Retro Porsche 914 EV

6297 Views 11 Replies 6 Participants Last post by electro wrks, Jan 8, 2018

Jonny Retrofit

Discussion Starter · Dec 31, 2017

Hi all,

I have 'lurked' for quite a while on this site but thought it time I posted.

I'm the technical director of Classic Retrofit - we make upgrade goodies for classic cars, particularly Porsches. In the last couple of years we have redesigned ignition units, fuse panels and other parts for these cars. We have also designed an aftermarket electric air conditioning system for classic cars. I monitor the EV world closely to see what tech can be adapted to classics and naturally drivetrain is the next step!

Two years ago I purchased a Porsche 914 from California. It had already been converted to electric power a few years previously. You could call it a vintage EV conversion. The spec was as follows:

42 x CALB Thundersky 180A cells
FB4001 motor adapted to manual 5 speed gearbox
Curtis 1231 controller.
Bogart Engineering 'Trimetric' SOC meter (must be vintage!)
Homebrew charger.

I knew the tech was a little out of date but the car was solid and a very good price so I thought it would be a good starting point. I had it shipped to the UK and amazingly it switched on with 99% charge and it drove along quite ok (albeit up and down my driveway).

In order to learn, I thought I'd strip the car and build it back up. There were some 'interesting' features. E.g. an undertray that was serving to store falling leaves and pine needles and about a mile of wiring, all the same colour - pink! It was a mobile tinder box so I am quite glad I took the decision to give it a refresh!

Plan A consisted of keeping the batteries and motor but updating the controller and the charger. I bought an Evnetics Soliton Jr off a guy on here and a couple of bits from the States. I ran the orange DI cable and started to remake the battery boxes.

We suddenly got very busy with the electric air conditioning systems so the project has frozen rather. Since I've been away, the tech has moved on again so I am now considering a different approach using Tesla parts. I need a little guidance:

Once the engine, transmission and fuel tank have been removed and with some 'lightweighting', I can get the rolling shell down to around 750kg

I have read that each 5.3kWh tesla battery pack weighs around 25kg and that the large Tesla rear Drive Unit weighs 132kg. The small DU weighs 88kg.

Ideally I would like a 200 mile range and sports car performance if required. Not insane (like that Evora) but need to keep up with old 911s (0-60 in 6 secs)!

So my first questions are:

1) How many battery packs should I aim for? I can accommodate 8 quite easily but 6 would be lighter obviously.

2) What would be the battery configuration for 6 vs 8 cells?

3) Am I crazy going for the large DU or would the small one give me adequate performance?

Cheers,

Jonny

p.s. Happy New Year!

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Duncan

· #2 · Dec 31, 2017

Hi
Your problem is going to be getting enough voltage
A Tesla uses about 16 of those packs in series and that is the voltage that the motor is designed around

You may be better off with Volt or Leaf modules

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brian_

· #3 · Jan 2, 2018

Jonny Retrofit said:
1) How many battery packs should I aim for? I can accommodate 8 quite easily but 6 would be lighter obviously.

2) What would be the battery configuration for 6 vs 8 cells?

I assume that you mean for 6 versus 8 modules, not cells. Any number of Tesla modules up to the number used in the original vehicle would be connected in series. With 8 or fewer of them, you'll be running at half (or less) of the original design voltage for the inverter and motor; I don't see any reason to run at even lower voltage (unless of course using a different and lower-voltage motor).

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Jonny Retrofit

Discussion Starter · #4 · Jan 2, 2018

Yes, I meant modules. I saw on another thread someone saying that the Tesla modules could be reconfigured internally to give 48V. Can't find any details though!

Another hypothetical question. Two battery packs with the same energy, say 48kWh but one was 150V and the other 300V both matched to respective voltage motors.

Heavy right foot notwithstanding, is the 300V system potentially capable of the same range as the lower voltage system?

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Duncan

· #5 · Jan 2, 2018

Jonny Retrofit said:
Another hypothetical question. Two battery packs with the same energy, say 48kWh but one was 150V and the other 300V both matched to respective voltage motors.

Heavy right foot notwithstanding, is the 300V system potentially capable of the same range as the lower voltage system?

I would expect MORE range with the 300v system as you will have lower current and lower resistive losses
But I don't think the difference would be noticable

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Kevin Sharpe

· #6 · Jan 3, 2018

Jonny Retrofit said:
Heavy right foot notwithstanding, is the 300V system potentially capable of the same range as the lower voltage system?

One of the advantages of the 300V system is the ability to use public rapid chargers

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Kevin Sharpe

· #7 · Jan 3, 2018

Jonny Retrofit said:
I saw on another thread someone saying that the Tesla modules could be reconfigured internally to give 48V. Can't find any details though!

I think Boekel can produce modified modules;

http://www.diyelectriccar.com/forums/showthread.php?t=185161

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Kevin Sharpe

· #8 · Jan 3, 2018 (Edited)

Duncan said:
Your problem is going to be getting enough voltage
A Tesla uses about 16 of those packs in series and that is the voltage that the motor is designed around

The Open Source Tesla controller (here) can use lower voltage. iirc Damien will use 11 Modules in his conversion (here) and I'm using 12 (here)... obviously if you modify the modules you could use just six

I understand the old bus battery Damien used for testing in this video is ~235V nominal but on the day of the recording had low SOC and significant sagging

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brian_

· #9 · Jan 3, 2018 (Edited)

Jonny Retrofit said:
Yes, I meant modules. I saw on another thread someone saying that the Tesla modules could be reconfigured internally to give 48V.

Internal reconfiguration is serious work. Since the goal is to increase the operating voltage, the modules would still be wired in series, so the answer to how to interconnect the modules is still all in series.

Jonny Retrofit said:
Another hypothetical question. Two battery packs with the same energy, say 48kWh but one was 150V and the other 300V both matched to respective voltage motors.

Heavy right foot notwithstanding, is the 300V system potentially capable of the same range as the lower voltage system?

Yes, the range depends only on stored energy and wiring/inverter/motor efficiency. If in these two scenarios the motors really are right for the voltages, the battery voltage won't change the range. As Duncan mentioned, higher voltage means lower resistive loss, and that's just one of the elements of the efficiency assumption.

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bawfuls

· #10 · Jan 8, 2018

Kevin Sharpe said:
One of the advantages of the 300V system is the ability to use public rapid chargers

By this do you mean Level 3 chargers, i.e. CHAdeMO?

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Kevin Sharpe

· #11 · Jan 8, 2018

bawfuls said:
By this do you mean Level 3 chargers, i.e. CHAdeMO?

Correct, CHAdeMO and CCS rapid chargers.

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electro wrks

· #12 · Jan 8, 2018

Jonny Retrofit said:
Yes, I meant modules. I saw on another thread someone saying that the Tesla modules could be reconfigured internally to give 48V. Can't find any details though!

Another hypothetical question. Two battery packs with the same energy, say 48kWh but one was 150V and the other 300V both matched to respective voltage motors.

Heavy right foot notwithstanding, is the 300V system potentially capable of the same range as the lower voltage system?

This company has them. Although they seem to be out of stock.http://edisonmotors.net/shop/product/12s-converted-model-s-module-5-4-kwh-50-4-v-max-140?category=7

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