Passenger Compartment Batteries

[Homer]

807 / C7 battery is in the compartment under the driver's footwell but then I used to drive a Mitsubishi van where the entire engine was under the middle passenger seat which doubled up as bonnet when you needed to do anything with the oily bits. IIRC the battery was behind the driver's seat.

[Peter.N.]

I had a Commer van like that many years ago, can't remember where the battery was though.

Peter

[Hell Razor5543]

The Vauxhall Movano has the battery in a cavity underneath the passenger footwell. I would not be surprised if the Renault and Nissan clones have the same layout.

[Michel]

I had a Commer van like that many years ago, can't remember where the battery was though.

Peter

Passenger footwell I think Peter. We had a Commer camper when I was a kid. It had the auxiliary battery under the bench seats/beds at the back!

[Michel]

The Vauxhall Movano has the battery in a cavity underneath the passenger footwell. I would not be surprised if the Renault and Nissan clones have the same layout.

As long as it's vented it's the safest place for it. It's not in the engine compartment, and it is inside the crash structure. In an accident it's likely to stay put and not become an acid-spraying projectile. I think the risk of being gassed by one or having it explode is minimal anyway.

[lexi]

5 years with a 64 Landy. Battery under passenger seat. 10 gallon of petrol under the drivers backside. It never stayed in the tank long enough to cause concern.

[CitroJim]

It never stayed in the tank long enough to cause concern.

[Hell Razor5543]

Henri Toivonen and his co-driver Sergio Cresto were killed when the fuel tank und the driver seat was ruptured and exploded in a bad accident. If it had not exploded it is possible that they would have escaped, but as it was they had absolutely no chance of survival. This accident sealed the fate of Group B rallying (there had already been serious accidents resulting in injuries and deaths).

https://en.wikipedia.org/wiki/Henri_Toivonen

[Mandrake]

Given that batteries for electric cars won't fit under the bonnet (or that it isn't sensible to, given the centre of gravity problems) batteries in/under/around the passenger compartment is the inevitable future!

To put this in context though, the traction battery for an EV is Lithium Ion not Lead Acid, so doesn't vent Hydrogen gas or Hydrogen Sulfide. Unlike even "sealed" lead acid batteries, it truly is a fully sealed, closed cycle battery which except in a catastrophic failure (catching fire) does not vent gas or material of any kind.

Unlike the very rudimentary charging system used for the Lead Acid 12v battery in cars (basically a constant voltage charge applied across the entire series string of cells) which can overcharge a 12v battery and lead to failure, the battery charging system on a Lithium Ion traction battery is very sophisticated and precise - every single cell in the series stack is individually monitored by the ECU's so a fault of an individual cell would be detected, and it's impossible to overcharge or over discharge an individual cell. Full charge voltage of individual cells is regulated to +/- 0.01 volts. The ECU's also "balance" the cells as part of the charging process to avoid any cell ever being over charged, over discharged, or pushed into a negative charge by adjacent cells discharging.

There is no such protection in a 12v Lead Acid battery - you have a string of 6 cells in series internally and the external 14.4v charger has no idea whether the cells are balanced or not - if one cell becomes weak or discharges due to an internal short it will still attempt to charge at 14.4v even though this is now effectively across 5 cells - the result is that the remaining 5 good cells are overcharged and will start venting hydrogen and overheating. The charging system is unaware of this since it doesn't monitor individual cells and that is where the danger is.

Although a Lithium Ion cell is theoretically more dangerous in an overcharge condition than a Lead Acid cell, due to the extensive monitoring of every individual cell the system is much safer because the cells are never pushed outside their safe regions. The ECU's not only measure the voltage and state of charge of every individual cell - the temperature of every cell is measured as well. Partly so the system knows how fast it can charge based on the temperature of the cells, but it's also a great safety feature that individual cell temperatures are monitored which would immediately flag up a faulty cell if one cell started overheating.

A Lithium Ion traction battery is also not in the passenger compartment as some 12v batteries now are. It's underneath the floor pan so is entirely outside the car, despite being "under" the passengers. Even if the cells did rupture and start gassing, its not gassing into the passenger compartment. Some EV's (Tesla's in particular) have a very strong chassis around the cells that forms part of the structure of the car, but also forms a "firewall" between groups of cells and the interior of the car. If an individual cell did catch fire it is designed to vent the gas pressure down onto the road and it is prevented from coming into the passenger compartment or into adjacent cell group firewalls.

Some other EV's are not quite so stringently designed for safety - for example in the Ion the battery pack has groups of cells stacked that are not firewalled off from each other, and the top cover for the battery pack is just a plastic air fairing used to duct cooling air, however like all other EV's the battery pack is still underneath the floor pan so there is no risk of gasses entering the passenger compartment. The only real risk is a full blown fire, which seems to be very rare - certainly less common than petrol tank related fires.

To be honest I'd much rather have a well designed lithium Ion traction battery sitting under the floor than a 12v Lead Acid battery inside the cabin under a seat!

One other note - EV's also still have a 12v Lead Acid battery to power the ECU's and ancillaries, however it is usually under the bonnet, I'm not aware of any current EV's that place the 12v battery inside the passenger compartment. Because electric motors are so compact the "engine" bay on most EV's has plenty of room for a 12v battery so there is no real impetus to place it anywhere less desirable like inside the cabin.

[CitroJim]

Simon, a most interesting dissertation there

So, given that in modern cars the demands on the battery are so heavy and critical why do they still have lead-acid batteries as standard and why have Li-Ion batteries not been developed for this application?

And why do EVs have an auxiliary lead-acid battery too when there's (presumably) ample power available from the main traction battery? It seems illogical...

[Mandrake]

Simon, a most interesting dissertation there

Sorry Jim, it started off as a short post but ended up rather long.

So, given that in modern cars the demands on the battery are so heavy and critical why do they still have lead-acid batteries as standard and why have Li-Ion batteries not been developed for this application?

Interesting question - I think there are a few factors including cost, inertia and backward compatibility.

Lead Acid batteries have been with us for over 100 years, are cheap and easy to make (lead, plastic, a bit of sulphuric acid!) and are well suited to being a "starter" battery. They last the warranty period of the car which is all the manufacturer really wants!

Lead Acid has a very low energy density compared to Lithium Ion but the power density (cranking amps) is pretty decent for its size. You'd need a reasonably large Lithium Ion battery to be able to crank a starter motor, so it wouldn't be physically much smaller to reach the same peak amps, although the payoff would be the Ah rating would be several times higher, maybe 200Ah or more, but that is unnecessarily high.

Lead Acid will tolerate short periods of overcharge/overvoltage without permanent damage whereas significant overcharge of Lithium Ion that pushes the cell voltage too high can cause serious damage or for it to catch fire. (Hence the close monitoring required when charging)

Lead Acid can also tolerate short term extreme loads that pull the terminal voltage below normal levels - like a starter pulling the voltage down to 9 volts even though full discharge terminal voltage is 11.6v. Lithium Ion is unhappy about being pulled down below the full discharge cutoff voltage even for a short time, so you'd need to manage the load to prevent the cells going below the cutoff voltage, which would be inconvenient to do with a stalled not yet turning starter motor.

I think the real deal breaker though is that you can't build a 12v Lithium Ion stack without a full battery monitoring and balancing ECU. On a lead acid battery you can "get away with" just sticking 6 cells in series and charging them all together without really caring about whether the cells are balanced. (Although as in my original post, this can lead to a shorted cell causing other cells to get dangerously overcharged)

The end of the charging process for lead acid has a point where only heat is produced, this allows the slow out of balance cells to "catch up", so as long as you fully charge and top off a lead acid battery, and as long as none of the cells fail, it will stay relatively balanced in use.

It's not safe to do this with a Lithium Ion pack - if you have multiple cells in series you must measure the voltage of every individual cell and provide a means of balancing the individual cells so they can't get out of step with each other, because the risk of fire if even one cell is overcharged is severe. Without some form of balancing eventually the cells would get out of balance and be at risk.

Balancing is performed by a transistor switching a shunt resistor across the individual cell. When charging this slows the charging rate to the individual cell slightly relative to other cells, or if you are not charging it discharges it slightly. By doing this the ECU's can get all cells in the series string perfectly in balance. They attempt to balance the terminal voltage of the cells within 5-10mV!

You could use a Lithium Ion battery in a starter battery role but it would be more expensive and need a completely different charging and control system - you could not simply hook it up to the existing alternator, and it would need an ECU to balance the cells. As it would not be backward compatible and have to be designed into the car from the beginning I suspect inertia will prevent it ever being adopted on an ICE car unless a ban was put on lead based batteries! It would probably last the lifetime of the car instead of 3-4 years but the higher cost and complexity probably doesn't justify it.

And why do EVs have an auxiliary lead-acid battery too when there's (presumably) ample power available from the main traction battery? It seems illogical...

That's also a good question.

The traction battery in an EV is 330-400v DC, and that traction battery is kept within a sealed enclosure under the car partly for safety reasons. There is a large contactor inside the battery pack enclosure that cuts off the supply to the rest of the car when the car is "off". So while there are orange HVDC cables going through the car to get to the inverter and motor, they are not energised when the car is in an off state and the car is safe to work on. (There is usually an additional mechanical cutoff link plug somewhere as well - on my Ion it's through an access hole under the passenger seat of all places!)

To charge the 12v battery there is a DC-DC inverter that runs from the traction battery but is mounted outside of the battery enclosure - so for that DC-DC inverter to be powered the contactor in the main traction battery has to be engaged and the high voltage system "Live", which again on the Ion, only happens when the car is either in Ready mode (ready to drive) or plugged in charging.

So if you were to try to eliminate the 12v battery and just run the electrical systems from the traction battery via the DC-DC inverter alone it means the contactor would have to be closed all the time leaving the car live, and the DC-DC inverter would have to be running all the time even for the very light load of ECU's sleeping for example central locking and alarm. The contactor coil would use a significant amount of power relative to a car on standby, and the efficiency of a 1kW DC-DC inverter to power a standby drain of say 100mA would be pretty terrible as well!

Hence the 12v battery is there to power all the ECU's that need to be operational when the car is asleep (central locking and alarm mainly) and power from that 12v battery is required to power the contactor to enable the HVDC supply.

As to why EV's still use Lead Acid batteries instead of Lithium Ion for the 12v systems I'm not sure - they don't have to supply a high discharge for a starter motor - they only run ECU's and ancillaries, and you don't have a legacy alternator to worry about being compatible with, so instead of a DC-DC inverter designed to charge a 12v Lead Acid Cell you could just as easily have had a DC-DC inverter and charging system designed to work with a 12v Lithium Ion battery since you're starting with a clean sheet of paper.

I suspect the reason is simply inertia and "sticking to what you know" but ironically the 12v battery on EV's can be a point of failure - the Leaf is well known for the 12v battery discharging as there is something wrong with the design of the 12v battery charger/management system that allows it to be discharged too low or not charged sufficiently (nobody really seems to know for sure) and early Tesla's also ate their 12v batteries for lunch due to cycling them excessively and deeply due to the relatively heavy ECU load they put on the battery. (They changed from a starter battery to a deep cycle AGM after the first couple of years which helped a lot)

Scuttlebut is that Tesla have eliminated the 12v battery entirely on the Model 3 to reduce costs and increase reliability - and that the approach they took was to integrate the 12v DC-DC inverter directly into the main traction battery assembly - so that 12v comes directly out of the enclosure.

This allows them to run the DC-DC inverter all the time without the main high voltage contactor running that normally powers the drive system. So no power wasted running the contactor 24/7 and no high voltage provided externally when the car is not being driven.

While a 1kW DC-DC inverter is not particularly efficient at low loads, Tesla's have a very high 12v "vampire" load anyway even when the car is off, as all the complex computer systems are running all the time and don't have a good low power standby mode. While my Ion has a standby load of less than a watt, on Tesla's the constant 12v load while parked is more like 20-30 watts, so requires periodic replenishment by the DC-DC inverter on existing models that still use a 12v battery.

On the Model 3 they can get away with just running the DC-DC inverter all the time and dispensing with the 12v battery altogether.

[CitroJim]

That is most excellent Simon Thanks for the very detailed answers and do keep the dissertations coming - I love them and learn a very great deal from them

And luckily, just recently my cognition has been well up to reading them and more to the point, absorbing the knowledge therein!

[Michel]

Henri Toivonen and his co-driver Sergio Cresto were killed when the fuel tank und the driver seat was ruptured and exploded in a bad accident. If it had not exploded it is possible that they would have escaped, but as it was they had absolutely no chance of survival. This accident sealed the fate of Group B rallying (there had already been serious accidents resulting in injuries and deaths).

https://en.wikipedia.org/wiki/Henri_Toivonen

The main issue here was Henri shouldn't have been driving due to his blackouts. Furthermore, the Lancia was made of cheese and cardboard, and they'd taken the undershield off to save even more weight.

[van ordinaire]

There is a move away from lead acid batteries but they are expensive (well, to me they are) & I gather that the latest products of the leading manufacturer are not a patch on the original gold plated Rolls-Royce products.