Whats the difference between a battery charger & Transformer to power a 12 V 3 A bilge pump?

Question

I made a common mistake, identical to an earlier post "Using a 12 V 3 A bilge pump on a 12 V 8.5 A LED power supply".

The pump was 3.8 amp so I purchased a 5amp led transformer, which just worked, but failed under load ie (under water). So I understand the problem of initial toque surge load requiring a higher amp transformer (I Think).

The pump works fine on a car battery, but I need a permanent "plug & play" solution. I connected my battery charger to the pump, & it also worked fine. The transformer needs to be on full time so as to automatically empty the sump when water enters.

These are my questions;

1) Why does my 20 year old 4amp 12volt Battery charger (see image) work fine as a power supply for the 3.8amp 12v pump?

2) & can I use the battery charger as a permanent power supply for the pump, ie could it overheat & cause a fire?, as they are not normally left on permanently.

3) As I don't want to waste money buying another unsuitable transformer. Will a " 40amp DC12V Driver AC 230V Power Supply Transformer for LED Strip Light" see image be a suitable power supply for the 3A 12v pump?

It can get expensive buying a standard transformer, but led transformers & battery chargers etc are mass produced, so they are much cheaper options.

thanks for any input.

Answer 1

1) Why does my 20 year old 4amp 12volt Battery charger (see image) work fine as a power supply for the 3.8amp 12v pump?

Probably because the battery charger has different heavy-load behavior then your power supply. i.e. it might have a slow-blow fuse and also respond much slower to being overloaded.

2) & can I use the battery charger as a permanent power supply for the pump, ie could it overheat & cause a fire?, as they are not normally left on permanently.

Regularly overloading any type of equipment is a bad idea. You are then starting to rely on the manufacturer's fail-safe implementation, if there is any!

3) As I don't want to waste money buying another unsuitable transformer. Will a " 40amp DC12V Driver AC 230V Power Supply Transformer for LED Strip Light" see image be a suitable power supply for the 3A 12v pump?

It should be sufficient for the job but then you can have very high currents running.

That by itself is a dangerous situation.

e.g. Your 20 meter cables might have an resistance of about 0.3 ohm. If the far end is short circuited your supply would happily pump 40 Amps through the cable heating everything up.