Hi Gents,
I'm trying to build a battery wich is going to used on salty water, the package is waterproof, no issue, but the charge contacts are getting oxydated due to battery getting discharged trought the salty water.

to avoid that I've firstly used a simple diode to avoid current to flow between battery contacts, but obviously when I'm charging the battery there is a voltage drop due to the diode :


I'm wondering if there is a solution to avoid any voltage drop during charge?
I've tried to use a low Rdson MOSFET, but I still got 200mV of voltage drop which makes the battery full charge not possible.

If anyone got an idea / commentI will appreciate.
regards
Bruno

 

What is the primary voltage source? Can it be changed? Adjusted? Modified? The reason for asking is because if you're dropping 200mV a simple increase of 200mV will overcome the forward voltage of the MOSFET.

Another question is: "What's the tolerance?" You might already be running at a slightly higher voltage than you suspect. Numbers don't lie, but reality often differs from numbers. If a full charge on your battery is 4.2V and you're only charging it to 4.0V that doesn't seem (to me) to be much of a loss. Can you live with that?

Off hand I don't know of a way of avoiding a voltage drop during charging. My solution would be to provide more than enough voltage and then let an on-board charge controller manage the battery voltage.

Now: I don't know what you're doing exactly. If you're thinking you need 4.2V to charge a 4.2V battery - that's not correct. You need a higher voltage than the battery voltage otherwise your full charge time would be on the order of months. I'm no expert on battery chargers so exactly how you achieve this is up to someone else with more experience and knowledge than what I know. Perhaps sharing more of what you're doing will help us answer your question in a way that best serves your plan.

 

You can't use a MOSFET as an ideal diode in this situation without some additional control circuitry.
Do you want to consider that?

What charger are you using?

 

Hi gents,
@ ThePanMan
Unfortunately I cannot increase the charger voltage to much, I've got some room to adust it but not more than 50mV.
About the loss of 0.2V on the battery charge it is a bit tricky, because the battery capacity is displayed to the user and will never show 100% battery capacity after a full charge.
You are right the output voltage of the charger with no loads is arround 4.25V, but the charger is detecting a full charge when the current injected into the battery drops under a certain treshold and reach +/- 4.2V on our case.

@ crutschow
I'm using a mascot 2241 LI :
https://www.mascot.no/catalog/medical/2241li/c-24/c-1323/p-168
What do you mean by additional control circuit?

Regards
Bruno

 

You have not clearly stated what problem You are trying to solve and the circumstances surrounding it.
You have only briefly described the first complication that You are wanting to try, to solve the problem.
It's not going to solve your problem.
Salt-Water on exposed Battery-Contacts ?????
That's not going to to survive no matter what You do.

What type of device are You working with ?
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First, we don't know what kind of battery you're trying to charge. Only that you're calling it a 3.6V battery. Different chemistry batteries require a different charge profile. So by way of example I'm going to talk about a 12 volt lead acid battery. You don't charge a 12 volt battery with 12 volts. You'll never get a full charge. You need to make at least 14 volts available for the battery to charge. Then it should be maintained at a float voltage of 13.8V. So assuming you're charging a 3.6V battery with 3.6V - that'll never charge.

So - what kind of battery are you trying to charge?
What voltage are you using to charge it?
How fast do you want to charge it?
That's important too because some batteries can not handle a high rate of charge without damage or danger of bursting.

 

The system is a kind of waterproof cell phone,
The battery is a single cell li-ion battery we are using it on the range from 3 to 4.2V, the charge current is +/- 1.3A.
the issue is that the charge connector is waterproof even without a mating connector, but in salt water, as the contacts are near to each other there is a current flowing between them causing oxydation of the contacts and unwanted battery discharge.
in clear water there is no problem, and using the diode is solving the issue in salty water but with a voltage drop will charging.

I've tried a kind of protection with the connector to avoid water to reach the contacts but users are loosing them so...

 

What's the voltage of your charging source? It's got to be higher than 4.2V

 

Why not use a connector with a sealed plug to protect it, something like this:

 

What do you mean by additional control circuit?

A MOSFET with an op amp, example circuit below:
(The SW is just for simulation purposes).

 

Eliminate the connector,
then charge the Phone exclusively with an Inductive/Wireless Charger.
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