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Converting from lead acid to LiPos

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    Converting from lead acid to LiPos

    Hi all,

    came here through the "Paralleling BMS protected Li-ion packs." thread, and I think you might be able to help me.

    Forgive me if this is the wrong subforum, please move into a better one if necessary.

    What I'm looking to replace:
    2x 12V Lead Acid batteries.

    What I have:
    3x 4s LiPos. (alright, writing this, it may be easier with 4. I also have 4.)

    Other stuff:
    - Peak Current 20A @24V (internally, the LA cells are wired in series at some point, but the circuit I'm trying to power runs on 12V of one of the cells)
    - Charging is/was done by supplying voltage to the battery over the same connection the battery discharges. This could be disabled, however.
    - Either external charging (I plug in 24V somewhere) or hot-swapping of the batteries must be provided.

    I added a picture (see below) to explain my current thoughts. The top is the situation as it was - 2 12V cells, that could also be charged by the magic box (that I'm not aiming to replace, sorry... we'll have to work with that for now)
    (I can disable the charging, however, if the new magic box supports being charged by an external 24V connection.)

    Now I think getting from 3x 14.4 Volts to 2x12V is rather tricky, so my first thought was to instead fake the central 12V connector by a stepdown converter, just a couple amps to run the magic box and disconnect everything that is connected to the 12V line on the right.
    Then we just need to convert 3 LiPos to 24 volts.

    OR, alternatively, we get.... 4 BMS, 1 for each 4s. Then we step down the Voltage to 12V, while also protecting from reversing current, add a fuse etc. Then we have 4x12V. Two of which we could run in parallel, and those two packs in series to get 24V. But will that work with charging?

    Okay, at this point, you may have noticed that my level of understanding electronics is barely high-school, which is why I need your help :)
    Oh, right, and of course, I'm on a budget. $100-$150 should be doable though...

    Every suggestion is welcome!