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🔥 BMS Hotswap

HotSwap is used in two situations:

  • Exchange of an engaged BMS pack/set from a copter by another pack/set
    • e.g. When currently active battery set has a low voltage (battery fill state low). It gets replaced by a set with higher voltage (full capacity). The copter should stay on (voltage level on external bus stays powered).
  • BMS internal HotSwap controller brings the external voltage level near to the voltage of the battery pack during turn on sequence

Copter Hot Battery Swap

It SHALL be possible to exchange drained batteries with full ones without powering off the copter.

Especially important when payload is powered by the drone, which may have long initialization and warm-up cycles Nowadays this is an industry standard feature, which should be support.

Requirements:

  • The existing (drained) set is fully engaged before starting the hot swap procedure.
  • The available batteries SHALL provide at least 25W of power (combined).
  • The hot-swap procedure SHALL work in both directions: the new packs may have higher or lower voltage than the existing ones.
  • No pack has an open circuit voltage of less than 15 V.
  • Exchange occurs always in two pack steps
    • Within one stage, two packs from different sets MUST NOT be engaged at the same time.
    • Any two drained batteries (SHOULD belong to different stages / neighbors) are replace with full ones.

Notes:

  • The copter MAY be powered entirely through the BMS’ hot swap controllers.
  • Mistakes are to be expected and MUST NOT cause damage to the copter or batteries. After any mistake, for the hot swap procedure to continue to work is OPTIONAL.
    • The customer may remove more than two packs at once.
    • The customer may remove two packs from the same stage.
    • The customer may insert “old” packs back into the copter.

Note

  • Battery packs B2, B6 and B10 form the lower stage, while packs B4, B8 and B12 form the upper stage.
  • The term "engaged" refers to a BMS’ main FETs (the "latches") being active.

BMS internal hotswap controller

  • Current is limited to 1A.
  • The HotSwap controller raises the external (pack) voltage slowly (30V/s).
  • If the current threshold is exceeded (or temperature above threshold), the hotswap controller stops and retries the process after ~512ms.
  • Even if all BMS activate synchronouz, the current is not distributed equally (manufacturing issue).
  • The HotSwap controller can be configured to current limiting mode. (but experiments showed no change as voltage gets completely reset).