How to charge batteries from PSU without blowing up PSU

[skillz21]

I have seen some videos online where people have broken their power supplies from charging batteries. My question is, if I wanted to change a battery with a lab bench power supply, how can I make sure that my power supply won't blow up?

[spiette]

Without knowing the battery type and chemistry the only thing I can offer is use a diode in series to block any reverse current into the PS.

[skillz21]

Ok, so would just adding a diode work for the following batteries?:
- 3.7v lithium-ion
- 12v lead acid

[JohnPen]

The best way is to set your PSU to the relevant voltage before connecting it to the battery. Typically 13.8v for the normal 12v car battery.  This will be roughly replicating what the alternator provides in normal driving conditions.  If your PSU has current limiting available you can control the charge rate with that providing you have set the PSU output voltage to ~13.8v or slightly above.  The use of a series diode offers protection of the PSU and prevents the battery voltage from possibly damaging the PSU.  If you have set the PSU output voltage to be greater than the battery voltage there is not a problem. Similarly for 3.7v Li cells etc the PSU output voltage should be ~4.2v.  Note if you do add a series diode you will typically need to increase the PSU output voltage a further 0.6v (schottky ~0.2v) to cover the voltage drop across the diode and the diode must be able to manage the maximum charging current. 

John

[soldar]

Set voltage to a voltage somewhat higher than battery. (Essential)
Set current to a value safe for battery and for power supply. (Essential)
Make sure PSU is protected against back voltage from battery. (The truth is that I never do this.)

[mikerj]

What often kills power supply's when charging batteries is connecting the battery whilst the PSU is turned off, or turning off the PSU while the battery is connected.  Some PSUs tolerate this, some don't.