EV Power Consumption

Technical discussion on converting internal combustion to electric
Magneto
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EV Power Consumption

Post by Magneto » Wed, 07 Jan 2009, 23:59

Great news (for me anyway), I now have my own EV, but being a real novice when it comes to electricity, I am trying to devise a method where I can reasonably accurately calculate how much power I'm putting into the batteries of the EV.

With a petrol powered car it is easy to calculate how much fuel you are buying to run it and thus how much it is costing one, per kilometre. But for an EV it is much more difficult.

There is now a domestic power consumption metre avaailable wher you can plug it into your power point and it displays exactly how much power the appliance has consumed. You can even enter the electricity companies rate and it will tell you in money terms exactly how much the power used by the appliance has cost you. (Available at Altronics $29.95 or Jay Car $39.95 if interested)

However this metre is only 10 amp and the charging system for the EV has 12 x 15 amp chargers, which makes that metre unsuitable.

My Ev has 12 x 12Volt 55AH Optima Yellow Top batteries Model D34.

Actually the manufactuers specifications says that they are actually 13.1 volts.

Therefore I calculate that each battery contains 13.1 x 55AH = 720.5 watts of electricity. Multiplied by 12 batteries makes a total of 8646 watts of electricity in the entire battery bank.

I have managed to drive 44 kilometres in the EV from a fully chare to a complete discharge. (not sure if some power was left in the batteries, but the EV wouldn't go forward any more)

I know I shouldn't have competely discharged the batteries, but it was one of the first times I had driven then EV and was not familiar with its range. I know what its capable of now and wont venture on any trips greater than 35 kms and that includes getting home.

Therefore assuming I have used 8646 watts to drive 44 kilometres, this means it has taken an average of 196.5 watts of electricity to drive each kilometre.

Is the manner in which I have calculated this correct and reasonably accurate?

If it is then I suggest the simplest method for me to estimate how much power the EV is costing me is to record the number of kilometres it travels, and multiply it by 196.5 watts.

eg 35 kilometre trip = 35 x 196.5 watts = 6877.5 watts or 6.877 kw.
6.877 kw at $0.1393 per kw = $0.96 which equals 2.7 cents per km.

(WA electricity cost)

Please can anyone tell me if I have calculated this incorrectly or has any suggestions as to an easier or more accurate method of calculating the power cost one puts into EV batteries.

Sincerley

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Post by Johny » Thu, 08 Jan 2009, 00:45

Hi Magneto.
Firstly, don't push your batteries below about 80% Depth Of Discharge (DOD) as this will diminish their life. If you can, stay at 50% or above. The power in and power out for lead-acid is not proportional due to a thing called "peukert effect". The higher current you draw, the less efficient the battery is in delivering power.
Apologies if you already know all this.Image

Measuring the power into the chargers is the easiest way to calculate wh/km (outlet to road). Could you use two of these meters and run half the chargers off one etc?

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Post by Electrocycle » Thu, 08 Jan 2009, 00:57

you may have some trouble using the Jaycar power meters to monitor your battery chargers.
I've found they don't work too well on switchmode devices, like most battery chargers.
According to the power meter, my 360watts of chargers on the bike are only drawing 130w, and my 1800w stereo amplifier only draws about 100w at full power (while making the lights dim in time with the music!)
It does work fine on the clothes drier, washing machine, heater, etc.
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Post by zeva » Thu, 08 Jan 2009, 01:06

Hi Magneto,

You're on the right track, but I can clarify a couple of things for you:

- Your math is correct but "watts" are units of power not energy. Energy is power multiplied by time, which is where the "watt-hour" unit of energy comes from. Basically wherever you've said "watts" above, you're really speaking in "watt-hours".

- Those nice cheap 10 amp power meters might actually do the job. Don't be distracted by the current they chargers are putting out to the batteries, as the current in is much lower.. That might need some explanation:

    - Neglecting efficiency losses in the charger, power in = power out.
    - Power is volts x current, so each charger is carrying about 12V x 15A = 180W of power.
    - 12 of them gives a total of 12 x 180 = 2160W.
    - Your mains power is 240VAC, so average current from the mains (current = power ÷ voltage) is 2160 ÷ 240 = 9A.

(The whole truth gets a bit more complicated, 12V lead acid chargers usually go up to about 13.8V at the end of the charge cycle, though by then they should be putting out less current.. 13.8V @ 15A x 12 may actually exceed the 240V 10A rating on the power meter! But then usually the current would have tapered off as the voltage approaches maximum. Another complication is the possibility of poor power factors in the charger, i.e they might be drawing 20A for half the AC cycle and 0A for the other half, may effect the power meter. But hopefully those meters don't suddenly explode at 10.01A... I think it'd be worth a shot!)
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Post by acmotor » Thu, 08 Jan 2009, 01:11

Having used those 'power' meters I can say that they are a bit average for loads that are not resistive i.e. unity power factor.

Most battery chargers are not unity power factor.
(Wiki that if it goes over you.)(Sorry, I hate posts full of Wiki links, it makes them out to be gospel !)

Having said that, those power meters can give a fair idea of power consumption with the right load. Are they SMPS or linear (trandformer type) chargers ?

To measure the power consumption of an EV from power point to km, you need to use an RMS mains 'power' meter. I have an old spinning disc power authority type that I use. It is not resetable so you have to do the sums but the measurement is the same as you will be billed in kWh.

Magneto, your estimates are a good start but the losses in charging are likely to be 30% or so from power point to fully charged lead acid. Exact efficiency is not the point, energy power point to km is, as it will include the peukert effect of the lead acid on discharge.

The optimas will be around 400Wh in EV operation. (someone will come in here, exact number is irrelevant, power meter will measure the real energy). The 13.1V (should be at least 13.8V) is a charging parameter and not a measure for Wh calculation. Image

All that aside, your 200Wh/km number sounds in the right ballpark.
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Post by Magneto » Thu, 08 Jan 2009, 02:02

Thanks everyone for all the good info.

I'm not able to accurately record from my household metre as to how much power the EV consumes when charging the batteries over say a 9 hour period, (when turning off all other power in the house that draws power) because Western Power have installed a digital display metre which only shows whole KW consumed, Therefore I dont known when I start measuring whether it is a part way to the next KW or the same when completing the charge.

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Post by acmotor » Thu, 08 Jan 2009, 02:51

The red LED pulses 1 Watt hour so you can start counting ! Image

Perhaps if you can calibrate the Jaycar power meter to the mains meter with your chargers as a load ? It may come out to be close but could well be a long way out. Mind you, either way, the cost of the power meter itself would pay for a lot of EV km !
Maybe don't worry too much ? Just keep driving the EV !
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Post by Magneto » Thu, 08 Jan 2009, 04:42

AC

It may sound a bit strange but I like to keep an eye of the actual cost of things and sometimes that requires a fair bit of drilling down to the cost elements of particular processes.

For example a late model very small 4 cylinder car I have uses 8 litres petrol per 100 kms so therefore without any other costs, each kilometre travelled costs 8 cents (with petrol at $1.00 per litre)

If my calculations are accurate with the EV then at 2.7 cents of electricity per kilometre it means my basic EV running cost is about 25% of a similar sized petrol car.

However trying to save monety is not the only motivation in having a EV.
Having an EV also provides an alternative method of transport so if anything interupts the source of fuel for petrol cars, then I still have a practical method of transport in our urban situation whic depends greatly on cars.

Many people dont remember the oil crisis of the early 70's when in some western countries there was such a shortage of fuel it had to be rationed. This could easily happen again. especially with the tensions in the Middle East and happen to a more severe extent than in the early 70's.

But us EV owners will be laughing, passing all those fat or unfit people who are trying to get used to their bikes again.
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Post by acmotor » Thu, 08 Jan 2009, 04:57

Admirable reasoning Magneto.

If you can gather some solid operational cost data it would be good to post it up for all to see.

Maybe a member's machines posting with pics and all.
We'd be keen to see your EV complete with real life operational data.

BTW, I remember the 70's. That's when I first got the EV bug.
I still have my first motor down the shed. It's a 32V compound wound marine generator with interpoles. It never made it into a vehicle though.
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Post by Magneto » Thu, 08 Jan 2009, 05:04

Ac

I will certainly share my experiences with EV's and any relevant Data.

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Post by antiscab » Thu, 08 Jan 2009, 10:13

hi magneto,

if you want to measure the kwh you are pulling from the grid to charge, i suggest buying one of these:
http://cgi.ebay.com.au/WATTMETER-ENERGY ... .m20.l1116
and wiring it in the middle of a extension cord.

since you are in Perth, if you arent confident in doing it yourself, id be happy to do it for you.

this is how i measure the energy my scooter pulls from the mains.

btw, those ms6115 meters from jaycars are rubbish.
they do correct for displacement power factor, but not for true power factor (which takes into account THD).
they cant be recalibrated as the reading isn't stable, even though the actual power drawn is.

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Post by Magneto » Thu, 08 Jan 2009, 15:14

Matt

Thanks for the tip.

Is it okay for 15 amp, as the 12 charges on the EV are 15 amp?

Brad

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Post by antiscab » Thu, 08 Jan 2009, 19:55

Hi Brad,

you can get a higher rated meter than the one i gave a link for.
ill have a look.

another option would be to take a 3-phase mechanical meter and put 4 of the chargers on each output phase, and combine the input lives together.
since the chargers are single phase chargers it wont matter if each phase isnt actually a different phase.

the meter should still work - correct me if im wrong here guys :).

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Post by Johny » Thu, 08 Jan 2009, 20:18

Umm. I don't think a 3 phase meter would work as you have described.
The meter you gave the link for is a 32 Amp device so will definitely cope with a standard 10 or 15 Amp wall outlet. It looks ideal.
The 12 chargers being 15 Amp must be their output current. That translates to around 1 Amp per charger on the mains side.

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Post by antiscab » Thu, 08 Jan 2009, 20:40

15A * 14.5v end of cc * 12 batts / 0.8 eff = 3262w = 13.6A on the mains side.

do you know why the 3-phase meter wouldn't work?
more a matter of curiosity really.
i would have thought the individual phases were isolated, except for the common neutral.

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Post by Johny » Thu, 08 Jan 2009, 20:51

On second thought a 3 phase meter has to cope with unbalanced load from phase to neutral so I reverse my statement. It would probably work. Image
But the one you linked would be cleaner and better IMO.

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Post by acmotor » Thu, 08 Jan 2009, 22:47

Mechanical 3 phase power meter is basically a 3 phase eddy current motor (spinning aluminium disk with parallel voltage and series current windings at 120 degrees on 'C' magnetic paths arranged as its stator). The rotating disk directly drives a mechanical counter.

The interaction between the phase of the magnetic field from the voltage and current windings results in the in phase component of the load(s) driving the disk. Any one phase can drive the disk.

It would require the 3 phases to be at 120 degrees or it will get the power reading wrong. So connecting the inputs together to a single phase would not work. This is from basic theory, I must admit to never actually trying what seems unlikely ! If anyone has a spare 3 phase meter it would be interesting to put theory to test.

You can use just one of the 3 phase inputs / outputs (remember to connect neutral to meter as well) and it will get the power right as will a single phase version of the meter.
Most mechanical meters are at least 30A (mine is).

Electronic meters (should) work by measuring the voltage and current of one (or three phases) within each mains cycle and calculating the power factor (cosine of the effective phase angle between V and A) and thus Watts from VA (W=cos(angle) x V x A) and then integrating this with respect to time to accumulate kWh (energy). The V and A must be the RMS (or effective) not average values.

Some low cost units cut corners and get the power wrong by assuming sinusoidal loads at only 50Hz etc. i.e. not making RMS measurements.

Switching or harmonic loads (as often produced by power supplies) will cause errors in the reading of these 'average' meters.
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Post by Magneto » Thu, 08 Jan 2009, 22:58

The Rail watt meter that Matt suggested (from Germany) looks good and the price seems reasonable when compared to whats around

I think I''ll order one of those?

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Post by antiscab » Fri, 09 Jan 2009, 06:44

hi Tuarn,

I actually do have a 3-phase energy meter sitting around, i bought it to install on my mr2. up for an experiment?

magneto - yeh buy one of those, and a long extension cord.
if you come to the next meeting, ill show you how i wired mine up.

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Post by acmotor » Fri, 09 Jan 2009, 08:32

Matt,
Go for it.
It would be interesting to see.

The bottom line is that it needs to read what the Western Power meter reads.

BTW, in my thinking they are power meters because the measure the power in Watts (not VA) and happen to integrate (mechanically or in software) w.r.t. time and display also the energy. I know, a rose by any other name...
The thing is, people should be charged for VA not Watts since the 'power' station generates VA and the distribution system has to handle VA.
Energy users should be more responsible for choosing unity power factor appliances.
That is probably too green a thought. Image
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Post by antiscab » Fri, 09 Jan 2009, 08:36

i was of the understanding that big industrial users already pay a PF penailty, aswell as a demand surcharge if their instantaneous power usage spikes for a little while.

im not sure how i could test it with respect to what the western power meter reads as there are other loads in my house that i cant actually turn off.....well i spose i could just pull *all* the fuses.
hmm, an experiment for another time perhaps.

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Post by Magneto » Fri, 09 Jan 2009, 13:33

Matt

Thanks very much for your generous offer. Hopefully I'll have the watt metre before the next AEVA meeting and I'll bring it in.

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Post by acmotor » Fri, 09 Jan 2009, 18:12

My interest to have the power meter reading the same as the Western Power item is obvious.... that is the one that matters as you pay for the power it measures ! (unless you have PV/wind/fusion)

Re PF,
It is the billions of domestic customers who buy their appliances with no regard (why would they) to PF and what it does to the grid.
The industrial users (mostly 3PIMs) already use VFDs, with near unity PF, as the economic reality is already in their face !

Even at the EV level, we should be concious of the PF of our charging system. (depending on our shade of green) Image
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Post by Magneto » Mon, 12 Jan 2009, 16:27


Dear learned electrical people in EV land (men of science).

Do I have my maths correct here regarding battery consumption?

After a 13 hours of charging my battery chargers are indcicating all 12 of my batteries are fully charged. The electric guage/metre in the EV is showing that the batteries have a total capacity of 156 volts.

(I find it strange that at one stage whilst charging the metre stated 170 volts, but it seems to settle at 156 volts)

I have been advise to never allow my batteries (Optima Yellow Tops) discharge more than 50%. Therefore if I am taking a drive, I should stop driving if the metre shows 78 volts. (1.2 x 156 volts)

Therefore if I venture away from home I shouldn't use more than 39 Volts on the away journey, so that I have 39 volts remaining, so I can get home with 78 volts capacity left in the batteries, then recharge.

Is this the correct way to calculate the power available to my EV?

Magneto (not a learned electrical person)



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Post by Johny » Mon, 12 Jan 2009, 17:04

The Depth of Discharge can be indicated by the no-load voltage of the battery pack but half voltage IS NOT 50% used. The reason your battery indicator show a higher voltage during the charge cycle is because your charger is "smart". It charges to ahigher4 voltage then after the pack is pretty much charged, chages to a "float" voltage which does nice thing to the batteries - like try to equalise voltage across all the cells.
Back to DOD.
50% DOD would be more like 144 Volts. If you drop your batteries below 11 Volts per battery (132 Volts) then your are beginning to damage them permanently.
The will be other comments on this thread but I would suggest getting some documentation on the Yellow tops and see if there is any details on SOC (State Of Charge) and how to measure it.
You should be able to come up with the standing voltage readings that give you a pretty good indication. From that you can observe your pack voltage over time and determine your safe range.

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