Hi.
Am building a device that it should turn itself on when wetted/submerged, off when not.

There is a 4V lithium cell inside. Needs to be recharged once every while.
The hermetic sealed-for-life case should have external contacts for charging the cell on a craddle and to sense water.

If contact 1 is cell +
If contact 2 is a pulled-down mosfet gate to turn the unit on,
Then, the turn-on feature could work.

But a contact 3 is also needed for common negative to recharge the cell. If there is a negative contact exposed to water/seawater; the gate may not turn on; and the power contacts may corrode by electrolysis.

Is there a circuit strategy that will avoid or greatly diminish the contacts erosion? I wonder if a diode at the minus terminal, allowing recharge but blocking electrolysis current works. Then the charger would have to supply just 0.7V extra.
Any suggestions ?

Edited : added - something like this ? :

 

I think you'll have problems with any exposed contacts, regardless of the internal circuit. This is one situation where inductive coupling of charging energy through the housing would be useful.

 

Are you concerned with the amount of moisture, or the level of water? Or both.

All connections will need to be water tight. Any photos? Size and environmental area of equipment?

And aside from the wetted/submerged function, you need a battery monitoring function also?

 

Thanks, alec_t. You think the diode will not prevent electrolysis of the contacts ? Good option to think about inductive recharging. If you know of suggestions to implement such, come forward.

BR-549 : The device should turn on when submerged only. But nothing is wrong if stays on if rain/splashes/moisture bridge the sense contact. No photos yet. Contacts can be stainless screw heads trough case. Size should be about a pack of cigarettes, scuba use. No battery monitoring needed. If it does not work, recharging need is assumed.

Any suggestions to polish the raw sketch ?

 

If your FET is a P-channel one the resistor should be a pull-up to keep the FET normally off.
You could eliminate one of the contacts, like this :-

Electrolysis could occur between either contact and any nearby metalwork, so I don't think the diode will help there.

Edit: Without battery monitoring, how do you plan to charge a lithium battery safely?

 

I think you'll have problems with any exposed contacts, regardless of the internal circuit. This is one situation where inductive coupling of charging energy through the housing would be useful.

The exposed contacts will be subject to electrolysis (corrosion) - there are plant watering alarm circuits on the web that show how to present symmetrical AC on the probe electrodes.

 

Excellent simple solution, alec_t. Thanks. Similar approach of some old Sony power jacks.
The (+) terminal senses the (-) terminal trough water conductivity and turns on the P gate that was kept off by the 2M resistor to (+).
When dry, applying fixed 4.0 to 4.1 + Vf to the (+) terminal charges the cell to a limit of 4.1 V or less. Do not want nor care to 'inject' more charge into the cell, even if its maximum chemistry allows 4.2V.
Then I consider it 'safely' a-bit-undercharged.

The wet electrolytic current between A and B will be limited by 4/2M + Ir. Should be in the range of minimal electrode corrosion specially for short wet times. I think.

 

Why not make the two sense contacts out of dis-similar metals and use galvanic current as an submerge indicator?
Or perhaps an exposed area of case and one sense contact.

The only marine sensors I have used were always submerged. Corrosion was a BIG and Expensive problem.

But your use seems intermittent. What are you controlling with this pack of cigarettes, may I ask?

 

Galvanic current would favor corrosion. Besides two sensing contacts, there have to be charging contacts, and should not impair functionality interacting. As alec_ t solved, replacing a Nchannel with a Pchannel to liberate/merge a contact.
The device will interact with magnetic fields.

 

Professional conductivity meters use a bipolar square wave driving a pair of electric motor style carbon brushes.

 

Why not utilize a mechanical solution?

Submerged operation only, but tolerant to wetting, just screams float sensor.
When submerged a hollow sleeve with a float inside comes into play. Any one of a number of hall type sensors could perform the on/off function and charge port openings could remain dry behind an o'ring type seal. No electrolysis concerns.