CellLogs 8M and Current Draw

Getting to the bottom of the imbalance

The CellLog 8M is a great little device for $15.  When configured properly and wired up, they can provide both HVC and LVC alarming.  However, their front-end design though clever, leads to some severe current imbalance (draw) from the cells.

Currently, the confirguation is 7 cells connected to a CellLog, set to alarm at 2.5V and 3.5V and if the pack goes above 25V (or below 21V though 20.5 may be a better set-point to detect Cell #7 is disconnected: 7 x 3.3 = 23.1, 6 x 3.3=19.8, and 7 x 2.5 = 17.5.  The trick here is to alarm under 6 cells with no load but not under nominal load with 7 cells.

After measuring the current draw from each cell with both the the ground (Pin 1) connected and disconnected, here is the data in mA.

Pin	Cell #(+)	Pin-1 On	Pin-1 Off
1	Cell 1(-)	33.00	0.00
2	1	0.03	0.30
3	2	0.04	0.26
4	3	0.03	0.22
5	4	-13.00	4.40
6	5	-0.03	0.03
7	6	0.38	0.45
8	7	-20.00	-5.00

From this data is became obvious the CellLogs are shuttling current between the different packs.  This CellLog as the D4-D5 current mod connecting Cell 7/8 to Cell 5/6 internally. I ran the same test with an unmodified (hacked) CellLog and the Cell 5 and Cell 7 currents were unchanged.  The one strange thing is the modified CellLog Mod powers up even with Pin 1 unconnected.  There appears to be insufficient power to fully boot up the device and make measurements, but the back-light does illuminate and the text is visible on the screen, though all of the voltages show zero on the screen.

The off-state current is the most worrisome since the device spends 95% of the time off (23 hours out the day).  So a 7mA draw for 95% of the month equates to about 5AH per month of imbalance.  Way too much and this explains why after 6 months, I was seeing a dramatic difference across the 60AH cells.  The 5% of on-time imbalance will still result in 0.7AH of drift per month or 8AH per year.  This is close to 10% of the pack capacity so perhaps annual balancing is the best strategy for now.

Without a full schematic for the CellLog it is tough to understand if the current deltas are a result of relative voltage levels or if it is a function of the overall design.

Next Steps - Full isolation while off, not just turning off Pin-1.  
Using 2 of these will allow for full isolation for the battery when the not charging or running.

Sainsmart 16 channel 12V relay module

$20 per 16 channels - 2 boards will provide full cut off of the 27 signals, though it probably isn't necessary to switch the ground, but for the sake of completeness and since there are extra channels, I go ahead.  There is not need to rebalance the pack until the isolation system is up and running, so until then, it will be prudent to just limit driving range and frequently check the cells.