The high quality lead acid battery charger circuits explained in this article are specially designed for charging all types of lead acid batteries very efficiently.
They are designed to automatically cut off the charging supply as soon as the battery is fully charged, thereby ensuring that the battery is never overcharged.
Recharging lead-acid batteries is sometimes believed for being an incredibly convenient issue.
And that is in fact the situation, assuming that no particular demands are being made on the life of the battery.
However, if one desires to make certain that the battery endures for a long time, then certain limitations are placed upon the charge cycle.
How the Circuit Works
Figure 1 shows the ideal charge current feature for a normal 12 V lead-acid battery which is completely released.
Through the first stage (A-B), a limited charging current is employed, until the battery voltage reaches around 10 V.
This limitation on the charging current is essential to make certain that the charger is simply not overloaded (excessive dissipation).
For the next phase (C - D), the battery is charged with the '5-hour charging current*.
The size of this current is determined by dividing the nominal capacity of the battery in ampere-hours (Ah) by 5.At the end of this period the battery needs to be charged to 14.4 V, whereupon the last stage (E-F) begins.
The battery is charged with a much smaller ' top-up' current, which slowly might reduce to zero if the battery voltage were to reach 16.5 V.
The circuit explained in this article (see figure 2) is designed to provide a charge cycle which employs are explained above.
If the battery is completely discharged (voltage < 10 V), so little current flows by means of D3 that T1 is put off.
The output of IC1 will probably be low, in order that the base currents of T2 and T3, and therefore the charging current, are established entirely by the position of P1.
If the battery voltage is between 10 and 14 V, D3 is forward biased and T1 is switched on.
The output of IC1 continue to remain low, so that the charging current is now determined by both P1 and P2.
If the wiper voltage of P3 surpasses the zener voltage of D1, then due to the positive opinions via R4, the output voltage of IC1 will swing up to a value determined by the zener voltage of D1 and the forward voltage drop of D2.
As a result T1 is switched off and the charge current is once more determined by the position of P1.
Contrary to phase A -B, on the other hand, the higher output voltage of 1C1 signifies that current through P1, and hence the charging current, is decreased consequently, Since D2 is forward biased, the result of resistors R2 and R3 will be to gradually slow up the charging current even more, as the battery voltage continues to rise.
To calibrate the circuit, P3 is modified so that the output of IC1 swings high when the output (i.e. battery) voltage is 14.4 V. By means of P1 the 'top-up' charge current is placed to the 20-hour value (capacity of the battery in Ah divided by 20) for voltages between 14.5 and 15 V.
Ultimately, with a battery voltage of between 11 and 14 V, P2 is altered for the nominal (5- hour) charging current.
The initial charging current (phase A-B) is set by the value of the 'top-up' current, and based upon the features of the transistors, will probably be around 30 to 100% greater.
Lead Acid Battery Charger Circuit using Relay Cut-off
Automatic battery chargers aren't especially affordable, although the safety they afford towards overcharging and possible battery destruction is extremely desired.
The circuit displayed in this article is meant to offer an affordable substitute for the commercially available fully automatic chargers.
The idea is to take a easy battery charger and integrate an add-on unit which will instantly keep an eye on the state of the battery and cut off the charge current at the preferred stage, i.e. once the battery is fully charged.
The circuit generally is made up of comparator, which monitors the battery voltage with regard to a fixed reference value.
If the battery voltage surpasses a presettable maximum level, a relay is actuated which interrupts the charge current.
If the battery voltage drops below a lower threshold value, the relay is released switching the charge current back in.
The comparator is formed by a 741 op-amp. The supply voltage of the op-amp is stabilised by R3 and D1, and is therefore not affected by variations in the battery voltage.
The reference voltage, which is fed to the inverting input of the op-amp, is derived from this stabilised supply via R4 and D2.
The reference voltage is compared with a portion of the battery voltage, which is extracted from the voltage divider, R1/P1/R2.
As the battery voltage increases, at a specific point (determined by the setting of P1) the voltage on the noninverting input of the op-amp will ultimately surpass that on the inverting input, with the result that the output of the op-amp will swing high, turning on T1 and T2, pulling in the (normally closed) contact of the relay and interrupting the charge current to the battery.
LED D3 will then illuminate to indicate that the battery is fully charged. In order to avoid the battery being reconnected to the charger at the slightest drop in battery voltage, a portion of the op-amp output voltage is fed back via P2 and R5 to the non-inverting input.
The op-amp thus performs in a fashion similar to a Schmitt trigger, the degree of hysteresis, i.e. the battery voltage at which the op-amp output will go low again, being determined by P2.
The circuit is best calibrated by using a variable stabilised voltage as an 'artificial battery'.
A voltage of 14.5 V is chosen and P1 modified in such a way that the relay just pulls in (opens). The ' battery' voltage is then reduced to 12.4 V and P2 modified until the relay drops out.
Since P1 and P2 will impact each other, the process is the most suitable repeated a couple of times.
One final idea: if the charge current is too large to be turned by the relay, the circuit can still be applied by connecting the relay in the primary of the battery charger transformer.
R1, R2, R6 = 10k
R3, R9 = 100 Ohms 1/2 watt
R4, R7 = 4.7k
R5 = 33k
R8 = 1k
P1 = 10k
P2 = 22k
T1 = BC547
T2 = BC140