Is there such a thing as a fast acting voltage or current sensor that can be put parallel, wired up to a relay to open the relay if the voltage drops to protect a 12v battery charger ?

 

To protect a 12V battery charger from what?
A 12V charger for what sort of battery?

 

Its a generator that gets used poss twice a year but needs testing every 6 weeks to make sure it starts, the battery is on a charger, sometimes they forget to disconnect the charger and cranking the generator over cooks the charger, Ive found this

https://www.panucatt.com/Current_Sensor_for_Arduino_p/cs-200a.htm

Something like that might do the job but would rather have something simple that doesnt need more electronic wizardy

 

It's the charger that needs sorting out. If it is a lead-acid battery, and the charger is correctly specified it will have a current-limited bulk charge, so loading it with the starter motor should do it no harm.

 

Simplest solution is to get a decent charger that is reverse current protected.

 

Are you sure it is the starter motor cranking that damages the charger? Does the generator have an alternator that charges its battery? Is it the voltage from the alternator that damages it? (As @Irving implied) If so, put a diode in series with the charger and increase the float voltage to compensate.

 

Problem is these people have no spare money so unfortunately the simplest solutions wont work, yes its definitely starter amp draw that is cooking the charger, I dont mind buying, building and installing a little circuit to protect the charger if I can to help them out

 

During the long cold NE Ohio USA winters I leave my motorcycle on a battery maintainer so I can periodically go in my garage and start my bike. Many times the maintainer/charger is left connected and I have never had a problem.

• CHARGE AND MAINTAIN: 12V / 6V 1.5A Battery Charger for 6 volt and 12 volt SLA / VRLA / GEL / AGM / WET Lead Acid batteries

Typically between $15 and $20 USD. I have never managed to cook one, if the load becomes too great it auto disconnects. There is no need to get complicated. No need to build something. Units like this typically can deliver 750 mA to 1.0 amp.

Ron

 

A Modern-Electronic-Battery-Maintainer will not be adversely affected by
any circumstance other than a direct-Lightning-Strike very close by,
and costs less than ~$50.oo at Walmart or your local Auto-Parts-Store.

I've had mine permanently attached to my Truck-Battery for around ~5-years now.
If the Truck will not be used for several days, it will be Plugged-In until the Truck gets used again.
My Truck gets used roughly twice a week, and spends all other time,
out-doors, on the Battery-Maintainer, with the same Battery.
.
.
.

 

If a much slower recharging is acceptable, then a simple series resistor can limit the current delivered by the charger during the engine cranking time. One option would be a series light bulb, such as a 12 volt backup light.When it is not lit the resistance is quite a bit lower, once it lights the resistance increases and limits the current..

However, it seems to me that if the charger is damaged by the temporary cranking overload, THEN THE WRONG CHARGER HAS BEEN SELECTED!!! I am guessing that the battery charger was selected exclusively by a lower purchase price. Suddenly that price has increased due to needing a replacement to be purchased.

Replacing an incorrectly applied part with another of the same incorrect devices is not any solution. Not ever!
Another option would be to add the instruction: "Switch off the battery charger." to the starting sequence instructions.

 

As an alternative, you could use a 12V SPDT relay to connect the charger to the battery using the NC contacts, and with the relay coil powered when the ignition switch is on.
That way the charger is disconnected whenever the motor is started/running.

 

As an alternative, you could use a 12V SPDT relay to connect the charger to the battery using the NC contacts, and with the relay coil powered when the ignition switch is on.
That way the charger is disconnected whenever the motor is started/running.

1st positive answer, Thanks,

yes good idea, it only needs when cranking, the charger dont seem to mind the alternator voltage when the engine is running

 

1st positive answer, Thanks,

yes good idea, it only needs when cranking, the charger dont seem to mind the alternator voltage when the engine is running

You mean the first time someone has given you the answer you wanted?
Go ahead, put a relay in it, and you have simply masked the fault with your charger which is that the current limitation doesn’t work. Then next time it is faced with a fully discharged battery it is going to fail because the current limitation doesn’t work.
What else is wrong with your charger? Is the float voltage correct? If not, you’ll be spending a fortune on replacing batteries.

 

the charger dont seem to mind the alternator voltage when the engine is running

But I see no reason to leave it connected when the engine is running.
Do you?

 

the fault with your charger which is that the current limitation doesn’t work.

How are you certain that's the problem?

 

How are you certain that's the problem?

By a process of elimination.
Two possible causes of damage: over voltage and over current.
TS says that it is not caused by the voltage from the alternator feeding back into the charger, so that rules out the over-voltage situation.
The only remaining possibility is that an overcurrent situation occurs when the battery voltage is low.

 

Its definitely overcurrent, Ive seen some of these big, on wheels charger / starter units shit out even on start setting when cranking a diesel engine as they do draw some, that why I was thinking of using something like I posted a link to in post 3, any thoughts?

 

A proper battery charger, when connected to a battery which is below float voltage, outputs a constant current. Therefore there is no possibility of overcurrent.

 

Its definitely overcurrent, Ive seen some of these big, on wheels charger / starter units shit out even on start setting when cranking a diesel engine as they do draw some, that why I was thinking of using something like I link to in post 3, any thoughts?

Whatever the cause for the charger failure (current or voltage spike), the relay connection I proposed will eliminate it.

 

Whatever the cause for the charger failure (current or voltage spike), the relay connection I proposed will eliminate it.

What makes you so certain?
Suppose the battery voltage dropped because someone switched off the charger for a few days. Wouldn't the charger be in the same situation as when the battery was loaded by the motor?