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| Figure 1: The as-built and working prototype constructed. Click on the image for a larger version. |
- Overcharge it.
- Overdischarge it.
With Lead-acid batteries:
- Lead-acid batteries - particularly the "flooded cell" types (e.g. those to which you can add water) can handle quite a bit of overcharging as long as the electrolyte level is maintained. "Sealed" batteries (e.g. AGM, or those that many mistakenly called "gel" cells) can handle some overcharging, but only to an extent before their pressure vents release accumulated gasses, reducing the amount of usable electrolyte, which is why they should not be "equalized".
- Lead-acid batteries can also handle being run 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 completely dead 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. In cold-weather environments, while the degradation (primarily sulfation) is 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 freeze.
- If a battery is overcharged, it will start to chemically decompose. Gross overcharging - while tolerated at least briefly by lead-acid batteries - may result in a lithium-ion battery venting and/or exploding, possibly catching fire.
- If a battery is overdischarged it will chemically decompose, often with the contained lithium changing into a more volatile - and not useful - state. Severe over-discharging (e.g. below 2 volts per cell - this voltage varies depending on chemistry) can mean that the battery can never safely be charged again or, in some cases - if the voltage is only allowed to get down to this general area and not lower (again, the voltage varies according to chemistry) some special charging precautions are required (e.g. a specific, very low-rate trickle charging regimen) to "recover" this battery. There are also some specific temperature-related restrictions with lithium-type batteries and these are noted by their respective manufacturers.
The avoidance of overcharging is usually pretty easy: Just use the appropriate charging system - but overdischarge is a bit more difficult, particularly if the battery packs in question don't have a "protection board" with them.
Lead acid batteries (almost) never come with any sort of over-discharge protection - one must usually rely on the ability of the device being powered (e.g. an inverter) to turn itself off at too-low a voltage and hope that the threshold is sensible for the longevity of a 12 volt battery system. For low-to-moderate loads (e.g. 1/10th "C" or so) a pretty safe "dead battery" voltage for a 12 volt lead-acid battery is around 11.7 volts - or somewhat higher for heavier loads. Again, after disconnect, it is not a good idea to keep it in a discharged state for any longer than possible.
Many larger (e.g. >10 amp-hour) lithium-iron phosphate (LiFePO4) do not routinely come with "protection" boards unless it is ordered specially or includes some sort of "Battery Management System": Batteries in this category can include the "Lead Acid" replacements sold for use with motorcycles and off-road vehicles and some of the "raw" LiFePO4 batteries available from many vendors, such as the 20 amp-hour modules made by GBS.
While it is also important to equalize LiFePO4 batteries when charging (refer to this post - Lithium Iron Phosphate (LiFePO4) batteries revisited - Equalization of cells - link) the more immediate danger in routine use is accidental over-discharge.
A simple low-voltage disconnect circuit:
Again, for lithium batteries one may install "protection" boards that prevent accidental over-discharge and, in some cases, provide charge equalization - but such things are much rarer for lead-acid batteries, but such a circuit is quite simple and is applicable to either Lithium or Lead Acid batteries.
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| Figure 2: Schematic diagram of the low-voltage disconnect circuit. Not shown is overcurrent protection (e.g. fusing) that should be present on the output of the battery - see text below. Click on the image for a larger version. |
These days it is rather easy to construct a low-voltage disconnect circuit using readily-available components: The diagram of one such circuit may be found in Figure 1.
How it works:
The RESET button is pressed, applying a positive voltage to the gate of N-channel power MOSFET, Q1, turning it on, which then connects the "BATT -" output terminal to the "LOAD -" terminal.
If the voltage at the "Ref" terminal of U1 is above 2.5 volts, as determined by the voltage divider consisting of R4, R5 and R6, the cathode of U1 is connected to its anode (e.g. "Load -"), pulling the base of Q2 down, making it negative with respect to its emitter, R2 limiting Q2's base current to a safe value and providing enough current for U1 to function, and turning it on. With Q2 turned on, Q1 is "latched" on, even when the RESET button is released.
If the voltage at the "Ref" terminal of U1, representative of the voltage across the LOAD terminals, drops below 2.5 volts, U1 turns off and the base of Q2 gets pulled positive to the emitter voltage by R1, turning it off. With Q2 turned off resistor R8 pulls the gate of Q1 down to its source, also turning it off and disconnecting the load. Because of the "latching" effect, once this has happened the load will never be turned on again until the RESET button is pressed. This happens because with Q1 turned off, U1 is without voltage (e.g. "Load -" rises to the same voltage as "Load +) and can never turn Q2 (and thus, Q1) back on again. Even though pressing and holding the RESET button will connect the load even if the voltage is below the threshold, until the voltage rises above the threshold the circuit will not stay "on" once the RESET button is released.
To accommodate a range of voltages, U1's "Ref" terminal is connected across the output (Load +, Load -) with R4 and R6 to "scale" the range of potentiometer R5 to have a threshold in the 8-16 volt range: Without R4 and R6 the usable range of R5 would be compressed to a very small portion of the overall rotation and make adjustment touchy, but with these resistors setting R5 at mid-rotation yields a threshold of around 11 volts.
Note the presence of capacitors C1 and C2: C1 provides a bit of filtering of the sampled output voltage to prevent brief current transients that might momentarily drag the voltage down below the threshold, "falsely" causing an undervoltage condition from being detected. Similarly, C2 slows the "fall" time of Q1's gate voltage, preventing it from shutting off instantly in response to a brief spike of current - and it also provides some degree of protection of Q1's gate in response to possible voltage transients.
While not tested, the presence of C1 and C2 should provide a modicum of RFI protection: If your environment includes high RF fields - such as powering a 100 watt amateur transceiver - this could be considered in testing and the construction/layout, knowing that such a transceiver can also impose very brief, high-current loads on the battery can causing momentary brown-outs due to I*R drops in the wiring and battery which could also trip this circuit.
Finally, the combination of R7 and LED1 provide an indication of power-on to the user.
Additional circuit notes:
The "high voltage" limitation of this device is primarily that of the gate voltage rating of Q1. Most power FETs are rated for only +/- 20 volts gate-to-source voltage which means that it is suitable for no more than a "12 volt" bus (e.g. 10-16 volts or so): If a higher operating voltage is required it will be necessary to add additional circuitry around the FET's gate to keep its voltage safely below its rating. For an example of such circuitry see this article: A Simple, effective, yet Inefficient Solar Charge Controller - linkand taking note of components D1, R7, R8 and C4 surrounding Q3 in Figure 3 on that page.
If a lowercut-off than 9 volts (or higher than 15) is required it will be necessary to recalculate the values of R4 and R6 (in Figure 2, above) to appropriately scale the adjustment range.
It should also be noted that if voltages below 10 volts are routinely required one should pay close attention to the saturation (e.g. "full on") gate voltage required for the FET that you plan to use: Typical FETs do not achieve their lowest resistance until 8-10 voltage of gate-source voltage is present but there are "logic level" FETs available that will be fully "on" at around 5 volts.
Construction:
None of the components are critical, save the possible exception of R4, R5 and R6 which are selected to scale the adjustment range of R5: While it is the ratios of these components that are important (e.g. one could use 4.7k, 1k and 1k for R4, R5 and R6, respectively) going much higher than the stated values may violate the minimum reference current specifications of U1 resulting in temperature/device variations of the set voltage thresholds.
The TL431 (U1) is a rather ubiquitous chip, found in practically every PC-type power supply made in recent years and is available in single quantities for well under $1.
Q2 may be practically any silicon PNP transistor with a rating of at least 30 volts while Q1 may be any N-channel MOSFET with a voltage rating of at least 30 volts and a current rating of at least 3 times the current that you plan to draw and an "ON" resistance of a fraction of an ohm. For the prototype I used an F15N05 FET - a 15 amp, 50 volt device, more than adequate for the 3 amp load that was to be used, but one could use as "large" a power FET as you wish. For "12 volt" operation make sure that the FET that you choose has at least a 20 volt gate-source voltage rating. Higher-current FETs include the IRFZ44 (50 amp max.) and the PSMN2R7-30PL (100 amp max.) to name but two out of hundreds of possibilities. If even more current is required one can parallel multiples of the same-type FET as needed.
Device layout is not critical aside from the use of appropriately heavy conductors to the source and drain leads of Q1 to carry the current. For most applications a heat sink is not even required for the FET - particularly if one chooses a device with milli-Ohm range "on" resistance. Note that the "Batt+" and "Load+" lead is straight-through and the wire connecting this circuit to that "through" connection may be of light gauge.
Finally, note that this circuit works by disconnecting the "BATT-" and "LOAD-": Your battery's negative terminal must be completely isolated from the load for this circuit to work properly and protect your battery!
(Comment: It is possible to reconfigure this circuit to disconnect in the positive lead, but this requires the use of a P-channel power FET.)
Adjustment and Operation:
For proper set-up an adjustable power supply is required and the procedure is as follows:
- Set the power supply to a volt or two higher than the desired drop-out voltage.
- Adjust R5, the potentiometer so that the wiper is closest to R6 to set the drop-out voltage to maximum (e.g. highest voltage at U1's REF terminal when measured between LOAD- while the RESET button is being pressed).
- Connect the device to the power supply using the BATT- and BATT+ connections. No load is required for testing.
- Press and release the RESET button: The LED should stay on, but if not, check the adjustment of R5 to verify that it is at the maximum possible value. If this checks out, check for proper resistor values of R4, R5 and R6 as well as proper wiring of U1.
- Lower the power supply to the desired drop-out voltage. The LED should stay on, but if not check the setting of R5. (Remember that the useful range of R5 with the specified values of R4 and R6 is in the 8-16 volt area.)
- Slowly rotate R5 until the LED just turns off.
- Increase the power supply voltage slightly, press and release the RESET button and verify that the LED turns on and then goes off again when the voltage drops below the threshold: Readjust R5 as needed.
Additional comments:
It is recommended that one NOT attempt to charge the battery "through" this device: In theory it should work, but the current will flow backwards through the FET. In this particular application Anderson Power Pole (tm) connectors were used on the input and outputs, allowing this device to be easily removed from the circuit and configured as needed.
This device should also not be left connected to a battery in long-term storage as it draws several milliamps when it is in its "ON" state due to the LED and the current consumption of Q1/Q2 and associated resistors, R4-R6 and U1. When in its "OFF" state its current consumption is negligible (likely in the nanoamp range) so if it is left connnected and the battery gets drawn down, it will still do its job, disconnecting the load - and itself - from the battery and protecting it. Note that if the load is "back-fed" from another source - say an AC power supply - and the voltage rises above the threshold, this will have the same effect as pressing the RESET button, turning the circuit on.
Not shown - but recommended - is the use of some sort of fuse or other overcurrent protection on the output of the battery. It is recommended that the fuse rating be no higher than a third of the current rating of the FET to increase the chance that the FET will survive the surge current required to blow the fuse in the event of a dead short on the output.
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