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I want to make a portable fridge more portable with Li ion batteries and I'm thinking about how to connect all the pieces.

Load is rated for 12V 5A / 24V 2.5A, there is AC/DC SMPS that outputs 13.2v and 12V car socket. I want to use 3s battery pack (higher voltage should be better but then the integrated power supply won't be able to charge the batteries without stepping up the voltage or switching out the power supply for higher voltage). User shouldn't have to do anything for it to switch from AC to battery, and automatically charge the batteries. All preferably using off the shelf things. If the automatic operation is not feasible, what other alternatives there are?

My preliminary circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

For charging and 24v conversion I'm thinking about this "300W" DC/DC step down converter with CC/CV. https://www.amazon.com/Aideepen-Constant-Current-Adjustable-Converter/dp/B0747QDRW9

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I see two potential problems with your circuit.

The first problem is battery protection. Li-Ion batterys need a proper charger and protection circuit or they can be a fire hazard. There are Li-Ion cargers that have a wide DC input range an should be used here. The battery pack should also be protected from overdischarge. There are Battery Management Systems (BMS) for that. Both the charger and the BMS can be found cheaply in the RC Hobby sections (e.g. HobbyKing.com).

The second problem arises when the user plugs the AC and the DC in at the same time. Either the powersupply or the DC in has the higher voltage and current flows in reverse. When the user plugs an almost empty car battery in the DC in and also plugs in the AC, the car battery will draw a lot of current, potentially killing the SMPS in the process. The simplest way to fix this is to add some diodes but they need to be very big ones due to the current. A better solution would be a switchover circuit.

Here is the updated diagam:

schematic

simulate this circuit – Schematic created using CircuitLab

not pretty but hopefully the point comes across

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  • \$\begingroup\$ The use of PCM/BMS is obvious. I was thinking of using mosfet as ideal diode instead of a silicon diode at least on dc as I would like not to charge the car battery from the internal battery pack. I'm not worried about simultaneous connection of AC and DC. \$\endgroup\$
    – Oxmaster
    Jul 22, 2020 at 8:09

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