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Ohmmu 12V Battery Feedback
- Thread starter LasVegas2019
- Start date
Their 12V "Group 31" deep cycle battery works great in my travel trailer.
But to use this thing in a Tesla seems absolutely foolish.
Stanwood Steve
2018 Performance Model 3
I believe I used a link on their web site to contact them (it's been a while), but subsequently have been dealing with "Queenie Tsui" at Ohmmu. The e-mail for Queenie is "[email protected]". Couple of notes to pass along, I eventually did receive the Tesla "battery requires replacement alert" after 25 cycles of the Ohmmu battery. I don't know how often the battery initiates a discharge cycle, but it felt like I had it for six months or so.
After taking screen shots of battery stats from an app Queenie had me download, they sent me a new battery and a return label so I could send the battery back that I received the alert on. The latest battery is also bluetooth but also had a sticker on it indicating that it is now "self heating". So far it's working fine, but it's still only on one cycle. We'll see if all is still well after 25 or more. I hope this helps!
My heated version continues working perfectly with the V29 firmware and Model 3 configuration. The battery will run at about 60F when 32 outside.
There are other guys on this thread who may have more detailed knowledge but my impression is that clearly you just never have resets or problems with this battery in warmer temperature the only time it seems to get into trouble with the 12 volt subsystem in the car is when it's cold probably because the BMS limits charge speed which the car then interprets as evidence the battery is defective, and then the car forces it to undergoes what we call its suicide self-discharge routine. The self-heating feature means essentially that you trade a little bit of energy for keeping the battery in a more desirable homeostatic zone. We've had some cold temperatures in Florida and I've had to do one or two resets in the last 2 months. But virtually none when it's above 75 during the day and 55 at night. Since I can do a reset in literally 10 seconds from the Bluetooth interface and with access to the dc/dc connectors under the rear seat, this little bit of extra work doesn't bother me
Sadly, I finally gave up on my Ohmmu and went back to lead acid. I went through several iterations of their battery (all warranty), with the last one being the BT enabled unit, While the Tesla 12v warnings initially slowed way down, a month ago, things got more weird. HAL started rebooting at random times (only while unoccupied). I would get in the car and have to wait for the reboot sequence to complete. I spoke with my service advisor (really helpful guy). He said the logs showed several references to 12v power. The only way they'd troubleshoot was to replace the Ohmmu first. Rather than pay them for the diagnostic AND the battery, I did the battery only to see what happened next. It's now been almost a month without random reboots.
It 's really a bummer that Tesla did not give older M3's software selectable battery type.
It 's really a bummer that Tesla did not give older M3's software selectable battery type.
The Ohmmu lithium is nothing like the Tesla lithium. There is no way Tesla is going to develop a completely custom profile for an aftermarket battery.
Agreed not, but the real attraction of an aftermarket lithium battery is to supplant the old lead-acid battery for the Tesla through 2021. Because the failure can occur quickly and with very troublesome consequences.
So, I think the issue is that Tesla has an unusually tight monitoring envelope, increasingly so over the past few of updates, that has made it very difficult to produce a non-lead-acid replacement.
Here in Arizona, I've come to expect car batteries to fail every 3 to 4 years and I'm not interested in experiencing a sudden failure. So I liked the idea of the Ohmmu but it just doesn't seem to be working out - I should probably just proactively replace the Tesla factory battery this spring.
(I've thought that there should be a way to develop a bidirectional buck/boost regulator to charge a relatively generic Li-ion stack while externally presenting a lead-acid-like charging curve. However this may be a step too far for companies like Ohmmu to take on and recoup profits from the effort.)
The latest Ohmmu with bluetooth and heating using V29 firmware and configured for model 3 is working well for months. After market battery technologies are changing and improving all the time.
Well, prior to this thread being revived a day or two ago, I had mostly gathered a problematic experience (with the understanding that the internet tends to greatly overrepresent problem reports compared to good experiences).
Looking at the new posts this week, I see both good and bad experiences. One of the concerns is that Ohmmu is constantly trying to react to challenges thrown down by Tesla firmware updates. (I don't think Tesla is actively trying to sabotage Ohmmu; I guess that Tesla is actively trying to mitigate the lead-acid battery failures because it's a real headache for customers and therefore for Tesla Service.)
But in any case, the concern is that the Ohmmu solutions are always one Tesla FW update away from another setback. I'm quite sympathetic to Ohmmu and I want to buy the product, but the whole point is to achieve worry-free reliability of the 12 volt battery. The user experiences, as reported here, are variable and so is the Ohmmu response and support.
Sometime back I perused website, looking for the magic FAQ page that would address all the issues reported here, along with the reassuring solutions and commitments. I found some information but not that. So I remain interested but today I'm leaning towards a proactive lead-acid battery replacement in 2024.
The Ohmmu is a LiFePO4 chemistry battery. It's about 30AH (they won't actually tell you), and it has a nominal voltage of 12.8V and a max voltage of 14.6V. This aligns nicely with a lead acid battery, which is about 12.2V to 14.8V.
The Tesla lithium battery uses an NMC chemistry, which has a higher voltage per cell. The "12V" Tesla battery is actually about 16V due to batteries which have a ~4V nominal voltage. The battery also is only 7Ah, and has a digital data connecton to the battery to manage it.
This change to 16V required changes to components in the car- such as the fact that the windshield wiper motors are different.
There simply is no way to retrofit a lithium to the car, and the Ohmmu isn't anything like the solution Tesla engineered themselves. You'd think they would have gone with the simple Ohmmu solution if it worked, and maybe even sell one themselves.
The irony here is that the tight monitoring envelope now catches lead acid batteries before they die, and allows the vehicle to keep the HV online along with the DC/DC so that the car will not die even if the 12V battery does. And if you look around, you will have a very hard time finding anyone who has been straned by their 12V lead acid in the last two years. I know on my car I got a warning about 2 weeks in advance, and even when it did "die" what I got was a warning that the 12V battery was dead and the car would not sleep and it would have much higher vampire drain, but could be driven. But the car worked fine.
Also, it's trivial to jump start a Tesla with a dead 12V battery. It's easier than an ICE car. Just carry a 12V jump pack in the frunk and tape a 9V battery to the inside of the bumper and move on with your life until the battery throws a warning. You'll never need the 9V or jump pack, but maybe it will help a friend someday.
As an EE I've considered this too, but this is just sillyness, and probably very, very hard. A lead acid battery is a very stiff source with a lot of capacitance and other interesting non-linearities. Remember, you need to both charge and discharge, and switch between these modes in the exact same way a lead acid does, with all the interesting chemical hysterysis that a real battery has. All of this just for Teslas that are no longer sold anymore, and don't really kill batteries anymore.
It's good to know that Tesla has greatly mitigated the sudden death syndrome of the 12 volt battery. My understanding is probably a year or so old, when I was still seeing quite a few messages from people who experienced the problem.
Regarding Ohmmu's market and what use case we're talking about here: you do seem to be concentrating on a second source for the Tesla 16 volt battery, whereas I (and I think many others) have been concentrating on a replacement for the lead-acid battery. At this particular moment, I would say that Ohmmu's largest Tesla market is probably more for the latter - not because I'm projecting my personal need onto everyone else (a common forun syndrome), but because the cars with batteries old enough to be aging out are the ones with lead-acid batteries. This will eventually change but not for a couple of years at least.
I have no comment on Ohmmu's or anyone else's potential market for replacing the Tesla Lithium low voltage battery. Certainly replacement lead-acid batteries are a big business in the automotive service and parts market, but the situation is likely to be quite different going forward with EV low voltage batteries.
As you note, the battery requirements as well as the fundamental construction and the operating environment are all very different from the traditional ICE cars. Also as we know, Tesla will be phasing in a 48 volt system and I have no details about whether that even uses a separate battery component*, or if so whether it interfaces directly to the 48 volt vehicle bus, as opposed to operating through some kind of more sophisticated PMU. I should probably look for a download of the booklet Elon sent to all his competitors.
*The current Tesla architectures have an important disconnect contactor (not to mention the Pyro fuse) to take the HV battery offline, separated from the LV section for several reasons; still I wonder if a future architecture could maintain the requisite safety and redundancy while making the low voltage system more as a part of the main battery pack. Simplification and cost reduction will continue be the name of the game, so I'd be looking for ways to eliminate the separate LV battery eventually.
Lead Acid 33Ah or greater will always work. But the latest complaints about Ohmmu were using the unheated model as temperatures have dropped versus the heated model with updated firmware which works well. Plus with the app it can be monitored constantly in detail. Batteries after Dec 2023 have the latest firmware and if you have an earlier heated model there is a bluetooth procedure to update the firmware. Once firmware updated the configuration software should be downloaded for the model tesla. Ohmmu is in Prescott. Drive up and see them have your questions and concerns heard. The battery has worked well through several software updates.
Not at all. I was answering @AZAV8R's question about how the Ohmmu battery is differnet than Tesla's, to answer the question of why Tesla can't just give us the battery type selection in the service menu and have their health monitoring work. It's important to understand that when you buy an Ohmmu you are buying a completely different solution than the one Tesla came up with when they switched to a Lithium LV battery.
The point remains here- If you go Ohmmu, you are sticking a LiFePO4 battery with a BMS on a system meant for a AGM Lead Acid with no BMS, that has very sophisticated monitoring of that AGM battery to detect health issues. Fundamental differences exist such as the fact that lead acid happily charges at 0F while lithium cannot charge below freezing. So now we have heaters in the lithium, wasting energy, just to trick the Tesla system. Problems should be expected, which is ironic as the primary selling point is to avoid problems.
Cybertruck derives 48V from a downconversion of the HV pack. There is no independent LV battery.
The question in 2024 is why would anyone buy an Ohmmu battery? Tesla does a good job monitoring and protecting the $120 lead acid now, and this thread should show you that the $498 Ohmmu is far from troube free. Ohmmu as a company has no reason to put resources into Tesla batteries in the future, since the last car made that can use their battery was made in 2021, and anyone in the future will go to Tesla for their lithium battery, or not need a battery at all.
Also, remember that when you buy an Ohmmu, this is what is inside, and this is the analysis of what it does to a Tesla electrical system by an expert:
Dysan911
Member
That guy's video really makes me question if I should continue using my Ohmmu battery which ironically has been pretty flawless lately on the V29 firmware. Before that, I had a really good streak of several months without any alerts despite the Tesla updates but watching that video makes me suddenly feel like it's not if but when my Ohmmu battery will burst into flames and burn down the car, garage, house, etc. 
.
Especially if the BMS is as bad as that guy seems to feel it is.
.Especially if the BMS is as bad as that guy seems to feel it is.
Thanks for the knowledge! My primary reason for going with the Ohmmu was concern over finding a bricked car in a mall parking lot in the dead of Summer (I’m in Phoenix). The second part of your response about the car being more tolerant to 12V failures gives me a little bit of comfort having going back to let acid.
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