Battery Management Systems: More options thanks to Open Source

Many battery packs used in EV conversions reuse the original manufacturers battery management system (BMS). This is also our recommendation as the original BMS is well matched to the battery cells and calculates current limits that have proven to guarantee longevity.

Sometimes we can’t reuse the original BMS though or we don’t obtain it in the first place e.g. because we just buy battery modules or even single cells.

In that case we must rely on a third party BMS. There are many around at various budgets but hardly any are open source. Some BMS are centralized requiring a lot of sense wires to be routed through the car – in case you battery pack is split into multiple boxes.

Those were reasons enough to develop a BMS that addresses most issues:

  • One module can measure up to 16 cell voltages and 2 temperatures – so a typical 96 cell 400V pack would require 6 modules
  • Modules can be distributed over multiple battery boxes and only require 3 low voltage wires between them
  • The first board in the row communicates with the rest of the car, no separate “head unit” is needed
  • An inexpensive but effective balancing scheme allows charging low cells and at the same time discharging high cells to keep all cells at the same top end voltage and get the most energy from your pack
  • The cell voltage measurement has been optimized for robustness and can survive wrongly connected tap wires
  • Only legacy components were used instead of specialized, single source BMS chips. This makes the design more robust against the next chip crisis

The BMS is currently tested by community members and will appear in the openinverter shop soon. You can find some more details in the openinverter wiki.

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