36V UPS 18650 Conversion

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    Thought I’d make a thread about my mini project since it’s powerwall related. I’m converting a free Ultra 2000VA UPS that was being thrown out from lead acid to Li-Ion 18650 since the original batteries were toast (0V and not taking a charge). It’s a 24V system so not ideal for Li-Ion voltages but after some testing it seems like it will work pretty well. The low voltage cutoff is around 21.5-22.3V depending on load, which lines up nicely for a little over 3V/cell. I’ve also tested it up to 30V on the input without any issues. The goal for me is to be able to run some 300-500W tools off the UPS which I could trickle charge from a car 12V socket.

    Here’s the inverter on the bench with a 7s4p pack I put together for testing. I’m currently capacity checking the parallel groups in the pack to see which ones need more capacity, since my discharge test showed the cells pretty out of balance at the discharge state and under load:

    The 7s4p pack can run up to 300W loads for 83WH out (measured at the 120V outlet). The battery gets quite hot after this though, up to around 140F in open air. Adding the extra 2p cells should help a bit but I think I’ll need some forced air to cool the cells in operation. I also added an XT-60 connector to the front panel so I can charge the battery externally (e.g. from a solar controller) or add more capacity.

    Next step is to get the 18650 pack up to the full 7s6p configuration, get it together more securely, maybe with some plexiglass sheets and screws, and modify the charger to charge the battery up to 29.05V instead of the current 27.25V.

    For More Details: online animated explainer

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