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Don’t ruin an expensive SLA, Li-Ion, Li-Po or LiFePO4 battery by overdischarging it. This small circuit will protect it by cutting off power before it reaches the danger zone. It has virtually no effect on available power or battery life. It’s also ideal for preventing devices like Uninterruptible Power Supplies and emergency lights from destroying their batteries in an extended blackout. LifeSaver for Lithium or SLA batteries by Nicholas Vinen R ings, we have quite a few emergency of death. Computer UPS (Uninterruptechargeable Lithium-based lights/exit signs in our office. While we ible Power Supplies) often have the batteries are great – they have only have the occasional black-out, we same problem which can make having high capacity, long service life, still have to replace several back-up a black-out quite an expensive event. high discharge current, light weight batteries a year which really should The Battery LifeSaver works with and fast charging. have lasted a lot longer except that 6-24V batteries and can handle curBut they’re easy to destroy if you run they were discharged to the point rents of up to 20A continuous them down below a particular voltage and 30A peak, making it suitalevel and in a lot of applicable for use with cordless power tions, all you have to do is leave ns tools, emergency lights, small the device on a bit too long and Features & specificatio to medium UPS (up to about your expensive battery is lost. Li-ion, Li-Po and , cid d-a Lea d ale Se h • Works wit 300VA) and a wide variety of Radio-controlled cars/ ) LiFePO4 batteries (6-24V other devices. planes/helicopters generally rent, <5A • Very low quiescent cur With a quiescent current have a low voltage cut-out feaable from 5.25 to 25.5V less than 5A, it has negligible ture built in to the motor speed • Cut-out voltage adjust s, capability – 20A continuou effect on battery life and as controller but if you use these • High current-handling ) rge cha dis long as the cut-off voltage is batteries in other applications, or e arg (ch 30A peak type and y ter set high enough, it won’t damyou definitely need this Battery bat on ing end dep • 0.3-2V hysteresis, age the battery even if left for LifeSaver. voltage quite a long time after it has As mentioned above, it’s also m) 5m in tight spaces (34 x 18. • Very small PCB, to fit activated – 4.3A continuous suitable for use with most leadaged ged once cut-out has eng discharge equates to less than acid batteries. As with most of• Battery can be rechar 38mAh per year. fices, factories and public build(maximum 1.5A) 64  Silicon Chip siliconchip.com.au It’s very small at just 46 x 18.5 x 5mm assembled and light too (about 5g) so it can be slipped into a tiny space in a battery compartment. It won’t cost a lot to build either, which is good since if you have use for one, chances are you will have uses for several. We certainly do! Operation and charging As shown in Fig.1, the unit connects between the battery and load so that it can stop the load drawing any further power once the battery voltage reaches its cut-off value. It is based on a Mosfet, shown here as a switch. When the Mosfet is off, the load can not draw any further power from the battery. The Mosfet’s intrinsic diode is reverse-biased in this condition so no current flows through it either. The battery can be recharged either by connecting the charger directly across the battery terminals (if they are accessible) or as shown in Fig.1, by connecting the charger across the load terminals, whether or not the load is still connected. Charge current flows in the opposite direction to discharge current and this path is shown in green. In many cases, it will be necessary or simply convenient to charge via the load side of the device. In this case, the positive output of the charger is connected directly to the positive terminal of the battery while the negative output is connected via the internal electronic switch and parallel diode. If the battery voltage is high enough then the switch is on and so charge current can flow through it and charging proceeds as if the charger was connected across the battery. With the switch off, current can still Believe it or not, this photo is actually larger than life size, just to show the detail on the tiny (46 x 18.5mm) module. It’s cheap to build but could save you a fortune in ruined batteries! Once we attached the input and output leads, we encapsulated it in some transparent heatshrink tube. flow from the charger to the battery but it must pass through the diode. There will be an associated voltage drop and power loss due to the diode junction, heating up the diode (inside the Mosfet). However note that because the battery voltage will appear to be near-zero at the load terminals, some chargers may refuse to deliver current in this situation. If the cut-out has activated, you should limit the charge current to 1.5A or else the diode could overheat. We tested charging under this condition using a Turnigy Accucell 6 charger and it worked fine as long as we turned the charge current down until the battery voltage had come back up a couple of volts. Once the switch is back on (as confirmed by a healthy voltage reading across the load terminals), you can proceed to charge at the full rate. CHARGING CURRENT DISCHARGE CURRENT L+ B+ + BATTERY LIFESAVER + BATTERY 6–24V CONTROL LOAD – B– L– – + CHARGER – Fig.1: block diagram for the Battery Lifesaver. The unit is connected between the battery and load and disconnects the two (at the negative end) if the battery voltage drops below a threshold. The battery can still be charged in this case, at a limited current, until the voltage rises enough for the cut-out to deactivate at which point full charge current can resume. siliconchip.com.au If your charger is too “smart” and refuses to supply current with the cut-out activated, it’s simply a matter of connecting some sort of current source (or current-limited voltage source) across the load terminals – a plugpack and low-value wire-wound resistor will generally do the trick. It usually doesn’t take long to raise the voltage of a flat battery by a volt or two. Circuit description The full circuit is shown in Fig.2. We have published similar circuits in the past that used special-purpose ICs but they can be hard to get so this one is based on general-purpose parts: a low quiescent current low-dropout linear regulator (REG1), an ultra-low-power comparator (IC1) and a very low onresistance Mosfet (Q1). REG1 has a dual purpose. It limits comparator IC1’s supply to 5V which is desirable since IC1 has an absolute maximum rating of 7V. The regulated 5V is also used as a reference for comparison with the battery voltage. IC1 has rail-to-rail inputs and this means that we can tie its inverting input (pin 2) directly to 5V. In fact, its common mode input range extends 0.2V beyond both supply rails. Pin 3, the non-inverting input, is connected to a resistive voltage divider that is connected across the battery. The upper leg of this divider consists of a fixed upper resistor (RU) and a trimpot (VR1) while the bottom leg is a single resistor (RL). RU and RL are September 2013  65 Fig.2: circuit diagram for the Battery LifeSaver. It’s based on a low-dropout 5V regulator (REG1), very low power comparator (IC1) and Mosfet Q1, which acts as the switch. Values for resistors RU, RH and RL are chosen to suit a particular battery cut-out voltage threshold and VR1 provides fine adjustment of this voltage. ZD1 is selected to keep the supply voltage to REG1 within its ratings. chosen so that VR1 can be adjusted this, as soon as the load is switched The 10nF capacitor across RL filters to give 5V on pin 3 of IC1 when the off, the battery voltage would re- out noise which may be picked up due battery voltage is at its lower operat- bound and this will cause the load to to the high impedance of the divider ing limit. be switched back on and the circuit network and smooths battery voltage With the battery voltage above this would oscillate. ripple. It also slows the action of this limit, the voltage at pin 3 of IC1 is Say the low voltage cut-out thresh- hysteresis considerably but IC1 has above that of pin 2 and so the com- old is set to 19.8V (for a 24V Li-Po a small amount of built-in hysteresis parator output (pin 6) is high, switch- battery). Once the output of IC1 goes (about 3.3mV worth) which helps ing on Mosfet Q1 via a 10 resistor. high, the switch-on voltage rises to compensate for this. This connects the load to the battery. about 21.4V. The battery is unlikely to REG1 has 1F ceramic input bypass When on, Q1 not only has a very low rebound this much – at least, not right and output filter capacitors for stabilon-resistance (about 1.3m) but is away – so the Mosfet will remain off ity, the minimum suggested value for fully on with its gate just 4.5V above until the battery is re-charged. This this part. Dual Schottky diode D1/D2 its source. hysteresis should be sufficient for most protects the circuit against reverse If the battery voltage drops too much, batteries but if necessary, it can be battery polarity although it won’t stop the voltage at pin 3 of IC1 goes below increased by lowering the value of RH. current flowing through Q1’s body dithat at pin 2, the ode and the load, if concomparator output nected. The other half of goes low and Mosfet D1/D2 clamps input pin Q1 turns off. The 3 of IC1 to the 5V supply only remaining load if the battery voltage is on the battery is the particularly high. circuit itself, drawZener diode ZD1 reing about 3.2-4.5A. duces the battery voltResistor RH, conage for REG1 and its nected between the voltage is selected to output and nonsuit the type of battery inverting input of used. REG1’s absolute IC1, gives a small maximum input is 16V. amount of posiFor batteries well below tive feedback which 16V, ZD1 is replaced provides 1-2V of with a link (see Table 1). hysteresis for the During operation, circuit. Its value is REG1 consumes about selected so that this 2A while IC1 draws hysteresis is about just 600nA. The rest of 8% of the battery    When we say tiny, we mean it: here is the LifeSaver sitting on top of a the quiescent current voltage. Without    12V, 7Ah SLA battery and it’s not even as high as the spade lugs! flows through the resis66  Silicon Chip siliconchip.com.au tive divider, hence the resistors used have as high a value as is practical to minimise this current. This is why we have used a combination of resistors and a trimpot to set the cut-off voltage; the highest value of trimpot commonly available is 1M. Optional buzzer/LED The PCB has a pair of pads so that a piezo buzzer or LED can be connected to indicate when the battery voltage drops below the cut-off threshold. However fitting this may be not a good idea if you are concerned about the extra current drain on a battery which has been drained to the cut-off voltage. A buzzer/LED could run the battery flat in a matter of hours so you will need to immediately recharge it once it sounds/lights up. If you do want to fit a buzzer or LED, it will be driven at 5V by the output of comparator IC1, which can sink a maximum of 30mA. LEDs will require a series current-limiting resistor. Component selection Since the battery voltage divider is formed from a combination of fixed resistors and trimpot VR1, we must change the values of these resistors so that the adjustment range of VR1 includes the desired cut-off voltage for your battery. High value input dividers for comparators pose a problem in that the hysteresis resistor typically must be a much higher value so we are limited by the highest value readily available. Luckily, it’s quite easy to get resistors up to about 22M in SMD packages which is higher than the typical maximum of 10M for through-hole parts. To determine which parts you need, first locate your battery or its closest equivalent in Table 1 and read off the value for ZD1. Next, decide which cut-off voltage you want to use; in very high current drain applications (10A+), especially when using a relatively small battery, you may want to set it a bit lower than specified. Once you have determined the cutoff voltage to use, find an entry in Table 2 which has a range covering it and then read off the values for resistors RL, RU and RH. These are chosen to give a hysteresis of about 8% of the battery voltage, thus the hysteresis is roughly proportional to the number of cells for a given battery chemistry. As mentioned earlier, you can adjust the value for RH if necessary – lower values give more hysteresis and higher values less. This will not affect the cut-off voltage although hysteresis does vary slightly as VR1 is adjusted. Construction The Battery LifeSaver is built on a PCB coded 11108131, measuring 34 x 18.5mm. Referring to the overlay diagram (Fig.3), start by soldering Mosfet Q1. It has a large pad on the underside Parts List – Battery LifeSaver 1 double-sided PCB, coded 11108131, 34 x 18.5mm 1 50mm length 25mm-diameter heatshrink tubing 1 length heavy-duty black wire (to suit installation) 1 length heavy-duty red wire (to suit installation) 2 female 6.4mm spade quick connectors (optional; for use with gel cell batteries) 2 male 6.4mm spade quick connectors (optional; for use with gel cell batteries) Semiconductors 1 MCP6541-E/SN ultra-low-power comparator (IC1) (element14 1439473) 1 MCP1703-5002-E/CB micropower LDO 5V regulator (REG1) (element14 1439519) 1 PSMN1R2-30YL 30V 100A Mosfet (Q1) [SOT-669/LFPAK] (element14 1895403) 1 BAT54 Schottky diode (D1) [SOT-23] (element14 9526480) 1 0.4W or 1W zener diode (see Table 1 for voltage) (ZD1) Capacitors (all SMD 3216/1206) 2 1F 50V (element14 1857302) 1 10nF 50V (element14 8820155 or similar) Resistors (SMD 3216/1206) 1 10 plus three resistors, 330k-22M, as per Table 2 1 1M 25-turn vertical trimpot (VR1) siliconchip.com.au Jaycar Electronics will be releasing a kit for the Battery LifeSaver shortly: Cat No KC-5523 <at> $29.95 ST Micro’s LFPAK series SMD Mosfets Mosfet Q1 is an ST Micro part with an incredibly low on-resistance – barely more than 1 milliohm. It is rated to carry 100A but it will dissipate around 1W at 30A (I2 x R) so without heatsinking (other than the PCB), it won’t handle much more than that. Its on-resistance is so low that losses in the Mosfet itself are a minor component of the dissipation, most of it being in the PCB and wiring. This is only really possible with SMDs since a TO-220 through-hole package has 1m of resistance in the package/ leads alone. By comparison, the LFPAK package (also known as SOT-669) has a resistance of just 0.2m. The semiconductor die is sandwiched between the metal drain pad on the bottom of the device (which also acts as a heat spreader) and a metal plate on top, which also forms the three source leads (pins 1-3). This gives a very large contact area between the device leads and the Mosfet itself, hence the low resistance possible. The LFPAK has roughly the same footprint as an 8-pin Small Outline Integrated Circuit (SOIC-8), a very common SMD IC package. There is a lot of equipment already designed to handle SOIC parts – pick and place machines, storage schemes, etc – and these can generally work with LFPAK Mosfets with little or no modification. At a pinch, SOIC-8 Mosfets can be substituted for LFPAK devices and can be soldered to the PCB without needing to modify it. However, losses will be higher in this case. Mosfets in LFPAK use the same pin configuration as typical N-channel Mosfets in SOIC packages. For more infor mation, see www.nxp.com/documents/leaflet/75016838.pdf September 2013  67 Table 1: battery types, voltages and values for ZD1 Battery type Nominal Fully charged 6V 12V 24V 6.6V 7.2V 7.4V 9.9V 10.8V 11.1V 13.2V 14.4V 14.8V 16.5V 18.0V 18.5V 19.8V 21.6V 22.2V 7.2V/7.35V* 14.4V/14.7V* 28.8V/29.4V* 7.2V 8.2-8.4V 8.4V 10.8V 12.3-12.6V 12.6V 14.4V 16.4-16.8V 16.8V 18.0V 20.5-21.0V 21.0V 21.6V 24.6-25.2V 25.2V Note: 2S/3S/4S/5S/6S refers to the number of cells in series 68  Silicon Chip paste underneath all melts and fills the gaps, forming a solid junction. Note that this will require a fairly hot iron as there is a large area of copper connected to this pad. Note also that you will need to put the PCB on a heat-resistance surface as the underside will get very hot indeed. To avoid overheating the Mosfet itself, stop after about ten seconds. You may need to let it partially cool down and then apply heat for another ten seconds or so, to ensure all the solder paste has melted. When this happens, the volume of flux smoke produced should drop right TO LOAD B– 10nF RL + (BUZZER) – – + L Q1 – BATTERY B+ + BATTERY 10 1F RU ZD1 VR1 IC1 which must be in intimate contact with the large pad on the PCB to ensure both low resistance (so it can handle high currents) and a good thermal bond for proper heat dissipation. To achieve this, first spread a moderately thin layer of solder paste evenly over the pad and a good dollop of it on the smaller pin 4 pad, at lower left. Position Q1 over its pads and press it down, then apply heat to the small pin 4 pad so as to melt the solder paste until Q1 is held in place. You may find you have to add some solder wire to get a solid joint. Check that Q1 can’t move, then examine its alignment. In particular, ensure that the other three pins are correctly positioned over their pads and the tab is not totally covering the pad to which it is to be soldered; there should be a thin sliver of pad visible although this may be obscured by solder paste. To adjust the alignment, re-heat the solder on pin 4. Once you are happy with its position, melt the solder paste along the edge of the large tab by running the tip of the iron along up and down along the exposed section. It may help to add a bit more solder. You will need to keep the tab heated for several more seconds so that the (Safe) 5.75V 5.5V 11.5V 11.0V 23.0V 22.0V 6.2V 6.0V 6.6V 6.0V 7.2V 6.6V 9.3V 9.0V 9.9V 9.0V 10.8V 9.9V 12.4V 12.0V 13.2V 12.0V 14.4V 13.2V 15.5V 15.0V 16.5V 15.0V 18.0V 16.5V 18.6V 18.0V 19.8V 18.0V 21.6V 19.8V * gel cell or AGM type lead-acid battery MCP6541 Lead-acid Lead-acid Lead-acid LiFe 2S Li-ion 2S Li-po 2S LiFe 3S Li-ion 3S Li-po 3S LiFe 4S Li-ion 4S Li-po 4S LiFe 5S Li-ion 5S Li-po 5S LiFe 6S Li-ion 6S Li-po 6S Cut-out (Best life) 1F RH REG1 D1/2 11108131 Fig.3: follow this PCB overlay diagram to build the unit. Most parts are SMDs and all mount on the top side of the board. VR1 can be laid over to keep the whole thing relatively thin, so it can be squeezed next to a battery. Heavy-duty wires to the battery and load solder directly to the large pads at top. The pads at lower-left are optionally used to connect a piezo buzzer for a low-voltage alarm. (Minimum) ZD1 5.25V 10.5V 21.0V 5.6V 5.4V 6.0V 8.4V 8.1V 9.0V 11.2V 10.8V 12.0V 14.0V 13.5V 15.0V 16.8V 16.2V 18.0V link 3.3V 15V link link link link 3.3V 3.3V 3.3V 3.3V 5.1V 5.1V 8.2V 8.2V 8.2V 8.2V 10V off. You can then solder the remaining pins one at a time and clean up any bridges between them using solder wick. If necessary, clean up using isopropyl alcohol. IC1 is a snack by comparison; it is the same size and has the same pin spacing but there is no big pad underneath so you simply pin it down by one lead, check the alignment and then solder the remaining pins once it is correctly orientated. For the rest of the SMD components, apply some solder to one of the pads, heat it, slide the part in place using angled tweezers, remove the heat and check the alignment. If it’s OK, make the remaining solder joint(s) and then refresh the first one with a dab of extra solder. Don’t get REG1 and D1 mixed up as they look very similar; the resistors will be labelled with their value (although you may need a magnifying glass to read it) but the capacitors won’t be. If you do get confused, you should be able to tell which is the 10nF as it will be thinner than the other two. With the SMD components in place, fit ZD1 with the orientation shown and then VR1, with its adjustment screw towards the bottom of the board. You can bend its leads over before soldersiliconchip.com.au ing, as we have, to keep the overall assembly thin so that it will fit into tight spaces. Note that if you are going to use the unit with a sealed lead-acid battery (“gel cell”), these are often fitted with spade lugs. So you could solder wires to the PCB and crimp female spade lugs onto those connected to the B+/B- terminals and male spade lugs to those connected to the L+/L- terminals. That would then allow you to easily connect the device in-line between the battery and device without any additional soldering. Testing and adjustment The easiest way to set up the Battery LifeSaver is using a variable voltage power supply (eg, a bench supply) but if you don’t have one, you can instead connect a fully charged battery (or power supply with a similar voltage) across a 1-10k potentiometer. The pot wiper connects to the B+ terminal on the PCB while the negative terminal of the power supply goes to B-. We used small hook probes to make the connection to these terminals, to avoid having to solder them initially (see photo) but if you do solder wires on, it’s probably a good idea to keep them long and use thick, heavy-duty wire so that you can also use them for the final wiring. Adjust the bench supply or pot to give the board close to the nominal battery voltage (measured across B+ and B-), then measure the current flow by connecting a multimeter, set to mA or A, in series with one of the board’s supply leads. You should get a reading of around 5A. If it’s more than 10A or less than 2uA then something is wrong and you will need to carefully check the assembly (note that not all multimeters can read such low currents with precision). Set the DMM to volts mode and measure between the + terminal of CON5 (upper) and the B- battery terminal. Assuming your DMM is accurately calibrated, you should get a reading in the range of 4.95-5.05V. Now adjust VR1 fully anti-clockwise (until it clicks) and measure the resistance between the L- and B- terminals. The reading should be close to 0, meaning Q1 is on. If not, check the supply voltage and try turning it up slightly but don’t exceed the fullcharge voltage of your battery. Assuming Q1 is on, reduce the supply voltage to the PCB until it is at your desired battery cut-off voltage, as measuring between B+ and B-. Confirm that Q1 is still switched on, then slowly turn VR1 clockwise until Q1 switches off and the resistance reading increases dramatically. It should be above 10M and may give a reading of “oL” (ie, effectively open circuit) on your DMM. To check this, we simply clipped the test leads connected to L- and B- onto Table 2: resistor values for different cut-out voltage ranges Cut-out range 5.2-5.6V 5.6-5.9V 5.8-6.4V 6.4-7.4V 7.4-8.7V 8.4-9.7V 9.6-11.0V 11.0-12.3V 12.2-13.6V 13.6-15.1V 15.5-17.1V 16.2-17.9V 17.7-19.3V 19.3-21.1V 20.6-22.6V 22.2-24.2V 23.7-25.8V Hysteresis RL (1%) RU (1%) RH ~0.3V 10M 330k 10M ~0.4V 10M 1.0M 15M ~0.5V 6.8M 1.0M 15M ~0.5V 3.9M 1.0M 15M ~0.6V 3.3M 1.5M 15M ~0.6V 3.3M 2.2M 22M ~0.8V 3.3M 3.0M 22M ~1.0V 3.3M 3.9M 22M ~1.1V 3.3M 4.7M 22M ~1.2V 3.0M 5.1M 22M ~1.4V 2.7M 5.6M 22M ~1.6V 3.0M 6.8M 22M ~1.6V 2.7M 6.8M 22M ~1.6V 2.4M 6.8M 22M ~1.6V 2.2M 6.8M 22M ~1.8V 2.2M 7.5M 22M ~2.0V 2.2M 8.2M 22M * Approximate quiescent current at cut-off voltage siliconchip.com.au Iq* 3.2A 3.2A 3.7A 4.4A 3.5A 3.8A 4.4A 4.3A 4.3A 4.6A 4.7A 4.6A 4.5A 4.6A 4.9A 4.9A 4.9A Quality Effects Pedal Enclosures www.rixenpedals.com our DMM probe tips and the used clip leads to connect the power supply to B+ and B-. This allowed us to vary the voltage while watching the Mosfet’s resistance. You can confirm that the board is working properly by turning the supply voltage up by the hysteresis voltage (a couple of volts should do); Q1 should then turn back on again. Installation Once you have soldered the leads to the PCB, it’s a good idea to sleeve the whole thing with 25mm diameter heatshrink tubing so that once it’s inside the battery compartment, or secured to the outside of a battery, it can’t short against battery terminals or any other exposed metal. Wire it up according to Fig.3. There are two different ways to connect the load’s positive terminal. Ideally, it should go straight to the battery’s positive terminal but since that will already be wired to the Battery LifeSaver board, in may be easier to connect it to the L+ terminal on the PCB instead. This means the full load current has to pass through the PCB twice which will slightly increase losses but should not cause any problems within the ratings we have provided. SC September 2013  69