kiev
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Miev cell monitoring board details

Tue Sep 29, 2015 6:37 am

Here is a thread to collect notes and findings about the circuit board from Martin’s used four cell LEV50-4 battery module unit (bmu).

The Factory Service Manual gives some details of the Battery Monitoring System (BMS) functions and the CAN Buss data available using the Mits Universal Tester (MUT).

The convention of the FSM/MUT designates each bmu from 1 to 12, and labels the cells A,B,C,D… starting from the most positive cell as A and the lowest potential cell is either D or H for the -4 or -8 bmu. The bmu 01-05 and 07-11 have 8 cells, bmu 06 and 12 are the shorties with 4 cells.

Based upon what i have read i assume that CanIon reports the cells from 1-88 along the same ordering as reported by the MUT.

Quirky numbering:

The LTC6802 ic is a battery stack monitoring chip that designates the cells C1,C2,C3… starting at the lowest voltage cell, which is opposite of the MUT convention. As a result, the lowest voltage cell at the bottom of the pack is bmu01 H, or canion cell #8, located at the right rear corner of the pack.


Temperature Sensing:

Although the ltc6802 chip has provision to measure temperature that feature is not used. Instead the cell temperatures are read directly by the NEC V850F3612 microcontroller thru thermistor voltage circuits and a 7kHz low-pass filter. The 5kohm thermistors are located on the flexible circuit tabs screwed to the cells and sense the temperature at cell terminals A+, C+, D-, F+, G+, H- . An interesting feature is the addition of copper traces on the flex board from the screw lands that wrap around the thermistor for a shorter and more direct heat path.

Since the -8 modules are laying over on their sides with the wiring terminals up, all but the F+ thermistors are towards the upper edge of the modules.


Discharge Balancing:

Each cell is connected thru a 1A fuse to a circuit that can be activated to bleed off excess voltage. The network consists of two 82 Ohm resistors in parallel switched on by a signal from the ltc6802 to the base of a pnp transistor. The maximum bleed current is about 0.1A (4.1 V/ 41 Ohms), which results in about 0.4 W of heating power per circuit, or 3.2 W if all eight are on.


CAN Buss Communications:

To be continued...
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