Tech Forum

Yes, there are relays that latch mechanically in the on or off position and draw no current except to change positions. They are listed as alternating or latching relays, and they depend on mechanical linkages to hold the contact position. They are often used in lighting controls and some have large current capacities. One manufacturer, Asco, comes to mind.

Aside from the load terminals there are three control terminals; Common, On, Off. For clarity consider a toggle switch with two solenoids and linkage. One solenoid pulls the switch on, the other solenoid pulls the switch off. Another design uses a single solenoid with an over-the-center linkage design. The one consideration is that the battery still has enough energy to operate the ‘Off’ solenoid when the cutoff point is reached.

One relay that you may seriously consider is made for the GE commercial and residential low voltage controlled lighting systems. The currently available relay is model RR7, it has a single 20A/277 contact and a pair of coils designed to operate on 24VAC, but they may work with 12VDC. Ebay has several listed as GE RR7 Relay, around $20.00 US.

A 16 amp relay will not be sufficient to handle engine starting load current, which can reach 200-300 amps.

A better and simpler solution, and one that I use on numerous vehicles that I have, is to use a Trickle Charger designed to be connected continuously. That type of charger uses a microcomputer to monitor battery state of charge and prevents overcharging and undercharging.

Some modern cars have low voltage disconnects but they typically disconnect too late and while retaining some charge in the battery, it is generally insufficient to start the vehicle.

You can have a relay that takes NO current. This is called a latching relay. It has two states which can be called SET and RESET and a separate coil for each state.

In use, you apply a pulse of voltage to one coil to put the relay in the SET state and likewise a pulse of voltage to the other coil to put it in the RESET state. In your application, your LVD circuit would pulse the relay to the reset state which would disconnect your electronics. You could use a push button to go back to the SET state or another circuit that would sense that the car has been started and the battery voltage is well above the low voltage disconnect value.

There are lots of listings for latching relays and circuits available on the internet.

A latching relay turns on with one pulse, and off with a different pulse. It will mechanically retain the last position (using no power whatsoever to do so) until the appropriate pulse is received to change it’s state.

A single coil latching relay requires a pulse with one polarity to turn it on and a pulse with the opposite polarity to turn it off. (This usually means using an H-bridge circuit to drive it.)

A dual coil latching relay has two separate coils: one that turns it on and one that turns it off.

DigiKey (www.digikey.com) has listings for both types of latching relays, including ones with contact ratings of 16A at 12VDC.

If you prefer not to build it yourself, LVDs have become popular in recent years in the Amateur (Ham) Radio community. You might check with Amateur Radio equipment suppliers to locate a pre-built LVD.

I recommend a latching relay. They come in a variety of styles and maintain their position after the power to the coil(s) has been removed: it is a mechanical memory.

You probably want a two coil style: one coil for on and the other coil for off. A short pulse to the desired coil will set it in that position until the other coil is pulsed. It will remember this position even if all power is removed form the vehicle.

I searched Digi-Key for an appropriate one (12V coil, 16 Amp and higher contacts) and got 41 results.

There are commercially made devices which provide both a low voltage disconnect as well as a timed disconnect. These are commonly used in public safety vehicles to remove the load of two way radios and other emergency equipment when the vehicle is powered off for too long.

I have provided a link for a company that uses MOSFET technology instead of a mechanical relay, thus the current draw for the device will be lower. The link is for a uni-directional relay that I believe may suffice for your application. They also make uni-directional relays for applications where there is a second "cabin battery" such as an RV (Caravan). www.perfectswitch.com/wp-content/uploads/2009/10/Specification-Sheet-Uni-Directional-Relay-GEN3.0.pdf

It must be a long time delay if your car battery can’t sustain a couple of hundred mA max during the on time!

However, to business: I assume that the idea is for your ‘misc’ circuit to come on when the ignition is on, then automatically go off after a specified time when the ignition is switched off. The circuit should handle 10-16A, but use almost nothing.

If that’s the case, then look at the relay C3-R20/DC 12V by RELECO. (RS Components part 350-377) This is a set/reset relay with DPCO contacts rated at 10A DC, so you will need two relays for the rating - preferably feeding 1/2 of the misc load each. (Paralleling contacts will result in burn-out as they are mechanical devices which will lead to one doing most of the work.) To use them, feed each relay’s SET coil through one of its contacts, from the ignition switched line. Thus, when the ignition is turned on, the relays will be SET, then their coils de-energized.

The trigger for the switch-off timer will be active-low, taken from the same ignition line. Add some suppression here to clean it up. Power your timer circuit from the unswiched battery line, with it providing a positive-going pulse to the RESET coils on expiry. (100mA per coil). I leave the details of the timer to you, but a 555 needs a pulse not a level, and the output may need beefing up? This way, the only consumption is that of the timer circuit except during switching.

A magnetic latching relay should do the job. You pulse it on and pulse it off, there is no current at all otherwise. I found a suitable relay at Newark, SPDT 12V, 16A, $5.18. Newark #26M2701, mfr: TE Connectivity #RT314F12. There may be others if you do a search for latching relay.

You might try using a DC-DC solid state relay. Input rages are typically 3 VDC to 24 VDC, so 12 volts DC will be sufficient to turn the relay on. I would get at least a 100 VDC unit due to electrical transients on the cars system, and sufficient current rating for the expected loads.

You can use a hidden switch or a tap off of a load that is switched with the key.

There are many suppliers of these types of relays, a search on Google or Ebaty should get you many suppliers.

Two solutions come to mind quickly.

1. Use a “latching” relay.
Would have to redesign your 555s to put out pulses - pulse to latch relay ON - another pulse to unlatch relay OFF.

2. Instead of using a relay - use a MOSFET.
Should be fairly easy to do with either a P or N channel mosfet.

To test, disconnect one battery lead, connect a test light or meter between the cable and the battery. If the light glows or the meter shows current draw more than 50 ma. then disconnect the voltage regulator and see if the light goes out or the current drops, if so the regulator is likely bad. I have seen this problem before on Fords.

I would suggest maybe a latching relay similar to this one from Digikey - www.digikey.com/product-detail/en/panasonic-electric-works/ADQ23Q012/255-1443-ND/560939.

A latching relay would consume no power at all except at the point of switching. Granted, that may be a little pricey, but if you search the surplus electronics sites (Electronic Goldmine, All Electronics, BG Micro, etc) you can often find one for much less.

You may use a relay that does not require constant power to maintain contact state. A “Latching relay” is a generic term that is used to describe a relay that maintains its contact position after the control power has been removed. They allow you to control a circuit by simply providing a single pulse to the relay control circuit.

Latching relays are also desirable when you need to have a relay that maintains its position during an interruption of power.

There are three main types of Latching relays. Magnetic latching, Mechanical Latching and Impulse Sequencing. Contact ratings vary with common ratings at 3 to 10A. If what you find can’t handle your current you can have it drive a slave high current contactor like the ones used in most vehicles.

Approach the problem from the other end. Put a battery maintainer on the car when it is not in use. Some Volkswagens even come with a solar panel to lay on the dashboard when the car is out of use.

You could use a mechanical latching relay triggered by a comparitor or also an SCR as a latching relay. An SCR will not change state until the power is removed, a transistor would momentarily cut the power and a reset button would gate it back on.

But there are two better solutions. Buy a battery tender to keep the battery charged, this will make the battery last much longer. The other possibility would be to buy a low voltage disconnect, they cost less than a new battery, have been engineered to work and are built into a rugged case.

Before recommending any high tech solution, have you located the electronics that caused the power drain? I recommend doing so by removing the power fuses of each circuit one-by-one until the drain disappears and measuring the current at the battery while removing each fuse. Then analyze if that drain is some important part of the vehicle, such as radio/entertainment/clock that may be on with the ignition off. That drain problem should be solved before using the electronic disconnect.

As for the LVD that you have now, I believe it is a timer based circuit. The relay current may now be acceptable or not needed after the drain problem is solved.