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I have a device that runs off SLA batteries that needs to be charged in a dirty environment. There is a possibility that the circuit may be shorted by accident during the charging cycle (this is unavoidable and must be protected against from a safety standpoint). I know that the smart chargers we are using will be able to handle the short but I'm worried about the batteries themselves shorting out.

I thought about a simple diode inline but with a 15A charging current I'm worried about heat buildup on the diode. Also, I don't know how the smart chargers will react to a diode inline when it expects only batteries (i am making the assumption that the smart charger wont see the diode).

What is the best way to protect the batteries from a short circuit in this situation?

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A diode won't work anyway because the load will be connected to the battery, not the charger.

It sounds like this can happen often enough that a fuse is not a good option because it would have to be replaced too often.

This sounds like a good application for a circuit breaker. A breaker may take a few 100 ms to trip, but nothing bad should happen in that short a time. There are different types of breakers. The basic kind needs to be manually reset, but most of those have the best characteristics in terms of reaction time, on resistance, and accuracy. They usually work on the current causing a magnetic field that trips a mousetrap type of latch that opens a switch. The automatically resetting type is usually thermal with a bi-metallic element activating the switch. They may take a little longer to trip, and usually stay tripped for a few seconds. Check the specs. If you can tolerate the little extra on resistance, this type is probably the most convenient if you regularly expect the power wires to be shorted.

There are electronic ways to do really fast overcurrent switches, but it doesn't sound like your application requires this level of complexity.

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  • \$\begingroup\$ How about a resettable fuse? \$\endgroup\$ – Daniel Tork May 21 '18 at 18:27
  • \$\begingroup\$ @Dan: That could be a good option too. \$\endgroup\$ – Olin Lathrop May 21 '18 at 19:31

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