Good call. That could give you even better data.
Measuring voltage across it could tell you if it had enough voltage to fire and drew enough current to not be an open circuit, but actually measuring current could additionally tell you if it drew enough current to actually actuate (not weak) or drew so much current that something is wrong (shorted coil, plunger failed to move).


I would go with a latching relay but I've been known to favor overly primitive means with little justification. Others have suggested fets and that seems reasonable. Is there something holding you back from using fets? My apologies if this has been answered already, I might have missed it.

Nothing is holding me back from fets ... in fact I'd better much prefer to use them instead of relays. My question pertains the proper configuration for a circuit to kick in as fast as possible and with as little power draw while idle when battery bank B is connected/removed

 

Nothing is holding me back from fets ... in fact I'd better much prefer to use them instead of relays. My question pertains the proper configuration for a circuit to kick in as fast as possible and with as little power draw while idle when battery bank B is connected/removed

Well a latching relay would use zero power while connected, but I understand your preference, and space constraints. A capacitor with enough charge to ride through the changeover seems sufficient for either scenario, or do you not think so?

 

Well a latching relay would use zero power while connected, but I understand your preference, and space constraints. A capacitor with enough charge to ride through the changeover seems sufficient for either scenario, or do you not think so?

Your suggestion is a good possibility... and perhaps it's also the simplest one ... gonna mull it over

 

Your suggestion is a good possibility... and perhaps it's also the simplest one ... gonna mull it over

Take this one for example:




Coil requires 33.3mA for 10ms to switch. After that your power dissipation is just a function of whatever is the voltage drop across the contacts. Datasheet lists 75mΩ max, but if you check the charts further down you'll see its typically closer to 40mΩ which is equivalent to about 3" of 1mm PCB trace.



When you compare this to the RDsON spec of MOSFETS, keep in mind that semiconductor manufacturers almost always provide this spec for the semiconductor junction alone, and it does not include the additional resistance of the package. You'll usually have to look up the specs of the package and do some math for an apples-to-apples comparison.

This was just the first result in my search. There are probably lower power options with lower contact resistance. Here are some more options.

 

Ok, here's my first attempt at something simple and possibly workable:

​

The key to the schematic shown above is that the nFet I'd be using is of the Depletion Mode type.

The arrangement would draw 0.6 µA from the lower part of battery bank B, but that would be acceptable. My main concern is if the 10M resistor will be quick enough to pull the nFet's gate to ground when bank B is disconnected before the big fat cap runs out of juice while battery bank A kicks in.

 

I am assuming that the lack of comments so far about my last post mean that there's a general agreement that things might work the way I'd like them to?

 

A depletion-mode N-FET requires a negative gate-source voltage to turn off, so I don't see how your circuit will work(?).

 

A depletion-mode N-FET requires a negative gate-source voltage to turn off, so I don't see how your circuit will work(?).

And here I thought that it would turn off using a positive gate-source voltage

 

If you have a spare pin on your MCU, you can connect it to the Gate of an N-channel Mosfet. Keep it as a High Z input until you want it to quickly switch the Mosfet low.

 

Keep it as a High Z input until you want it to quickly switch the Mosfet low.

Why not keep the output low rather than High Z?

 

Ok, how about this? A little clumsy and inefficient, I'd say. But it's a start?

​

 

I think this is a better circuit, using a single nFet:

​

 

https://www.analog.com/en/products/ltc4412.html

https://www.analog.com/en/products/ltc4411.html

https://www.analog.com/en/products/ltc4358.html

 

I think this is a better circuit, using a single nFet:

So why do you need the diodes on top?

 

So why do you need the diodes on top?

Excellent observation. I hadn't considered that ... still, the diodes in the middle (6V) are a necessity, right?

 

the diodes in the middle (6V) are a necessity, right?

On second thought, you do need the top diodes since there is a sneak path through the Bank A battery to the MCU regulator.
And the middle diodes prevent a sneak path in the reverse direction.
Use Schottky diodes for minimum forward voltage drop.

But if you ran the MCU from the 15V, you wouldn't need any diodes.
I see no significant difference in battery life by doing that.

 

But if you ran the MCU from the 15V, you wouldn't need any diodes

I can't. The regulator I chose accepts a maximum input of 7V. And I chose it because it is extremely efficient, with only 0.5 uA of quiescent current.

 

Make sure your big fat cap is low leakage. That leakage can contribute to shorter battery life.

 

Use Schottky diodes for minimum forward voltage drop.

Doing that already... and thinking about using pFets configured as diodes instead.

 

thinking about using pFets configured as diodes instead.

Note P-MOSFETs cannot work as diodes for that scenario without some addition control circuitry.