Li-Ion Charging with Supplies in Series

[tegmine]

I'm wanting to experiment with partial charge regimes using a lithium ion battery pack configured for a nominal 36V. The pack does have a basic BMS for passive balancing but at the charge voltages and current I'm planning I don't expect it to be doing anything.

The only supply I have which will give me the control I'm looking for is a Rigol DP 832 so I willneed to run two channels in series to achieve the necessary charge voltage.

I've tried an electronic load connected to the supply to predict how things will behave with the supply in series CC mode, but obviously this isn't a battery and the voltage drops to near zero as soon as the load exceeds the current limit set on the supply.

Rigol do advise against connecting an active load to the supply, but from my limited understanding things would remain in the first quadrant so it should be ok?

Before I try blowing anything up, is this likely to work and are there any additional considerations regarding the setup or the supply?

[Daixiwen]

I've never had any problem with this setup, although it wasn't with Rigol supplies. I'm not sure it matters that much. The only problem I could see is an interference between the two power supplies if they are badly regulated/filtered.

Adjust your two supplies so that the sum of the output voltages is your maximum charge voltage, adjust the current limit of one of the supplies to your maximum charge current, and the current limit of the other one at a slightly higher value. At the beginning of the charge, when you are in the constant current phase, the supply with the lowest current setting will be in current limit mode, and the other one in voltage limit mode. During the constant voltage phase both supplies with be in voltage limit mode.

[Doctorandus_P]

You can easily build a "Vbe multiplier" with just 2 resistors and a bipolar resistor, and this acts as a power zener diode. Add a potentiometer to adjust the "knee voltage", so you can simulate a battery pack between "full" and "empty".
For a sharper knee, use a TL431, for a softer knee, add a series resistor, or tweak it in any way suitable to your needs.

Putting multiple power supplies in series is common, and any decent power lab power supply should be able to be used in this configuration.

When in CC mode it becomes more problematic, because only one of the power supplies will be in current limiting mode, and then if the total voltage drops below the output voltage of the other power supply, it's terminals get reversed.

[ledtester]

Another option is to another device control the current such as a one of those DC-DC buck regulator modules you can find on ebay/aliexpress. They range from the very cheap to the quite elaborate with uC-controlled graphic displays and input keypads.

Idea 2: Using one power supply will get you charged to around 30V. The rest of the way can be handled by a LM317 rigged up as a constant current regulator for a charge current < 1.5A. THe LM317 might even be able to control the current all the way since it can handle input voltages up to 40V.