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I want to build a 24v 7S 3.7-4.2 18650 lion pack. My concern is the parameters my BMS should have to keep in balance all the 7 cells, not over charge them or over discharge them. cells from laptops: ICR18650-26c.

Please open /check this spreasheet with the values I think the BMS should have to be on the safe side. If values are incorrect , kindly provide feedback and correct values.

BMS Specification with details BMS specification

Thank you!

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A battery protection module should not be used to prevent overcharge and discharge, but only for protection in case the normal charging and cutoff circuits fail. Your BMS specifies 4.28V and 2.8V as the upper lower limits. Those voltages are outside the range of healthy battery operation, but should be OK for protecting against catastrophic failure.

The balancer circuit should pass enough current to correct any imbalance condition. Theoretically that means it must match the charging current, because one cell could become fully charged before the others reach peak voltage. In practice a lower balance current is acceptable provided that the cells are not far out of balance.

Your cells have a capacity of 2.6Ah so they can be charged at 2.6A (1C rate). But the balance current is only 0.168A, so if you charge at 1C those cells better be in good balance! The answer to this issue is to measure the individual cell voltages and charge the battery slowly (ie. not much higher than 168mA) if they are significantly out of balance. Then you can safely use the BMS at full current for subsequent charges.

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To add to Bruce's solid advice, the cut-off and balance settings can be fine tuned to squeeze more capacity at the expense of more rapid aging and conversely, more conservative values for extended life cycle * Ah capacity. The exact tradeoffs depend on supplier quality and cell temperature determined by I^2ESR= Pd effects that rise rapidly when the battery becomes charge "saturated" >90% SoC (state of charge) and/or "depleted" < 10% SoC = 90% DoD ( depth of discharge )

When packs expire, the ideal is when all cells expire simultaneously with >300~ 500 cycles. The reality is that charge life cycles can range from 1 > 3000 cycles depending on rates used and degree of imbalance for over/under charge. When ESR rises when depleted <10% it accelerates heat rise and accelerates differential aging.

Thus monitoring ESR, state of charge (SoC) and temperature of each cell is a critical parameter for battery quality.

THis BMS looks like an excellent tool

  • try to measure this ESR parameter with your BMS and record after each cycle and/ or make a pulse charge/discharge tester with a Sample and hold.

Beware of measurement errors from inductive long wires and noise may affect perfect operation such as a CM Noise filter with a ferrite sleeve on the battery cable.

  • Understand that the incremental series resistance or ESR is typically < 10mOhm new and rises rapidly < 20% %DoD then restores when charged. ESR also lowers with rising temperature as well as increases Ah capacity somewhat, but also reduces the cycle life significantly.

    • heat is like adrenaline to the heart. Small amounts used sparingly are useful. Too much or sustained will kill the battery.
  • It depends on your budget and goals. Do you prefer max power or max life cycle?

  • Thermal pack conductance and temperature rise of the module is a user responsibility. Do you intend to add a heat sink?

Since the BMS has limited size Watt heatsink for balancing, this limits the charge rate when any imbalance occurs. So beware the stage 2 and 3 parameters need to be tuned to avoid overheating of BMS with aging because of need to dump power from mismatched cells rises with aging. 1=CC, 2=CV, 3=Cutoff, 4=Float or off. SO more conservative values are necessary as cells age. It's only natural. Ref my other post

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