Popular Post Chriull Posted March 22, 2020 Popular Post Share Posted March 22, 2020 The time course of the charge current: (from batteryuniversity.com) At stage 1 (constant current phase) one has the maximum current, then with stage 2 (constant voltage phase, saturation charge) it starts to drop until the charger should cut off at around "3% of rated current". So the highest burden for the cells and the BMS (charge side) is in stage 1(CC phase). As dissipated power is proportional to the square of the current burden raises quickly with the charging current. While stage 2 (CV phase, saturation charge) the cells "just" get their last 15-20% of charge and the cell/BMS burden gets less and less. (While this stage cell balancing occurs - so even if one has high capacity packs, one should not skip this stage (always)!) My personal list of shortcomings of most actual BMS/chargers: - they do not show/warn of bad cells! Once a cell drops for some time below 2/2.5V it is dead and is not to be used anymore! By the "internal shorts/platings" this cells are dangerous and have a higher risk of thermal runaway/fire while (dis)charging. At batteryuniversity.com is also some "polarity switching" mentioned - ?one of the paralleled cells gets "negative voltage" and destroys itself and the paralleled cells without any additional (dis)charging burden!? Especially dangerous with such deteriorated cells is unattended, long term (overnight) charging. As such cells do not reach (at some stage) never their 4.2V again the charger never reaches stage 2 (dropping current) and the cells are burdened with full current throughout the whole charging phase! Especially the deteriorated cells are now endangered for thermal runaway! And again - the higher the charging current the by the square higher dissipated power for this cells. There is a ?theoratical? security mechanism for this case - the charge input protection circuit which cuts off once any cell of the pack reaches a certain overvotage threshold (something aroung ~4.28V). This would work perfectly for single battery packs - but in our wheels there are mostly some packs in parallel. So if one pack with a bad cells cuts off the charger input because a cell reached the threshold this battery pack gets charged from the "backside" by the other pack as they are connected at the discharge side! A big hope is that the "synchronization wires" of the BMS used the last years are for exactly this case - if one pack shuts down the charge input, it signals to all other packs to do the same! But still - if this hope is true - every charging of a pack with a bad cell deteriorates the cell more, burdens it with the maximum of the chargers capabilities and increases the risk of a thermal runaway each time! - some thermal sensors well distributed throughout the pack for emergency shutoff! - the input protection circuitry has no heatsink and is wrapped in plastics with the whole pack. No real chance for cooling by convection for the mosfet! In older BMS just one mosfet was used for this input protection - so its borderline burdened by fast(er) chargers. Putting here a second or third mosfet in parallel would greatly reduce this burden! Especially if not put side by side on the board! - as it seems (quite some reports here) many stock chargers do not cut off once the end of stage 2 (CV stage) is reached (~3% of rated current). This is nothing "severely dangerous" but unnecessary deterioration of the cells by endless "trickle" charging. Li ion cells do not like this! 4 Quote Link to comment Share on other sites More sharing options...
Popular Post Chriull Posted March 22, 2020 Author Popular Post Share Posted March 22, 2020 Ps.: What to do? - no unattended charging - use a timer to limit charging time. - do not use a wheel that does not take a full charge anymore! Replace the bad pack! Afair chris from @1RadWerkstatt advertised/?offers? a new BMS for his battery packs / ?KS wheels? that fully supervises each cell and all charging and discharging! In ecodrifts interview with the wheel manufacturers KS mentioned new BMS? Hopefully chris did the "pretesting" of this new BMS and they will become standard?! 4 Quote Link to comment Share on other sites More sharing options...
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