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Forum Tesla Model S 12 V battery

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2017-08-16 03:33:32

Model S has a low voltage system, powered by a traditional 12 volt battery. The low voltage system operates the same electrical components found in conventional vehicles, including the supplementary restraint system (SRS), airbags, ignition, touchscreen, and interior, and exterior lights.

The low voltage system interacts with the high voltage system. The DC-DC converter supplies the 12V battery with power to support low voltage functions, and the 12V battery supplies power to the high voltage contacts to allow power to flow out of the high voltage battery.

Tesla Motors Model S -- Dead 12v Battery -- What to do to get back up and running (Roadside Service)  |  More info

Tesla Motors Model S -- Dead 12v Battery -- What to do to get back up and running (Roadside Service)


A Good Jump Box can be found here:

The Tesla Model S is one of the most advanced vehicles on the road. Yet, it still uses a old style 12v Lead Acid battery to power the Auxiliary Electronics, as well as used to "wake the car up" when it's time to drive off.

If your 12v Aux battery dies, your stuck. This video will show what to do to get your Model S running again to save you from a Tow Truck Ride.


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2017-08-16 03:37:40

BattMobile Batteries - $395.00

The C&D DCS-33RIT battery (OEM for TESLA) is rated for an "industry leading" (AGM) cycle life of 600 cycles at 80% Depth of Discharge (with a 5A load). Our batteries are rated for >2000 cycles in the same scenario! Direct fitment in Tesla Model S and Tesla Model X vehicles Cycle life 3x higher than OEM battery 35% of the weight of OEM battery (increase efficiency, 9.5lbs vs. stock @ 27lbs)

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Replacement 12 Volt battery on Tesla Model S by BattMobile Batteries (  |  More info

Upgrade your Tesla to an advanced technology Lithium 12V battery and stop worrying about when it is going to fail and leave you stranded due to low/no 12V power. BattMobile Batteries' Director Dr. Scherer walks you through the process of swapping your 12 volt accessory battery in the Tesla Model S, this is on one of the "tough" ones which is a single motor Model S "pre-facelift" (2014 model).

To purchase an advanced technology Lithium battery for your Tesla go to and "get the lead out" of your TESLA!

We also sell other 12V Lithium batteries including RV, Solar, marine, trolling motor and more! We can also make custom batteries if you are looking for something you haven't been able to find.

We have an installation guide on our website as well. Email with any questions to

2017-08-16 03:44:10

Why Tesla’s lead acid 12V battery needs to be lithium-ion based ?

It’s a prominent issue surrounding the electric vehicle market that the old-school lead acid battery just isn’t appropriate for new technology vehicles. Many users of electric vehicles, especially Tesla owners, have cited concerns with the poor performance of their 12V or low-voltage battery, oftentimes requiring annual replacement.

In contrast, a lead acid battery in a traditional internal combustion engine (ICE) vehicle generally has a 4 year life-cycle, but why?

First off, some of the most important factors to consider in longevity of a battery are “cycle-life”, environmental conditions, discharge/charge rates and calendar-life; cycle-life is how many times the battery can be drained and recharged in its life. Environmental conditions include temperature and humidity. Discharge/charge rates are the amperages going out of and into the battery respectively.

There are two major differences between the way an ICE vehicle uses its 12V battery and the way an EV uses its 12V battery:

“OFF” state discharge and cycling frequency

ICE Vehicle: generally has a very low 12V load while the vehicle is in the “off” state, often this load doesn’t exceed a few watts and doesn’t present a major challenge for the 12V battery to maintain.

Electric Vehicle: The 12V load while in the off-state is often much higher due to advanced computer systems that are running to maintain the high-voltage battery, keep vehicle “connected” (all EV have some remote access features), maintain charging and BMS (Battery Management System) communications, etc.

In fact a Tesla Model S/X puts about 50 Watts of load on the 12V system when the vehicle is in the “off” state. 50 Watts equals about 4.5 Amps of discharge on the 12V battery, this drains the battery down relatively rapidly and requires the 12V battery be “recharged” by the high-voltage battery regularly, this usage pattern results in many cycles being placed on the battery.

“ON” state utilization and purpose

ICE Vehicle: The 12V battery is used to initiate the ICE (start the car) and is designed for putting out large amounts of current to accommodate this process. Once an ICE vehicle is in the “on” state, it relies on an alternator to power all of the 12V sub-systems and also maintain the voltage of the 12V battery.

Electric Vehicle: The 12V is subjected to (practically) no additional load while the vehicle is being turned “on”, and although most vehicles are designed with DC/DC converters (which act as alternators) it is often an engineering design choice to reduce load on the DC/DC converter by minimizing the frequency with which it is utilized. This also extends the driving range of the vehicle because none of the precious high-voltage battery capacity is being shunted to non-driving tasks. Due to this usage profile the 12V battery is subjected to relatively low discharge and recharge currents.

When you combine the high number of cycles and the low current requirements of the electric vehicle 12V battery system you arrive at a completely different battery need than that of an ICE vehicle. Lead Acid batteries are very good at high discharge and low cycle count life-styles, this is their bread and butter and this is where they last a long time and provide the most bang for the buck (cheap cost and decent product life-cycle), but they aren’t lasting in electric vehicles.

The electric vehicle 12V battery system is one that is best suited by a battery capable of tremendous cycle-life as the main design goal. The battery chemistry that suits this usage scenario best? Lithium! Lithium battery technology is specifically very good at being cycled many times and continuing to provide minimal capacity loss and degradation. This, along with reduced weight, is why these batteries are used for the high-voltage battery packs, cell-phones, laptops, medical equipment and cars where batteries are being cycled frequently and longevity is important.

Editor’s note: This post was submitted into our network by Tesla Model S owner Sean Scherer. Having suffered an unfortunate incident in his Model S that left him stranded because of a faulty 12V battery, Sherer began on a mission to create a lithium-ion based 12V battery solution that was not only more reliable than the traditional lead acid battery, but better suited for the demands of a Tesla Model S, Model X, and electric vehicles in general. He began BattMobile Batteries, who have made it their mission to improve adoption of electric vehicles by solving some of the small details that has been missed by EV manufacturers.

by teslarati

2017-08-16 03:51:18

The Tesla Model S actually has two battery systems:

1 - Primary - Used for propulsion consisting of Lithium Ion Cells in a Battery Pack Enclosure.

2 - Secondary - Used to power the 12 Volt DC electronics in the car such as the Navigation System, Audio Equipment, Dash and Headlights, Wiper Motor, Window Mechanism, etc.

This secondary battery is a 12 Volt 35 Amp hour AGM maintenance-free design.

2017-08-20 11:33:00

Tesla Model S 12V Battery Analysis

The Model S uses the 12V system to run pretty much everything. The computers run off the 12V system, the interior electronics (locks, windows, seats, etc) run off 12V, and the main contactor for the big battery underneath requires a functioning 12V system. If the 12V battery is dead, so is the car. There are jump start terminals (which, by the way, you shouldn't use to jump another car), but the Model S is entirely reliant on this 12V lead acid battery to function.

If you replaced this 12V battery with a lithium battery, you'd have to add heaters and the like to it - you simply cannot safely charge a lithium battery below about the freezing point of water. Of course, if you never let your car below about 40F, a LiFePO4 drop in replacement would probably work fine, assuming the computers didn't notice and complain.

The battery is a C&D Technologies DCS-33RIT, at least in many of the cars. I don't know if they've changed to a new battery or not on the brand new vehicles.

Capacity is 33Ah (amp-hours).

General design is AGM - Absorbed Glass Mat.

Float charging voltage: 13.5-13.8V

Service charging voltage: 14.4-14.8V

Freshening equalization voltage is 14.5V

Recommended charge rate is C/5A at the 20h rate (6.6A)

Cycle life at 50% depth of discharge is 1800 cycles, at 20% depth of discharge is 5700 cycles.

This is a standard valve regulated battery (it vents overpressure).

So... pretty much a bog standard AGM battery. Nothing exciting, though the capacity (33Ah/12V - it's only ~400Wh fully drained) seems quite low for a car that pulls so much standby current.

My Analysis

So... why does the Model S chew through 12V batteries? Here are my thoughts on the issue.

First, the car is cycling the battery furiously - 5+ cycles per day. Lead acid is damaged by cycles, by chronic undercharging, and also by overcharging. It's a remarkably robust chemistry in that it shrugs off a lot of abuse and will keep functioning, but you pay in longevity. A well maintained lead acid battery can last 10 years or longer, while the same battery, abused, will die (be murdered, really) inside a year. Tesla is putting an insane number of cycles on this battery, so even in the best case, where everything else is right, it's going to die sooner than most people would expect. 5000 cycles at 5 cycles a day isn't even 3 years, and it's clear that the batteries are dying far sooner than that, around 1500-2500 cycles, in many cases. That's not an unreasonable lifespan for a lead acid battery, but a year is an unreasonably short time for a car battery.

Second, I'm not convinced that the charge time per cycle is enough. The initial charge current is fine, and the taper off is normal, but I'm not convinced that this battery is ever really 100% charged. Being fully charged on a regular basis is essential to prevent sulfation of the negative plate, and it helps balance charge between positive and negative plates as well. It's hard to tell state of charge by voltage, and mostly impossible to tell by voltage on a system with constant drain - you have to check specific gravity, which is more or less impossible on an AGM. But there's no sustained charge period I've seen, other than the float window while the main battery is charging - and that's not enough to really reset things. Tesla might be trying to accomplish a full charge with those voltage spikes, but...

Third, those voltage spikes. Whiskey Tango Foxtrot. At no point in the data sheet do I find anything that says, "Hey, blast me with almost 16V on a daily basis." They look like an attempt to either equalize the battery or fully charge the battery, but what they're most likely doing is creating more gasses than the battery can recombine internally. When this happens, the valves ("valve regulated" - remember?) go into action and the battery vents some hydrogen and oxygen - but that's lost to the battery forever. Venting a sealed battery is a great way to permanently shorten its life. I've also seen reports that the failed batteries are bulging, which is consistent with high internal pressure from this.

I don't think Tesla is deliberately trying to kill this battery quickly, but given this data series, I'm not one bit surprised that they die so rapidly. It kind of looks like they're trying to treat it like a lithium battery - "close enough to fully charged" and repeated cycling is more or less fine for lithium, but it's a great way to kill a lead acid battery in a hurry. They have sit at a higher voltage for a long period of time, somewhat regularly, if you want anything resembling rated life.

Were one to tear down a dead 12V battery that the Model S killed, I expect you'd find badly sulfated plates from chronic undercharging, as well as dry glass mats and a very high specific gravity acid remaining (the water will separate and vent, but the acid does not).

What Can Tesla Do?

If someone were to tell me, "Fix the Model S 12V battery longevity issues," I'd take the following steps. Remember, I'm not a world class battery expert, but I do have an interest in lead acid longevity. Some tweaks are software, some are hardware.

Put a bigger battery in. This battery is just too small for the car. Double or triple the size, pay the weight penalty (this is a 27lb battery, so 3x the size would only be an extra 50 lbs), and put something decently sized in - then adjust the software for it. You can reduce the cycle count significantly with a larger battery, and if you change nothing else, that should take the battery from a 1-1.5 year life to a 3-5 year life - much more reasonable.

Find a way to perform longer absorb cycles, at least once a day. Lead can handle partial state of charge cycling decently, as long as it gets a good absorb cycle (so around 14.5V) regularly - you have to get it up there and let it sit to reabsorb the sulfate crystals into the acid and balance the plates. You can do this while plugged in at night, and I don't think it would hurt anything to run the higher voltage (14.5V or so) while driving. That's how a normal car charges the battery.

Keep cycles off the battery whenever you can - whenever shore power (the charger) is connected. If the car is connected to a charger, you shouldn't be cycling the 12V battery up and down. Charge it, let it absorb for a while (a few hours is good), drop to float. Stay there until disconnected from the charger. I understand if they're reluctant to pull any current before the charge time, but I'd do that and add an advanced option to cancel it. Float time on an AGM battery is "free" - it doesn't do anything to degrade the battery.

Eliminate those 15.8V spikes entirely. If you have longer absorb times (around 14.5V), the battery should be in good shape without any need to zap it like this. As a bonus, you won't have as much gassing and loss of water. Bulging batteries mean gassing, and high voltage causes gassing. AGM is much less forgiving of a high voltage than flooded lead acid.

Sleep the computers when possible. The car is constantly drawing 30W and regularly pulls more - when turned off. I don't know what exactly goes on, but maybe some of it could happen when charging. In the realm of greater hardware redesign, a low power standby computer for handling stuff that always has to happen would help.

Or maybe just leave the DC-DC converter on. I have no idea what the losses running that are, but lead acid charging is far from 100% efficient. It may be easier to just hold the battery at a float charge most of the time.

I expect those changes would radically improve the 12V battery longevity. Even if you can't put a larger battery in, changing the behavior to reduce the cycles on the battery should help a lot, as should letting it charge for longer when possible. I'm pretty sure that can be done in software.

Again, I'm not a professional battery engineer - just a guy who makes a living in an office with a big bank of lead acid batteries I'm not eager to replace. These are my suggestions, based on what I see in these few days of data and what I know about lead acid. I've spent an awful lot of time reading up on the chemistry, though.

by Syonyk's Project Blog