# Thought experiment



## sjc (Oct 28, 2008)

My son and I are converting an '86 911 Targa (almost all parts in now;
he's eager to be done with the de-ICEing and cleaning and get to the
constructive phase of the project).

We will have a 144v system, 160 Ah Thundersky LiFePO4, ADC FB1-4001-A,
and a Soliton1 controller (that's the major part we don't yet have).
On paper, this should have a range in the 70's, but I don't really
expect that much if we want to preserve the batteries.

Nearby to us there's a 78-mile road rally for alternative-fuel
vehicles (www.greengrandprix.com). AFAIK, to date, all entries have
been some form of ICE (hybrid, propane, etc.)---no pure BEV. He and I
were talking about how we could enter this, given that the length of
the race is right at our max threshold of travel. We talked about a
range extender, more batteries, etc.

Today I thought of something I haven't seen discussed here before: a
reconfigurable battery pack. If we could switch from full
series-wired 144v to parallel 72v (approx) subpacks, we could double
our range (or would we?). In this particular race, there is a
mandated 10 minute stop about 1/2 way through. This leads to
interesting possibilities, such as running the first part of the race
at 144V and the second half at 72V (or vice versa), changing
configuration during the 10-minute stop.

Now, with 45 TS cells, we can't exactly split the pack into two 72v
subpacks. Rather, they would be 70.4 and 73.6 volts (unless we had one
more cell and then have 73.6v subpacks or a 147.2v single pack).

So, what problems should we anticipate if we ran off of two
slightly-different voltage packs? Are there dire implications?

72v is the low end of where the motor is rated. The controller is
fine with it, so things should work, if not as efficiently as we would
like.

Are there dragons lurking here? Anything obvious we're overlooking in
thinking about this approach?


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## Overlander23 (Jun 15, 2009)

Splitting the pack wouldn't really buy any more range. A battery pack has an amount of storage. How you use that storage is where amps and volts come in, series vs parallel.

Since the storage capacity of a pack is basically determined by multiplying its voltage potential by its amp-hour capacity it doesn't matter if the pack is wired in series or parallel as far as range goes.

For example:

45 160AH Ts cells in series. That's, 45 x 3.2v x 160Ah = *23040* watt-hrs of storage. Total system output is 144v nominal at 160Ah capacity.

45 160AH cells in two parallel strings of 22.5 cells (just to make the math easy). That's (22.5 x 3.2v x 160Ah) x 2 strings = *23040* watt-hrs of storage. Total system output is 72v at 320Ah capacity.

If anything, the higher voltage, series configuration has the potential to be more efficient.

Keep in mind that with the same motor, a series or parallel configuration of the pack won't change performance for what you have in mind. The motor will still need the same number of volts to achieve a particular level of performance. You're not using full pack volts all the time in this case. Under partial throttle, you're using less volts. Gven a specific speed, you're using the same volts regardless of whether they're delivered from a pack in series or parallel.


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## Guest (Oct 5, 2010)

Your best off with the single pack setup. Just keep off that throttle a bit. You may then want to increase your AH and get some 180 or 200 AH batteries and get your extra range you desire. Dropping to a low voltage will just increase your amp draw to get the speed you need. Keep the voltage as high as you can. I have driven both 72 and currently 96 volts. Both go about the same distance but the higher voltage one has better performance. I am on the throttle more so my distance has not changed much. I'd suggest you have as high of voltage system as you can and have as high of AH as you can afford. I'd say you could go with higher AH like the 180 or 200 AH battery. You will be a mucho happy camper that way and you can join in the rally. 

Pete


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## sjc (Oct 28, 2008)

*head slap* It seems so obvious when you say it that way, and the question is foolish  Of course I should have focused on the total pack capacity.

So, we're back to considering methods to maximize our driving efficiency, reducing weight and resistance, and thinking about range extenders and auxiliary battery packs. First, we need to finish the build and take some measurements.


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## Duncan (Dec 8, 2008)

Hi sjc

It may be too obvious but the watts/mile is heavily dependent on speed - you should be able to complete the 78 miles - but you may have to do it at a lower speed

you have 23040 whrs at 100% or 18,434 whrs at 80% that gives you 236 whrs/mile for your 78 miles

With something slippery like the Targa I would hope to be able to drive at 60 mph at 236 whrs/mile (14.178 Kw power output)

But if you can't you certainly should be able to at 50 mph


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## Woodsmith (Jun 5, 2008)

sjc said:


> *head slap* It seems so obvious when you say it that way, and the question is foolish  Of course I should have focused on the total pack capacity.
> 
> So, we're back to considering methods to maximize our driving efficiency, reducing weight and resistance, and thinking about range extenders and auxiliary battery packs. First, we need to finish the build and take some measurements.


It does seem obvious but there are other factors that might have an effect.

I²R losses. Power(W) lost due to the resistance(R) in the system is proportional to the square of the current(I) flowing in it.
So if you can reduce the current you reduce the losses.


One way of reducing the current is to increase the voltage for the same power given that W=VI


Running at 144V, instead of 72V, would halve the current flow and result in a quarter of the I²R losses.


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## DIYguy (Sep 18, 2008)

The range seem very doable at lower speeds... .unless it will be cold wherever this is...??? My first thought was to arrange charging at the stop over. 30 minutes isn't much, but if you have a high current charger (or even borrow one) you could keep the worries away.  ... If you had to, you could even set up a gen-set at this location. Just a thought. Buying different batteries for a one-off journey seems .... ... 

Good luck!

Oh yes... and most importantly... inflate tires to max!


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## JRP3 (Mar 7, 2008)

It's also quite likely your cells will all come in above 160ah, my 100ah SE cells are all 110-114ah, which gives you some extra capacity. If you can keep your wh/mi. low enough I think you could make it. Batteries would need to be warm when you charge and discharge them. A single deep discharge won't kill the pack as long as you don't take any single cell too low. A coroplast belly pan and any other smoothing techniques should help you out, maybe temporary rear wheel covers as well.


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## JRP3 (Mar 7, 2008)

I didn't realize this was going on at Watkins Glen, that's about 45 minutes away from me. I don't have enough pack to do it myself  You'll have some hills to climb so any weight reduction will help keep the wh/mi lower. I don't know the route but if they start in the city at lake level the big climb will be at the beginning when your pack voltage is highest which will help a bit.


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## DavidDymaxion (Dec 1, 2008)

Sounds like a great project (as a fellow 911 EVer  ).

Take every pound/kg out of the car you can (passenger seat, tools, floor mats, etc.). Run with the windows up. Make sure each wheel spins freely. Use coroplast (old campaign sign) to make a belly pan. The first year of 911 had 4.5 inch wide rims. I'm planning to buy a set for less rolling resistance and better aerodynamics.

If you have regen, you can do prodigious amounts of charging in 1/2 hour via tow regen.


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## sjc (Oct 28, 2008)

JRP3 said:


> I didn't realize this was going on at Watkins Glen, that's about 45 minutes away from me. I don't have enough pack to do it myself  You'll have some hills to climb so any weight reduction will help keep the wh/mi lower. I don't know the route but if they start in the city at lake level the big climb will be at the beginning when your pack voltage is highest which will help a bit.


Think in stages! Start in the ATV, take it out until you hit its max range, at which point you have parked your fully-charged car, and then...hmm. 

It will certainly be easier than the last time I did it---by bicycle, in 1989. While not this exact route, my wife and I camped at Taughanoock Falls, rode through Ithaca up the east side of Cayuga Lake, and around the top for a stop at Cayuga Lake State Park for the night. Next day we rode over to Geneva, then down the east side of Seneca Lake through Sampson State Park, and back to Taughannock Falls. Not the same route, but close.

Thanks for the pointers on efficiency. I was already planning a belly pan to keep out muck, so it's good that it will help with aerodynamics too.


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## JRP3 (Mar 7, 2008)

That was quite a bike cruise. At least you know the type of terrain you'll be traveling, you can probably coast the last two miles back into Watkins Glen if you have to. I grew up a few miles away from Taughannock Falls, spent a lot of time in the park.


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## sjc (Oct 28, 2008)

DavidDymaxion said:


> Sounds like a great project (as a fellow 911 EVer  ).
> 
> Take every pound/kg out of the car you can (passenger seat, tools, floor mats, etc.). Run with the windows up. Make sure each wheel spins freely. Use coroplast (old campaign sign) to make a belly pan. The first year of 911 had 4.5 inch wide rims. I'm planning to buy a set for less rolling resistance and better aerodynamics.
> 
> If you have regen, you can do prodigious amounts of charging in 1/2 hour via tow regen.


No regen; series-wound motor.

I'm thinking of starting a thread over at Pelican Parts called "What's that thing?" in which I'll post photos of the bits I'm thinking about taking off the car. I have the full shop manuals, which are great if you know you need to replace the fromnambulator, but the problem is I'm working from the opposite direction. I received the car as a glider with the engine pulled and no tranny (and most of the wiring very helpfully just sheared off in certain spots), so I'm spending a lot of time looking at a tube and figuring out whether it's part of the fuel system (goes), A/C (goes), brakes (stays), etc. Most of it is pretty obvious, but still, it takes me a LOT longer than it would a Porsche aficionado. I'm currently staring at the plastic tank of some kind just ahead of the left front wheel, wondering, "What's that thing?" Could be meat, could be cake. It's meatcake (with apologies to George Carlin). Fuel system, A/C, not sure...I'm positive it's coming out, but it's a lot easier to remove when I can find it in the manual.

It's time for me to start a real build thread and get some pictures posted, as that is where this thread is headed.


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## GerhardRP (Nov 17, 2009)

sjc said:


> I'm thinking of starting a thread over at Pelican Parts called "What's that thing?"


I wonder if the Pellican admins would allow the creation of a new forum "EV conversions"
Gerhard


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