Automatically re-connecting low-voltage cut-outs for 12 and 24 volt battery systems

In a previous post I described a simple circuit that provided a low-voltage cut-out that could be used in a battery-operated system - see the article "A latching low-voltage disconnect for 12 volt lead-acid and lithium batteries".  That circuit - intended mostly for lithium-based batteries - required manual intervention to "reset" the device, intended for those situations where you wanted to provide manual intervention to prevent causing harm to the battery.

Figure 1: The completed 24 volt low-voltage cut-out.  R5, the cut-off voltage adjustment, is the blue 10-turn potentiometer near the middle and R8 is the black, single-turn potentiometer near the top, to the left of Q1 and its heat sink.  U1 is the lower transistor-looking device near the left side of the board. This device was installed in an outdoor enclosure that houses a relay point to bridge a wireless Internet connection over a hill that has a 24 volt back-up battery system.  In the event of an extended power outage, this device prevents damage to the batteries, particularly in the winter where this device will prevent the batteries from being damaged by freezing. Click on the image for a larger version.

This circuit is different from that described in the link above, intended mostly for lead-acid based systems where an automatic reconnect of the battery is required upon application of charge current.  An example of where this would be useful is a system in which the battery is charged by a main-powered DC supply that is used to keep the battery charged where the battery will power the load in the event of a power failure.

The problem arises when there is an extended power failure and the back-up battery has been depleted.

Here are several important points when taking care of lead-acid batteries:

• For best longevity, the shallower the discharge, the better.  A classic example of this is a solar-powered system where there is a daily discharge.  If the battery were sized such that it was discharged to 30% every day, it would last only a year or so at best, but by increasing the battery size such that it was only discharged to 75% worst-case, the cycle would be much shallower and the battery would last much longer.

• Lead-acid batteries can also handle being run (almost) completely down - as long as you don't keep them in that state for very long(a few days at most - as little time as possible) and don't do it very often.  In other words, if you run an otherwise healthy lead acid battery completelydead and immediately recharge it, little actual damage is likely to have been done other than taking a bit of life off it farther down the road.  Most lead-acid batteries can only handle a few hundred of such cycles before they are significantly and permanently degraded.  (The previous statement applies only to "Deep Cycle" lead-acid batteries:  Automotive "starting" batteries do not fall into this category:  They can be permanently damaged by only a dozen or two deep discharges.)

• In cold-weather environments, while the degradation (primarily sulfation) is dramatically slowed, extremely deep discharge also reduces the specific gravity, raising the electrolyte's freezing point, increasing the possibility of the battery being damaged/destroyed at very low temperatures if it does get cold enough to freeze.

• Important:  A general rule of thumb is that above approximately 70F (approx 20C) for every 10F (approx. 5.5c) increase in temperature, the expected lifetime of a lead-acid battery is approximately halved.  Remember this if the battery is to be in a location that might get warm.

For these reasons a low-voltage disconnect circuit is essential in a system where there is the distinct possibility that the battery could be completely discharged - both to prevent the loss of battery capacity due to over-discharge as well as prevent freezing of the electrolyte in areas where that is a concern.

A simple low-voltage cut-off circuit:

While there are many ways to build such a circuit, the circuit below - designed for a 24 volt battery system (values are included for use with a 12 volt battery) - demonstrates a simple, yet flexible device.

Figure 2: A low-voltage battery disconnect, designed for 24 volts.  It has a useful adjustment range of between 22 and 28 volts. For 12 volts:  R2=4.7lk, R4=62k, R7=47k, R11=47k, R12=10k, R13=10k. Not shown is appropriate fusing on the battery side of the circuit to protect against short circuits and/or catastrophic component failures. Click on the image for a larger version.

This circuit, somewhat similar to that described in the other article linked above, works as follows:

The battery voltage is measured using R4, R5 and R6 which form an adjustable divider to take the (nominal) 24 volt battery potential down to the 2.5 volt comparison voltage required by U1, a TL431 "programmable" zener diode.  When the voltage at the wiper of R5 goes above 2.5 volts, U1 will conduct.

When U1 conducts it pulls down the voltage at the base of Q1 via R3 causing it to turn on.  This voltage, via R12, is fed to the gate of Q1 - an N-channel power MOSFET in the negative battery lead. In this way, as long as the voltage is above the threshold set by R5, Q1 is turned on, connecting the load to the battery.  While U1 is turned on and pulling the voltage at the bottom end of R3 close to ground, the voltage at the gate of Q3 is low, keeping it turned off.

If the divided battery voltage drops below the low-voltage cut-out threshold U1 turns off and its cathode is pulled high by R2 with resistor R1 being used to allow Q2's gate to go high and turn it off.  When Q2 is turned off, the gate voltage on Q1 goes away, being pulled to ground by R11.

At this same time, the rise in voltage at U1's cathode also causes Q3 to be turned on and the voltage at the junction between R4 and R5 is pulled slightly down, requiring a higher battery voltage to turn U1 back on than was required for it to be turned off, thus providing a bit of hysteresis.  It is by adjusting R5 that the amount of hysteresis (e.g. the amount by which the turn-on voltage must be higher than the cut-off voltage) may be set.

There are some "extra" components included to make the circuit more robust:

• C1, across the reference pin of U1, reduces this circuit's susceptibility to being triggered by power glitches and stray RF energy.

• Capacitors C2 and C3 are across their respective FET's gates to provide RF energy protection as well as to smooth transients that might cause false triggering or damage.

• ZD1 is placed between Q1's source and gate to protect it against excess voltage that would damage the device - possibly caused by transients.

• Finally, C4 is placed across the drain and source leads of Q1 to provide both resistance to stray RF energy as well as to suppress transients that might appear across it when the power is off.

Also included in the circuit is the "power on" indicator consisting of LED1 and R13 which provides an indication that voltage is present between the "Load +" and "Load -" terminals.  The LED2/R14 combination shows when Q1 is turned off (e.g the load is disconnected) by detecting when there is a voltage drop across it.

Comments:

• Important:  To simplify the design, it is the "Batt -" lead that is switched to disconnect the load.  What this means is that the negative battery lead must not be connected anywhere else or the battery disconnect will not work properly.  It is possible to design the circuit to use a P-channel FET in the positive lead, but high-current N-channel devices are more plentiful and less expensive.  Instead of a power FET, a slight modification will allow a relay to be used, instead - but at much higher quiescent current owing to the required current for the coil.

• LED1 and LED2 will only light appropriate when there is a load connected across the "Load +" and "Load -" terminals.  If there is no connected load, LED2 (the "Disconnected" LED) will light because of the current through LED1.

• Even when the load is disconnected this circuit will draw several milliamps of current due to the quiescent load of U1, its voltage divider and LED2, the "Battery Disconnected" indicator.  Keep this in mind if you are going to use it in an application where there is a reasonable probability that battery charging may be unavailable for long periods of time.

• The current capability of this circuit is limited by the current-handling capability of Q1, the power FET and the amount of heat-sinking attached to it.  When choosing a power FET, not only should the current rating be considered, but also the specified "on" resistance.  For example, if a 50 amp FET is used and it has an ON resistance of 0.01 ohms, at 20 amps it will dissipate at least (20 * 20 * 0.01 = )  4 watts and will drop (20 * 0.01 = ) 0.2 volts.  The heat sink shown in Figure 1 is capable of dissipating that much power under all but the hottest ambient temperature.

Comment about Q1, the main power FET:

The power FET is not specified as practically any N-Channel power FET will work - with a few caveats:

• Pick a device with a current rating of at least 3 times the maximum that you expect to draw.  100-150 amp devices are fairly inexpensive, so don't be afraid to pick a far "heftier" device that you might otherwise expect.

• Pick a device with a voltage rating of at least 3 times the battery voltage, which would imply at least a 125 volt device for 24 volts and 75 volt device for 12 volts.

• With a properly functioning circuit, a heat sink is likely not required, but a small heat sink doesn't hurt - particularly if the circuit will be exposed to high temperatures.

Adjustment:

Important:  Verify that the circuit works and is properly adjusted before placing it into service!

The procedure is as follows:

• Set R8for maximum resistance. 

• Connect "Batt +" and "Batt -" to an adjustable DC power supply that is capable of providing between 20 and 30 volts.

• Connect a known-accurate voltmeter to the DC power supply to measure the "Battery" voltage.

• Connect a load to the output terminals ("Load +" and "Load -").  A small lamp of the correct voltage or resistor (1k at 1 watt or more) will do. 

• Increase the voltage to 30 volts:  The load will turn on. (15 volts for the 12 volt version.) 

• Decrease the voltage slowly:  The load will turn off below a certain voltage.

• Increase voltage again to turn the load back on and then adjust R5 so that the load turns off at the desired voltage  The lowest cut-off voltage that is suggested is 1.92 volts/cell for a state-of-charge of 20-25% at "room temperature", which translates to:
• For a 12 volt lead acid battery, 11.5 volts
• For a 24 volt lead acid battery, 23.0 volts. 

• This concludes the adjustment of the cut-off voltage.  Adjusting R8 should not affect the cut-off voltage.

•  Set the power supply voltage below the cut-off voltage and slowly increase it until the load turns back on, noting the voltage at which that happens.

• Adjust R8 so that the cut-in voltage is adjusted as desired.  The cut-in voltage is much less critical, but should be well above the cut-out voltage.
• A typical cut-in voltage for a 12 volt lead-acid battery is 13 volts, or 2.17 volts/cell, but it may be set lower - to 12.5 volts (2.083 volts/cell) if quicker power restoration is required in a system that has limited charge capacity. 
• A typical cut-in voltage for a 24 volt lead-acid battery is 26 volts, or 2.17 volts/cell, but it may be set lower - to 25 volts (2.083 volts/cell) if quicker power restoration is required in a system that has limited charge capacity. 
• Note:  A higher cut-in voltage may delay restoration of the output load, but it will allow the battery to charge more before the load is restored.  An example where this may be important is in a solar-powered system where a low output from the PV array may be sufficient to slowly charge the battery, but may not be able to sustain the load:  This can happen in the morning, when the panels are covered with snow, and/or on very dark and cloudy days.

•  Re-check the cut-off and cut-in voltages:  When adjustment is needed, remember to set the cut-off voltage first and then the cut-in voltage.

Comment:

This device could be used for similar-voltage Lithium-based batteries with appropriate adjustment of the cut-in and cut-off voltages, but please note that any Lithium-based battery system should already have a "protection" circuit that protects against over and under voltage.  Particularly with LiFeO4 batteries, it should also have an equalization circuit.

This page stolen from ka7oei.blogspot.com

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