Charge Indicator

Thread Starter


Joined Aug 9, 2006
I am having a mental Blank,

I have a 12v battery
This I charge from a small wall charger.
I need to put an indicator light on the battery to indicate when it is charging.

I can't put it across the circuit as it will remain on when I remove the charge plug.

I can't put it in series as it will restrict the amp flow to the battery.

I know this is easy but my brain is not functioning today.



Joined Jan 10, 2006
More info..... What sort of Battery..(lead acid/ NiMH/ Nicad/Lithium etc etc) AH rating...What is the correct charge current and voltage.... what is the plug pack actually providing (V and A).... does the plugpack have an internal regulator..?? are you using a separate charging regulator, are you using any charge regulator..... etc etc...


Joined Jan 17, 2007
This is not as easy as you make it sound. If the battery is charging, the current flows into it. If the battery is full, the current doesn’t flow into it whether the plug is connected or not. So, you need a current-sense amplifier that outputs voltage proportional to current – V(I). Then you can compare V(I) to some threshold value and drive the LED with a comparator output.


Joined Sep 21, 2007
If it is just a transformer plus rectifier charger it will have a higher voltage when in open circuit and the voltage will decrease when connected to the battery. You can deduce the current output indirectly by measuring the voltage or you can just have a LED switch on or off when the voltage is below, say, 14 V.

That answers your question as worded but I wonder if you may be under the impression that the charger stops charging when it considers the charge is complete and therefore you want to know when the charging has stopped. My guess is that this is most probably not the case and the charger just continues charging forever. To be 100% sure you'd have to do some measurements though. Does the voltage of the battery fall at some point? That would be an indication that charging has diminished or ceased.