Gel Cell Battery Charger Circuit [Constant Current, Constant Voltage]

In this post we will learn what are gel cell batteries and also learn how to build a specialized charger for charging a gel cell battery, with full calculations.

The proposed circuit works in two modes: it starts charging a discharged gel cell battery with constant current, until the full charge voltage is reached across the battery terminals. As soon as the full charge is reached, the circuit changes over from constant current mode to a constant voltage mode.

This changeover from constant current to constant voltage is important for gel cell batteries which protects the battery from overcharging.

What's a Gel Cell Battery

The gel-cell is quite identical to a contemporary automobile battery. The gel-cell supplies high energy density within a sealed multi-cell, maintenance-free, lead-acid battery.

Gel-cells are generally not created in tiny enclosures such as the common AA, C, and D cells.

Instead, these are designed in bigger enclosures which can be available in sizes from a cigarette-pack to a car battery, and in many cases much larger.

Typical gel-cell batteries are available with voltage specs that may vary from 2 to 24, and in Ah capacities which range from 1.2 to 120 Ah (Amp-Hours). The Ah rating relates to the quantity of current which could be supplied by the battery within a length of time.

For instance, a battery may be specified with 2 volts and 30 Ah. This signifies that the battery must be capable of supplying a current of 1.5 amps consistently during a period of 20 hours.

A correctly handled battery could actually survive for years, however an incorrectly handled battery might remain operative just a few months, or maybe weeks.

The proposed gel-cell battery charger circuit is actually not designed to restore a ruined or mistreated gel-cell battery: it's under your control to take care of your batteries properly.

The quantity of cells in a gel-cell battery can be equivalent to the battery's nominal voltage divided by 2.

A 12 volt battery as a result features six (12/2) cells. Every single cell includes a 2.3 volts output while it is thoroughly charged. Likewise a 6 cell battery nominally specified at 12 volts, in fact provides a fully charged output of 13.8 volts.

It is possible to detect when a gel-cell battery is almost discharged from the simple fact that, when it is without any load or low load situation, it provides an output voltage which is close to its 100 % rated output, but as soon as the battery is subjected to a reasonable to heavy load, voltage falls by approximately 4.6 volts.

The reason behind the two-cell decrease is that a discharged cell basically reverses polarity and starts functioning like a load that "cancels out" the voltage of the good cell.

Therefore you could possibly measure no more than 9.2 volts (13.8 - 4.6 = 9.2) for a 12 volt battery that should be now recharged. And talking about charging, we will now find out how this may be precisely done.

Charging Methods

Gel-cell batteries through various suppliers are created in numerous ways, and these have diverse charging demands.

Many of these batteries could be charged making use of the circuitry explained in this article. That said you need to verify with the manufacturer of your battery in order to be certain. A widespread and trustworthy approach to charging can be as explained below.

Initially, a regulated constant current source which is corresponding to 10% of the battery Ah is given to the battery.

As an example, a 12 volt 7 Ah battery could begin using a charging current of 700 mA. Voltage has to be supervised; as soon as the battery terminal voltage gets to 90% of rated output. At this stage the circuit disconnects the constant-current source and switches to a regulated voltage in order to accomplish the full charging of the battery.

This changeover is actually important in order to protect against over-charging just in case the battery remains attached with the charger for an extended length of time. The battery could go through a float-charge in this manner forever.

You could work with a charging current other than 10% for instance, intended for "fast-charging." But, if you are using a different current, make sure to comply with the manufacturer's suggestions cautiously.

To figure out how much voltage the gel cell charger circuit needs to produce, multiply the number of cells in your battery by 2.3, then add 5 to account for circuit losses. In order to charge our example 12 volt battery, we'll need a 19 volt unregulated IC supply.

Circuit Description

The constant-current charger circuit is straight from the data book of the manufacturer. The heart of the charger, as depicted in Figure 1 below, is an LM317 adjustable regulator. If properly heatsinked, an LM317K can deliver up to 1.5 amps of current and can withstand up to 37 volts.

If your battery demands a greater charging voltage, an LM317HV, which can take up to 57 volts, can be used instead.

You might incorporate an LM338, which can generate five amps of current at a maximum of 32 volts, to boost current.

Let's now calculate the value of R1 based on the desired charging current (Icc) and the LM317's 1.25 volt bias:

R1 = 1.25 / Icc

Icc = 0.7 A for a 7 Ah battery, hence R1 = (1.25/0.7) = 1.78 ohms. The wattage of R1 is calculated as follows: 0.7 A x 1.25 V = 0.875 W. Use a 1 watt metal film resistor to just be on the safer side.

Here, the current is taken care of, but what about voltage? Take a look at Fig. 2 below to see what this means. An LM317 is used as a traditional constant-voltage regulator in this application. R1 should be 240 ohms in most cases, according to the manufacturer.

The output voltage is determined by the value of R2, which may be calculated using a complicated formula.

It's normally easier to wire up the circuit using a 5K or 10K potentiometer, select the output voltage, and then replace the potentiometer with the nearest standard fixed resistor.

We now have a voltage regulator and a current regulator. But how do we assemble them to form a combined circuit? Figure 3 shows the situation.

The Complete Gel Cell Battery Charger

Let's go through the circuit's general functionality before looking at how to compute resistor values. 

Initially, because SCR1 is turned off when power is connected to the circuit, there is no bias current channel to ground, and the LM317 works as a constant current regulator. 

The steering diode D1, limiting resistor R1, and bias resistor R2 connect the LM317 to the battery. This section of the gel cell charger is identical to the circuit depicted in Fig. 1 above.

Once power is disconnected from the circuit, the steering diode prevents the battery from draining through the LED and SCR.

The voltage across the TRIP-POINT potentiometer R5 increases as the battery charges, eventually turning on the SCR.

When the SCR is turned on, it also offers a route to ground for LED1 (through R3).

Because current from the regulator may now pass to ground, the regulator will now operate in the voltage-mode.

When LED1 is turned on, the circuit is in voltage-regulating mode; when LED1 is turned off, the circuit is in current-regulating mode.

Calculating the values of resistors

Let's look at how to compute resistor values now. Consider we're still dealing with a 12 volt, 7 amp-hour battery. R6 is the voltage adjustment potentiometer, so let's start there. To begin, we must determine a multiplication factor F, which may be calculated by using the following formula:

F = (Vcc / 1.25)+ 1

Vcc is the full charge output voltage of the battery; in our example, Vcc = 13.8, hence:

F = (13.8/1.25) + 1 = 12.04.

The value of R6 is then calculated as follows:

R6 = F(R1 + R2)

We already know that R1 is 1.78 ohms and that R2 is 220 ohms, hence R6 = 12 x (1.78 + 220) = 2661 ohms. This figure is close to what we'll need to get the intended end-of-charge voltage.

To make it simple, just replace R6 with a 4k7 or a 10K pot, that's all, no fuss and no confusions.

Because the voltage drop across the SCR is not taken into consideration, the value is an estimate. So we just use a 5K potentiometer for R6 and round up to the next highest value. This will allow you to adapt the circuit so that it can be used with different voltage batteries.

R2 should be around 240 ohms, according to the manufacturer of the IC. The series resistance of R1 and R2 is within 5% of 240 ohms, which is acceptable. If you utilize the high current LM338, you may need to change the value of R2 to accommodate a different charge current, voltage, or both.

The value of TRIP-POINT potentiometer R5, which controls the voltage at which the SCR switches on, now needs to be determined.

For R5 a 5K potentiometer will work nicely if the end-of-charge voltage is less than 20 volts. A 10K potentiometer will work for voltages greater than 20 volts.

R3 is the current-limiting resistor for the LED, and its value is simple to calculate:

R3 = (Vcc - 3) / 20 mA

R3 = (13.8 - 3) / 0.02 = 540 Ω

R3 might be perhaps replaced with a 1K 1/2 watt resistor. This will protect the LED from harm when this gel cell charger circuit is hooked up with batteries with voltages more than 12 V.

The last parameter to compute is R4, which limits the current that may be applied to SCR1's gate. If the TRIP-POINT potentiometer was rotated too much in the direction of the regulator's output, that current might destroy the SCR1. R4's value may be therefore calculated as follows:

R4 = Vcc / 50 mA

Therefore, R4 = 13.8 / 0.05 = 276 Ω in this scenario.

To give further current limiting, round up to the nearest standard figure of 300 ohms, which should work nicely. For the indicated SCR, it must be capable of handling the bias current of the LM317K while it is in voltage mode, as well as the full, no load voltage given by your DC source (19 volts in our case). The SCR mentioned is rated to tolerate 200 volts at 800 mA, thus it should be able to handle any battery you may have.

How to Set Up

In simple way, you can set the gel cell battery charger circuit by directly adjusting the R6 and R5 pots in the following manner:

• Do not connect any battery to the output initially.
• Keep the wiper of R5 pot grounded.
• Connect a 4k7 dummy load at the output of the circuit.
• Connect a volt meter at the output of the circuit, across the 4k7 resistor.
• Switch ON the input supply 19 V to the circuit.
• Adjust the R6 pot until you get an output voltage of 13.8V on the meter.
• Next, adjust the R5 preset until the LED just illuminates at this 13.8V at the output.
• Your circuit is all set now.

What this means?

This means that as long as the battery voltage is below 13.8V, the SCR will be OFF and the circuit will work like a constant current charger. Meaning, during this time voltage is not controlled, and the battery gets the full voltage that is available at the LM317 output. This voltage may be around 19V which is equal to the input supply.

When the battery terminals reach 13.8V, the SCR triggers ON, and the LM317 IC starts working like a constant voltage regulator, which means now the output voltage is restricted 13.8V.

If you think 13.8V limit is too low, you can raise it to 14V, no issues with this.

While actually charging a gel cell battery, remember to connect the battery first to the circuit and only then switch ON the input supply. If you switch ON the input supply before connecting the battery, then the SCR will detect the 13.8V and switch ON, preventing the initial constant current mode, which will cause the battery to charge slowly, and ineffectively.