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Protect your expensive batteries from discharge damage with this mini-sized electronic cutout switch. It uses virtually no power and can be built to suit a wide range of battery voltages.

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Back in May 2002, we (Silicon Chip) 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 schemary 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.

Source: Silicon Chip 27 July 2004

The schema described here can be used to  ensure  that  a  12 V  sealed  lead  acid  (SLA)  gel battery isn’t discharged too deeply. The  principal part of the schema is a bistable relay,  which is driven by the output of an op amp. 

The battery voltage is first reduced via D1, R1,  P1 and R2, and then continuously compared  with a reference voltage set up by diode D2.  When the battery discharges too much and  its terminal voltage drops below the level  set by P1, the output of the opamp becomes  High, which causes the relay to toggle. This  in turn isolates the load from the battery. The  battery can be reconnected via S1 once the  battery has been replaced or recharged. 

The relay used in the prototype is a 5 V bistable type made by Omron (G6AK-234P-ST-US  5 VDC). The two windings of the relay each  have a resistance of 139 Ω (for the RAL-D 5  W-K made by Fujitsu this is 167 Ω). When the  battery voltage starts to become too low and  the relay is being reset the current consumption of the schema is about 45 mA. Shortly  after the load has been disconnected, when the battery voltage rises above the reference  voltage again, the reset coil will no longer be  powered and the current consumption drops  back to about 2.5 mA. 

The range of P1 has intentionally been kept  small. With a reference voltage of 5.6 V (D2)  and a voltage drop of 0.64 V across D1, the schema reacts within a voltage span of 11.5 V and  11.8 V. This range is obviously dependent on the zener diode used and the tolerance. 

For a greater span you can use a larger value  for P1 without any problems. With the potentiometer at its mid setting the schema switches  at about 11.6 V.

Author : Jürgen Stannieder