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BMB Pinouts

My roadster drained it's battery while I was away. I had it plugged in, but it stopped charging for some reason. Since it was plugged in it kept running the cooling system and killed the battery pack. It refused to charge, so I looked in the service menu. I saw VMin was under 2V, so I immediately pulled the service plug to prevent further drain.
I pulled a BMB and sourced the connector. I am hoping I can save the pack since it went to zero miles only a few days ago.
Can anyone post the pinouts of the BMB's? I looked at the board and using the fuses I can see which pins are to bricks, but I don't know which is to which brick. It would save me some time if someone has the pin info.


Well-Known Member
Apr 13, 2018
Buford, GA
Not sure of details for roadster, but is the 12V battery operational? Will things turn on? If not, I'd think about charging the 12V battery first. I suspect that once it is charged, you may then be able to start charging.

I had a Ford Probe many years ago. One day the 12v battery was dead, so, being a straight shift (and on top of a hill) I just tried to jump start it. It wouldn't start. Finally realized that if the battery isn't dead, the electronic ignition wouldn't turn on and the car wouldn't start.

If the 12V isn't on in the roadster, then very possible it can't even engage the contactors to connect the batter to the cable.
I probed the pins and found the ones needed.
Pin 24 is sheet negative. Pins for positive of each brick, in the order from 1 to 9 are:
3, 23, 1, 19, 5, 20, 6, 22, 4
The connector is a Molex 44428-2402. Mouser has plenty in stock.
Note that the pins are spaced on 3mm centers, not 0.1"(2.54mm) so to use it on a standard proto board you will need to do some pin bending.
I found two bricks in sheet 8 that were critically low, 1.43v and 1.66v on bricks 1 and 2. I charged them individually to 2.75v and they are holding there, so they are probably OK, at least for now. They will probably have reduced capacity though.
I'm charging all of the sheets individually in stages up to 32v. This should be done by tonight, and tomorrow I'll recheck all of the bricks to see if any have dropped significantly in voltage, which would indicate a shorted cell. If they're good I'll reconnect everything and start charging using the on board system.
I'm very fortunate I caught this just as the pack went to zero. If it had sat or if I had fiddled with it more with the service plug in those two bricks would probably have been gone. From now on when I leave for more that a week I will shut off the APS and pull the service plug. The car won't restart charging after a temporary power problem, but apparently will continue to run the cooling system, killing the battery. A serious flaw.
Any idea why it stopped charging? When I’m away for more than a week, I run it down to about 50% charge and put it in storage mode. I try to keep an eye on it via OVMS, but I’ve never had a problem where it dropped too low.
I have no clue yet. I can't get to the logs until I'm done with the battery pack. I do know that power was good the whole time, and it showed 241v on the screen when I looked.
Figures. I didn't see that data sheet, so I used the more standard numbering. I wrote it on my board with a permanent marker too. Oooops.
So going by the data sheet numbering I come up with...
24 is still 24
2, 12, 1, 10, 3, 22, 15, 23, 14
Have you figured out the difference between pins 10 and 11?(data sheet numbering). They always have the same voltage, and test as shorted on the battery, but they have separate fuses and transistors on the BMB.
  • Informative
Reactions: chrisro


May 22, 2012
Perfect - that matches my notes from looking at the BMB and looking at the internal structure of the sheet.

Pins 10 and 11...

Every second sheet has a large fuse in the middle of the series chain. The sheets that don't have a fuse have the same structure but with a metal bar installed instead of the fuse.

Pin 10 is one side of the fuse and pin 11 is the other. You can use either pin interchangeably to access the +ve of the lower brick and the -ve of the brick above it.

I built 11 boards that bring the 10 points out to an exposed connector...

1) Good to check the status of each brick. If you have any bricks that are near 0V then the brick is dead - don't try to charge!

2) After determining the battery in this Roadster was dead I purchased a second-hand battery and used a 400V bench power supply to charge the complete chain via these boards at 100mA for a few weeks. I stopped once the highest-voltage brick reached 3.95V (figure 4.15V is fully charged, 3.95V is a reasonable level to be sure the battery will power the car so the car can take over charging).

3) To improve balancing I wired up 36 fully isolated charging circuits so I can charge each brick in 4 sheets at once. They are all set at 3.95V/100mA and slowly charging.

Note that the BMB interface wiring inside the battery is very light weight - figure the BMB will pull 100mA when balancing - I wouldn't recommend exceeding that when testing or charging via the BMB interface.
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Reactions: dhrivnak
Makes sense on the fuse. My 1.5 has fuses on all sheets. Wouldn't hurt to have both pins available to enable fuse checking.
No bricks were at or near zero. Lowest voltages are above.
I used 300mA to get the voltages up quickly to avoid further damage to the cells. They are all at around 3v now, so I've turned it down to 200mA. No problems so far. I've read where some have put as much as 1A through. Doesn't sound smart at all. Not only is the wiring light, but the cells are potted and without the cooling system to help they'll get mighty warm mighty easily, especially if there's a marginal cell.
Thanks for the PCB files. If I have to do this again a full set of boards would be better.
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Reactions: dhrivnak


Developing Tesla Roadster 4.0 Battery
Jul 29, 2018
Some of my batteries where discharged to 0.3v. After charging in chain up to 31v so 3.43v per brick they drop to 3.1v after 3 days. Temperature in my garage is quite low now even around 0 deg. at night. Should they hold voltage or 0.3v after 3 days is acceptable at this temperature? Service plug is removed.
Thanks a lot.


Active Member
Supporting Member
Feb 18, 2018
Was the 31v hot off the charger or did you wait an hour to read your voltage. You may find the individual cells are balancing causing the differential, I would charge up in stages once you are sure they are holding voltage. Some drop is expected from an initial reading of 0.3v
Last edited:
  • Helpful
Reactions: chrisro


Active Member
Supporting Member
Feb 18, 2018
Not exactly balancing but each brick will act as one cell. The cells in each brick will absorb energy depending on their condition, so if one cell has low resistance and charges to 4v quickly it will show as if the whole brick had reached 4v. Over a few hours each brick will even out as it’s acting as one cell. I balance all my cells for my e-bikes by taping them together and placing them on some tin foil. the other end I put foil on, then a towel and then weigh it down so all the cells become charged/discharged to the same voltage, this takes a few days. So if you have cells in the brick absorbing energy at different rates it may take a while for them to settle after charging from such a low state. I would wait and see where they even out to then raise the brick voltage and see. By bypassing the bms boards you have to monitor brick by brick without being able to balance other than manually. If you can charge each brick to 3.8 and then after a few hours look at the differential it will give you a clue as to which bricks may be at risk of overheating.

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