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External Battery Charger Control

External Battery Charger Control

This External Battery Charger Control applies line power to the motor of a 1kW motor-generator set via a high current relay contact. Upper and lower voltage limit (UL & LL) potentiometers provide full control of the On /Off voltage thresholds thus preventing both battery overcharging and over-discharging. Note that this can also control a static battery charger instead of the M-G set, or even an actively generating solar panel.

Schematic of the battery charger control circuit

External Battery Charger Control

System diagram

Battery Charger System

Bench setup

bench setup

On the bench, I simulated a motor-generator set with a simple current limited power supply that is switched on at the lower limit and off at the upper limit. Inverter load is simulated with a power resistor.

Voltage reference

The LM317LZ may be applied as a very inexpensive 1.25V reference. R5 provides a fixed load to bring its current up to 5mA, the specified minimum operating current. The lower limit (LL) and upper limit (UL) pots are adjusted for the required threshold voltages.

Voltage comparators

U1C & U1D compare the feedback voltage from the battery voltage attenuator (R6 & R7) with the voltages from the two potentiometers. C2 provides a 0.47sec time constant (delay) in order to prevent noise disturbances—the delay is a non-issue in real life as the battery voltage changes very slowly.

R-S Flip Flop

U1A & U1B are wired as an R-S flip flop. One unique feature of the LM339 comparator is the use of open collector outputs. The open collector feature enables the use of the wired /OR function. When either of the comparator outputs goes low, it either sets or resets the R-S flip flop. It is about as simple as possible.

Relay Driver

Q1 is the relay driver—it gets its gate drive signal directly from U1-C. To protect the MOSFET gate from excessive voltage, zener diode D2 clamps the voltage to 10V.

Relay

The Omron G8P series relay provides both high current (30A) and low cost ($4.57). The relay in my bench setup is an open version of the same relay. Why a relay? While this can be done with a solid state switch, relays have numerous advantages as follows: low cost, inherent isolation, high power, low losses and robustness.

Setup

It is easiest to set up the control using a variable voltage power supply. Simply set the UL pot to max and the LL pot to min. Adjust the power supply voltage to the desired lower threshold voltage, then rotate the LL pot slowly CW until the LED comes on. Then adjust the power supply voltage to the desired UL threshold and rotate the UL pot CCW until the LED extinguishes. Done!

Photos of the external battery charger control

35 Comments

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  • Allan

    Hi Jim. There’s a small error in your description of the circuit. Transistor Q1[2N7000] actually gets its gate drive from U1-A and not from U1-C. Given that it’s always easier to read a power supply schematic from left to right, especially where component alphanumerics are concerned, I think it would have been better if you’d labelled U1-D as U1-A, U1-C as U1-B, U1-B as U1-C and U1-A as U1-D. Just the same as with other components, like the resistors. Resistor R14 could have been labelled as R1, and R13 as R3 given that R2 down below sits between R14[R1] and R13[R3] etc. when reading [searching actually] from left to right. R20 also appears before R18 and R19 scanning from left to right. You’ll notice that U4’s -A, -B and -C all read from left to right alphabetically making it so much easier for novices to find a component when referred to in text etc.

    Thanks for taking the time to show what you’ve done and to keep the interest in electronics alive. Looking at your gear on the bench I’d hazard a guess that you’re somewhere in the good ol’ US of A.

  • prasad

    I wired the dump load (lamp) side with thicker and separate wires, but the MOSFET heating problem is resolved only to some extent. At times the MOSFET heat sink still heats up.
    Also one observation I have is as below – when I try to measure the voltages at various pins of 339 chip by touching the digital multimeter probes, the circuit triggers and changes the ON-OFF state of the dump load lamp. Is this expected or is there some short-circuiting in the there or is the 339 chip likely to be faulty? I am anyways decided to make one more circuit and try out.

    Thanks.

    • Allan

      Hi Jim. It’s always a good move to use a ‘latching relay’ if one wishes to keep excess power usage [from solar panels etc.]to an absolute minimum.

    • Jim Keith

      Yes, it is oscillating –touching any part of an oscillating circuit affects the oscillation. Since this type of high power circuit can be a real challenge, I recommend that you drive the load with a relay as it was originally intended.

  • prasad

    Hi Jim, Thanks for the quick and detailed response. I will try to use separate wires for the load and signal lines upto the controller circuit and update back on this forum, most likely by coming weekend.

  • kumar

    hai prasad which way u r mosfet incoprated pre set all-ment very simple there is no critical

  • Prasad

    Thanks for sharing a useful circuit.
    I built it for my setup with a 12V, 88Amp-Hr lead acid battery charging with 120 W Solar panels.
    I have set the Q1 to drive a MOSFET STP55NF06 which turns on a 12V/100W automtotive lamp. The circuit seems to works fine in the sense that it regulates the battery voltage between the set range, but seems to dribble/drag when between the two setpoints. The mosfet does not switch on fully, but keeps getting heated. I have installed a good heatsink so it is safe, but still want to try and get the mosfet switch on/off sharply. Do you have any suggestion?
    Also how narrow can the difference in the hi-low be? can it be 0.25V?

    With many thanks,
    Prasad

    • Jim Keith

      The circuit is oscillating so that the MOSFET is always in transition –this causes substantially increased power dissipation. The resistance of the wiring and the source resistance of the battery are all contributing to the problem. My suggestion is that you connect the resistance load and MOSFET power terminals directly to the battery as in a Kelvin connection. I think that the MOSFET gate threshold voltage is high enough to avoid additional ground loop interaction, provided that the lead to the control is as short as possible. Also, get rid of D1 by shorting it out –reverse polarity protection could be adding to the problem.

      Read the paragraph in this discussion about oscillation:
      http://www.electroschematics.com/9462/battery-discharge-cut-control/

  • Suresh

    Hi Jim,

    Thanks. I will try that.

    Regards

  • Suresh

    Hi Jim,

    Until a solution to the Auto start/stop is found, could you recommend a circuit that would give a buzzer sound when the battery voltage drops (LL) to 11 volts and UL reaches to 14.5 volts? This will at least help me for the time being, manually start and stop the AC motor as and when required, upon getting the buzzer sound.

    If a digital display of the current voltage and ampere of the battery can be included it will be grand.

    Thanks again for your cooperation.

    Regards

    • Jim Keith

      Simply rewire it so the motor starts upon closure of a manual switch. Then slave a buzzer off the (now unused) relay contact.

  • Jim Keith

    If anyone intends to build this, please comment because I have an adjustable current limit circuit for the alternator that I can publish– this will solve the motor overload issue.

  • Jim Keith

    The alternator is configured with an electromagnetic field winding and a three-phase rotor connected via slip rings. The three-phase is rectified and provides a good output current waveform to the battery. The output current and voltage are controlled via the same means– adjusting the field current.

    If the field current is set too high, the alternator will generate excessive voltage & current to the battery and will likely stall the motor. The crudest means of adjusting the field current is via a rheostat. For experimentation, it may be operated open loop while you are watching and controlling the output current. By limiting the current, you will be limiting the shaft torque and thus make it easier to spin.

    One common way of connecting the field is with one end permanently connected to the positive output terminal– since this sources the field current, all the regulator must do is to ground the other end of the field winding via adjustable means– generally PWM for efficiency. You must determine how it is actually wired– an ohmmeter may be all that is necessary.

    For proper operation, alternators absolutely must have a regulator. Note that the 12V regulator absolutely will not function properly in your 48V system.

    • Suresh

      Hi Jim,

      ### Make sure you check out my new current limited alternator charge control.

      Thanks Jim, I have already gone thru’ it. I will test that also. Further, just few min. ago I had conducted some other tests, that failed earlier but now they also sailed thru’ smoothly.

      Without your exceptional support I don’t think, being a novice I would have reached this far.

      I will keep you updated.

      Regards
      Suresh

    • Jim Keith

      Make sure you check out my new current limited alternator charge control. The circuit is essentially the same, but the alternator field control is modulated in order to limit the torque load.

    • Suresh

      Hi Jim,

      Wish to share with you a new development in my project. The problem of my AC motor coming to halt, connected via a V-Belt with a Car Alternator to charge the battery when routed thru’ the Charge Controller, HAS BEEN SOLVED.

      I used your original 12 Volt External Charger Control and also another one made by a local friend of mine for testing. Once clearly understood the problem was caused by the alternator not either by the relays or by the AC motor, I set out to identify the particular action that makes the alternator to stop.

      Day before yesterday finally I managed to do that and then hooked first your Charger Control, it worked well and followed by that the one constructed by my friend. Both worked very well without causing the alternator to stop or making the Ac motor come to halt. I tested the circuits with 12 Volt 30 Amps and 12 Volts 65 Amps Alternators run by 1.5HP and 0.5 HP Ac motors and 100 AH and 55 AH batteries.

      As soon as I take a video I will send you the link.

      Regards

      Suresh

    • Jim Keith

      There is also a possibility that the shaft speed may be too low to generate 48 to 58V (full charge). I have heard of modifications generating 120VDC, but this requires high rpm– note that the std engine shiv ratio increases the rpm by about a factor of 2 or 3, so at 5000rpm engine speed, it must be running over 10,000rpm. Hard to believe that they do not fly apart…

      So your direct drive system (if indeed it is that) may be too low a speed.

    • Jim Keith

      Correction: The alternator has a rotating field rather than a 3phase rotor.
      Check out this article that is very good on the basics, but it does not mention the older 3phase half-wave rectifier configuration:
      http://www.autoshop101.com/trainmodules/alternator/alt135.html

      also an interesting thread:
      http://forums.anandtech.com/showthread.php?t=155626

      also an interesting article including regulator modification for grounded field config:
      http://www.poiesisresearch.com/assembly.php

      If this doesn’t meet your needs, I will design one for you to build and test.

    • Suresh

      Hi Jim,

      Thanks for the elaborate reply.

      I have already ordered for another Alternator and I will update you on the outcome.

      ## Since the only interface between the control and the motor is a relay contact, it is very unlikely that the control is the problem.

      One of my friends who is an electrical engineer also emphasized this point.

      ## Note that the 12V regulator absolutely will not function properly in your 48V system.

      For this testing I have changed the set up to inverter below 1Kv with 12 volt 185AH and 55AH battery, used any one battery only.

      Thanks for your patience and cooperation.

      Regards
      Suresh

  • Jim Keith

    My guess is that you need a current regulator for your alternator. Since the only interface between the control and the motor is a relay contact, it is very unlikely that the control is the problem. The fact that it ran once without the control is inconclusive at this point.

    A good current regulator turns on (or increases to the desired current setting) only after the motor reaches base speed– this allows the motor to accelerate to base speed where it may run continuously at rated torque. The 0.5HP motor probably has more starting torque than the 1.5HP motor– this may be the reason why it spins the shaft– starting torque is a wild card here. When overloaded, the motor thermal switch will automatically disconnect the motor– that is probably why it stopped. Also, the 1.5HP motor may required a phase shift capacitor in order to develop significant torque– such motors have low starting torque.

    I know how to do a current regulator, but am unsure how the field winding is internally connected and also if it already contains an integral regulator for the original 12V application. Also, I have no apparatus for testing such. You may have to disassemble the alternator to see what is inside.

    I toyed with the idea of putting a current regulator block on the application diagram, but chose to simplify and not show it.

    Hope that this is helpful.

    • Suresh

      Hi Jim,

      Thanks for your prompt replay, as always.

      Although I am not that technically qualified, I managed to understand the concept in your message.

      Nevertheless, would you suggest any other alternative method, which is less complex, wherein I will be able to achieve my objective of controlling the start/stop of the AC motor at the respective voltages? I have no issue in replacing 1.5HP with 0.5HP motor. 0.5HP will as well suit my purpose.

      Regards
      Suresh

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