then has to be Vref/VR1 for the selected battery. Gain in this case=2.5/.105 =
    23.81. To make this adjustment without a battery connected for a 7AH battery, connect a 200 ohm 2W resistor on the output terminals and adjust the 1K potentiometer on indicator board until the LED just comes on. For other size batteries calculate the charge current and value of resistor based on the charge voltage of 13.7 volts.
    This circuit is also built on a Radio Shack 276-159B proto board. It also uses some of the preformed breadboard jumpers, the grey and white have been bent to connect some traces. The longer wires on the board are connected to the charger board.
    The minimum input voltage for proper operation is 17.5 volts.
  4. baysidebecca

    New Member

    Jul 14, 2009
    Here's a good article about gel cells over at;

    Also, National Semi has a design for a charger on page 10 of it's LM350 datasheet;

    I found this charger worked better without the 500 ohm resistor across the input and output of the LM350. I also changed R1 to 2.7k in series with a 1k ohm pot, and R4 to 13k in series with a 5k pot to allow precise trimming of the charge and float voltage states.
  5. Bernard


    Aug 7, 2008
    Talk is cheap, so here is some more:

    SgtWookie suggests C/10,which I would follow. If the batteries were discharged in series so that they were in same state of charge, then " I " believe you could charge them in series; re-adjust some values and have at it. 12V batteries are already 6 cells in series.
    Last edited: Jul 18, 2009