I was recently asked to take a look at a neighbours
boat where he is having issues with his
electric propulsion system - 10kw
motor driven by 72v (24s) 280Ah
lifepo4 bank.
On 1st
inspection I found that the Daly 100A BMS was reporting cells 1>4 at 5v and cells 5>24 at +- 3.28v. I had trouble believing this and on testing I found cells 1>4 to be effectively 0V! (low mV) and cells 5>24 correctly reported at +- 3.28v with very little variance.
He had the 19v controller taking
power off 0v and Cell 6 giving approximately 19v which would have caused an imbalance over time that the BMS may have had problems dealing with, however if this had been the cause of the failure I would expect cells 1>6 to have been compromised, not just 1>4 (5 and 6 are testing similar to the rest).
The above issue will be addressed as part of the
repair process so that no
power is being taken off mid-bank.
After some
head scratching I have arrived at the theory that some form of BMS failure has resulted in the BMS over reading the voltage of cells 1>4 (5v), and attempting to balance this by drawing power off of these cells through passive balancing resulting in 0v with 5v still reported.
While importing a replacement BMS via Amazon is possible, I suspect that importing 4x 280Ah cells may be very difficult and hellish expensive!
So here are a couple of questions where I would appreciate some input;
1. Is my mode of failure theory reasonable, or do you see other considerations?
2. My plan is to attempt to recharge Cells 1>2 and then cells 3>4 using a DSP5020 power supply set to 6.56v:5A using the
lithium house bank as source, but I have never attempted recharging an
LiFePO4 cell from 0v - it this even feasible and are there any associated dangers (bearing in mind that charge power is limited to 33w)?
3. In the unlikely event that recharge is accomplished, how likely is it that these cells may pose a
safety risk going forward?