Fast charging from an outboard battery pack

1382 Views 9 Replies 5 Participants Last post by toddwcarpenter, Apr 25, 2017

toddwcarpenter

Discussion Starter · Apr 24, 2017

Excuse me if this has been talked about before. I've been lurking on this forum for months and the search has answered all my questions to date.

I'm in the "should I do this" phase and trying to come up with a plan. I want a lightweight build (using a Miata) with a range of 50-75 miles. Keeping the weight down is really important. I plan to autoX the car.

At the "track," I want to be able to fast charge the car from an identical battery pack mounted on a small trailer. I want to be able to tow the trailer to the track, charge, race, charge, race, etc.. then charge and drive home.

From all I've read about balancing battery packs, just connecting the two seems like a bad idea. In addition, I want to be able to deplete the outboard pack during the charging process.

Has this been done before? Would I use an inverter? Could I use the re-gen feature in the existing controller in the EV?

Thanks for your time.

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Ampster

· #2 · Apr 24, 2017

toddwcarpenter said:
.......

At the "track," I want to be able to fast charge the car from an identical battery pack mounted on a small trailer. I want to be able to tow the trailer to the track, charge, race, charge, race, etc.. then charge and drive home.

Thanks for your time.

Need to know Voltage of packs, Amphours. You wont be able to deplete one while charging other hooking them in parallel if they are same voltage. You might need a DC to DC converter . Inverting to AC to then use charger would be inefficient. It could work depending.......

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onegreenev

· #3 · Apr 25, 2017

I have sent you a private message.

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toddwcarpenter

Discussion Starter · #4 · Apr 25, 2017

Ampster said:
Need to know Voltage of packs, Amphours. You wont be able to deplete one while charging other hooking them in parallel if they are same voltage. You might need a DC to DC converter .

Defiantly the same nominal voltage for both packs. Thinking two complete Volt packs. On for the car. One for the trailer.

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Ampster

· #5 · Apr 25, 2017

Two similar packs will end up at the same voltage when connected in parallel. To deplete one you would have to use a DC to DC converter to get one to discharge into the other. Doing it through inverter and charger is going to be more lossy. The trick will be finding a DC to DC converter that is adjustable. The higher the voltage difference the greater the Amps.

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toddwcarpenter

Discussion Starter · #6 · Apr 25, 2017

Found this. Looks expensive. http://reneweconomy.com.au/got-oil-cooled-100kw-dc-electric-fast-charger-ev-racer-yep-42944/

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Ampster

· #7 · Apr 25, 2017

What we don't know is what power options you have at the track to charge. It all boils down to kwhrs. Give us a clue as to how many you need to get there and how much you intend to use at the track. We can assume the same to get home.

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brian_

· #8 · Apr 25, 2017 (Edited)

I don't understand the idea of using two identical packs. If you use most of the energy in the car's pack in a single run, having two packs only gets you - at best - two runs. If you only use a small part of the energy in the trailer to recharge the car's pack between runs, then the car's identical pack is much larger (and thus heavier) than necessary.

If recharging between runs, the car only needs a hybrid-sized pack - enough for a couple of minutes of full power, or a couple of kW-hours (maybe a few kWh). To repeatedly recharge the car, a large battery pack pulled along to do the recharging needs to be EV-sized - 15+ kW-h? A Chevrolet Volt pack seems barely adequate for the trailer pack and much too large for the car pack in competition. I suppose the intention is to have enough capacity in the car to make the trip one-way between home and the event while towing a heavy trailer... but then recharging between each run seems superfluous.

I certainly see the appeal of an electric vehicle for autoslalom competition.

In contrast, I don't see any appeal in carrying extra energy along in a battery pack, but I don't know what's important to you - buying "green" power instead of burning gasoline, maybe?

Why not a generator trailer? If you have five minutes of recharging time for each minute of run, and a 100 kW motor in the car, then you only need about 20 kW of generator; that's big on the scale of general purpose generators (such as the ubiquitous Hondas), but not big on the scale of hybrid vehicle hardware. I can't imagine anyone at this type of event objecting to a well-muffled generator... unless it's an EV-only event.

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Duncan

· #9 · Apr 25, 2017

Hi Todd
I'm using most of a Volt pack in the device - with two drivers a day's Tarmac Autocross used about 1/2 a charge

http://www.diyelectriccar.com/forum...dubious-device-44370p15.html?highlight=duncan

If you want to have a separate battery for a "top up" then use one more module in the "main" battery

I have used the two 1Kwhr modules and six of the 2Kwhr modules - which gives me 340v Full and 295v empty
If I had a full pack it would be 390v to 336v

I would connect the two together with a large enough resistor to limit the current
Probably make a resister bank so I could maintain the current when my main bank was low

So - how much current? - say 40amps - nice safe 1C for the batteries
The max voltage difference is 100v - so a 2.5Ohm resister - an electric fire??
In practise with the internal resistance of the batteries that would not give as much current

I'm thinking of a number of electric fires an ammeter and some contactors - you won't be able to use switches because you are breaking high voltage DC

Connect up with all contactors "open" - then switch on a fire - when you want more current switch another one on

With that layout you would be able to dump all of the charge from the "trailer battery" into the "car battery" - you are wasting some power - but not a lot - and a LOT less than using a generator

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toddwcarpenter

Discussion Starter · #10 · Apr 25, 2017

Duncan said:
Hi Todd
I'm using most of a Volt pack in the device - with two drivers a day's Tarmac Autocross used about 1/2 a charge

........

With that layout you would be able to dump all of the charge from the "trailer battery" into the "car battery" - you are wasting some power - but not a lot - and a LOT less than using a generator

Thanks Duncan. Yeah, I don't imagine I would use the whole pack in a day. It's the 30 miles to and from the track that are the kickers.

Thanks everyone for the advice. It looks like this would be an option. As would a portable generator. I also think the solution could be as simple as making the car capable of fast DC charging, then finding a charger near the track for before and after.

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