battery drain using a float charger after power failure

[Brucehaslam]

I have a Harbor Freight cheapo float charger which I have used to maintain my motorcycle battery.  The red light is on when the polarity is correct and the power is supplying the charger.  However, when the power is disconnected (ie  my grandkid pulled the extension cord out of the wall) the red light remains on and, over time, drained the battery.  My thought is to use a diode on the positive cable to prevent the battery from back charging the float charger.  First of all, will this work?  Secondly, is it a good idea and thirdly, if the previous two questions are a yes, what type of diode should I use?  Note:  I probably don't need to tell you that I am dumber than a sack of hammer handles when it comes to electronics!

[Peabody]

If your charger needs to read the current battery voltage, then the diode would mess that up.  And even if it doesn't, there would be a voltage drop across the diode of about 0.6 - 1.0V, so the battery might not be fully charged.  Maybe it's time to move up from Harbor Freight to Autozone, and see what a more capable charger would cost.  But first, check Google and Youtube under the model number of your charger to see if anyone has already solved this problem.

[floobydust]

Welcome to the forums

What model is the Harbor Freight charger?
This guy did mods (add diode and voltage trim) Battery Charging Part 2  Harbor Freight item 42292.
Part 3 went to LM317 and missing an output diode, it looks unfinished.

[Brucehaslam]

Thanks for the reply.  I think you are right.  I need to upgrade to a better float charger. 

[David Hess]

It is possible to make an ideal diode circuit using a power MOSFET and comparator which will not drain the battery and allow the charger to work properly, however I think it would be better to buy a better charger.  I have used and can recommend both of these linked below, or similar chargers from these companies:

https://www.amazon.com/gp/product/B07W3QT226/
https://www.amazon.com/gp/product/B01FG7NW60/

I cannot test the MOTOPOWER charger because I gave it to my father, however I tested the NOCO GENIUS10 which I keep in my pickup and with the power removed, it draws 14 microamps from the 12 volt battery which is insignificant.  I would assume that the cheaper NOCO GENIUS1 will work just as well.