Tell me: Why do we still run lead-acid batteries in every car?

Tell me: Why do we still run lead-acid batteries in every car?

Why why why. all the way home.

If Li-ion cells are so good, why havent they replaced lead-acids already?

I think the GM EV from the 90s actually used L-A batteries, so they must be reasonably effective.
But why not Li-Ion for starters? (pun).

Ross1 | October 15, 2016

Then you can do the sum:
No. of cars produced globally x Li-ion cells needed for starters etc, does that equal how many hamsters?
2 gigafactories?

(Is this EM's new unexpected product?)

jordanrichard | October 15, 2016


Dramsey | October 15, 2016


If Li-ion cells are so good, why havent they replaced lead-acids already?

Because the usage scenarios are different.

Different battery chemistries have different strengths and weaknesses. Lithium-ion cells have higher energy density, but:

• They operate efficiently over a relatively narrow range of temperatures
• Their voltage declines as they discharge
• They don't like spending time at or near full or empty. Doing so can damage the battery irreparably.

Our Teslas work to mitigate these disadvantages with elaborate battery temperature control systems and electronics to compensate for voltage declines and safeguards to help prevent damage from edge-case charge scenarios.

Lead-acid batteries have lower energy density, but maintain a relatively constant voltage as they discharge, are less temperature sensitive, and aren't as easily damaged by being run down. They don't require the fancy extras that Li-ion batteries do to keep them running optimally.

McLary | October 15, 2016

Lead batteries also can provide huge cold cranking amps. Lithium ion starter battery, in winter, in Minnesota? Hah!

There is a company that provides marine and motorcycle batteries. Both warm weather, and low amperage needs, mostly.

Watt fun | October 15, 2016

I do understand why not li-ion as a 12 volt battery in a Tesla or an ICE, but specifically in a Tesla are there no better alternatives than lead acid for its 12 volt batteries? Due to the constant draw and cycling, the environment for a 12v lead acid battery is not ideal either. A battery such as a NiMH as used for Prius hybrids has a near unlimited discharge/recharge life and would seem to be perfect for the Tesla 12v cycle. Lots of Prius in taxi use get far more than 500,000 km/300,000 miles with ease on the original battery pack and 50% more than even that is not unknown.Yes a lead acid battery has more cold cranking amps, is cheaper and likely smaller but the 12v but surely the existing 12v battery is a very unnecessarily fragile weak link for an expensive vehicle

UnshodBob | October 15, 2016

@Watt fun - lead acid batteries are SO last century. We should have better! :)

johndoeeyed | October 15, 2016

I created a thread about this a few months ago.
There already are lithium 12V lithium batteries, including OEM batteries.
I expect Tesla will soon replace the lead acid battery with a lithium battery.

johndoeeyed | October 15, 2016

It may even be their new product announced on Monday. | October 15, 2016

Lithium Ion 12v makes absolutely zero sense in the Tesla. It would require connection into the liquid hvac loop for cooling and heating, or your car could not be run cold weather. It would also affect the range in cold weather as it would need to be heated all the time. Because of the number of discharge cycles - vastly more than the main battery, depending on the design you may need to replace it every 6-12 months.

Hi-mh is also a lot more expensive than lead acid batteries. It has a better fit than Lithium-Ion, but I expect it's properties are also not ideal for the 12v battery in a vehicle. Ni-mh batteries have a higher internal impedance than lead acid batteries, so you need higher amperage than needed in the lead acid battery (more cost). Then the cycle life is about the same as Lead Acid. It does have a better power density, so you should be able to save some weight. Hard to see if the cost is worth the small weight savings for just a 12v battery. Makes more sense in a larger battery, as typically used in Hybrids.

oldgoat1968 | October 15, 2016

LiFePO4 (the chemistry most of us use in home made EV conversions) works just fine without liquid cooling and they are less likely to catch fire than some other chemistries, even with simple, or no BMS installed. They have higher energy density, more charge-discharge cycles than Lead Acid, and their price is coming down all the time. I would think it would be relatively easy to substitute a four cell Lithium Iron Phosphate 12V battery for the LA one that comes stock in the Tesla 12V system. My guess is the main advantage would be that it would outlast almost any Lead Acid 12 V (flooded, AGM or gell cell) battery in that application. BMS (battery management system) could probably be quite rudimentary, no where near as complex as the traction battery system in the Tesla.

Efontana | October 15, 2016

$ and peak current capability.

johndoeeyed | October 15, 2016

It would not require "connection into the liquid hvac loop for cooling and heating". As I said, there are already lithium 12 volt batteries, including OEM batteries.
It does not need to be replaced "every 6-12 months."
The life cycle is not "about the same as Lead Acid"

brando | October 15, 2016

Seems I often get 7 years min. on lead acid, friends truck going on 12 years Ram Cummins Diesel.

Watt fun | October 16, 2016

I have driven diesels (VW, mea culpa) in the past, and the type of usage even a diesel gives a lead acid battery is different than what the 12V battery in a Tesla has to endure. In a Tesla, it is constant small load (no starting ICE high compression engines or any other significant load) but also very frequent charging. Very hard on lead acid that endless in and out, surprisingly. LA can start a diesel at -25 but then it isn't normally called on to do much else or any other huge load for a long time, and just sits there being slightly charged, often for hours at a time, and therefore even with a diesel can last a long long time. LA is of course better in several ways than Li-ion for the 12V, but I am still surprised that there hasn't been another chemistry used for the 12V would could take constant in and out cycles and last as long as the main pack appears to be able to, rather than the year or maybe a bit more. It has always seemed to me to be the one weak-ish link that perhaps deserves a bit of thought. The 12V issue is one of those things that has contributed to some dissatisfaction/inconvenience to owners, and extra cost to Tesla, especially as it is not set up/placed to be easily addressed when it does fail, as most 12V in other cars are. | October 16, 2016

You can't equate Tesla's 12v application with how 12v batteries are used in ICE cars - totally different use cases.

Using Li cells for EV main power is ideal. It rarely needs to go from 0 to 100% charge - in fact Tesla doesn't even allow the Li main cells to go to 0 or 100% (they keep a small buffer on each end) to maintain cell longevity. Many thousands of cycles are possible using this technique. They further extend the life and performance by maintaining a fairly narrow temperature range.

LI's lifetime of current generation cells is not so good when you need to do many full 0-100% cycling like the 12v battery as used our Teslas. If Li was used, a lifetime of 500-1000 cycles is expected. So a Li in this specific high-frequency 0-100% discharge cycle might last 6 months (3-5 full cycles a day). Now Li doesn't just die, it loses capacity. So in this specific 12v application, the number of cycles will increase as it ages, further speeding up it's demise.

I may be wrong, but while Lead Acid works fine at -20C, Li cells do not. They really need to be heated. They have far less capacity and would require far more charge cycles at low temperatures.

Anyway, those that want to replace their current 12v with a Lithium replacement can do so. Just don't be too surprised when the battery doesn't last very long, or you get stranded if you leave the car in very cold weather.

johndoeeyed | October 16, 2016

There is no need whatsoever for the 12V battery in the Tesla to go from 0-100%. Nor does the 12V battery need to be cycled to the depth of the main battery. Lithium batteries can be lightly cycled for many years using the correct chemistry.

Ross1 | October 16, 2016

Li-Ion batteries in my home tools seem to be lasting about 8 years:

Paslode nail gun: 4 years for two, longer for the replacement.
Makita driver: maybe 8 years (3 batteries)
Braun toothbrush: Maybe 10 years.

Frank99 | October 16, 2016

I don't fully understand the purpose behind the 12V battery in the Tesla. There may be a need for 12V (to supply radios, marker lights, etc), but a DC-DC converter supplying 12VDC from the 400VDC battery is a simple circuit that's cheaper than a lead-acid battery. The Tesla already has a DC-DC converter to charge the lead acid battery, so it seems like eliminating the battery is a no-brainer. There must be a use case that I don't understand that requires the second battery system.

And, as jde noted, even if you do include a 12V battery, you don't have to cycle it from 0-100%.

jon_b | October 16, 2016

I think the 12v battery powers things like airbags and hazard lights that still need to work even if the main battery fails.

syd | October 17, 2016

The 12v battery drives everything that has to run with the high voltage battery connector in 'disconnected mode'. This includes all the computers (so it can start charging, unlock the car, upgrade the firmware, connect the HV battery to recharge the 12v battery, etc). When the Tesla is off, the HV battery is 'not connected' to the car, and is only connected when needed for charging the 12v, charging the HV battery, or when the car turns on. Thus the vampire load of a Tesla is running the radios and the computers, plus the load of the lights/doors, etc when accessing the vehicle and the system is still 'off'. | October 17, 2016

In addition to safety related items as Jon points out, it also powers the contactors to connect/disconnect the high voltage main pack. Without the 12v battery this critical safety system would not work.