Right now I have a device that works with a 24V NiMH battery. This device has a connector where the charger plugs so the battery can be charged, obviously, the problem is that recently a piece of metal got into this connector making a short circuit and showing a big pretty shiny light (an electric arc) leaving a big nasty black mark on the connector.

Now I'm having trouble finding a solution to prevent this. The idea was to put a protection circuit that disconects this connector from the battery when it the carger isn't connected.
I tried a relay to use only enable the circuit when the charger is on (and powering the coil) but once the circuit energized, the battery keeps the coil energized so it never shuts down.

Right now I have no idea which kind of circuit could disconnect the connector pins from the battery when charger is not plugged in.
Ideas ?

 

how about a fuse or circuit breaker?

 

how about a fuse or circuit breaker?

Yes and no.
The problem with the fuse would be to replace it, the idea is to just prevent the short circuit but with the possibility to function after the short is fixed.
The problem with the circuit breaker (as far as I know) is that it has a voltage drop which could prevent the battery to charge to its maximum capacity, this is why I was searching for a circuit with no voltage drop. Am I mistaken here?

 

I use AirPax circuit breakers for my chargers. The 30 amp unit that I use has .003 ohms resistance (DC).

 

I use AirPax circuit breakers for my chargers. The 30 amp unit that I use has .003 ohms resistance (DC).

Do they exist with auto reset?
I need it to be able to reset itself without human interaction.

 

Post a picture of the plug and socket.

Any room for a reed or prox switch to activate relay?

 

The problem with the circuit breaker (as far as I know) is that it has a voltage drop which could prevent the battery to charge to its maximum capacity, this is why I was searching for a circuit with no voltage drop. Am I mistaken here?

This is not true in practice. The first charger phase is a constant current bulk charge until the battery reaches the transition voltage. At this point the charger then goes in to a constant voltage mode until the current drops below a threshold (1/10 to 1/20 of max). Even though there is a voltage error due to IR loss, the charger will stay in constant voltage mode and as the current drops, the errors due to IR drops with it. Your charger IR losses should not be evaluated at max current, they should be evaluated at the "end of charge current, which is normally 1/10 to 1/20 the max current.

 

How about a self-resetting PolyFuse?

 

Here is a data sheet for the AirPax CBs I use. You can get a remote trip feature but I do not see a remote reset feature.

MikeML - A polyfuse is worth looking at.

Tglaria - What is the max current?

 

I would agree with everyone here so far.
And Say you would be well served with a PTC Resettable Fuse.
Digi-Key has them under the breadcrumb trail of: Product Index > Circuit Protection > PTC Resettable Fuse

They go from 14ma to 50A trip current.

 

Post a picture of the plug and socket.

Any room for a reed or prox switch to activate relay?

The socket is this one:

I don't see space to put something in it, tough I'm willing to evaluate any option.

This is not true in practice. The first charger phase is a constant current bulk charge until the battery reaches the transition voltage. At this point the charger then goes in to a constant voltage mode until the current drops below a threshold (1/10 to 1/20 of max). Even though there is a voltage error due to IR loss, the charger will stay in constant voltage mode and as the current drops, the errors due to IR drops with it. Your charger IR losses should not be evaluated at max current, they should be evaluated at the "end of charge current, which is normally 1/10 to 1/20 the max current.

The battery is a 24V 4.2Ah NiMH one, during charge, the current measured with a multimeter is around 4A.
Since I haven't been able to find an auto reset circuit breaker that would work between 24-40V, would a simple diode work in this case?
I'd have a constant voltage drop and should work with 4A through it, but other than this voltage drop, acording to what you wwote and understand, it should still work ok, right?

How about a self-resetting PolyFuse?

I'll check it out, don't know them.

Here is a data sheet for the AirPax CBs I use. You can get a remote trip feature but I do not see a remote reset feature.

MikeML - A polyfuse is worth looking at.

Tglaria - What is the max current?

I couldn't find a auto or remote reset, if all else fails, I guess I could go with a manual reset one, but it's not the ideal.
The battery is a 24V 4.2Ah one, last times I charged it, the current measured with a voltmeter was around 4A during charge.

I would agree with everyone here so far.
And Say you would be well served with a PTC Resettable Fuse.
Digi-Key has them under the breadcrumb trail of: Product Index > Circuit Protection > PTC Resettable Fuse

They go from 14ma to 50A trip current.

I'll check them out, thanks.

 

I'll look at the data sheet.

Is the design fixed?

Could two shorted pins be added for relay activation? ie. 5 pin.

Oh wait. This is three pin. Do you use all three?

 

I'll look at the data sheet.

Is the design fixed?

Could two shorted pins be added for relay activation? ie. 5 pin.

Oh wait. This is three pin. Do you use all three?

The connector is a three pin one and all three are used.
The idea was originally to not change the connector (I was thinking in using 4 pin connector, have two of them connected to the power line and use one to activate the realy coil and the other one to feed the battery, keep the ground pin as shared one).

 

A fuse/CB is the "preferred" method.. And may be required depending on the nature of this device and if "safety approvals" may be required.
Fuses also have a Vdrop due to the resistance of the element.
I call fuses "little heaters"

A PTC can be used but they have "cons" too..

http://www.littelfuse.com/technical-resources/education-center/fuses-vs-ptcs.aspx

 

The connector is a three pin one and all three are used.
The idea was originally to not change the connector (I was thinking in using 4 pin connector, have two of them connected to the power line and use one to activate the realy coil and the other one to feed the battery, keep the ground pin as shared one).

If possible, the extra pin seems a good idea. +1

 

If possible, the extra pin seems a good idea. +1

Apparently it's possible, I'll go this way.

I'd still like to know if there's an alternative using a circuit rather without this extra pin.
Oh well, apparently not.