Originally Posted by dougingraham
Everyone thinks about doing their BMS this way at first. The big problem is that you are powering the PIC and LED's from the cell it is monitoring and every cell has a little different load on it. This will imbalance the pack.
I admit that I do not have any direct experience with this, but I am used to dealing with precision instrumentation and measurement technology, and one of the main principles is the effect of the measurement on the system being measured. While in theory a current draw of 5 mA will affect the voltage of a cell, and there might be tiny differences in the current draw based on tolerances of components, there is no way that it will cause any perceptible imbalance on the cells of the pack.
In fact, if the battery monitor circuit draws more current from a cell with a higher level of charge, it would tend to balance the cells. I don't know the circuitry of a commercial BMS, but I would venture to guess that its current draw would be on the same order as the circuit I showed. And since 2.5mA is 0.01% of the one hour discharge rate of a small 25 A-H cell, it is probably well below the internal self-discharge current. In any case, it would take 10,000 hours, or over one year, to deplete the cell if it were just stored and unused and not on a float charge.
Moreover, the circuit I would use for a commercial version (or even for my own purposes) would use a microcontroller such as the PIC10F320, which has an operating current of just 25uA at 1.8V, and can also operate in a standby mode with just 500nA for the watchdog timer which can awaken it every 10 seconds or so to perform its measurement and display functions, such as flashing a status LED, and then resume sleep. If the status LED flashes for 0.25 seconds every 10 seconds, and uses 10 mA, the average current is 0.25 mA, which would deplete the cell after about 11 years of unused storage, at which time it's probably dead anyway.
I don't want to start an argument, but I'd just like to hear an explanation as to why the ideas I presented would cause problems. Thanks!