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Fix or workaround for BMS issue

2651 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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re: batt-bridge, I went with individual (cheap) voltmeters because the batt-bridge needs a few volts imbalance before it triggers, and knowing the voltage of each pack half to 0.1 volts or so is more useful info.

re: diode drop, it works, only caveat I can think of is if your charger has a "trickle charge" or otherwise tries to top-off the pack repeatedly. If left plugged in it may overcharge since at very low currents there is almost no voltage drop in the diode.

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The battery bridge will tell you something is wrong, but likely too late to save the low cell from being damaged unless you're willing to pull over and stop driving the instant it goes off.

The cheap individual voltmeters are fine on groups of cells, say 4 cells per group. I have six for my 32 cells- three groups of five cells compared to one another and two groups of six compared to one another. But I also have a cell by cell BMS which gives an alarm. The cheap voltmeters are just for finding problems quickly (isolating them to a particular group of cells) and for monitoring imbalance if it develops.

The burnt out thing you bought was poorly made. But its function, if I understood the little reading I did about the kit you bought, included a means to cut back the controller when any one cell went into low voltage cutoff range, i.e. automating the process of protecting your pack.

Personally I think the BMS is most needed during CHARGING, to take care of any cell which, as one of mine did, loses capacity for some unknown reason through its life. Overcharging a cell like that is dangerous and can lead to a fire. Personally I think a BMS on charging is a minimally necessary piece of safety gear and I wouldn't operate a Li-ion battery pack of any substantial size and power without one.
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So I did managed to find a replacement part for the burnt piece. It was a shoktty diode code 12cwq03fn.

The replacement costed only 2$ (but costed a few more for missing soldering tools and flux etc...) so I gave it a try.

I am kinda happy to have been able to replace it:

But... all these hours of work for nothing :(

There is still something wrong. All my cells sits around 3.5V now (i manually top balanced them one by one) but bms read cell 13 at 3.94V and cell 12 at 3.14V. That is a pretty huge voltage difference, I cannot use that BMS.

Back to square one :(. I have ordered some celllog 8s and an arduino . I might try that once i receive it.

The bat bridge will not be enough for unattended charging .

I was so much hoping just replacing the diode would have been enough, oh well.


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Hi Ben
The Batt Bridge is for driving - it will tell you if a cell has failed

As far as I can see what happens is a cell fails - and then when it is charged you get the problems

You charge at much lower currents than you drive - so I don't think cells fail when charging

BUT if a cell has failed and you then charge........
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