DC Power Disconnect Alarm System

@farzad_k BINGO. 

4AWG from alternator to the charger (longer distance) and 8AWG from charger to the battery (short distance). The battery sometimes needs to be handled because it is built into a bed frame I have designed for my long-distance roadtrips for astronomy. If while handling the bed frame or the battery we are not careful we could accidentally trip the breaker, causing the battery not getting the charge that it is supposed to get while I am driving. I have learned my lesson and now am more careful handling the battery, but most of the time when I am doing this I am too busy to remember and I become careless.

Hi @farzad_k,

  You haven't identified a specific relay, but if I understand your sketch correctly, you are proposing to pass all of the current from the charger to the battery through the coil.

In principle, one could design/specify a relay coil for this purpose, but it is not the normal design.

Normally, the relay is a voltage, not current, driven device. That is the relay coil will be specified to reliably actuate when a certain voltage is applied across the coil ...  that is, the coil will have a resistance chosen to allow the the minimum steady state current required to reliably actuate the mechanism.

As an example, this is a (randomly chosen) datasheet for a relay:

https://static.rapidonline.com/pdf/60-4600e.pdf

From this sheet, the 12V relay coil has a coil resistance of 270 Ohm

Hence, when the current flow will be 12 (V) / 270 (Ohm) =  83 mA, when when 12 V is applied to the coil terminals.

If this coil was placed in series with the connection between the charger, then only a small current will reach the battery, probably insufficient to actuate the relay or charge the battery.

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As I said above, in principle, a relay could be designed with a coil of minimal resistance, to actuate when sufficient current flows through the coil ... some circuit breakers work on this principle, although of course they are mechanically designed to open the contacts if the current exceeds the trip level ... but I have never seen a general-purpose relay on sale for the use I think you are suggesting.

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Common ways of measuring current are based on either measuring the voltage drop across a low value resistor or a Hall-effect device. Both methods usually require some additional electronics.

In addition, depending upon the load on the battery, there might be periods of time when the battery is approaching or at full charge, when the charging current will approach zero. Could this be an issue?

Best wishes, Dave

@davee I arrived at much the same conclusion after I fully understood the circuit. The better solution is as @farzad_k finally came up with, just build a cover for the breaker so it doesn't get accidentally tripped. Us techies sometimes don't see the simpler solutions right away.