I cannot understand what has happened but I have a single 18650 mains operated battery charger which I've used for several years to charge the 18650 battery in a LED torch. Whenever the battery needed to be charged I would of course place the battery in the charger with the positive end of the battery towards the positive connection pad and toward the + sign in the charger and negative against negative against the negative spring.
The battery needed charging the other day so I placed the battery in the charger as I do and left it to charge but the green LED light remained on as if the battery was already charged. I tested the battery with a multimeter and the battery only had a few mVolts. So I placed the battery again in the charger. After about 12 hours and with the green light constantly on, I took the battery out but again the battery had no charge. I couldn't work this out.
I then tested the charger and the positive terminal is now negative and the negative terminal is now positive. I placed the battery in the charger the other way around and the red LED came on and after 12 hours the green LED was on and the battery is now fully charged and usable.
The charger shows about 4.2v on a multimeter.

Why has the positive and negative terminals of the charger now got themselves reversed?

 

I hate to say this, but I find that very hard to believe. The only way can think of that makes any sense is that a prankster opened it up and reversed the wiring. Is there someone around who might have dine that? Can you post a photo of the charger so we can see what we are dealing with?

 

Why has the positive and negative terminals of the charger now got themselves reversed?

Please get an AA, C or 9V battery and test it with your meter. It is common for the leads on a meter to be wrong. (Reversed Red and Black.)

 

This is the said battery charger.
I HAVE NOT opened it up.

I HAVE NOT opened it up

 

Please get an AA, C or 9V battery and test it with your meter. It is common for the leads on a meter to be wrong. (Reversed Red and Black.)

He put the battery the other way and it charged fine. Not meter leads.

 

I hate to say this, but I find that very hard to believe. The only way can think of that makes any sense is that a prankster opened it up and reversed the wiring. Is there someone around who might have dine that? Can you post a photo of the charger so we can see what we are dealing with?

The other possibility is that the battery leads are wrongly wired internally.

 

This is the said battery charger.
I HAVE NOT opened it up.

I HAVE NOT opened it up

Did someone claim you did?

The other possibility is that the battery leads are wrongly wired internally.

Read the initial post again. It used to be correct and then magically reversed. I suppose it is possible that the charger senses the battery polarity and automatically corrects and the that function started failing.

 

Sounds fishy to me ........
A complete impossibility.
.
.
.

 

A real puzzle. Did you try @ronsimpson 's suggesting of measuring a different battery? Not sure what it would tell us, except perhaps whether the meter had mysteriously switched its polarity. That has not been completely ruled out.

 

A real puzzle. Did you try @ronsimpson 's suggesting of measuring a different battery? Not sure what it would tell us, except perhaps whether the meter had mysteriously switched its polarity. That has not been completely ruled out.

How could a misbehaving meter cause the charger to charge a battery inserted incorrectly and not change one inserted correctly? Because that is what TS is claiming. Additionally, he claims it used to work correctly.

 

The meter is working perfectly and fine. If someone does not believe me then I can post photos.
It's just I do not understand why the polarities have reversed. I have never seen this before and cannot understand it... And this is not a hoax!!
I used to plug in the battery the correct way with the negative end of the battery towards the spring, but now I have to plug it in with the positive against the spring.

 

Where is the rest of the unit?
Does the charger come directly out of a wall-wart etc.?

 

How could a misbehaving meter cause the charger to charge a battery inserted incorrectly and not change one inserted correctly? Because that is what TS is claiming. Additionally, he claims it used to work correctly.

That would rule out one problem. If he wants to get to the bottom this he probably should not ignore leads as to where the error(s) are.

I believe some batteries can be charged in reverse, especially if they are dead, though I don't know about Lithium cells.

 

And this is not a hoax!!

This is the second time you have denied something no one accused you of. Now I am suspicious.

I did give you two possible explanations, neither if which you have commented on. Both of these suggestions could be further investigated by you, so why no effort on your part?

 

Both of these suggestions could be further investigated by you, so why no effort on your part?

If the OP is driving a Ford Prefect as his name suggests, he could have a lot more problems on his mind. !

 

Let's give Ford Perfect, a long-time member in good standing the benefit of the doubt as long as he tries to find the reason for this strangeness.

Not following up on your ( @BobTPH ) questions is a cause for some concern, but in my personal opinion I think it is way too early to start tossing accusations around.

 

Just a wild thought. I assume the charger operation is controlled by an IC. If (a big IF !) that IC is one with some setting for selecting either a positive controlled output or a negative one (I don't know if such ICs even exist), perhaps it's lost the setting and gone into negative output mode ?

 

I'm certainly not one to mess people around and make fictitious jokes as BobTPH (impolite comment deleted by moderator) suggests and no thanks for his unkind comments, suggestions and accusations that I'm a fake!

I've been a member here for 12 years (longer than BobPTH) and I'm not the sort of person who gets my kicks making fools out of people.

And thanks DickCappels for your kind support.

Sometime when I have time, I will connect the meter to a reliable power source or battery and take a photo.
I will take a photo of the meter connected to the 18650 battery.
And yes, the charger is plugged into a 240vAC power supply
Also my comment saying ' I HAVE NOT opened it up' is to emphasise that I have not opened the charger and rearranged the wiring just in case anyone suggests this.
Is there anything else that you would like?
Maybe I'm not seeing or not realising something, I'm totally baffled .. I do not know.

Below is the battery charger without anything attached and is also connected to the 240vAC power supply.
I have circled the positive end .. but which is now the negative end.

 

I think BobPTH has the right idea with this being a polarity sensing device. Either it is broken and stuck in reverse polarity, or it has latched into last used state and is just waiting for you to put a reasonably charged battery in the correct orientation to set it back to top positive. Maybe that first battery was just too low of a charge and it got confused?

 

Below is the battery charger without anything attached and is also connected to the 240vAC power supply.

So there is nothing else but what is shown in the pic?
When you say "plugged into the 240vac" is there anything else in between, i.e. SMPS etc.?