The EV fact thread

[Bobby Dazzler]

Quite apart from choosing a Li battery the mentioned capacity of just 60 Ah sounds very low for reliably starting a big engine in a car that's unlikely to be a daily driver. The battery in my R129 is 100 Ah .... even my W205 (which is just a 2 litre 4 cyl) has an 80 Ah.

I am far from an expert however my limited understanding is that the energy density is far greater in a lithium battery than a lead acid battery, which means that a smaller, lighter, lower capacity lithium battery can outperform a larger, heavier, higher capacity lead acid battery. As a result the Ah rating can be much lower, whilst offering additional performance.

 

[BTB 500]

I am far from an expert however my limited understanding is that the energy density is far greater in a lithium battery than a lead acid battery, which means that a smaller, lighter, lower capacity lithium battery can outperform a larger, heavier, higher capacity lead acid battery. As a result the Ah rating can be much lower, whilst offering additional performance.

Energy density just refers to the weight/size for a given amount of stored energy. So a 100 Ah Li battery would be smaller & lighter than a 100 Ah lead acid battery, but both would store the same amount of energy (assuming they are the same voltage ... otherwise you need to compare capacity in Watt hours rather than Amp hours).

 

[190]

Modern Mercedes have very large batteries but I don't think it's because of the demands of starting the engine.

My C180K has a piddling 1.6L engine but an 84 amp hour battery. Now I know the Ah rating is not a direct indication of cranking amps but in general a bigger battery will have more cranking amps. No way is such a big battery needed for the starter, it's big to cope with the high drain current of a complex electrical system that wakes up every time you open a door and takes time to go back to sleep again.

So back to the discussion above, a small lithium battery may easily have the cranking amps to start the engine but does it have the Ah to cope with the demands of a complex electrical system and will it work after two weeks in an airport carpark.

The same discussion goes on with motorcycles where really tiny Lithium batteries will start the bike with ease and provide a very useful weight saving into the bargain, but they run flat in no time at all because they lack Ah capacity so it better not need too many attempts to fire up.

 

[Ted]

If the starter/generator is using the 48volt battery to start, then the 12 volt battery only needs to boot up the electronics and can be low capacity.
The Lexus has a 45Ah battery as the starter is driven by the traction battery via an inverter.

A side effect of this is that the 12v battery is more likely to go flat if left for long periods.

 

[Bobby Dazzler]

Energy density just refers to the weight/size for a given amount of stored energy. So a 100 Ah Li battery would be smaller & lighter than a 100 Ah lead acid battery, but both would store the same amount of energy (assuming they are the same voltage

I am far from an expert however my limited understanding is that the energy density is far greater in a lithium battery than a lead acid battery, which means that a smaller, lighter, lower capacity lithium battery can outperform a larger, heavier, higher capacity lead acid battery. As a result the Ah rating can be much lower, whilst offering additional performance.

It’s perhaps my limited understanding but I think you just said the same thing as me, even though it didn’t feel like it when I read it.

Again I am far from an expert on batteries, but I recall that a lithium battery can use much more of it’s capacity than a lead acid battery before it becomes flat and unusable.

Let’s say the lithium battery has double the usable capacity, then very roughly speaking the “total” capacity of a lithium battery only need be half that of a lead acid to allow a similar real world capacity.

 

[Bobby Dazzler]

So back to the discussion above, a small lithium battery may easily have the cranking amps to start the engine but does it have the Ah to cope with the demands of a complex electrical system and will it work after two weeks in an airport carpark.

I can only speak from my own experience which is limited to just a few cars with lithium batteries, however a lithium battery can easily deal with being left for very long periods without charging.

Unless it’s a modern Ferrari, in which case, once parked, the battery will almost immediately be discharged completely regardless of whether it’s lithium, AGM or connected to a charger.

 

[ChrisPDuck]

I've made a 300Ah Lithium Iron Phosphate (LiFePO4) leisure battery for my camper van, from four individual cells, a BMS and an active cell balancer, temporarily housed in a wooden box, and in my experience Mr Dazzler has it exactly right. I was able to fit the whole lot into a box that's only slightly bigger than a 110Ah lead acid battery. And not only do you get more Ah rating per unit volume, but more of those Ah are usable. A conventional lead acid battery really needs recharging at around 12V, for best longevity, at which point it is approximately 50% discharged. The LiFePO4 battery can be taken right down to virtually empty. So, roughly, a lead acid 110Ah battery will actually give, let's say, 60Ah, whereas a similar volume of LiFePO4 can give much closer to 300Ah.
The BMS on my LiFePO4 battery has a bluetooth interface that allows me to see the state of charge. I brought the battery indoors for the winter and left it disconnected at approximately 70% capacity for about six months, and there was no discernible drop in charge.

 

[poormansporsche]

If the starter/generator is using the 48volt battery to start, then the 12 volt battery only needs to boot up the electronics and can be low capacity.
The Lexus has a 45Ah battery as the starter is driven by the traction battery via an inverter.

A side effect of this is that the 12v battery is more likely to go flat if left for SHORT periods.

FTFY

Yes, the Lexus RX. Much to the amusement to the original bunch of EV haters, whacking great battery in the boot but the car won't start because the tiny sized battery up front goes flat in 3 days !

 

[BTB 500]

It’s perhaps my limited understanding but I think you just said the same thing as me, even though it didn’t feel like it when I read it.

Again I am far from an expert on batteries, but I recall that a lithium battery can use much more of it’s capacity than a lead acid battery before it becomes flat and unusable.

Let’s say the lithium battery has double the usable capacity, then very roughly speaking the “total” capacity of a lithium battery only need be half that of a lead acid to allow a similar real world capacity.

The rated capacity of any battery is based on its ability to deliver current over time. So very simplistically a 60 Ah battery could deliver 1A for 60 hours, or 60A for 1 hour (both are 60 Ah).

In reality though the usable capacity varies depending on how much current is being drawn. The convention for car batteries is that rated capacity is based on a continuous 20 hour discharge ... so a 60 Ah battery would deliver 3A for 20 hours, and a 120 Ah battery would deliver 6A for 20 hours. At lower currents the usable capacity will be higher, and at higher currents it will be lower. All of this is true regardless of the battery type.

The relationship between usable capacity and current is driven by something called the "Peukert constant". A value of 1.0 would be a perfect battery which gave the same capacity at any current - this doesn't exist of course. Where Li batteries score is that they have a lower (better) Peukert constant than lead/acid batteries, so in a constant high current discharge they deliver closer to their rated capacity. So a 60 Ah Li battery will crank the engine over for longer than a 60 Ah lead/acid battery. But on the flip side at low currents (sitting idle in a car park for weeks) the lead/acid battery will last longer.

All of this is based on constant discharge though. The reduced capacity at higher currents isn't actually 'lost' - some of this is recovered when the battery is allowed to rest. We've all seen this when trying to start a car with a low battery. You crank till it 'dies', but in a minute or so it will crank again.

The Peukert constant for any battery changes with temperature - that's why they perform worse in winter (it's not all down to thick engine oil and more use of headlights etc.). It's also affected by age, and I think that's where Li loses out. I strongly suspect they degrade faster than lead/acid.

 

[markjay]

Lexus slashes price of its electric cars by as much as £7,000

With today's EV market being propped up by huge fleet demand , the manufacturer has slashed its prices to increase appeal for private buyers.

mol.im

Gran a deal while they last....

 

[BTB 500]

In the news today:

BERLIN, July 9 (Reuters) - Volkswagen on Tuesday warned it may close the Brussels site of its luxury brand Audi due to a sharp drop in demand for high-end electric cars that has hit Europe's top carmaker, forcing it to cut its margin target for the current year.

Volkswagen has not shut down a plant since it closed the Westmoreland site in Pennsylvania in 1988, and the last VW brand chief to threaten closures in Europe stepped down months after doing so, according to a labour source.

Automakers have been hit hard by lower than expected EV demand after investing heavily in capacity and technology development, with Audi warning earlier this year its sales would dip in 2024 as it worked on introducing new models while also cutting costs.

Volkswagen said the costs of finding an alternative use for the Brussels plant or closing it, as well as other unplanned expenses, would have an impact totalling up to 2.6 billion euros ($2.8 billion) in the 2024 financial year.

https://www.reuters.com/business/autos-transportation/vw-group-lowers-forecast-possible-closure-audi-brussels-site-other-expenses-2024-07-09/#:~:text=BERLIN%2C%20July%209%20(Reuters),target%20for%20the%20current%20year.

 

[markjay]

In the news today:

https://www.reuters.com/business/autos-transportation/vw-group-lowers-forecast-possible-closure-audi-brussels-site-other-expenses-2024-07-09/#:~:text=BERLIN%2C%20July%209%20(Reuters),target%20for%20the%20current%20year.

It shouldn't come as a surprise, it has already been said many times that in the wake of the Model-S, car manufacturers all rushed in with expensive high-end EVs to compete in the same price tag bracket as the Tesla, which in turn made this small marketplace very crowded.

It took a while before the penny dropped, and car manufacturers started making smaller and cheaper EVs.

 

[BTB 500]

The rated capacity of any battery is based on its ability to deliver current over time. So very simplistically a 60 Ah battery could deliver 1A for 60 hours, or 60A for 1 hour (both are 60 Ah).

In reality though the usable capacity varies depending on how much current is being drawn. The convention for car batteries is that rated capacity is based on a continuous 20 hour discharge ... so a 60 Ah battery would deliver 3A for 20 hours, and a 120 Ah battery would deliver 6A for 20 hours. At lower currents the usable capacity will be higher, and at higher currents it will be lower. All of this is true regardless of the battery type.

The relationship between usable capacity and current is driven by something called the "Peukert constant". A value of 1.0 would be a perfect battery which gave the same capacity at any current - this doesn't exist of course. Where Li batteries score is that they have a lower (better) Peukert constant than lead/acid batteries, so in a constant high current discharge they deliver closer to their rated capacity. So a 60 Ah Li battery will crank the engine over for longer than a 60 Ah lead/acid battery. But on the flip side at low currents (sitting idle in a car park for weeks) the lead/acid battery will last longer.

All of this is based on constant discharge though. The reduced capacity at higher currents isn't actually 'lost' - some of this is recovered when the battery is allowed to rest. We've all seen this when trying to start a car with a low battery. You crank till it 'dies', but in a minute or so it will crank again.

The Peukert constant for any battery changes with temperature - that's why they perform worse in winter (it's not all down to thick engine oil and more use of headlights etc.). It's also affected by age, and I think that's where Li loses out. I strongly suspect they degrade faster than lead/acid.

As a follow-up, I've done some quick & dirty calculations for 60 and 100 Ah lead/acid batteries and a 60 Ah Li Ion. Three scenarios are shown for each: 75 mA parasitic drain (typical of a parked & locked modern car), 350 A discharge (guesstimated cranking current for a V8), and a 20 hour discharge (just to verify my calculations - the delivered capacity should then match the rated capacity).

As noted below this takes no account of the natural self-discharge over time that any battery suffers, although this shouldn't be significant here. Energy lost as heat at high currents (due to the internal resistance of the battery) is also excluded, and this would likely affect the results ... reducing the capacity delivered and cranking time shown.

The Peukert Constant values used are also just examples. 1.05 is typically quoted for Li cells, but Lead/acid batteries have quite a range (AGM is actually very similar to Li, gel is in the middle, and wet is the worst). As mentioned they are all affected by ambient temperature and the age of the battery. Of course Li batteries will be significantly smaller and lighter (but more expensive) than any type of lead/acid of similar rated capacity.

With these points made, for continuous engine cranking a 60 Ah Li battery seems to out-perform (just) a 100 Ah lead/acid. But at very low currents (parked car scenario) even a 60 Ah lead/acid will last a lot longer. I suspect exotic ICE cars with Li starter batteries are intended to be kept on a maintenance charger if not driven regularly?

If you'd like to see the results with different values for the Peukert Constants or other battery capacities I can easily do that.

 

[Bobby Dazzler]

I've made a 300Ah Lithium Iron Phosphate (LiFePO4) leisure battery for my camper van, from four individual cells, a BMS and an active cell balancer, temporarily housed in a wooden box, and in my experience Mr Dazzler has it exactly right. I was able to fit the whole lot into a box that's only slightly bigger than a 110Ah lead acid battery. And not only do you get more Ah rating per unit volume, but more of those Ah are usable. A conventional lead acid battery really needs recharging at around 12V, for best longevity, at which point it is approximately 50% discharged. The LiFePO4 battery can be taken right down to virtually empty. So, roughly, a lead acid 110Ah battery will actually give, let's say, 60Ah, whereas a similar volume of LiFePO4 can give much closer to 300Ah.
The BMS on my LiFePO4 battery has a bluetooth interface that allows me to see the state of charge. I brought the battery indoors for the winter and left it disconnected at approximately 70% capacity for about six months, and there was no discernible drop in charge.

Thank you, said so much more clearly than me

 

[Bobby Dazzler]

As a follow-up, I've done some quick & dirty calculations for 60 and 100 Ah lead/acid batteries and a 60 Ah Li Ion. Three scenarios are shown for each: 75 mA parasitic drain (typical of a parked & locked modern car), 350 A discharge (guesstimated cranking current for a V8), and a 20 hour discharge (just to verify my calculations - the delivered capacity should then match the rated capacity).

As noted below this takes no account of the natural self-discharge over time that any battery suffers, although this shouldn't be significant here. Energy lost as heat at high currents (due to the internal resistance of the battery) is also excluded, and this would likely affect the results ... reducing the capacity delivered and cranking time shown.

The Peukert Constant values used are also just examples. 1.05 is typically quoted for Li cells, but Lead/acid batteries have quite a range (AGM is actually very similar to Li, gel is in the middle, and wet is the worst). As mentioned they are all affected by ambient temperature and the age of the battery. Of course Li batteries will be significantly smaller and lighter (but more expensive) than any type of lead/acid of similar rated capacity.

With these points made, for continuous engine cranking a 60 Ah Li battery seems to out-perform (just) a 100 Ah lead/acid. But at very low currents (parked car scenario) even a 60 Ah lead/acid will last a lot longer. I suspect exotic ICE cars with Li starter batteries are intended to be kept on a maintenance charger if not driven regularly?

If you'd like to see the results with different values for the Peukert Constants or other battery capacities I can easily do that.


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You clearly know your onions, thank you for taking time to explain it and for backing it up with the calculations.

I struggled to follow it all but I think the conclusion of your calculations is that a smaller, lighter, lower capacity lithium battery can outperform a larger, heavier, higher capacity lead acid battery. As a result the Ah rating can be much lower, whilst offering additional performance. Is that right, roughly speaking?

PS Your theoretical explanation says that it shouldn’t be the case, however in practice lithium batteries barely lose any charge even over extended periods. Whether it’s down to the chemistry or the built in BMS I don’t know.

 

[BTB 500]

In a nutshell, at high continuous currents Li batteries typically deliver more of their rated capacity than wet lead/acid ones do. In my example of pulling 350 A while cranking a big engine, a 60 Ah Li battery would give 47 Ah and a 60 Ah Lead/acid one would give 23 Ah.

A 100 Ah lead/acid battery (as in my 5 litre R129) would match the high current performance of a 60 Ah Li one for a fraction of the cost, but at a slight weight penalty in terms of the total vehicle weight. Against that it would supply much more capacity at lower currents, and would almost certainly have a longer lifespan (the one in my SL is still fine at 20 years old). Swings and roundabouts.

Where weight is absolutely critical (electric powered light aircraft, for example) then Li is the only option. But it's a less obvious choice for the starting battery in an ICE road car. As mentioned AGM lead /acid batteries perform very similarly to Li ones of the same rated capacity, so are arguably a better all-round solution (used in many high-end cars now).

 

[Bobby Dazzler]

In a nutshell, at high continuous currents Li batteries typically deliver more of their rated capacity than wet lead/acid ones do. In my example of pulling 350 A while cranking a big engine, a 60 Ah Li battery would give 47 Ah and a 60 Ah Lead/acid one would give 23 Ah.

A 100 Ah lead/acid battery (as in my 5 litre R129) would match the high current performance of a 60 Ah Li one for a fraction of the cost, but at a slight weight penalty in terms of the total vehicle weight. Against that it would supply much more capacity at lower currents, and would almost certainly have a longer lifespan (the one in my SL is still fine at 20 years old). Swings and roundabouts.

Where weight is absolutely critical (electric powered light aircraft, for example) then Li is the only option. But it's a less obvious choice for the starting battery in an ICE road car. As mentioned AGM lead /acid batteries perform very similarly to Li ones of the same rated capacity, so are arguably a better all-round solution (used in many high-end cars now).

Got you, thank you for fining another way go explain it to a numbnuts like me. So a smaller, lighter, lower capacity lithium battery can outperform a larger, heavier, higher capacity lead acid battery.

Back to your previous comment (below) do you still think that 60 Ah is still very low for a big engine unlikely to be used every day?

Quite apart from choosing a Li battery the mentioned capacity of just 60 Ah sounds very low for reliably starting a big engine in a car that's unlikely to be a daily driver. The battery in my R129 is 100 Ah .... even my W205 (which is just a 2 litre 4 cyl) has an 80 Ah.

 

[MikeInWimbledon]

Lexus slashes price of its electric cars by as much as £7,000

With today's EV market being propped up by huge fleet demand , the manufacturer has slashed its prices to increase appeal for private buyers.

mol.im

Gran a deal while they last....

Although the UX300e with £7k off has been on discount for quite a while and is generally viewed as being a comfy but crepe motor...

Lexus UX 300e 2022 long-term test | Autocar

Lexus arrives at the EV party with a converted version of its compact crossover

www.autocar.co.uk

"Beware salesmen bearing headline discounts."

 

[markjay]

Dacia Spring EV review: The UK's CHEAPEST sub-£15k new electric car

The all-new Dacia Spring is the cheapest EV in the UK by almost £10k. We test drove it to find out if this budget EV really can put the country on cheap electric wheels...

mol.im

"It's proof that EVs can be cars for needs and not wants and for that reason it has to be applauded."

I think that in future we'll see more of this type of EVs, and less of £80k+ models.

 

[MikeInWimbledon]

I see they're relaunching Ford Capri as an EV

Let's hope it's light, nimble, and affordable, like the old one.