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This was just touched on in another thread here very briefly. I've wondered the same myself and every time I show people the motor and bay they always think it's an odd thing to have in an otherwise revolutionary package.

A quick google search found this result and lots of others that basically link back to it: http://www.plugincars.com/why-do-el...lt-batteries-when-lithium-lighter-129118.html

Is it just cost? Is it regulatory in some way (old school rules that don't cater for new-school all-in-one battery tech), or what is going on here?!

It's really two questions. a) Why do EVs have two batteries, not one? b) Why is that battery lead-acid?

Thoughts, facts, conspiracy theories all welcome, especially if someone has some cold hard facts or insider info?
 

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You need a separate battery for the brakes, power steering and lights etc so a flat traction battery results in a flatbed recovery and not a hospital visit. A good old lead acid battery means they can use the current off the shelf units.
 

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You need a separate battery for the brakes, power steering and lights etc so a flat traction battery results in a flatbed recovery and not a hospital visit. A good old lead acid battery means they can use the current off the shelf units.
Is there no way to stop a battery from going completely dead, so such vital systems always have enough power?

So you can listen to the radio while waiting for tow truck ?
Harsh. :D
 

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There are systems that need to run all the time, such as central locking and detesting when a charger cable is plugged in. These could be run by DC-DC conversion from the main pack, but converters are probably really inefficient when run at the very bottom end of their power range. The car could be using 5W for running bits and bobs, but 50W overhead in the converter! Better to just top-up a low voltage battery from time to time.

I just wish that EVs would top-up the 12V battery even when turned off (as long as the main is not very low). This is because if the 12V runs flat you can't even charge the main pack!
 

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T

It's really two questions. a) Why do EVs have two batteries, not one? b) Why is that battery lead-acid?
All of the above plus:

Most EVs are based on or share parts with ICE cars. There is a huge economy of scale savings by sticking with existing 12-volt accessory systems.

b) Primarily cost. Manufacturers buy them in bulk. Also the existing systems are all designed around lead-acid batteries. Even with lead-acid there are better choices then an automotive start battery. Pure EVs don't require the high current needed to start a petrol motor. A SLA, traditional deep-cycle or spiral wound battery would be better. With proper charge management, an Enersys Cyclon battery would last the life of the car.


But those all cost more.
 

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One thing I do wish is a little more intelligence applied to charging and reporting on the 12V.
My leaf 12V was a dud. had next to zero capacity. hard to notice as car takes so little power to boot up.
did the car tell me it was dud? Nope. Did it try and keep it topped up more frequently to compensate? Nope I don't think so.
The SW algorithms could be improved to better cater for this situation!
 

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Modern cars have quite a significant ongoing parasitic load, radios, alarms, ECUs, Remote locking,BMS even extra demands like sidelights etc. Result is a constant load, that having a Dc to DC converter on line all the while , just isnt an option, as the high voltage systems would have to be permanently live. Thats no such a good thing. DC to DC converters can be made to produce quite a high current but they are expensive and seem to burn out a lot when under continuous use. You dont want to tap off the main pack as that will severely unbalance the pack. So having a 12v battery , be it lead acid or any other chemistry, means that 12v systems can be kept live, but also the 12v system that charges that battery doesnt have to be so large,as the DC to DC + battery will except under really extreme situations cope with peak 12v demand. A problem I have with the Citroen C1 EVies is that the 12v batteries can go flat in a week or so if unused or uncharged, when this happens the car wont even charge. So I have developed a system that monitors the 12v battery and at 12.2v (Good voltage to limit sulphation due to over discharge) it turns on the DC to DC for two hours, long enough to keep the systems alive but not permanently on so hastening on the demise of the DC to DC coverter.
I worked on a US/chinese made car that had a constantly on Sevcon DC to DC and to my knowkedge it ate at least 3 of the devices. At about £2to300 each thats a lot of cash. The 12v battery on that car was also far too small, so couldt keep up with the peak power needs.
I dont have any issues with having a 12v battery, the fact they are lead acid is I am sure cost related. You can buy a complete 12v battery for the price of one Lifepo4 cell. So its a no brainer really.
What would make a real difference would be if we didnt have remote locking, radios and CDs , computers etc that need back up power, so they are instantly alive and working as soon as we get into the car. Moving to LED lights (Headlights at the moment on LED consume huge current to get enough brightness) will make a difference, and I can see the time when a small Lifepo4 12v battery could be possible. As long as you dont have a starship enterprise dash, hard drives, charging for your Ipad etc etc.
 

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Modern cars have quite a significant ongoing parasitic load, radios, alarms, ECUs, Remote locking,BMS even extra demands like sidelights etc. Result is a constant load, that having a Dc to DC converter on line all the while , just isnt an option, as the high voltage systems would have to be permanently live. Thats no such a good thing. DC to DC converters can be made to produce quite a high current but they are expensive and seem to burn out a lot when under continuous use. You dont want to tap off the main pack as that will severely unbalance the pack. So having a 12v battery , be it lead acid or any other chemistry, means that 12v systems can be kept live, but also the 12v system that charges that battery doesnt have to be so large,as the DC to DC + battery will except under really extreme situations cope with peak 12v demand. A problem I have with the Citroen C1 EVies is that the 12v batteries can go flat in a week or so if unused or uncharged, when this happens the car wont even charge. So I have developed a system that monitors the 12v battery and at 12.2v (Good voltage to limit sulphation due to over discharge) it turns on the DC to DC for two hours, long enough to keep the systems alive but not permanently on so hastening on the demise of the DC to DC coverter.
I worked on a US/chinese made car that had a constantly on Sevcon DC to DC and to my knowkedge it ate at least 3 of the devices. At about £2to300 each thats a lot of cash. The 12v battery on that car was also far too small, so couldt keep up with the peak power needs.
I dont have any issues with having a 12v battery, the fact they are lead acid is I am sure cost related. You can buy a complete 12v battery for the price of one Lifepo4 cell. So its a no brainer really.
What would make a real difference would be if we didnt have remote locking, radios and CDs , computers etc that need back up power, so they are instantly alive and working as soon as we get into the car. Moving to LED lights (Headlights at the moment on LED consume huge current to get enough brightness) will make a difference, and I can see the time when a small Lifepo4 12v battery could be possible. As long as you dont have a starship enterprise dash, hard drives, charging for your Ipad etc etc.
Just like on a gas motor an alternator runs all that and charges the battery. They last a long time and take very Little effort to turn
 

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Just like on a gas motor an alternator runs all that and charges the battery. They last a long time and take very Little effort to turn
Only when you spin them by hand with no load. At typical engine speeds and loads it takes over 1hp to run an alternator.

Why?

The biggest single contributer is the power coming out the Alternator. The rest is "lost"

Typical alternator efficiencies are around 60% to 70%

For a 40Amp load, the alternator is putting out 552Watts, but taking about 910Watts of power from the engine. Since 746watts is 1 hp. 910Watts is 910/746 = 1.22hp

Where does the rest go? Resistance in the copper wires turns some of the power into heat. The alternator fan creates drag at high rpm. There are also small losses from bearing drag and in the diodes.


This is backed up by real testing on a dyno:



Link here:
http://www.carnut.com/ramblin/dyno.html


More here:
http://www.instructables.com/answers/How-powerful-of-a-motor-do-you-need-to-run-an-alte/
 
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