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Thread: Charging 2 Lifepo's in serial.

  1. Charging 2 Lifepo's in serial.

    This is gone probably be a very stupid question but here goes:

    I have two 12v Lifepo's connected in series to give me 24V.
    Now i have been recommended that i should charge the batteries individually.
    The Lifepo's each have their integrated balance control circuit.

    For each charge cycle can i expect problems if i connect my 12v charger to each battery in sequence without removing the series connection ?

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    Re: Charging 2 Lifepo's in serial.

    It depends on the details of the balance system used and also the charger used. If the internal balancers are simple 'per cell' discharge circuits with each connected only to a single cell (basically a small discharge MOSFET and some passives to dump power off the cell when the cell voltage exceeds 3.4~3.6V), then you should be safe with a 'dumb' charger (CC/CV for a given cell count without using balance leads) as each circuit will only ever see the voltage across the cell to which it is connected. If it is a much more advanced balance system where all the cells in the pack are connected to a central microcontroller that handles the balance discharging and/or disconnects the battery in over/under-voltage events, then the total voltage that can be placed across the pack is limited by the components used in it.

    A 12V LiFePO4 'drop-in replacement for lead-acid pack' with integrated cutoff and discharge balancer (only gives you two power connections and no balance connections) using 30V MOSFETs would be limited to charging two packs in series with an 8S 'dumb' charger or risk exceeding the voltage ratings of the MOSFETs during an error event. If the pack gives you access to all the cells through a balance harness, then you should be able to use a smarter 8S charger with the balance harness to charge both packs at one time without any issues.

    If you are absolutely certain that the batteries are completely isolated from any connections other than the batteries and the charger, then you could probably be able to charge the packs in sequence with a single 4S 'dumb' charger while the batteries are still connected in series (they MUST be completely isolated from anything else). You would absolutely not be able to connect a 4S charger to each pack and charge them simultaneously while they are still connected in series unless the chargers are of the uncommon/expensive/fancy variety that possess completely isolated outputs (the charger on the 'upper' pack would connect the top cell in the 'lower' pack directly to ground and cause lots of heat/smoke/sparks).
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

  3. Re: Charging 2 Lifepo's in serial.

    They are indeed simple lead acid replace batteries with only a contact for the battery, no separate balancer leads.
    The charger is a dumb charger giving about 15v.

  4. #4

    Re: Charging 2 Lifepo's in serial.

    If they each have a separate balancing circuit, then it is best to charge them individually. Otherwise, when one battery fills up, it will prevent charging of the other -- over time, this will actually exaggerate any initial imbalance, as the "fullest" battery keeps getting charged to full, and the "emptier" battery never charges full.

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