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Fix or workaround for BMS issue

2650 Views 23 Replies 6 Participants Last post by  Duncan
I have been working on installing an aftermarket kit on a Prius to makes it into a plug gable hybrid. I have as input a 16s LiFEPO4 battery.

I have a problem however with the BMS that come with the kit. Cells #12/16 and cell #13/16 both read incorrect voltage (cell 13 reads .2V too high compared to the multi meter reading and cell 12 reads .2V too low). I am worried that the BMS will then "burn" these 2 cells by overcharging cell 12 and over-discharge cell 13.

I was wondering if there is any way I could re-adjust the BMS reading so it properly detect the correct voltage. Otherwise, would it be possible to maybe insert a low resistance on one or many balancing cable to compensate for this reading ?

I could also just buy a cheap BMS circuit without monitor but then I have no idea how I could trust such circuit without feedback.

The kit in question is an Enginer kit (bought a used one)
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How are you measuring the voltage on the cell? At the cell? or at the Enginer BMS?

What kind of multimeter are you using? Is it calibrated? How accurate is it and on what scale? i.e. if it's 0-600VDC and 1% accurate, it could technically be off 1V. If the scale is 0-10V and it's 1%, it could be off +/- 0.1V.
That's a cheap multi-meter without calibration. From the manual for it, it's +/- 0.5%. On the 20V setting (which you'd be using), it could be off 0.1V from the REAL battery voltage. Comparing the same meter readings between cells won't tell you anything. It could be off 0.1V and you'd never know it. You can't compare a meter to itself. It's good for getting a rough idea of the voltage of a device, but still 0.5%.

As you're seeing, the BMS not likely a calibrated piece of equipment either.

How long are the BMS to Battery cables? Are they all the same exact length? Are all the crimps good on the ends? It's possible there's some resistance and the voltage reading at the BMS is wrong (Although it shouldn't measure high unless there's noise).
frodus, a multimeter like that should read the same reading (+/- 0.1V) on the same voltage every single time. Its accuracy may not be very good, but it should be very repeatable.
Correct, but he's also comparing the voltage on the multimeter to that of the BMS, with a different accuracy. I'm just saying, it's not going to be exactly correct.

As far as the difference between cells, you're right, the accuracy doesn't change when the multimeter looks at diff cells. So if it's off 0.2V between 2 cells, then those cells are likely off 0.2V from eachother.

BUT, the way that BMS's are usually done, is that each cell gets it's own voltage sense, so it's like having one multimeter per cell, not one that gets switched between each cell. So with resistors, and other components and their tolerances, the sensed voltages might vary more between cells, esp if the tolerances of the components aren't tight (>5% resistors for example).
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