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Protect your expensive batteries with this mini-sized, micropowered electronic cut-out switch. It uses virtually no power and can be built to suit a wide range of battery voltages. By PETER SMITH MICROPOWER BATTERY PROTECTOR B ACK IN MAY 2002, we presented the “Battery Guardian”, a project designed specifically for protecting 12V car batteries from over-discharge. This unit has proven to be very popular and is still available from kit suppliers. This new design does not supersede the Battery Guardian – at least not when it comes to 12V car batteries. Instead, it’s a more flexible alternative that can be used with a wide range of battery voltages. In this new “Micropower Battery Protector”, we’ve dispensed with the low-battery warning circuitry and the relatively cheap N-channel MOSFET used in the Battery Guardian in favour of a physically smaller module that steals much less battery power. It costs a little more but can switch lower voltages, allowing it to be used with 6V & 12V lead-acid batteries and 4-cell to 10-cell NiCd and NiMH battery packs. Most battery-powered equipment provides no mechanism for disconnecting the batteries when they’re exhausted. Even when the voltage drops too low for normal operation, battery drain usually continues until all available energy is expended. This is particularly true of equipment designed to be powered from alkaline or carbon cells but retro-fitted with rechargeables. Another example is emergency lighting and security equipment designed to be float-charged from the mains. In an extended blackout period, the batteries can be completely drained and may not recover when the mains power is finally restored. Death by discharge Fig.1: the cut-off voltage for lead-acid batteries is dependent on the rate of discharge. This graph enables you to determine the correct voltage for your application. Although representative of “Panasonic” brand 1.3Ah - 33Ah VRLA batteries, all good quality sealed lead-acid batteries will exhibit similar characteristics. 22  Silicon Chip Over-discharge is undoubtedly one of the main causes of early battery failure. How well a particular battery can cope varies according to type and application. Some “gel” electrolyte leadacid batteries will not fully recover after a discharge right down to 0V. On the other hand, batteries designed for deep-cycle use can usually withstand such treatment, albeit with a reduction in maximum cycle life. The latest generation of NiCd and NiMH cells can be completely discharged without damage. However, when connected in series to form a siliconchip.com.au Fig.2: the circuit is based on a MAX8212CPA voltage monitor IC (IC1), which controls Mosfet Q1 to switch the power to the load. Resistor R2 selects the cutoff voltage (see Table 2), with fine adjustment provided by VR1. battery pack, unequal cell conditions mean that some cells will reach 0V before others. These “weaker” cells are then reverse-charged until all of the energy in the pack is expended. This results in heat damage and electrolyte loss, or worse. In most cases, the battery will be functional again after a recharge but the reverse-charged cells will have been weakened. And that makes the problem even worse the next time around. Obviously, the solution to this problem is to disconnect the batteries at some minimum terminal voltage, allowing enough headroom for cell imbalances. For NiCd and NiMH batteries, this is typically 0.9V per cell. For lead-acid batteries, the minimum voltage is dependent on discharge current. Fig.1 shows the relationship between discharge current and the minimum recommended terminal voltage for both 6V and 12V VRLA batteries – also commonly referred to as “SLA” (sealed lead-acid) batteries. The discharge capacity of SLA batteries is measured over a 20-hour period and normalised to an amphour (Ah) rating. In theory, a 7.2Ah battery can deliver 7.2A for one hour. This is referred to as the “C” or “1C” discharge rate. In practice though, the battery will be exhausted before the hour’s end, due to inefficiencies in the electrochemical process. The horizontal axis represents the discharge current, expressed as a frac- Fig.3: block diagram of the MAX8212CPA voltage monitor IC. It contains a 1.15V reference and a comparator which drives complementary FET output stages. siliconchip.com.au tion of the “C” rate. For example, a 6V 7.2Ah battery discharged at 3.6A corresponds to a 0.5C rate, with a recommended cut-off voltage of 5.05V. Note that high-capacity lead-acid car batteries have different characteristics to SLA batteries. Where possible, refer to the manufacturer’s datasheets for the recommended cut-off voltage. We’ve listed a cut-off of 11.4V in Table Main Features • • • • • • • Disconnects load at preset battery voltage Automatically reconnects load when battery recharged Ultra-low power consumption (<20µA) Miniature size 10A maximum rating Suitable for use with 4.8-12.5V batteries Transient voltage protection (optional) Suitable for use in . . . • • • • • • Cars, boats & caravans Security systems Emergency lighting Small solar installations Camera battery packs Many other low-power applications July 2004  23 Fig.4: install the parts on the top of the PC board as shown here. Resistors R2 & R3 must be chosen from Table 2, to suit the battery pack. Fig.5: the optional transient voltage suppressor (TVS1) is soldered directly to the copper side of the PC board. It’s non-polarised and can go in either way around. 2 simply because at this voltage, there should still be enough energy in the battery to start the engine! side via P-channel MOSFET Q1. The gate of this MOSFET is controlled by IC1, a MAX8212 micropower voltage monitor. Power for the MAX8212 is derived from the battery input, which is filtered using a 100Ω resistor and 100µF & 100nF capacitors before being applied to the V+ input. A 16V zener diode (ZD1) ensures that the supply Circuit description The circuit diagram for the module appears in Fig.2. Battery voltage is applied to the input (lefthand) side of the circuit and switched through to the load on the output (righthand) rail can not exceed the maximum input voltage of the IC (16.5V). Fig.3 shows the basic internals of the MAX8212. The voltage on the threshold (THRESH) input is connected to the inverting input of a comparator, while a 1.15V reference is connected to the non-inverting input. When the threshold voltage is below 1.15V, the comparator’s output is driven towards Table 1: Resistor Colour Codes o o o o o o o o o o o o No.   1   1   1   1   1   1   3   1   1   2   1 24  Silicon Chip Value 3.9MΩ 5% 3.3MΩ 5% 2.7MΩ 5% 1.8MΩ 5% 1.5MΩ 5% 1.2MΩ 5% 1MΩ 820kΩ 620kΩ 470kΩ 100Ω 4-Band Code (1%) orange white green gold orange orange green gold red violet green gold brown grey green gold brown green green gold brown red green gold brown black green brown grey red yellow brown blue red yellow brown yellow violet yellow brown brown black brown brown 5-Band Code (1%) not applicable not applicable not applicable not applicable not applicable not applicable brown black black yellow brown grey red black orange brown blue red black orange brown yellow violet black orange brown brown black black black brown siliconchip.com.au Table 2: Selecting Resistors R2 & R3 Parts List Number of Cells Recommended Cut-Off Voltage Reconnect Voltage (nominal) 4 3.6V 5.1V 820kΩ 620kΩ 5 4.5V 6.5V 1MΩ 820kΩ 6 5.4V 7.8V 1.2MΩ 1MΩ 7 6.3V 9.2V 1.8MΩ 1.2MΩ 8 7.2V 10.8V 1.8MΩ 1.5MΩ 9 8.1V 11.7V 2.7MΩ 1.5MΩ 10 9V 13.4V 2.7MΩ 1.8MΩ 6V SLA 5.4V 6.8V 1.2MΩ 470kΩ 12V SLA 10.8V 13.4V 3.3MΩ 820kΩ 12V Car Battery 11.4V 13.4V 3.9MΩ 820kΩ R2 R3 Table.2: select R2 & R3 according to battery type and number of cells. The cutoff voltages shown for SLA batteries are for low-drain applications only. Refer to Fig.1 for more realistic cut-off voltages in higher power applications. Fine adjustment of the cut-off voltage is achieved with the 1MΩ trimpot (VR1), as shown in more detail in Table 3. the V+ rail and the two FETs are off. Conversely, when the threshold voltage is above 1.15V, the comparator’s output is near zero volts, switching the FETs on. Now back to the circuit – a string of resistors (R1, R2 & VR1) divide down the positive rail such that 1.15V will be present on the “THRESH” input at the desired lower threshold voltage. We’ve also called this the “cut-off” voltage because this is the point at which Q1 is switched off, disconnecting the battery from the load. The lower threshold voltage (VL) can be determined from the formula VL = 1.15 x ((R2+VR1)/R1 + 1). Using the values shown and with VR1 in its mid position, the load will be disconnected at approximately: 1.15 x ((3.9MΩ + 500kΩ)/470kΩ + 1) = 11.9V. You will recall that when the threshold voltage is above the trip point, both FETs in the MAX8212 are switched on. This means that the “HYST” output is connected to the positive (V+) rail, shorting out the top resistor in the string (R3), so it is disregarded in the above calculation. However, when the threshold voltage falls below the trip point, the “HYST” output goes open-circuit, adding R3 into the equation. The rail voltage must now rise higher to gen­ erate 1.15V on the “THRESH” input than it did before R3 was in-circuit. This is called the upper threshold or siliconchip.com.au 1 PC board, code 11107041, 58 x 46mm 2 2-way 5/5.08mm 10A terminal blocks (CON1, CON2) 1 Micro-U TO-220 heatsink (Altronics H-0630, Jaycar HH-8502) 2 3AG PC-mount fuse clips 1 3AG 10A slow-blow fuse 4 M3 x 10mm tapped spacers 5 M3 x 6mm pan head screws 1 M3 nut & flat washer Semiconductors 1 MAX8212CPA voltage monitor (IC1) (Farnell 205-278) 1 SUP75P05-08 75A 55V P-channel MOSFET (Q1) (Farnell 334-5348) 2 16V 0.5W (or 1W) zener diodes (ZD1, ZD2) 1 15V 0.5W (or 1W) zener diode (ZD3) 1 SMCJ24CA transient voltage suppressor (TVS1) (Farnell 167-563) (optional) Capacitors 1 100µF 16V PC electrolytic 2 220nF 63V MKT polyester 1 100nF 63V MKT polyester Fig.6: this is the full-size etching pattern for the PC board. “reconnect” voltage, and it ensures a clean, positive switching action at the output. The upper threshold (VU) voltage can be determined from the formula: VU = VL + ((R3/R1) x 1.15V). Using the values shown, the reconnect voltage will be approximately 11.9V + (820kΩ/470kΩ) x 1.15) = 13.9V. We’ve used quite a large hysteresis value (2V) because the battery voltage will “rebound” somewhat when the load is disconnected. Ideally, the load should only be reconnected once the battery is recharged or the input power is cycled. The “OUT” pin of the MAX8212 drives the gate of the P-channel MOSFET (Q1). When the internal FET driving this pin switches on, Q1’s gate is pulled towards ground via a 1MΩ Resistors (0.25W) 1 3.9MΩ 5% 1 3.3MΩ 5% 1 2.7MΩ 5% 1 1.8MΩ 5% 1 1.5MΩ 5% 1 1.2MΩ 5% 3 1MΩ 1% 1 820kΩ 1% 1 620kΩ 1% 2 470kΩ 1% 1 100Ω 1% 1 1MΩ 25-turn trimpot Note: the above list includes all values for R2 & R3 shown in Table 1, so you’ll have some resistors left over after assembly. Farnell have discontinued the MAX8212CPA (IC1), alternatively Wiltronics have this part listed in their catalog. Check their website at www. wiltronics.com.au. The MAX8212CPA is also available direct from the manufacturer at www.maxim-ic.com resistor, switching it on. Conversely, when the internal FET switches off, Q1’s gate is pulled up to the positive rail via a second 1MΩ resistor, switchJuly 2004  25 Fig.7: this scope shot shows the rise time of the voltage at the output terminals when a 12V battery is connected to the input. The rounded edge at the top of the waveform is probably due to the battery’s response as full load is applied. ing it off. Two zener diodes protect the gate-source junction of Q1 (ZD3) and the drain-source junction of the internal FET of IC1 (ZD2) from potential over-voltage conditions. Circuit protection Output overload protection is afforded by a slow-blow fuse (F1) at the input. For light load switching, the size of the fuse can be reduced accordingly, to provide increased protection for the MOSFET. No reverse polarity protection has been provided. Due to the 10A current rating of this circuit, a series protection diode would reduce the output voltage by as much as 1V and generate considerable heat. Momentary reversal of the battery leads will probably not damage either IC1 or Q1. However, the intrinsic drain-source diode in the MOSFET will conduct, allowing reverse current flow through the load. For use in a car or other noisy electrical environments, an optional bidirectional transient voltage suppressor (TVS1) can be installed. These devices behave like back-to-back zener diodes but are faster acting and can absorb much more energy. The specified device will clamp the input rail to ±39V peak, protecting the MOSFET and load from all but the most severe high-voltage transients. Assembly The assembly is quite straightforward, with all parts mounting on a small PC board coded 11107041 and measuring 58 x 46mm. Install the low-profile components first, using the overlay diagram (Fig.4) as a guide. Take care to align the banded (cathode) ends of all the zener diodes (ZD1-ZD3) as shown. The values shown for R2 & R3 are suitable for use with a 12V car battery. For other applications, select the appropriate values from Table 2. Table 3: Max. & Min. Cutoff Voltages R2 Max. Cut-Off Min. Cut-Off 3.9MΩ 13.1V 10.6V 3.3MΩ 11.6V 9.2V 2.7MΩ 10.2V 7.7V 1.8MΩ 8.0V 5.5V 1.2MΩ 6.5V 4.0V 1MΩ 6.0V 3.5V 820kΩ 5.6V 3.1V 26  Silicon Chip Fig.8: again captured at the output terminals, this waveform shows the voltage fall time when a 4-cell battery pack drops below the preset 3.6V level. Note that it’s much longer than the rise time because the MOSFET’s gate must be discharged through two 1MΩ resistors. Table 2: by selecting an appropriate value for R2 and adjusting VR1, cut-off voltages from 13.1V to 3.6V are achievable. Note that with a value of 820Ω for R2, it is possible to achieve a cut-off of 3.1V. However, you should not adjust VR1 for less than 3.6V to avoid overheating Q1. Note that the MAX8212 (IC1) should be installed without a socket. Make sure that the “notched” (pin 1) end of the IC goes in as indicated on the overlay diagram. A small “micro-U” style heatsink is needed to keep MOSFET Q1 cool. It is sandwiched between the MOSFET and the PC board, with both items held in place with a M3 x 10mm screw, nut and flat washer. Bend the MOSFETs leads at 90° about 5mm from the body and trial fit it in position. If the lead bend is correct, the hole in the metal tab will line up with the hole in the PC board without stressing the leads. Apply a thin smear of heatsink compound to the mating surfaces before assembly. Be sure to tighten up the mounting screw before soldering the MOSFET’s leads. The optional transient voltage suppressor (TVS1) can be left until last. It mounts on the copper side of the board and must be positioned precisely as shown in Fig.5 before soldering. Finally, for operation in high-humidity environments, we recommend that the board be cleaned, thoroughly dried and then coated with a circuit board lacquer. This will prevent problems associated with leakage currents that could affect the accuracy of the threshold voltage setting over time. Setup & test In order to set the cut-off voltage accurately, you’ll need an adjustable DC bench supply, a multimeter and a small load for the output. A 680Ω siliconchip.com.au Switching Capacitive Loads & Incandescent Lamps Capacitive loads can cause huge instantaneous currents to flow at switchon. One way of reducing this in-rush current is to reduce the switching speed of the MOSFET. To this end, we’ve used a 1MΩ resistor in series with the gate, which acts with gate capacitance to slow MOSFET turn-on. The result (see Figs.7 & 8) should be sufficient for most general-purpose applications. In-rush current is an even bigger problem for lamp loads and can not be solved by simply slowing gate turn-on. Tungsten-filament incandescent lamps, for example, exhibit a very low cold-filament resistance – as much as 10-12% of the hot resistance. This means that when an incandescent lamp is switched on, at least 10 times the normal current flows through the filament. After about 5ms, this reduces to about twice the normal level, decreasing slowly until full brilliance at over 100ms later. We therefore recommend a maximum lamp load of 3.5A (3.4W <at> 12V) for use with the Micropower Battery Protector, as higher power lamps may well damage the MOSFET switch. Note that it is possible to increase lamp load handling by connecting a positive temperature coefficient (PTC) resistor in series with the lamps(s). For example, to switch a 10A lamp load, a 30A PTC with a cold resistance of 0.5Ω and a hot resistance of 0.01Ω would be suitable. Farnell stock a suitable part, Cat. 606-832. This will protect the MOSFET switch and your lamps will last much longer to boot! 0.25W resistor in series with a LED makes an ideal load (see Fig.9). Hook up the bench supply to the battery input terminals and the load (resistor & LED) to the output terminals, observing correct polarity. Initially, set the input voltage a couple of volts higher than the desired cut-off level. Now wind VR1 fully anti-clockwise and then power up. The LED should illuminate, indicating that the MOSFET has switched power through to the output. Next, monitor the input voltage while you carefully adjust your bench supply to the desired cut-off level. That done, wind VR1 slowly clockwise until the LED goes out, indicating that the MOSFET has disconnected the load. To check the “reconnect” voltage level, slowly increase the input voltage. The MOSFET should switch on again at the expected level, illuminating the LED. Note that there will be some deviation from the listed voltage due to resistor tolerances. In use, the battery cut-out level will also vary slightly from that set above due to the resistance of the fuse, battery connections, cabling and any other in-line connectors. Housing & wiring The small size of this module means that, in many cases, it can be built right siliconchip.com.au Silicon Chip Binders REAL VALUE AT $14.95 PLUS P & P These binders will protect your copies of S ILICON CHIP. They feature heavy-board covers & are made from a dis­ tinctive 2-tone green vinyl. They hold 12 issues & will look great on your bookshelf. H 80mm internal width H SILICON CHIP logo printed in gold-coloured lettering on spine & cover H Buy five and get them postage free! Price: $A14.95 plus $A10.00 p&p per order. Available only in Aust. Fig.9: a 680Ω 0.25W resistor in series with a LED makes an ideal load when setting the cutoff voltage – see text. in to the equipment it protects. Alternatively, it can be installed in a “UB5” size Jiffy box and these are available from all the usual parts suppliers. All wiring to and from the terminal blocks on the PC should be sized to suit the intended application. When operated at or near the maximum rating, be sure to use extra-heavy duty automotive-type cable. For use in a car, the unit can simply be wired in-line with the cigarette lighter plug that’s connected to the appliance. Alternatively, power should be sourced from a fused terminal in the fuse box. Do not connect the Micropower Battery Protector directly SC across the vehicle battery! Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or call (02) 9939 3295; or fax (02) 9939 2648 & quote your credit card number. Use this handy form Enclosed is my cheque/money order for $________ or please debit my  Visa    Mastercard Card No: _________________________________ Card Expiry Date ____/____ Signature ________________________ Name ____________________________ Address__________________________ __________________ P/code_______ July 2004  27