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12Volt EV using inverters

Posted: Wed, 06 Jun 2012, 08:38
by acmotor
Just to throw another idea around... Image
This sort of thing has been done before, but here is another take.
In the quest for lower voltage battery packs...

Remember the old blue gemini that had a 1.1kW 3 phase 230V delta emotor and 20:1 gearbox with a 1.5kW danfoss vfd and 330V of 7Ah SLAs around 2005 ? No ? Oh well.

Anyway, the Gemini has been dragged out of the hay shed and fitted with a 2.2kW 4 pole 3 phase 230V delta emotor with 5.63:1 integral gearbox direct to tailshaft.


VFD is a danfoss FC302 7.5kW 380-500V unit running in torque mode open loop and the DC bus is fed by 12V inverters from 12V TS LiFePO4.

Motor nominal torque is 14.9Nm but delivers around 40Nm before pullout, enough to climb a kurb or our steep driveway.
This is not a road going EV, just a farm hack.

The inverters are tapped for their 280VDC bus internally and 4 x 3000W inverters are arranged 2S2P for 560VDC at 20+A (12,000W).
Insulation megga'd out at 1000VDC fine.

Testing DC takeoff point. Multiple HF DC-DCs feed discrete UF5408 diode bridge for common 280VDC. This is then H brigded for modified square wave AC (this part not used for this setup, just DC taken).

There are 2 battery packs, first is 4 x 160Ah TS, the other 4S4P 40Ah TS. (300A TCBs on the parallel connections for safety) Each pack feeds 2 inverters. Total capacity around 4kWh (320Ah 12.8V).
2 x 50mmsq cable or 4 x 25mmsq used + 350A andersons. Voltage drops are around 160mV at 300A battery to inverter.

Emotor is set up for 6000RPM max (35kmph) though is of course torque limited above 3000RPM.

Logged data from FC302 shows >11kW at shaft of emotor around 3000RPM at the point where the DC bus sags.

Battery clamp meter shows 646A peak(4C)drawn from each 12V pack. (646 x <12 x 2 = <15.5kW so overall some 70% efficient to motor shaft. Inverters claim 90% efficient, VFD 95% and emotor 85% implies around 70% overall so that looks reasonable.

Emotor and inverters etc still cool (<10 deg rise) after 10km driving around at 10-30kmph. Mains power meter on recharge works out to about 140Wh/km. (the inverters have built in 10A LA chargers ea. that work fine with 12V lithium packs and can float at 3.45Vpc) 4 IEC leads to a plug board covers the charging.

There is cell top BMS on each TS but almost not required once cells are balanced.
Inverters shut down at 10.5V

Just goes to show, V and A are only numbers if the electronics is there to push it around !

This is a long way from 50 or 100 + kW but cost may be less than the BMS etc for higher voltage packs ?

No regen is offered here, but relatively simple to add via DC-DCs back to 12V battery. (no negative torque demanded of VFD in this torque mode so there is little if any regen in this setup. Brake pedal could be used to apply -ve torque demand)

Battery / inverter setup in the boot

Screen grab of log data, values are at cursor

Anyway, just food for thought.   Image

edit:added pics, converted some text to English

12Volt EV using inverters

Posted: Wed, 06 Jun 2012, 20:40
by Richo
So I guess the question will be is the 10% loss in the inverter worth the extra saftey of using a 12V battery pack and reduced cost on BMS and charging systems?
Also how big do you think the inverter would be for 100kVA?

12Volt EV using inverters

Posted: Wed, 06 Jun 2012, 22:16
by acmotor
Using an ultra cap bank on the DC bus would be the way to go so the inverter is only supplying the average power and the say 100kW peak is only for seconds from the cap bank.
Most motor arrangements would at best work at peak power for 60 seconds or less anyway.

Managing the regen into the cap bank would probably make up for the inverter loss.

If an EV had a 20kWh battery pack then average power consumption would only 10 to 20kW. That's all the inverter you'd need.

Mind you, if you are prepared to use all that inverter silicon then a lower voltage emotor and VFD would be the go anyway ? Its a question of what is off the shelf at present although the old higher voltage / lower current factor for VFD and emotor is still there.

12Volt EV using inverters

Posted: Thu, 07 Jun 2012, 21:07
by Richo
I don't think an ultra cap bank is really a way to go.
$3.5k would be ~200kJ of caps alone.
And I doubt 200kJ would be enough for most cars.

I might think about this somemore...

12Volt EV using inverters

Posted: Thu, 07 Jun 2012, 21:11
by Johny you need some kind of boost converter to get the available energy from ultracaps. All sounding a bit complex to me. Then again I wouldn't have OKed the Prius design either Image

Edit: converter

12Volt EV using inverters

Posted: Thu, 07 Jun 2012, 23:53
by acmotor
200kJ is 20kW for 10 seconds... at start at least.
This would be + 20kW continuous from inverters.

A possble line of thought....


Ultracaps are fully cycled at least.

Regen may need further handling and DC Bus is down at 400V after 10 seconds.

Maybe a prius could use an ultracap injection to its booster circuit ?

12Volt EV using inverters

Posted: Fri, 08 Jun 2012, 15:09
by Johny
I seem to recall that idea being posed once before but with a battery pack rather than inverters.
The problem with the above sketch is that the inverter outputs are diode bridges so the regen current would just cause the inverter outputs to momentarily climb (until some high voltage limit stopped regen) and not actually charge the caps.
You would need a PWM/inductor clamping system on the inverter(s) output.
i.e. A PWM zener.

12Volt EV using inverters

Posted: Fri, 08 Jun 2012, 20:44
by Richo
I think I'd rather put the $3.5k trying to achive peak power out of the inverters.

400V bank is ~150caps.
650F are usually the best value for money.
That would be ~$5k and 340kJ.

The limiting factor will be the toroid inductor/transformer to do the inverting.

12Volt EV using inverters

Posted: Fri, 08 Jun 2012, 23:17
by Simon
I saw this and thought, oh acmotor must of had these parts lying around and that they would be big $ for the 3000W inverters if you got them new.
But it looks like they are only $208USD.

I wonder if you could use 4 of these inverters, along with a 3-400AH 12V Lithium battery to replace the HV battery in a NHW10 Prius?

There is a company in New Zealand that sells the Power Jockey which does a similar thing by feeding around 1000W from the 12V accessory battery into the HV battery through an inverter. Props up the aging Nimh battery apparently.

Let acmotor loose on a dead Prius! Image

12Volt EV using inverters

Posted: Sat, 09 Jun 2012, 05:57
by acmotor
Image AUD$160a. delivered off ebay actually for 3000W (6000W peak) with inbuilt charges. It was worth it for the experiment.

Yes Johny, I suggested a similar circuit a few years back with a battery pack. The 100% cycle depth of ultra cap being the big advantage. This one...

Part of the idea of incorporating ultracaps somewhere is to raise the specific POWER of a battery pack (or inverter system) without higher C / more / higher voltage packs.

Simon, It could be said that a prius was dead from the start, but then that would show me up as anti hybrid. Image I'll leave that experiment to Matt.