# 172 Wh per mile!



## jondoh (Sep 8, 2008)

I just came back from a 20 mile spin around town. It's late so I was able to travel 30 something mph on 40mph speed limited roads. 

With my thundersky batteries and '92 tercel weighing in at around 1800 lbs, I was able to manage 172 Wh per mile!

Here is my car:

http://www.evalbum.com/2975


See attachment from Jack Richard. 

My 1 minute rest voltage was 77.7 v. This is about 3.237 v per cell which according to jacks chart is 45% discharged. So with my relatively small (7.68 KWh pack) i could go a theoretical 44 miles or about 35 miles to 80% DoD. 

I'll stick with my 20 ~ 25 miles range of normal driving but it's nice to know I could coax 40 miles if I had to.


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## ahambone (Jan 13, 2009)

Jondoh,

Congratulations. Thanks for posting that chart. My system is setup for Low-Voltage-Cutoff at 2.1 volts - it's good to see that under load that was a reasonable number for somewhere between 95% and 100% depth of discharge.

Cheers,
--Adam


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## Bowser330 (Jun 15, 2008)

wow thats great! congrats!

1800lbs fully converted? wow!


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## Voltswagen (Nov 13, 2008)

Ahambone
Whose batteries are you using?
When you say your LVC is set at 2.1 volts.....what does your BMS do at that point? Shut Down Controller? Sound an alarm? Reduce voltage by 50%?
Does the system bounce back?
2.1 volts on a TS or SE LiFePo is dangerously low. Look at Jacks chart.
Notice how sharply the voltage drops off in the high 2's.
At 2.1v you have about 50 feet of range left before cell damage.

I plan to set my LVC at about 2.7v - 50% power.


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## jondoh (Sep 8, 2008)

voltswagen,

2.7 volts may be too high for load conditions. Jack's chart (which is only for TS) shows the voltage under load at only 1 C. Since I can go up to 4.5C, i might trip 2.7 volts at 60 ~ 65% DoD under hard acceleration. Depending on the size of your batteries, you might want to consider something less than 2.7v. Under load conditions, 2.5v per cell may even be slightly conservative-- all though you definitely want to SLOW DOWN if you hit 2.5v per cell under load. If my meter hit 60v under load, I'd start babying it right away. Even with a full charge, i'll hit 2.8v per cell under hard acceleration.

All I have is my volt meter. What the chart also shows is that a volt meter is "ok" for letting you know when you're at around 80% DoD.


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## gss99 (Dec 31, 2009)

I'm not familiar with cars that run on lifepo4. How much would it cost on electricity to go 20 miles in a car? I got lifepo4 for my electric bike and it cost about 1 cent per mile. Nice to know we could possibly go back to joy riding like we did in the 80s and 90s where we can ride all day for real cheap.


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## roflwaffle (Sep 9, 2008)

Those are some great numbers! What speed is your motor at when you're cruising at ~35mph?


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## jondoh (Sep 8, 2008)

> How much would it cost on electricity to go 20 miles in a car?


At $0.14/kwh it would cost between $0.35 to $0.70 to go 20 miles.




> What speed is your motor at when you're cruising at ~35mph?


I'm in second gear which can go up to 40 mph on a flat with no wind. It does take time.


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## frk2 (Jan 2, 2009)

jondoh - you are using almost the same setup as I am - here is my car :

http://www.diyelectriccar.com/garage/cars/195

However, on full acceleration I can get to maybe 35-38mph in 3rd gear (my car is geared higher). I have FLAs weighing about 360kgs in the car. I am able to get only 130-160Wh/km (250Wh/mile) 

My car is probably much smaller and lighter than yours. You think the batteries are making the huge difference? That IS a large difference 

I wonder what makes you do only 110Wh/km.

How fast do you accelerate? I am usually pedal to the metal or half way there so i let it suck 180-250amps no problem  maybe my driving style needs to improve


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## Voltswagen (Nov 13, 2008)

jondoh
I rarely push much above 1C on acceleration with my LA Pack. 
Under acceleration, I try to keep my amp draw under 200 and the gearing for a stock VW trans is such that this is easily done without crawling.
Normal acceleration.
I can cruise at 35 - 40mph in 4th gear, level road, at 50-60amps.
I live in a flat area with very few hills.

If you are regularly drawing 3C or more then your pack or the motor is undersized for the vehicles weight. A 7.68KWh is not very big. My LA pack is 21KWh....with Lithiums it will be 18.4KWh.

Using the TS 160ah LiFeYPo I would assume even less amp draw as my Beetle will weigh in at about 2200 lbs with Lithiums. It's now about 2700 lbs with LA.

My plan for an LVC will utilize some type of Power reduction....say 50%.
For my vehicle, setting the LVC at 2.1v is living on the edge. Consider this scenario:

I am fully charged, I travel 10 miles from home and my LVC begins to engage. My SOC meter says 85% charged but none-the-less I have one cell near death which is not indicated by the overall SOC. Had I set the LVC higher, I would have been notified earlier and might be able to limp home. But at 2.1v, I can't get very far before permanent damage results. I'm going to have to dig out my AAA card.


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## frk2 (Jan 2, 2009)

Voltswagen said:


> jondoh
> I rarely push much above 1C on acceleration with my LA Pack.
> Under acceleration, I try to keep my amp draw under 200 and the gearing for a stock VW trans is such that this is easily done without crawling.
> Normal acceleration.


Im assuming you shift a lot? Ive been lazy in shifting ever since I removed my clutch. So i pick it up in third and draw way lot of amps. I think that was a bad idea - going to get it put back in!

If I do the 2nd-3rd combination I can also restrict the amps to under 150


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## Voltswagen (Nov 13, 2008)

FRK2
Yes, I use all 4 gears and I am clutchless. My trans has about 127K on it and yet I only have to pause for maybe 1 second between gears. It's not the big deal everyone says it is.


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## ahambone (Jan 13, 2009)

Voltswagen said:


> Ahambone
> Whose batteries are you using?


I'm using a custom pack of Headway 38120 cells, configured in a (5p) x 24s pack. They are available from EVComponents. 

That's nominally a pack at 76.8 volts x 50 amp-hours. I plan to run two of these in series for ~154 volt pack, right now I am running on one for short distances while I finish off the other parts of my car. You can find the stats and some pictures of my VW Cabrio here: 

http://www.evalbum.com/2619 

There are a couple of videos of the pack in he car here: 

http://www.youtube.com/profile?user=txhambone#g/u



Voltswagen said:


> Consider this scenario:
> I am fully charged, I travel 10 miles from home and my LVC begins to engage. My SOC meter says 85% charged but none-the-less I have one cell near death which is not indicated by the overall SOC. Had I set the LVC higher, I would have been notified earlier and might be able to limp home. But at 2.1v, I can't get very far before permanent damage results. I'm going to have to dig out my AAA card.


If you have once cell near death the pack should detect that and stop you from driving the car. In this case pulling out the cell phone and calling AAA or a friend to pick you up is exactly what should happen. If you have cell-wise voltage monitoring and prismatic cells then you might be able to identify the weak cell in the field and disconnect/jumper around it provided you have the tools on board. Most BMS systems won't take kindly to this kind of in-the-field hacking. Since I don't use prismatic cells this kind of field-fix isn't really an option for me. 

I'm expecting the 2.1 volt LVC to be hit under load. Whether the 2.1v is hit at a load of 10 amps, 100 amps or 300 amps my goal is to treat it as a point-of-no return. I want the system to strongly discourage any driving of the car past that point - something like make the pack and the vehicle shut down until the key is switched off and back on. The cells will of course bounce back to a higher voltage once the load is removed - but once a cell under any load has hit LVC at 2.1 volts it's time to recharge the pack. 

In my opinion once LVC has been hit by one cell it is still not safe to drive again until the pack gets charged. Lithium packs cost several thousand dollars - an investment that should protect itself at all costs. I'd rather have the car make the decision for me that it's not letting the pack get killed than have my adrenaline influenced decision making when I'm stuck on the side of the road thinking "nah, the pack's got 1 more mile in it". The pack should be smarter than me and much less influenced by emotions.  This should also prevent the pack from getting killed by other drivers who might be driving my EV (read: this safety mechanism prevents the wife et. al. from killing the pack).




Voltswagen said:


> When you say your LVC is set at 2.1 volts.....what does your BMS do at that point? Shut Down Controller? Sound an alarm? Reduce voltage by 50%? Does the system bounce back?


When any one cell reaches 2.1V my BMS just triggers a 5 volt logic signal telling me that I've hit LVC. What I do with that signal is up to me; I'm still in the process of designing and wiring it up right now. 

My plan is LVC will do two things: (1) Trigger circuit latch that lights a big, bright LED on my dash saying that LVC has been hit. (2) Interrupt the main relay/contactor so that the controller is disconnected and shut off. I want my LVC to cause the main relay/contactor to stay off until the key switch is restarted. This discourages the use of "creep home" driving and should allow getting the car off to the side of the road with a restart or two.

My pack only has about 7 cycles on it at this point. I haven't taken the total under-load voltage down below 69 volts total. I was pulling 300 amps flooring it when the voltage dropped from ~78 to ~69. That's about 2.87 volts per cell, well above my LVC. 

I haven't put more than 10 miles per charge on the pack yet since I don't yet have instrumentation in place showing how many amp-hours I've pulled from the pack. I'm thinking about using a Cycle Analyst (http://www.ebikes.ca/drainbrain.shtml) setup for high-current and high-voltage as my "amp-hours-used" gas gauge. (There are other parts of the car that need work as well... like the power brakes, registration, etc. )

That's my plan for the LVC. I'll post more data about how the system works as I get more of it built.

Cheers,
--Adam


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## Voltswagen (Nov 13, 2008)

Adam
Now I understand why you're hitting 3C. The pack is small even for an 1800 lb vehicle. I think once you add the other pack your C number will decrease but I would still suggest raising the LVC a little.
You said that at 2.1v your plan is to shut down the controller. What if you are in the middle of an intersection making a turn with traffic coming at you . The sudden loss of power might put your vehicle in jeopardy.

I have thought on this and it's why I want to set my LVC at 2.7v and have it trigger a 50% power reduction. In that way I can keep the vehicle moving and possibly limp home or as you suggested bypass the sick cell right then and there on the side of the road.
I did that once with my LA batts. I had to bypass a battery with a long jumper cable and limped it home.
Bet ya can't wait to get that second pack . I'm anxiously waiting for my order to arrive.......I might even pop a cork once they drop them in my driveway.
Good Luck - Roy


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## jondoh (Sep 8, 2008)

Adam

I'm in agreement with voltswagen regarding not shutting down the controller. 

I originally tried something similar where, if pack voltage were to fall below minimum required (which is very close to 2.1v per cell), under load to keep a dc-dc converter powered, then a normally open relay would keep the controller from powering back on again unless a start button was pressed. In reality, relays are a little touchy and a slight blip (like from a vacuum pump) from the dc-dc converter would cause it to de-energize for just a spit second-- but enough to turn off the car. So the car was constantly turning off. It was annoying. I eventually took out the relay and replaced it with a simple rocker switch. 

Thunderskys do sag noticeably under load past 70% DoD. If you keep one eye on the volt meter, you will see it clearly. Unless one cell is way off, you aren't in any danger of killing a cell at 70% DoD. You still have time to slow down or stop and charge up. You can limp home using less than 100 amps at a time for up to 5 miles (at 25 mph) if you're on surface streets.


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## Voltswagen (Nov 13, 2008)

Once I get my cells I will test them to determine 8 of the weakest cells.
These cells will hit high voltage first and discharge the deepest under load. Since weak cells will never get stronger....these will be the weak link in my pack. Every pack has a few. Right now I am planning on using this small BMS system to monitor those 8 cells. 

Here is the BMS:
http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=9282&Product_Name=Cell-Log_Cell_Voltage_Monitor_2-8S_Lipo

Tomofreno bought 2 of these and he is monitoring his Lithiums on his "SwiftE" (see garage). http://www.diyelectriccar.com/garage/cars/223
According to him they work amazingly well and have built in alarms.


I could do worse things with $28.00


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## tawright (Dec 30, 2009)

Okay this may seem like a stupid question but isnt there some relay that can go on each cell so that if the cell goes back you can signal the relay to remove it from the pack and continue to run? 

Kind of like when the Starship Enterprise needs to reroute power jordie would pull up the screen in the hall and reroute the power around the bad spot 

But seriously why cant this be done?


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## rmay635703 (Oct 23, 2008)

tawright said:


> Okay this may seem like a stupid question but isnt there some relay that can go on each cell so that if the cell goes back you can signal the relay to remove it from the pack and continue to run?
> 
> Kind of like when the Starship Enterprise needs to reroute power jordie would pull up the screen in the hall and reroute the power around the bad spot
> 
> But seriously why cant this be done?


This is how I would like to see things, sadly you need relays that would handle 100-500 amps on each cell one for cut one for bypass, lot of bus bars, you would need the associated logic and obviously an awefull lot of space and money.


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