Float charger output voltage rapidly fluctuates when there is no load, is that normal?

Thread Starter

yourminky

Joined May 31, 2019
28
I just bought a float charger from Harbor Freight for my car battery. I hooked up a voltmeter with no other load and the output voltage reading from the float charger was rapidly fluctuating back and forth from 0 volt to 2V and sometime up to 8V but never reached 13+ volt like it was supposed to. However, when I hooked up the float charger with the car battery (load), the output voltage was no longer fluctuating and it slowly and steadily rose from 12.6 volt to 13.7 volt like it was supposed to and 400 mA current was flowing steadily through it.
Is that no-load fluctuating voltage a normal behavior? What is the reason?
 
Last edited:

Alec_t

Joined Sep 17, 2013
11,276
Is that no-load fluctuating voltage a normal behavior? What is the reason?
It's normal. Chargers/power-supplies/voltage-regulators and their ilk usually rely on voltage fed back from their output to compare with a fixed reference and so provide a difference signal which controls the output. If the output is left floating then the difference signal is arbitrary or out of tolerance and the control is erratic.
 

jpanhalt

Joined Jan 18, 2008
9,391
I just bought a float charger from Harbor Freight for my car battery. I hooked up a voltmeter with no other load and the output voltage reading from the float charger was rapidly fluctuating back and forth from 0 volt to 2V and sometime up to 8V but never reached 13+ volt like it was supposed to. However, when I hooked up the float charger with the car battery (load), the output voltage was no longer fluctuating and it slowly and steadily rose from 12.6 volt to 13.7 volt like it was supposed to and 400 mA current was flowing steadily through it.
Is that no-load fluctuating voltage a normal behavior? What is the reason?
How do you know the no load voltage should be +13V? That is not in the manual.

In fact, I have about half a dozen of those chargers for my equipment during the Winter and one car that is not used. You will also note that the LED doesn't come on without a load. I consider both safety features somewhat. The LED confirms that the charger is connected correctly.

Here is one version of the schematic in the Internet (Source: http://www.desert-home.com/2012/02/battery-charging-harbor-freight-item.html ):

upload_2019-6-16_5-12-31.png
 
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Thread Starter

yourminky

Joined May 31, 2019
28
How do you know the no load voltage should be +13V? ......
You will also note that the LED doesn't come on without a load......
View attachment 179824
My old HF float charger always has a 14V output even when not connected to a load.
Both float charger LEDs light up even without a load, just by connecting to AC.
I opened one of the power adapters and it has a significantly more complicated circuit than the one you published above.
Different models, one has a fat power adapter, the other one has a skinny adapter with switching symbol.
 
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