Ohmmu Model S lithium battery

[Zuikkis]

Not directly related, but in the news here in Finland today, "electric car exploded while charging in a parking lot"...

VW id.buzz

Latauksessa ollut sähköauto räjähti Lahden toriparkissa

Auto savutti räjähdyksen jälkeen voimakkaasti. Pelastuslaitos varmisti tilanteen ja hinasi auton pois parkkihallista. Räjähdyksen syy on epäselvä.

yle.fi

Google translation: (hopefully direct link works)

Latauksessa ollut sähköauto räjähti Lahden toriparkissa

Auto savutti räjähdyksen jälkeen voimakkaasti. Pelastuslaitos varmisti tilanteen ja hinasi auton pois parkkihallista. Räjähdyksen syy on epäselvä.

yle-fi.translate.goog

Picture taken from the article for your convenience:

Anyway. I think it's pretty obvious it was the 12V battery that exploded. Owner has reported he was having 12V issues before the incident. Lead acid batteries tend to generate hydrogen when (over)charged. Apparently VW doesn't detect broken 12V battery properly and keeps trying to charge it, instead of throwing some error? ICE car simply would refuse to start, but on electric car there is a real risk of overcharging/explosion if car keeps trying to charge the 12V battery from main battery.

Now, lithium battery like Ohmmu is actually better in this respect that it doesn't generate hydrogen that could explode. But lithium batteries can have other kind of defects which again can result in fire and smoke. However there is a big difference.. With the original battery in place, you are probably covered by warranty or insurance. With a chinese lithium battery, hmmmmh..

 

[MP3Mike]

Anyway. I think it's pretty obvious it was the 12V battery that exploded. Owner has reported he was having 12V issues before the incident. Lead acid batteries tend to generate hydrogen when (over)charged. Apparently VW doesn't detect broken 12V battery properly and keeps trying to charge it, instead of throwing some error? ICE car simply would refuse to start, but on electric car there is a real risk of overcharging/explosion if car keeps trying to charge the 12V battery from main battery.

It seems like VW's bigger mistake is that they don't vent the hydrogen outside the vehicle.

 

[vcor]

Now if they just harnessed that hydrogen to run the vehicle, they could start to compete with Toyota's hydrogen cars!

 

[cmaster]

Where do you buy the battery? My local Tesla Service Center quotes me $200 self-install no warranty. Including install has 12 months warranty is $250.

Wow! I got mines at Watertown, Massachusetts service center.

 

[NewbyMaybe]

No problems with my ohmmu, oem before only lasted a year.

 

[geordi]

So far in my X, the Ohmmu is doing well. I may not have needed to swap it, the OEM was 2 years old by my estimates... But I'd rather that it was providing strong voltage for the heavier uses than risking power sags.

 

[Ostrichsak]

No problems with my ohmmu, oem before only lasted a year.

Plenty of Ford Pintos didn't blow up too.

 

[Ostrichsak]

So far in my X, the Ohmmu is doing well. I may not have needed to swap it, the OEM was 2 years old by my estimates... But I'd rather that it was providing strong voltage for the heavier uses than risking power sags.

Watch the video in post #15 and see if chief among your concerns is still "risking power sags" which is a non-issue in Tesla's anyway. Sounds like another Ohmmu victim.

 

[ucmndd]

… risking power sags.

Because that might… cause your blinker to blink 4% more slowly or something?

 

[NewbyMaybe]

Plenty of Ford Pintos didn't blow up too.

? Paranoia will destroy ya!

 

[Ostrichsak]

? Paranoia will destroy ya!

Who's paranoid? I didn't pay a premium to install a potentially ticking time bomb in my car. I'm good.

 

[geordi]

Show us on the 9v where the evil battery wires touched you.

We get it - you don't like them, and want people to watch your videos complaining about how they are made in China like 90% of everything isn't already made there. Maybe knock off the hyperbole and stick to the facts.

 

[Ostrichsak]

Show us on the 9v where the evil battery wires touched you.

We get it - you don't like them, and want people to watch your videos complaining about how they are made in China like 90% of everything isn't already made there. Maybe knock off the hyperbole and stick to the facts.

My video? Nope. The fact that they're made in China isn't the issue.

 

[ucmndd]

I’m more concerned that they look like they were made in China by the lowest bidder using junk-bin parts and assembled by toddlers.

Lots of good stuff is made in China. This thing isn’t one of them.

 

[n2siast]

My experience, take it for what it's worth. My car was throwing some weird electrical/battery messages on my 14MS RWD. Went down a few rabbit holes that ultimately landed on needing to replace the 12v.

Against my better judgement, I decided on getting OHMMU battery, primarily for the ease of install and removal if needed. First few weeks went fine, but then all of a sudden, the car would die after sitting unplugged for a few hours, never wanting to "wake" up.

Popped the nosecone, check voltage and it was completely dead. Jumped the 12v and the car instantly wakes up. After a few of these events, I decided to get an OEM and ditch the OHMMU. Installed the OEM and haven't had an issue since.

Something leads me to believe that the car is not "sensing" the voltage drop, and therefore, the DC-DC converter does not engage to keep the 12v charged. My story, take it for what it's worth/grain of salt, knowing that all experiences are different.

 

[Zuikkis]

, check voltage and it was completely dead. Jumped the 12v and the car instantly wakes up.

Issue here is, Ohmmu is using a generic BMS board (battery management system) to take care of cell balancing and overcurrent and overvoltage protection etc.

And by "generic" I mean off-the-shelf unit that costs $10 in Aliexpress and is usually found in electric scooters, RC planes etc.

When this kind of BMS detects some fault, it cuts off the negative wire from the battery. Voltage drops to 0V instantly. The fault is cleared when external power is applied, that is, when you charge.

This works well with small devices with separate chargers. Your scooter dies; just plug it in and it's fine.

All Tesla charging logic and the high voltage contactors are run from the 12V battery. If 12V is completely dead, there is nothing that could enable charging. You need an external 12V charger to wake it up.

 

[n2siast]

When this kind of BMS detects some fault, it cuts off the negative wire from the battery. Voltage drops to 0V instantly. The fault is cleared when external power is applied, that is, when you charge.

@Zuikkis - what events do you think would/could trigger the Battery's BMS to cut off the negative, causing the rapid voltage drop?

 

[Zuikkis]

@Zuikkis - what events do you think would/could trigger the Battery's BMS to cut off the negative, causing the rapid voltage drop?

BMS usually has high and low limits for voltage, and some overcurrent limit. Perhaps also temperature limits.

Voltage limits are per cell. It's possible your battery was so out of balance that one cell hit the high voltage limit...

Tesla charging voltage is quite high, can be even 14.6V... that's 3.65V per cell on Ohmmu. 3.6V is usually considered 100% on a Lifepo4, and actually charging above 3.4V offers little benefit as 3.4V is already 98% or so. BMS overvoltage limit is probably very close to the 3.65V.

However Lifepo4 batteries can take a lot of abuse without breaking. We have tried discharging one 18650 cell to 0V, then leaving it short circuited over the weekend, and it charged happily back to 3.6V and showed no degradation when we measured the capacity. Same for overcharging, they tend to heat a bit but not explode like Lipos.

 

[cmaster]

BMS usually has high and low limits for voltage, and some overcurrent limit. Perhaps also temperature limits.

Voltage limits are per cell. It's possible your battery was so out of balance that one cell hit the high voltage limit...

Tesla charging voltage is quite high, can be even 14.6V... that's 3.65V per cell on Ohmmu. 3.6V is usually considered 100% on a Lifepo4, and actually charging above 3.4V offers little benefit as 3.4V is already 98% or so. BMS overvoltage limit is probably very close to the 3.65V.

However Lifepo4 batteries can take a lot of abuse without breaking. We have tried discharging one 18650 cell to 0V, then leaving it short circuited over the weekend, and it charged happily back to 3.6V and showed no degradation when we measured the capacity. Same for overcharging, they tend to heat a bit but not explode like Lipos.

lithium iron posphate batteries drains quicker in the cold I found out