Neato D5 li ion battery failure

[masterx81]

Hi!
I have one neato battery pack that have draind out a little bit too much (2.7v/cell), and the onboard battery control logic have switched off the output.
I've tried recharching slowly the cells with an appropriate 18650 battery charger (monitoring temperature, current and tension during the charge) and all of the 8 battery have charged fully with good capacity and good voltage/current matching, allowing me to reassemble the battery pack.
Now i've connected the batteries back on the control logic, connected first ground, then the intermediate connections for checking the series voltages, and for last one the positive of the last 2 batteries of the series (that give vcc to the control ic), but the control logic still not enable the output fets (no voltage output on red/black wires). The control ic is a FM3DXY, that i can't find any PDF over the net, but i suspect that have built in a flash memory that remember the state of the battery also if powered off...
The FETS are 2 AON7426, and over them there is a strange 3pin thing.
Someone have "played" with the safety circuit of the battery pack?
I attach a photo of the control board.

[valamilamasag]

That strange 3 pin thing is probably an electronically blowable fuse. Check it for continuity. One of its pin is highly probably connected to the fets or the output or one of the battery terminals.
If you find such connection and its blown then sadly its not repairable. Even if you replace it, the IC will blow it again or simply never enable the output.

[masterx81]

Yes, one side it's connected to the battery output, and the other to the fet. Newer seen this type of fuse
Normally in the BMS systems that i've seen, the only protection were the 2 fets. I suppose that this will help protect in case of fet in short circuit... I'll try to see what control that fuse...
Thanks!

[masterx81]

Tried to bypass the fuse (as was blown), but the bms doesn't close the fets.
And also, i get a constant voltage to the control pin of the blownable fuse, so i suppose that the control ic try to blown it.
So it has an eeprom o flash for remember the battery state (as now the batteries are fine, at right voltage)... Maybe can be reset via i2c, but will need a pdf of the device, that can't be found...

[valamilamasag]

These blowable fuses are common in smart batteries. Not in the cheap copy ones, but the proper ones that gets all the required certificates to be sold outside of china. Btw this fuse is just a normal fuse plus a heater element.
Datasheet is not required, these talk via SMBus (similar to i2c) and all the commands are standardized (smart battery protocol). Laptop batteries use the same.
If you dont have the keycodes then you cant reset the IC. Its usually a 128 or 256 bit RSA key-response challenge, not crackable. Once the manufacturer locks it and that's it. If you are lucky then they used one of the common codes which are already out in the wild. There are 3rd party tools to work with these IC-s, but they cost far more than a single new battery.
Also even if you know the keycodes to unlock the IC the manufacturer might have uploaded a custom firmware to it which complicates the repair.

[glnc222]

Old NIMh XV model batteries had automatically resetting polyfuses for short circuits etc., simple. These fancy fuses here are probably due to the fire hazards associated with faults in lithium ion battery usage, taking no chances with anything irregular, over loading, short circuits etc. Normally the system power management software should keep the battery in a normal range of everything, with a buffer of unused capacity etc.
Some robots such as lawn mowers with more room for lower energy density batteries use safer LiFePo4 cells lacking the fire hazard. A DIY LiFePo4 battery was made for the original Botvac with NiMh cells (see Mods & Repairs list top of forum). The higher voltage in the lithium equipped Botvac's would require more LiFePo4 cells than would fit.

[masterx81]

These blowable fuses are common in smart batteries. Not in the cheap copy ones, but the proper ones that gets all the required certificates to be sold outside of china. Btw this fuse is just a normal fuse plus a heater element.
Datasheet is not required, these talk via SMBus (similar to i2c) and all the commands are standardized (smart battery protocol). Laptop batteries use the same.
If you dont have the keycodes then you cant reset the IC. Its usually a 128 or 256 bit RSA key-response challenge, not crackable. Once the manufacturer locks it and that's it. If you are lucky then they used one of the common codes which are already out in the wild. There are 3rd party tools to work with these IC-s, but they cost far more than a single new battery.
Also even if you know the keycodes to unlock the IC the manufacturer might have uploaded a custom firmware to it which complicates the repair.

Yes, i've read a pdf of this blowable fuses and seen how this works, sometimes i open old (2/3 years out of use) nb battery packs for recover the good 18650 cells and reuse them in other projects, but never seen it.
Really thanks for the explaination about the protecion in the BMS chip. The last BMS chip that i've seen was resettable simply disconnecting/reconnecting, as it not had any memory of the failure.

[valamilamasag]

but never seen it

You just missed it Even 20 year old notebook batteries have them.
Here is a picture where I encircled the fuses (4 pin variants are more common than 3 pin ones):

[ghelec]

Bonjour,comment neutraliser fusible chargeur batterie li-ion neato d5?Merci

[gemcor28]

The three-terminal fusible resistor/fuses combo will always blow because the control circuit is stuck at power fail mode. But for testing purposes, just leave the center lead not connected to prevent the resistor/heating element from blowing the fuses to complete the circuit and do the testing.