Charging a Deep Discharged Battery

In the following post we discuss an advanced battery charger circuit that can be used for charging a deep discharged lead acid battery in a stage wise manner.

Figure 1 below demonstrates possibly the perfect charge current characteristic that can be achieved for a standard 12 V lead acid battery in its fully or deeply discharged condition.

At the initial charging phase (A-B), a controlled low charging current is used, until the battery voltage attains around a 10 V level. This low current controlled charging is necessary to make sure that the devices of the circuit does not get too much hot initially.

During the next charging stage (C-D), the charging to the battery is done with a 5 hour charging current rate. This current level can be estimated by dividing the printed Ah spec of the battery by 5. This charging phase can be assumed to be complete when the battery voltage reaches 14.4 V. At this point, the final charging phase is executed (E-F).

In this phase battery is charged using a significantly scaled-down top-up current, which slowly and automatically drops down to zero when the battery terminal voltage touches the 16.5 V mark.

How the Circuit Works

The circuit concept explained here for charging a deeply discharged lead acid battery is designed to deliver a charge cycle which is exactly as per the steps discussed above.

If you have a 12V battery that is fully or deeply discharged at below 10V, that will allow a very small current to pass by means of D3. Due to this the transistor T1 will remain switched off.

Because of this IC1 output will stay low, which will enable the transistors T2 and T3 to get its base current from the low op amp output.

With T2, T3 switched ON the charging current to the battery, can be entirely set as required through the preset P1.

While the battery charges, as soon as its voltage reaches a value of 14 V, causes the diode D3 to get forward biased, which in turn causes transistor T1 to switch ON.

Even at this point the IC1 output continues to be low, which means now the charging current is decided by the presets P1 and P2 both.

In this situation, the setting of P3 causes the voltage at the non-inverting pin of the op amp to increase beyond the zener voltage of D1, then because of the positive feedback arriving from the resistor R4, the IC1 op amp output voltage now switches high, up to a level as determined by the D1 zener voltage, along with the forward voltage drop of diode D2.

With the op amp output turning high, switches OFF the transistor T1, which yet again allows the charge current to be determined by the adjustment of preset P1.

However, contrary to the charging phase A-B, the IC1 output turning high signifies that current would flow by means of the preset P1, and as a result the charging current to the deeply discharged battery would be decreased proportionately.

Because the diode D2 is forward biased, the resistors R2 and R3 will help to progressively minimize the charging current further and further as the battery slowly charges and its voltage gradually increases.

How to Calibrate

In order to adjust the circuit, you must first begin setting up preset P3 so that the IC1 output goes high in a situation when the output of the circuit or battery voltage attains 14.4 V.

Next using the preset P1 the 'boost' charge current must be adjusted to the 20-hour level (which is determined by dividing the battery in Ah by 20), for the battery voltage levels between 14.5 and 15 V.

Lastly, using a battery voltage between 11 and 14 V, start setting up the preset P2 until a nominal 5 hour (battery Ah divided by 5) charging current is fixed.

The starting charging current (phase A-B) can be adjusted depending upon the value of the top-up current, and also according to the specifications of the transistors, wherein the transistors current handling specs should be approximately 50 to 100% greater than the top-up current.

Parts List for the Deep Discharge Battery Charger

PCB Design