Power Consumption for Dummies

How do you store and manage your electricity?
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Post by EV2Go » Fri, 08 Aug 2008, 03:32

Ok I am officially confused. .. I am not used to working in Ah and it is starting to do my head in.

Found this on Wiki…

An ampere-hour (symbol A h or A•h, sometimes abbreviated as Ah) is a unit of electric charge. One ampere-hour is equal to 3600 coulombs (ampere-seconds), and is the amount of electric charge transferred by a steady current of one ampere for one hour.

So taking a LiFePO4 3.2v 90AH battery as an example

The battery is capable of drawing 90 (amperes, amps, A) consistently for 1 hour @ 3.2V but because you can’t drain the battery 100% you would get 90Ah @ 3.2v for 70%-80% of an hour or roughly 45 minutes.

You times volts x Ah to get Wh… So that is 3.2v x 90Ah (0C) = 288Wh or .288kWh

And you then multiple the amount of batteries (say 90 in series) x 288Wh which gives 25,920Wh or 25.92kWh @ 288v

Multiply that by 70-80% actually usage without damaging the batteries so 25.92 x .75 = 19.44kWh (usable) @ 288v

So you could draw 25.92kWh @ 288v for 45 minutes or 19.44kWh @ 288v for 1 hour

You can draw more from the batteries by asking for more current from the controller but it reduces the battery charge duration

Matt said the battery will drop roughly .2v per C, so at 3C that would be 3.2v - .6v (3 x .2v) or 2.6v (3C) but I times the 90Ah x 3 as a result I now have 270Ah @ 2.6v

Following the same formula as before 2.6v x 270Ah (3C) = 702Wh x 90 batteries = 63.18kWh for a third of the time (20 minutes, killing the batteries)

Or playing it safe 63.18kWh @ 288v for 15 minutes (20 mins x .75) or 63.18 x .75 = 47.385kWh @ 288v for 20 minutes

Because the motor will take what the batteries throw at it (most of the time), say 63.18kWh the motor will make that power in electrical kW and factoring in mechanical loss roughly kW become the same amount of HP (63.18HP).

If I know what usable kWh are available in the battery pack, and I calculate the power requirements for a given load using a power calculator, I should be able to determine how long the battery pack will last at the energy consumption.

Which is what I think Matt said here “energy per km * average speed = continuous A draw * pack voltage. “

But I am not sure how I would calculate energy per klm because the calculator I use only gives a power to maintain a set speed figure (gives power to accelerate too).

Anyway I think I am on the right track can some please confirm… Thanks

Forgot this bit "You have to iterate the last bit as the pack voltage sags with greater current."
Last edited by EV2Go on Fri, 08 Aug 2008, 05:33, edited 1 time in total.

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Post by woody » Fri, 08 Aug 2008, 14:41

You're nearly there, Most of the time above you've written kW when you meant kWh (kilowatt hours). I.E. Batteries hold kWh, motors produce / consume kW.
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Post by EV2Go » Fri, 08 Aug 2008, 15:34

Thanks Woody, I have updated what I think you were refering to.

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Post by hyper24 » Fri, 08 Aug 2008, 16:32

The way you calculate the effeciency of your car in terms of Wh/km, is
A. You drive your car and see how much energy it uses over a given distance
or
B. Look at other conversions roughly the same to yours and you can estimate what your effeciency can be.

There are ways to do it through formulas of course, but it would be way to complicated.
You can try and estimate it through power consumption at given speeds but even then this is way off because it doesnt take into account acceleration and the different driving cycles one could possibly do.
Before you start thinking "outside the box" you better have a good understanding of what's "inside the box".

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Post by EV2Go » Fri, 08 Aug 2008, 17:20

hyper24: I am using this to calculate the power requirements http://www.cameronsoftware.com/ev/data/EVDrivePower.xls

I extrapolated the formulas to workout the requirements up to 160kph.

Most of the specs it requires can be found here http://www.nology.com/pdfandzipfiles/carsdata.pdf

I don’t know how accurate it is any would appreciate any one in the know giving it a go.

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Post by woody » Fri, 08 Aug 2008, 18:35

It matches my spreadsheet, which matches the Uve's EV calculator, so I think it's good theory.

As for practical I don't know. I have a hair-brained idea to write something which would calculate the power required by a particular journey based on GPS logging of that journey, which would mostly capture the accelleration/speed etc of the car as well as changes in height.
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Post by EV2Go » Fri, 08 Aug 2008, 18:48

Check with Albert who I believe has already done something like that. I am not sure if he is currently a member, but you might be able to contact him through Kearon.

I found that .xls fairly handy in trying to work out what different donor car may be suitable.

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Post by a4x4kiwi » Fri, 08 Aug 2008, 22:17

For anyone that would like an excellent guide to the physics of energy and motion see http://www.cameronsoftware.com/ev/Welcome.html

There is also a GPS spreadhseet here

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Post by EV2Go » Fri, 08 Aug 2008, 22:51

I had already downloaded that but didn't understand how to use it, I should have looked at the other tabs first. Image

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Post by acmotor » Sat, 09 Aug 2008, 01:44

Another less theoretical way to go..

The way I started (some years ago) was to tow the donor vehicle with a load cell (10kN - 1 tonne) in the tow line.
Towing vehicle probably disrupts good aerodynamic flow, but this component is miniscule compared with force to accelerate the donor.
I drove up the steepest slope I would expect to have to climb as well and recorded the loads.
Once I had the Newtons of force required I calculated Nm torque required on the back wheels, Nm going to the diff (tailshaft torque) and through gearbox back to motor torque required.
e.g. on 15deg slope, 1500 Newtons pull x .33m rad wheels = 500Nm required at the axle.
divided by 4 for diff ratio gives 125Nm required at tail shaft.
divided by 2 for 2nd gear gives 50N at the motor. This will push a 1000kg vehicle up a 15deg slope at any speed until wind drag amounts to something. 60kmph+

At 60kmph this will be 60,000/60 = 1000m/min
Wheels are .33m rad so ~ 2m (2.073451151m) per turn so doing 500RPM
Tailshaft is doing 500 x 4 = 2000RPM
Motor would be doing 2000 x 2 = 4000RPM

This will require motor kW of 50Nm x 2 x pi x revs/sec (RPM/60) = 21kW.
So need to select a motor capable of 50Nm+ at 4000RPM
A 10kW motor will only manage half that speed on the (very steep) 15deg slope. Clearly the torque and RPM must match the drive train. A 100kW motor won't do if it only develops 30Nm of torque at a high RPM.
... change the gearing. My trial of a 1.1kW motor clearly links top speed to motor kW not torque ! http://www.evalbum.com/1173

All this allowed for rolling resistances etc but not drivetrain losses.

Peak force during acceleration could also be measured and calculations done as above. Spreadsheets can either confirm or be calibrated by this technique. There are always plenty of assumptions regarding wind drag and rolling resistance and mechanical efficiency etc.

This approach may help some people understand what is going on.

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Post by hyper24 » Sat, 09 Aug 2008, 21:08

That spreadsheet is great, ive used it myself but its use is limited.

Working out your Wh/km is best done through looking at other conversions. Have a look through the evalbum

I know if I use a barina my efficiency will be 150Wh/km, thats if I use lithiums so the weight is still under 900kg.

so if you had a battery pack that was 15kWh

15000(Wh/km) / 150(km) = 100 km range

But EV2go don't forget to factor in your DOD% and also the properties of the batteries, things like for Lead Acid batts you can only use around half of its rating due to thr Peukert effect.
Before you start thinking "outside the box" you better have a good understanding of what's "inside the box".

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Post by Mesuge » Sat, 16 Aug 2008, 14:19

Very good ideas in this thread as always on this forum :@ )
I'd like to offer just a few hints about the GPS logging:

1. For this you don't have to own/operate any GPS unit or it also might come handy in situations you would like to have approximative model for several basic routes, which could be tedious to collect by driving around like crazy in ICE. For instance, in my case/place I can choose from 4-5 different routes with relatively same length (km) but very disimilar tax on kWh performance, which is kind of pointless in wastefull ICE driving but quite a different subject in sub 15kWh pack EV system.

2. Log on/browse your favorite online/offline map website/nav program. Select several waypoints alongside your desired route/s, and waypoints, the more points the better. For each route export the data into .gpx format, (provided this option is available/should be user selectable).

Edit:
a/ for selecting route, waypoints and export in .gpx anyplace globaly you can start with GoogleMaps but you need this browser plugin/utility "GMapToGPX": http://www.elsewhere.org/journal/gmaptogpx/

b/ for adding the OPTIONAL feature of insertion/moving waypoint positions like in autonavigation system, you have to also use this utility "ConvWPT"
http://www.linuxtechs.net/kruch/tb/foru ... c&start=45


3. Logon to "GPS Visualizer" website and firstly convert your newly saved waypoints from .gpx into a textfile, most likely don't un/check any other options there: http://www.gpsvisualizer.com/convert

4. Open second browser tab/screen and log on "GPS Visualizer" website again and copy/paste your converted data from (step 3) into their "Elevation Lookup Utility". Hit "start" button and when finished after some number crunching select on the bottom right corner "draw a map as waypoints" and "format: google maps" (you can also save your new elevation data enhanced .gpx log, etc.)http://www.gpsvisualizer.com/elevation
http://www.gpsvisualizer.com/map

5. As the new Googlemaps window pops up, watch your waypoints now featuring the elevation dimension as well..
--

After some route optimisation for elevation "smoothness" hence practicing lowkey kWh spender tactics you might also try experimenting to fit those lat,long,elevation data into the "Cameron EV" spreadsheet, link posted above.

-

Related to this "EV-gps logging fun" is another tip for this very interesting application on mobile phones and handhelds "TrekBuddy", it works on relatively old phones as well, you can map/track/log in the open. Should be usefull for collecting EV charging points, plan ahead for high degree slope with warnigns etc. http://linuxtechs.net/kruch/tb/forum/viewtopic.php?t=23
http://linuxtechs.net/kruch/tb/forum/index.php
http://wiki.trekbuddy.net/index.php/About

Old phone, no bluetooth? Connecting phone<->gps module on serial data cable, faq here: http://www.wayviewer.de/en/faq.html

.
This gps logging and energy *modeling stuff deserves its own thread..
.
I hope that based on *knowing every location related data for given route, (except for time) and selecting more or less stable cruising speed, we can by known formulas ("CameronEV" spreadsheet) compute back the unknown variable time and hence derive the rest of energy modeling data for the trip. In second run one could optimize the cruising speed for better efficiency etc. Another higher logic, would be to incorporate "zones of possible regen" into the route. In the end 5-10% accuracy for simulation of future EV routes would make for super efficient travel, and less >80% DoD stress. The times of 35-50kWh of onboard storage are still far away for most of the people.

I'm not sure Excel/OpenOfficeCalc type of spreadsheet is the best tool for it, but this idea is software doable in principle.
.
Last edited by Mesuge on Sat, 16 Aug 2008, 20:18, edited 1 time in total.
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Post by Mesuge » Sun, 17 Aug 2008, 22:43

Just to give you a first glimpse of things to come..

This is route/waypoint plus added "elevation lookup" model as described
above. The "route profile" graph was drawn without using actual gps.
You can clearly see the difference and derive most likely conclusions
for EV driving on that trek:


Image

1. Route option - highway:
shortest travel distance, but have to maintain "safe" higher speed, clearly more energy demanding profile featuring one hill with 4% slope for 7kms. Also note the higher max elevation point in comparison to #2, 45m difference! Luckily, this is no major trucking route, so with modest lithium/advanced pack it's very doable under 90km/h.

-

Image

2. Route option - local roads:
Just a few % longer route, can maintain safely very low speeds 45-55km/h
for most of the trek, so this is lead acid friendly. Also this route
takes place in deep river valleys, so one doesn't have to face strong
headwinds as on the open highway. Factor in the possibility of better
regen zones, positive factor is much nicer scenery as well..

.
Last edited by Mesuge on Sun, 17 Aug 2008, 13:17, edited 1 time in total.
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Post by Mesuge » Tue, 19 Aug 2008, 12:02

Almost there for the first crude beta, in Cameron's spreadsheet
one has to deal with different formats:

The issue here is converting latitude and longitude into X,Y
=> convert WGS84 coordinates (from GPS) to UTM (XY) coordinates

Good intro and links here:
Also linked Python library to handle LatLongUTMconversion..
http://discussion.forum.nokia.com/forum ... p?t=134065

Several good online convertors, even a spredsheet (not working in batch mode):
!http://www.bivouac.com/MultiConvert.asp
http://www.google.cz/search?hl=cs&q=Lat ... ersion&lr=
hopefully could be repaired with UTM examples/formulas here:
http://www.uwgb.edu/dutchs/UsefulData/UTMFormulas.htm
http://www.posc.org/Epicentre.2_2/DataM ... cs34h.html

To verify the conversion, select the point on the map:
http://www.itouchmap.com/latlong.html

-
Batch processing capable convertors:
http://www.gpsy.com/gpsinfo/geotoutm/
http://www.kolumbus.fi/eino.uikkanen/ge ... riefgb.htm
!http://earth-info.nga.mil/GandG/geotrans/index.html
-
!Excel function addin plugin(won't work in OOCalc though):
http://www.dartmouth.edu/~renshaw/geogt ... gitud.html
!Spreadsheet easy:
http://www.sulross.edu/pages/3889.asp
-


"UTM Coordinates: UTM coordinates are preferred over latitude-longitude or any other types of coordinates. Use UTM Zone( ) according to your place.

Record UTM Easting coordinates to 6 digits (leave off any preceding zeros), and UTM Northings to 7 digits. Record whole numbers only; no decimal places (which represent fractions of a meter), no commas (thousands separators), and no letters (such as ‘E’ or 'N’). In case your software does not have an option for the WGS84 datum, so instead select NAD83 (the resulting error should be less than a meter."
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Post by Mesuge » Wed, 20 Aug 2008, 12:18

Disclaimer, in the above described "no own GPS/data" approach take into account the fact that these mapping utils & GPS derived & estimated data sample is obviously skewed in one direction or other by the non-regular data collection (x,y,z, time), at the moment the error margin seems to be consistent in shorter routes and in populated areas (frequent waypoints) around 10-15%.

Which is ok for back of the envelope league, and as the the tools and understanding increases it will make easier the full GPS logging approach or even some future *fancy modeling.

--
* I know that guys at the transportation/physics uni department play with this kind of deep analysis/modeling soft but that's almost OEM grade, too big guns for us..
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Post by Mesuge » Wed, 20 Aug 2008, 16:19

Battery Modeling
The following is an excerpt from the HEVsim Technical Manual. It reviews the history of battery modelling for HEV applications:

Very comprehensive and usefull set of formulas and approaches it covers:
-State of charge (SOC)
-Battery storage capacity
-Rate of charge/discharge
-Temperature
-Age/shelf life

http://www.thermoanalytics.com/docs/batteries.html
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Post by MikeG » Fri, 07 Nov 2008, 15:34

I was with you up to the point you stated "Matt said the battery will drop roughly .2v per C..."

I had to google a bit to get back on track here, so I thought I'd provide my findings here for other dummies wanting to learn more about batteries.

According to the eBay battery buyers guide, "The capacity number of a battery is often referred to as "C" when describing the maximum safe current you can draw from it", so this means a 20Ah battery rated at 1C can draw 20A safely, but one rated a 5C can draw 100A. Obviously higher C rated batteries will be more expensive.

A real world example, I'm seeking a min. of a 12Ah battery that can draw 26A normally, and up to 100A startup.

If I were to look at a 20Ah battery for this solution, I'm not sure if I would need to spec it to 5C or if 2C would do... (1/3hp induction motor for an electric lawn mower)
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Post by Richo » Sat, 08 Nov 2008, 00:26

A battery has an equivelent resistance inside it.
So when you draw current from a battery it goes through this resistor.
And as a result a voltage drop is developed.
So when drawing 20A 1C from a 12V battery you may have only 11.8V at the terminals.
But when drawing 100A 5C from a 12V battery you may have only 11.0V at the terminals.

So when trying to work out your maximum power you need to take into account the loss of voltage from your pack.

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Post by MikeG » Mon, 10 Nov 2008, 15:58

It's probably worth noting that a 12V battery that is not under load is actually around 13VDC when fully charged. There's probably a spec that states that a 12V battery will be rated at 12V when fully charged under x Amps load. If we're talking about voltage drop under load, we should probably identify the baseline voltage under minimal load (it's probably above 12V).

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Post by Magneto » Tue, 25 Nov 2008, 05:10

I have brought an EV where the engine and controller operate at 144 Volts.

It has 12 x 12 volt sealed Optima lead acid batteries.

Its been suggested that to increase the range of the car, I can replace the lead acid batteries with 45 x 90Ah lithium Ion Batteries which I believe have a nominal voltage of 3.2 vols each therefore (45x3.2 = 144 Volts.)

Now if I want to continue operating the car at 144 Volts but I fit more than 45 x 3.2 Volt lithium Ion batteries, say I fit 60 x 3.2 volt batteries, will this be a problem for the motor and contoller.

Will it result in me being able to opeate the car as before, but simply with an incraesed range over what it would have if I stuck with just fitting 45 x 3.2 volt batteries?
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Post by woody » Tue, 25 Nov 2008, 05:45

Hi Magneto,

depending on the controller, the rating on the parts in the controller may be as low as 200V. 45 x LiFePO4 batteries will be close to that when fully charged, so 45 is probably the highest you want to go.

Thankfully for you LiFePO4 batteries come in a large range of sizes, from 5AH up to 800Ah, so you may pick a useful size, e.g. Thunder Sky have 60, 90, 160 or 200 AH depending on the budget and range requirement you have.

The optimas will have an Amp-hour rating which will be based on a 20 hour discharge ("C/20"), of maybe 55 amp-hours, of which you can probably use 30-40 ah because you are using it in less than 20 hours.

The LiFePO4 you only want to use 70% of the capacity to get the longest cycle life (1500+ cycles), so a 90Ah battery will give you 63 Ah usable, a 160 Ah 112Ah usable etc.

So about double the range with 90Ah, almost quadruple with 160Ah, etc.

The 90 Ah LiFePO4s will be double the price, of yellow tops, but should do 1500+ cycles of double the range, instead of only 750 cycles, so will work out cheaper in the long run, and lighter in your EV.

But do your sums, see what suits.

You'll need to spend an extra couple of grand on a battery management system to look after your lithiums too. Lead acids just electrolyse their acid if you over-charge, lithiums do bad things.

ev-power.com.au sells the thunderskies and the management system to go with it, and is in WA to boot. bev.com.au sell them too, in victoria plus there are others around.

cheers,
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Post by scooterage » Mon, 02 Feb 2009, 21:04

i have an 800w electric off road skateboard that ran on 36v sla batts (3x12v 12ah)...i just bought a 36v 10ah lifepo4 batt which is a Hefei Gouxaun brand ..
http://helen1088.en.ec21.com/Lithium_Ba ... 22691.html

i set it up exactly the same as the sla's...but i found that the board now doesnt have as much power as the sla's did, especially going up hill, its top speed slightly not as fast either. Is there something ive missed? is it just that its a new batt and needs a couple of charges and discharges so the bms can work its self out? is there any way i can re wire this batt to power up hills better, the length of time it lasts is great, but it just doesnt like climbing hills like the sla's did (tho the sla's did last so long especially after climbing hills). or is there any other ideas anyone can give me to power this board for best capability.

i'm thinking it may be that it may have a low (C) continuous current rate which doesnt give the power going up hill but am not sure, can i fix it or am i better to look at just getting the 10C cylindreical cells?

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Post by scooterage » Mon, 02 Feb 2009, 21:06

but i really dont want to spend more money if i dont have to

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Post by MikeG » Mon, 02 Feb 2009, 22:39

If you don't mind me asking, what did the battery set you back? Does it include a charger?
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Post by scooterage » Mon, 02 Feb 2009, 22:56

it did come with the charger...i got it on ebay just last week from a new unused disassmbeled e-bike....$320...i know...very cheap that may be why but its the same sort of batts that most bought ebikes run from...they use the pack cells rather than the cylindrical ones... the girl had 2 she was selling

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