cell checker draw

Experts,
I am using three of these cell-meters to keep my 20S 50Ah LiFePO4 always under control. It has a ground pin and 7 cables to connect to a series of 7 cells to provide individual voltage reading: It works pretty well indeed. Considering a current draw of 35ma on the ground pin, I initially thought to leave it always on; it will take 1400 hours to discharge the battery ~59 days.
But then a doubt came into my mind after I noticed that after one week the voltage of the first two cells was consistently lower than the other cells: are we sure that it draws the same current from every cell?
So I decided to measure the draw from every pin and, surprise surprise, it basically draws almost entirely from the second pin !! So I have offset the first two cells by almost 6Ah during this week.
These are my reading:

• PIN microA
  ground 35000
  1 50
  2 33000
  3 150
  4 250
  5 300
  6 390
  7 410
  Total 34550

Thanks,
Luca.

http://eclipsebikes.com/cell-checker-voltage-alarm-lipoliionlimnlifepo4-p-1090.html

The measurements you have made are useful for people to understand the way in which these devices should (and should not) be used.

This is common behavior, and predictable if you understand how these devices are powered and how they sample the voltages.

Indeed, many a BMS will do this. Bestech BMS modules will draw power from the first two cells. Cell Log 8's will do it too.

So it's always a good move to put a switch on the negative if you plan on using them as a battery monitor.

now i find out after its done the damage.

Yes, unfortunately these cell checkers are not intended for permanent connection to the battery (connection via the ignition switch might work if you just want them on for charging & discharging). A BMS is designed for permanent connection and consequently has a much lower quiescent current draw.

Not sure it has done "damage" in this case, but it has happened.

This has been discussed before. Even lifting the ground does not completely correct it.