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Some Switched Mode Power Supplies offer a potentiometer to adjust their output DC voltage. Mean well's LRS-350-24 power supply for instance has an adjustable range of 21.6 ~ 28.8V. Can it be used to safely charge an 8 LiFePo4 prismatic cell battery pack (with BMS)?

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    \$\begingroup\$ If there's a BMS, then yes. If the PSU doesn't like back feeding, then use a diode in series with the V+. IF there's too much current when the battery if dead, a small power resistor inline with V+ can limit the current to the battery to keep it within safe operating limits. \$\endgroup\$
    – dandavis
    Commented Oct 2, 2023 at 0:05

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No, an adjustable constant voltage supply can't be used to charge batteries, because a power supply is not a charger.

A power supply like the LRS-350-24 tries to keep the output supply voltage constant. For example you can set it to 26V.

A somewhat empty LiFePo4 could have 22V.

When connecting a power supply to battery, the battery will try to pull the voltage down to 22V while sinking current from supply, and the supply tries to raise the voltage to 26V by pushing current into battery.

So as the battery can sink a large amount of current, the power supply cannot push enough current to rise the voltage to 26V setpoint.

The power supply overcurrent limit triggers and it shuts down to protect itself. This specific supply will go into hiccup mode so it constantly starts up momentarily until overcurrent limit triggers again.

Many power supplies also can't handle being connected to an external voltage supply, i.e. the battery, and will damage.

So a power supply cannot be connected to battery directly.

You mentioned in comment lab/bench supplies which are different. They have also a settable current limit which does allow for pushing charge into battery with constant current at battery voltage until battery no longer takes that much current and then battery is float charged at constant voltage.

But as lab/bench supplies are also not battery chargers, they are unable to detect when battery is full and stop float charging at constant voltage to prevent overcharging and damage to battery.

And that's why batteries need to be charged with chargers, not power supplies. The power supply can be used to provide a power to a charger between the power supply and battery.

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  • \$\begingroup\$ Well said. Well said. However, it is possible to use a current limited power supply (such as a lab supply) to charge Li-ion batteries IF: a) the battery is balanced, and b) the top voltage is set to the equivalent of 3.4 V / cell (for LFP). Indeed that is what base stations (in telecom towers) do. \$\endgroup\$ Commented Oct 1, 2023 at 22:45
  • \$\begingroup\$ Wouldn't the battery's BMS stop the float charging once full? \$\endgroup\$
    – dandavis
    Commented Oct 2, 2023 at 0:03
  • \$\begingroup\$ @dandavis No, the BMS will cut off when it detects overcharge, because the charger didn't end in time and damage or fire is about to happen. That's like leaving brakes off your car/bike because you have an airbag or a helmet for cases when you need to stop. \$\endgroup\$
    – Justme
    Commented Oct 2, 2023 at 4:43
  • \$\begingroup\$ Thanks for the detailed answer. In this YouTube video the host uses an SMPS to charge his lithium battery pack. Even though he calls it a "charger" the Aliexpress link in the description opens an SMPS product page. \$\endgroup\$
    – A.Jesin
    Commented Oct 2, 2023 at 16:14
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    \$\begingroup\$ @A.Jesin That's the problem, anyone can make a Youtube video, and claim anything they want in it. Things made in a video might be wrong or dangerous, done as a one-off joke just for the subs. It may even be fake. You don't know that. You should be aware that charging batteries incorrecely can make them explode or burn down your house. Trying what the video does is just wrong and the real problem is people not underatanding it's wrong because even the Youtubers did it. Learning if some source is credible or not is difficult, but always double check if something is true/real or false/wrong. \$\endgroup\$
    – Justme
    Commented Oct 2, 2023 at 16:27

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