Auxiliary Battery - Why?

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smithy2167
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Auxiliary Battery - Why?

Post by smithy2167 » Wed, 02 Jan 2019, 13:15

I notice that the I-Pace, Tesla and many other pure EVs still have an auxiliary 12V battery. For the I-Pace, it's an AGM battery, not Lithium, so it's likely to only have a lifetime of ~ 5 years. Worse, it appears that if the aux battery runs down, the whole vehicle is disabled.

Presumably, they've added the aux battery to run legacy 12V devices. However, with a cleansheet design like the I-Pace, surely they could have designed everything to run from the main battery, even if it required a dc-dc inverter and perhaps supercaps. There must already be an inverter/charger to charge the aux battery from the main battery.

Is there any real reason to still have a 12V lead-acid battery in the vehicle?

poprock
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Re: Auxiliary Battery - Why?

Post by poprock » Wed, 02 Jan 2019, 13:45

Hi; A valid question and it is covered in NCOP14 guidelines.
"2.12 Power Supply Priority... If safety equipment such as lights, brakes and windscreen wipers use the same power source as the traction motor, these services must be supplied in preference to the traction circuit. The design of any ancillary equipment supply should be such that satisfactory operation of all equipment, particularly brakes and headlights, is available throughout the discharge cycle of the traction batteries. 2.12.1 Auxiliary ELV An independent auxiliary ELV (nominally 12V) must be used to guarantee the supply of power to safety equipment such as lights, brake boosters and windscreen wipers in the event of a shutdown of the main battery system in the vehicle. (Typically this power supply is a 12V battery). The auxiliary supply must be capable of operating the hazard lights (four-way flashers) at normal duty cycle, for a minimum period of 20 continuous minutes. If the auxiliary supply is charged via a DC/DC converter from the main traction battery pack, then it must be supplied in preference to the traction circuit, i.e. the motor should stop working before the headlights. The auxiliary equipment supply must be designed so that the satisfactory operation of all equipment, particularly brakes and headlights, is possible throughout the discharge cycle of the traction batteries."
If through some mishap the traction pack were to become completely unusable a 12v is a simple way of maintaining safety for driver and passengers by ,as stated, operating emergency lights for at least 20 minutes. Ok for Suburbia but not for the M3. In my Suzuki Carry (just sold), I had two 12v in parallel which easily met the requirements. For DIY'ers the extra charging connection is the only burden.

T1 Terry
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Re: Auxiliary Battery - Why?

Post by T1 Terry » Wed, 02 Jan 2019, 14:59

My Prius had two 12v lead acid batteries fail in roughly 3 yrs. I slapped together a lithium battery made up from some old 90Ah cells that had been damaged after an over voltage charging incident so I at least had something to use until I bought another battery. All the fault problems that would occasionally pop up stopped occurring and that battery stayed in there for at least 2 yrs. I was given another 50Ah 4 cell lithium battery that had been built up out of failed electric scooter cells and that battery fitted better and is still in there and works a treat. The 90Ah battery is now in my Kombi and still going strong.
So, although there is a requirement for a separate 12v battery, there is no requirement that it be a lead acid battery that will fail. Swap it out for a lithium battery and it will probably outlast the vehicle. The lead acid batteries appear to only get charged to 13.2v or so and that causes them to sulphate and fail, LiFeP04 lithium batteries are happy to sit at between 13.2v and 13.8v without a problem.

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reecho
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Re: Auxiliary Battery - Why?

Post by reecho » Wed, 02 Jan 2019, 21:00

smithy2167 wrote:
Wed, 02 Jan 2019, 13:15
I notice that the I-Pace, Tesla and many other pure EVs still have an auxiliary 12V battery. For the I-Pace, it's an AGM battery, not Lithium, so it's likely to only have a lifetime of ~ 5 years. Worse, it appears that if the aux battery runs down, the whole vehicle is disabled.

Presumably, they've added the aux battery to run legacy 12V devices. However, with a cleansheet design like the I-Pace, surely they could have designed everything to run from the main battery, even if it required a dc-dc inverter and perhaps supercaps. There must already be an inverter/charger to charge the aux battery from the main battery.

Is there any real reason to still have a 12V lead-acid battery in the vehicle?
It's a design requirement currently. There must be a separate power source to operate separate contactors in the HV battery if there is a crash. Yes they could use fit for purpose 12V Lithium packs if they chose. Maybe they still want you to pop into the service centre every couple of years?

Even Tesla's Model 3 has one. Odd considering the ground breaking design with the rest of the power electronics.

Peter C in Canberra
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Re: Auxiliary Battery - Why?

Post by Peter C in Canberra » Fri, 11 Jan 2019, 10:14

Even a petrol vehicle could work without the 12V battery. That is what happened before automatic starters and you hand cranked or, in the case of motor bikes, you kick started. I had a motor scooter that had no battery. The lights ran directly off a generator. The headlight brightness varied with speed!
Once going, you could run all the ancillary stuff of a petrol car from the generator. In the case of an EV from a DC-DC converter from the traction battery. However, in either vehicle type, the standards require being able to keep lights flashing and horn sounding etc even if the traction system has failed - hence the battery sitting across the 12V system.
In either vehicle type, you could save some weight with a lithium battery.
Daihatsu charade conversion 2009-18, iMiEV 2013-2019, Holden Volt 2018-present, on the ACT's 100% renewable electricity. Kona on order.

poprock
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Re: Auxiliary Battery - Why?

Post by poprock » Sat, 12 Jan 2019, 17:24

Interesting point from T1 terry about the auxiliary FLA only being charged to 13.2v causing sulphation.Surely a small inverter could remove this issue? Ah, but then the point was raised about 12v failure requiring a regular service visit. Onselling at it's best. " DO YA WANT FRIES(12v battery) WITH THAT? "

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