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Bench Power Supply Battery Charging Circuit

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steve1515:
Hello Everyone.  :)

Occasionally I use my bench top power supply to charge 12V batteries. When I've done this in the past, I've put a diode in series (10A02-T) with the battery so I don't damage my power supply should the battery voltage rise above that of the supply. Basically, I want to protect against pushing current back into the supply. I usually put a meter at the battery terminals and set the voltage. Usually 13.8V or 14-15V depending on the battery markings. The problem I have is that the voltage at the battery terminals changes depending on the current going into the battery. My power supply does not have sense terminals, so I have to keep adjusting the voltage to compensate for the drop across the diode.

Is there a way to protect my power supply with some kind of simple MOSFET circuit instead of the diode? I'm trying to get it so I can just set the voltage and the power supply can regulate the voltage without having to deal with a diode drop.

Any tips?  :-//

MarkF:
Here is a circuit I found on the internet.  Never built it.
There is a YouTube video demonstrating it somewhere.

The resistor in 'red' adjusts the charging current.
There are a few resistor values shown at the bottom with the corresponding current.

The 5K pot adjusts the charging voltage.

The LED indicates when the circuit is charging.




This isn't the video.  But, here is a description:  https://www.eleccircuit.com/gel-cell-battery-charger-circuit/

Ian.M:
There is such a thing as a single chip ideal diode controller, designed to be used with a MOSFET to replace a Scottky diode in many power rectification applications.

Take a look at these two datasheets:
http://www.farnell.com/datasheets/85443.pdf
http://www.farnell.com/datasheets/78928.pdf

The low voltage one is viable for single cell LiPOs and the HV one starts off at a useful start of charge voltage for 12V and higher Lead Acid battery packs.  If its below their minimum voltage you'd want to trickle charge to bring the voltage up and the MOSFET body diode can handle that.  The controller will cut in and eliminate the diode drop before the battery reaches a normal CV mode float voltage.  If you need to charge single cell (or even two series) NiCd or NiMH, they wont do the job as there is insufficient gate bias supply, but as NiMH are ill suited to CC/CV charging (even a C/100 trickle charge sustained past full charge degrades NiMH life), and need some 'smarts' in the charger to terminate the charge before you cook them, you shouldn't be trying to use a diode + CC/CV PSU as a charger for them anyway.

Another simple option is an old laptop PSU, or any other convenient power source + a cheap CC/CV buck or boost module* + a relay running off your 'raw' power source to isolate the battery under charge if the mains supply fails.

* e.g: https://www.ebay.co.uk/itm/DC-DC-Step-Up-Down-Boost-Buck-Voltage-Converter-Module-CC-CV-LM2577S-LM2596S-/281777880409

ledtester:
I understand that the LM317 can be damaged by reverse voltage, i.e. V_out > V_in, and that it is advisable to put a shunting diode across pins 2 and 3 to mitigate that situation.

So how about putting a diode on the + output of PS1 and a load resistor from the LM317's IN to GND? That would route current from an overvoltaged battery around the LM317 and prevent it from entering the power supply.

Something like:



Jwillis:
Depending on the battery it may be advisable to place in a current pass transistor since the LM317 max current is only about 1,5 amps. For example a 10Ah lead acid will usually bulk charge at around 3 amps which would quickly destroy the regulator.

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