“A small study shows that automotive lead-acid batteries are different from deep-cycle or stationary batteries. Car batteries are designed to maximize cranking current capacity and do not respond well to deep discharge or float charge (also known as a stage 3 charge cycle). The plate structure of the starter battery maximizes surface area and has a higher electrolyte specific gravity (SG) than other batteries to provide high start-up current. Like stationary batteries, automotive batteries that are allowed to remain in a deep state of discharge undergo permanent sulfation, in which small lead sulfate crystals produced during discharge transform into stable crystalline forms and deposit on the negative plate.
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A small study shows that automotive lead-acid batteries are different from deep-cycle or stationary batteries. Car batteries are designed to maximize cranking current capacity and do not respond well to deep discharge or float charge (also known as a stage 3 charge cycle). The plate structure of the starter battery maximizes surface area and has a higher electrolyte specific gravity (SG) than other batteries to provide high start-up current. Like stationary batteries, automotive batteries that are allowed to remain in a deep state of discharge undergo permanent sulfation, in which small lead sulfate crystals produced during discharge transform into stable crystalline forms and deposit on the negative plate. Float charging, on the other hand, can easily cause oversaturation in car batteries, which can lead to oxidation of the positive plates and shorten battery life. Therefore, the charge voltage and charge cycle are critical and are different for automotive and deep cycle types; furthermore, the charge voltage should decrease with ambient temperature at a rate of 3mV per degree Celsius above 25ºC.
Figure 1 shows the charging cycles for Phase 1 and Phase 2. Stages 1 and 2 can be accomplished with the circuit of Figure 2, in current limiting the charge current for stage 1 is relatively constant and as the charge current decreases below the current limit for stage 2 constant voltage mode. A good rule of thumb here is that the battery is fully charged when the current is no longer decreasing.
Figure 1 Phase 1 and Phase 2 charging cycles
Figure 2 The raw power supply unit (PSU) operates in constant current mode (CCM) until the load current falls below the current limit threshold. The adjustment sequence is: Adjust the VR2 10k potentiometer to make Vout = 14.1V under no-load condition.
Hard or permanent sulfation is a function of time and state of discharge, so if the vehicle is not in normal use it is advisable to have some means of monitoring the battery voltage and restarting the charging process if the voltage drops to some point below the full charge voltage. In Take the vehicle’s discharge rate into account when setting the set point to initiate Phase 1 charging.
Exact value data for charge rate, current, voltage, and float voltage varies by source. However, the main takeaway from most sources is that to charge the battery optimally without shortening its lifespan, don’t let it overheat, don’t hard sulfate, don’t outgas, don’t oversaturate. This design idea attempts to do this as easily as possible using any equipment other than a soldering iron, screwdriver, and multimeter.
how does this work
Figure 3 shows the complete circuit that provides constant voltage current limit operation to complete charge phases 1 and 2, once the charge current has decreased to a steady value of approximately 200mA, the charge voltage is removed and the battery is discharged to a point below 12.6V. Using a trimmer potentiometer allows some freedom in setting up the charger so that it can meet the charging requirements of most 12V car batteries.
D4 is a completely optional attempt to reduce the charging voltage based on ambient temperature. It worked well in the lab, but it remains to be seen how it will perform in the hot Texas summer! It is generally accepted that if the ambient temperature exceeds 49ºC/120ºF, it should not be charged to prolong the life of the battery.
U1 and Q1 form a constant voltage current limiting power supply, VR2 sets the maximum charging voltage, and VR4 sets the current limit. D4 provides some thermal derating at about 4mV/ºC.
Differential amplifier U2 conditions the signal on current sense Resistor R1 and applies the conditioned signal to the inverting input of U3. U3 is connected as a comparator with its set point at the non-inverting input provided by VR1. As long as the load (charging) current signal from U2 is above the set point, the output of U3 will be low, energizing RL1 and supplying charging current to the battery. The set point should be set to represent 3-5% of the maximum charge current. This can be done with a resistive load or by monitoring the battery charge cycle and seeing at what current the charger flattens out (Figure 1). Depending on the charge current and initial state of charge, this method can take up to about 13 hours, but it’s a better method. Once the charge current falls below the set value, the output of U3 will go high and reverse bias D1, turning off Q2 and thus de-energizing RL1.
Figure 3 Completely modified PSU circuit
The PSU operates in the CCM mode described above until the load current drops below the current limit threshold. When the battery voltage drops below 12.96V, the charge cycle begins, causing RL1 to turn off. When the charge current falls below 200mA, the charge cycle ends, causing RL1 to open.
Adjust the order
Step 1: Adjust VR2 10k pot Set Vout = 14.1V Set constant voltage under no load condition
Step 2: Adjust the VR4a/b 1k potentiometer to set the current limit to the desired value under short circuit conditions
Step 3: Adjust VR1 10k potentiometer to open Relay 1 (RL1), disconnect the battery, when the load current is lower than 3-5% of the charging current (or saturation current)
Step 4: When the battery voltage drops below somewhere between 12.5 and 12.6V, adjust the VR3 10k potentiometer to close Relay 1 (RL1).
U4 monitors the battery voltage and is also connected as a comparator; however, its set point is connected to the inverting input, so when the battery voltage falls below the set point, U4’s output will go low, turning on Q2, energizing RL1 and apply charging current to the battery. When the battery voltage is higher than the set value, the output of U4 will go high and reverse bias D2, thus turning off Q2 and de-energizing RL1. VR3 is used to adjust the battery voltage to the setpoint provided by VR1. Using one set point for current and voltage saves a few resistors!
The outputs of U3 and U4 are diode-OR’ed so that U3, U4, D1, D2, Q2 and RL1 form a control loop with the battery to provide automatic control of the charge cycle. The components in the circuit containing RL1 and Q2 need to be adjusted to accommodate the coil resistance of RL1.
Component values can be changed to suit the situation at hand, but resistor ratios should be maintained where they interact to allow for a similar range of adjustment. A good choice for RL1 is any high current automotive relay, but the component values around Q2 and RL1 will depend on the coil resistance of RL1. The relay used is a 10A, 12V 1000Ω type.
A single pole switch toggles the input of U5 to Display current output or battery voltage on the meter.
Any op amp can be used as long as its output swing is within about 200mV of either rail. The LM358 was used as the comparator for the U3 and U4 locations as they were on hand and the roughness of the application allowed this, but any single supply comparator could be substituted if desired. The maximum current can be increased if Q1 is a Darlington and the value of R1 is decreased. The LT1413 used in the simulation is a replacement for the LM358 used on the board. U2 can be replaced with an integrated current sensor such as the LTC6102.
The control circuit upgrade was initially simulated using LTspice, then built on a solderless prototype board for evaluation and is being added to existing chargers.
It should be noted that different sources provide significantly different values for battery and charging voltage. Since the difference between low and high voltages that cause hard sulfation or corrosion is very small, it is worth checking the battery manufacturer’s data on the specific battery being maintained. Different sources also give a rule of thumb for stopping charging at 0.1ºC or 3-5% of maximum charge current. When the correct charging voltage is applied, the point at which the charging current tapers off and stops falling is the best way to determine when to stop charging. One charge cycle should provide the required measurements.
A small study shows that automotive lead-acid batteries are different from deep-cycle or stationary batteries. Car batteries are designed to maximize cranking current capacity and do not respond well to deep discharge or float charge (also known as a stage 3 charge cycle). The plate structure of the starter battery maximizes surface area and has a higher electrolyte specific gravity (SG) than other batteries to provide high start-up current. Like stationary batteries, automotive batteries that are allowed to remain in a deep state of discharge undergo permanent sulfation, in which small lead sulfate crystals produced during discharge transform into stable crystalline forms and deposit on the negative plate. Float charging, on the other hand, can easily cause oversaturation in car batteries, which can lead to oxidation of the positive plates and shorten battery life. Therefore, the charge voltage and charge cycle are critical and are different for automotive and deep cycle types; furthermore, the charge voltage should decrease with ambient temperature at a rate of 3mV per degree Celsius above 25ºC.
Figure 1 shows the charging cycles for Phase 1 and Phase 2. Stages 1 and 2 can be accomplished with the circuit of Figure 2, in current limiting the charge current for stage 1 is relatively constant and as the charge current decreases below the current limit for stage 2 constant voltage mode. A good rule of thumb here is that the battery is fully charged when the current is no longer decreasing.
Figure 1 Phase 1 and Phase 2 charging cycles
Figure 2 The raw power supply unit (PSU) operates in constant current mode (CCM) until the load current falls below the current limit threshold. The adjustment sequence is: Adjust the VR2 10k potentiometer to make Vout = 14.1V under no-load condition.
Hard or permanent sulfation is a function of time and state of discharge, so if the vehicle is not in normal use it is advisable to have some means of monitoring the battery voltage and restarting the charging process if the voltage drops to some point below the full charge voltage. In Take the vehicle’s discharge rate into account when setting the set point to initiate Phase 1 charging.
Exact value data for charge rate, current, voltage, and float voltage varies by source. However, the main takeaway from most sources is that to charge the battery optimally without shortening its lifespan, don’t let it overheat, don’t hard sulfate, don’t outgas, don’t oversaturate. This design idea attempts to do this as easily as possible using any equipment other than a soldering iron, screwdriver, and multimeter.
how does this work
Figure 3 shows the complete circuit that provides constant voltage current limit operation to complete charge phases 1 and 2, once the charge current has decreased to a steady value of approximately 200mA, the charge voltage is removed and the battery is discharged to a point below 12.6V. Using a trimmer potentiometer allows some freedom in setting up the charger so that it can meet the charging requirements of most 12V car batteries.
D4 is a completely optional attempt to reduce the charging voltage based on ambient temperature. It worked well in the lab, but it remains to be seen how it will perform in the hot Texas summer! It is generally accepted that if the ambient temperature exceeds 49ºC/120ºF, it should not be charged to prolong the life of the battery.
U1 and Q1 form a constant voltage current limiting power supply, VR2 sets the maximum charging voltage, and VR4 sets the current limit. D4 provides some thermal derating at about 4mV/ºC.
Differential amplifier U2 conditions the signal on current sense resistor R1 and applies the conditioned signal to the inverting input of U3. U3 is connected as a comparator with its set point at the non-inverting input provided by VR1. As long as the load (charging) current signal from U2 is above the set point, the output of U3 will be low, energizing RL1 and supplying charging current to the battery. The set point should be set to represent 3-5% of the maximum charge current. This can be done with a resistive load or by monitoring the battery charge cycle and seeing at what current the charger flattens out (Figure 1). Depending on the charge current and initial state of charge, this method can take up to about 13 hours, but it’s a better method. Once the charge current falls below the set value, the output of U3 will go high and reverse bias D1, turning off Q2 and thus de-energizing RL1.
Figure 3 Completely modified PSU circuit
The PSU operates in the CCM mode described above until the load current drops below the current limit threshold. When the battery voltage drops below 12.96V, the charge cycle begins, causing RL1 to turn off. When the charge current falls below 200mA, the charge cycle ends, causing RL1 to open.
Adjust the order
Step 1: Adjust VR2 10k pot Set Vout = 14.1V Set constant voltage under no load condition
Step 2: Adjust the VR4a/b 1k potentiometer to set the current limit to the desired value under short circuit conditions
Step 3: Adjust VR1 10k potentiometer to open relay 1 (RL1), disconnect the battery, when the load current is lower than 3-5% of the charging current (or saturation current)
Step 4: When the battery voltage drops below somewhere between 12.5 and 12.6V, adjust the VR3 10k potentiometer to close Relay 1 (RL1).
U4 monitors the battery voltage and is also connected as a comparator; however, its set point is connected to the inverting input, so when the battery voltage falls below the set point, U4’s output will go low, turning on Q2, energizing RL1 and apply charging current to the battery. When the battery voltage is higher than the set value, the output of U4 will go high and reverse bias D2, thus turning off Q2 and de-energizing RL1. VR3 is used to adjust the battery voltage to the setpoint provided by VR1. Using one set point for current and voltage saves a few resistors!
The outputs of U3 and U4 are diode-OR’ed so that U3, U4, D1, D2, Q2 and RL1 form a control loop with the battery to provide automatic control of the charge cycle. The components in the circuit containing RL1 and Q2 need to be adjusted to accommodate the coil resistance of RL1.
Component values can be changed to suit the situation at hand, but resistor ratios should be maintained where they interact to allow for a similar range of adjustment. A good choice for RL1 is any high current automotive relay, but the component values around Q2 and RL1 will depend on the coil resistance of RL1. The relay used is a 10A, 12V 1000Ω type.
A single pole switch toggles the input of U5 to display current output or battery voltage on the meter.
Any op amp can be used as long as its output swing is within about 200mV of either rail. The LM358 was used as the comparator for the U3 and U4 locations as they were on hand and the roughness of the application allowed this, but any single supply comparator could be substituted if desired. The maximum current can be increased if Q1 is a Darlington and the value of R1 is decreased. The LT1413 used in the simulation is a replacement for the LM358 used on the board. U2 can be replaced with an integrated current sensor such as the LTC6102.
The control circuit upgrade was initially simulated using LTspice, then built on a solderless prototype board for evaluation and is being added to existing chargers.
It should be noted that different sources provide significantly different values for battery and charging voltage. Since the difference between low and high voltages that cause hard sulfation or corrosion is very small, it is worth checking the battery manufacturer’s data on the specific battery being maintained. Different sources also give a rule of thumb for stopping charging at 0.1ºC or 3-5% of maximum charge current. When the correct charging voltage is applied, the point at which the charging current tapers off and stops falling is the best way to determine when to stop charging. One charge cycle should provide the required measurements.
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