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I have a Silverline Trickle Charger that reads 'Output 12V DC - 500mA'. https://www.amazon.co.uk/Silverline-634004-Trickle-Charger-500/dp/B001C827ZA

I heard the battery bubbling after a few minutes on charge so I took the charger off the battery and put a multimeter across the crocodile clips and the voltage read 20V. I realise that to charge a 12V battery (motorcycle battery in my case) the voltage needs to be a bit higher to overcome the resistance, but this sounds like too much to me, then again I don't know much about electricity.

  1. Is this charger faulty?
  2. Am I testing it in the correct manner? or should I test voltage whilst under load on the battery or something?
  3. Will 20V damage the battery?
  4. Also, I know how to test amps, i.e inline, but when testing the amps, this should be at or below the 500mA listed?

Thanks.

  • Welcome to Motor Vehicle Maintenance & Repair! – Pᴀᴜʟsᴛᴇʀ2 Oct 15 '18 at 21:51
  • There is a chance that the multimeter is faulty. – Steve Matthews Oct 16 '18 at 13:25
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  1. Is this charger faulty?

20vdc seems excessive. It has to be over 12vdc to actually charge, but that should be something like 13.5-14v, not 20v. That doesn't, however, mean the charger if faulty (see Q2).

  1. Am I testing it in the correct manner? or should I test voltage whilst under load on the battery or something?

You should test it while charging. Testing it directly to the charger may have produced erroneous results for you.

  1. Will 20V damage the battery?

If it truly is charging at 20vdc, then yes, it can damage your battery.

  1. Also, I know how to test amps, i.e inline, but when testing the amps, this should be at or below the 500mA listed?

You can test amps inline, but you can also use a clamp on ammeter. That said, with the device rated at 500mA output, that's all you can expect. It shouldn't actually go over it, but it might by a little bit. It will only produce what it will produce. If it puts out a little more, it's because the charger can do it, not because the battery is drawing more than the charger is designed to put out.

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If the charger is not loaded while testing, there is an apparent voltage increase due to the reflection of AC in the transformer then rectified into DC.

This is common behavior for this type of circuit, and the unit must be tested under load, not OCV.

  • These types of "wall wart" power supplies and battery chargers don't use transformers any more. A transformer is much bigger, heavier, and more expensive (lots of copper wire!) than a cheap and tiny integrated circuit chip. – alephzero Oct 16 '18 at 13:18
  • Agreed for your cell phone charger, but I'd really like to see an automotive battery trickle charger that is all IC. Those tiny chips can't handle the current. Even a measly 500 mA at 14 volts is 7 watts - you're into laptop power supply range. The Amazon page says the shipping weight is around 13 ounces, and I doubt the packaging weighs anything. Methinks there's more in the "wart" than an IC chip. Line voltage is 200 in this case, I think. Of course I could be wrong. – SteveRacer Oct 17 '18 at 3:00
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The Amazon web page says this is a constant current trickle charger. If that is an accurate description, it will try to supply 500mA and adjust its voltage to whatever it takes to do that.

When you measured the voltage with no battery connected, it was trying to push half an amp through your voltmeter, which is impossible because the voltmeter as a very high resistance. The 20V you measured is probably the maximum voltage that the charger can produce.

If your battery is so "damaged" that it needs 20V to send half an amp of charging current through it, then it is already useless (i.e. replace it), and the charger can't make it "worse than useless".

To get a useful voltage reading, measure the voltage with the battery connected. That should give about 14 to 15V, when the battery is fully charged, as other answers have said.

FWIW the voltage of a fully charged "12 volt" battery which is not supplying any current is about 13.2V, not 12V. If you measure the voltage when it is not connected to anything else, a meter reading of 12V is actually bad news, not "OK".

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Thanks for the answers all. I tested whilst charging as advised... 13.29 V, and 0.33 A, so appears all is good.

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