Partially charging a Smart Electric Fortwo car

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telarium

New member
Joined
Sep 30, 2017
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1
My wife drives an electric Smart Car to her job, which uses about 50% of the car's charge per day. This was great when we were renting a house and could plug in the standard Level 1120v AC charger, but now we're being displaced to an apartment and won't have access to a power outlet near the parking space or access to any charging stations.

Since I've recently started experimenting with my own 18650 battery packs, I'm wondering how feasible it would be to at least partially power the car with these packs. The idea is that I'd charge some packs (3S? 4S? Not sure yet). I would then use those fully chargedpacks with an AC inverter, attach the Smart Car's 120v AC charger to the inverter, and plug the charger into the car while it's parked.

Based on what information I can find, the Smart Fortwo uses a17.6 kWh / 52.0 Ahbattery consisting of 93 cells. I take it this mean the nominal voltage is 344v DC.

The Smartcar's AC charger runs at 120v * 12a = 1,400w

It's a shame to have to go from DC to AC and back to DC to charge the car's battery, but I'm guessing it's not a wise idea to try and charge the car's battery directly from my DC source.

So my question is... how feasible would this be to charge half of the car's battery from some 18650 packs connected to an inverter which is plugged into the car's AC charger?
 

jdeadman

Member
Joined
Mar 28, 2017
Messages
963
if you need about half the charge per night you could make a 10kwh li-ion pack and an inverter to 120. To charge the car when plugged in. To charge the 18650's you could bring them to a power source but it would be heavy. 10kwh is about 14s80-90p so just in cells we are looking at 60kg
 

DarkRaven

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Joined
Sep 2, 2017
Messages
1,223
Welcome!

Let's do some maths:

The battery seems to be something like 93S16P then to get 344V and 52Ah. You could charge this directly using DC, in theory, however this is like neither practical nor advisable. You would need an external 390.6V CCCV power source and this usually isn't something you can buy off the shelf. Also, the car has something like this built in already. So yes, unnecessary AC-DC conversions, but probably still the better option.

Let's assume that you can charge the battery at 95% efficiency, the inverter inverts at 85% and the charger runs at 95%. To get half a charge (8.8kWh) into the car you have to put 9.3kWh into the charger and to do that you need to put 10.9kWh into the inverter and these 10.9kWh have to come out of your battery. You probably want another 20% or so capacity so you don't have to run your cells from 3.0V to 4.2V all the time. That means you are looking at a 13kWh battery and you are using 11.5kWh from somewhere in you appartment per 8.8kWh in your car.

You don't want to go 3S or 4S, neither will give you a 12V battery with lithium cells. 7S is the minimum. Assuming you are buying new 3Ah cells you need 7S167P = 1169 cells for a 13kWh battery. As jdeadman was saying, at 45g per cell we are looking at 52.6kg cell weight, plus additional material. 14S84P would be the same number of cells and same weight, just double voltage and half capacity. The battery has to supply about 70A (24V) or 35A (48V) which is no problem for a 167P or 84P configuration.

7S can be charged at home with available off the shelf balance chargers like bigger units from SkyRC, Hitec, Junsi, and so on. Everything above 10S usually needs some kind of specialized solution in this context. But realistically everything from the RC world smaller than a beefy Junsi 4010 is not powerful enough to charge such a battery. A SkyRC D400, which is already a bigger unit in RC context, would take more than 24h to put the needed 10.9kWh into the battery. A Junsi 4010 can charge at 40A, given a big enough input and can reduce the charging time to around 12h or so.

So, yes, it is possible. However, it is up to you to decide whether this is feasible
 

Larry Smith

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Joined
Nov 8, 2017
Messages
1
Hello... I have a 2008 Gas Smart Car and I am in the process of buying an Electric Smart Car. I, also, have had ideas of alternate ways to charge the battery and after reading the other responses I think my idea has some merit. With the weight of the charging batteries, inverter and etc., an 8000w(+ or -) gas electric generator towed someway like a small trailer or stowed in the back. It would probably weigh about half the weight of a passenger. I have to admit that I haven't compared cost of gas over 110vac...........?
 

DarkRaven

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Joined
Sep 2, 2017
Messages
1,223
Holy cow, time travel seems to be a thing now as we seem to be back in the 1970s :)

http://www.bus-bild.de/1024/zwischen-1975-1988-setzte-rheinbahn-97608.jpg
http://www.bus-bild.de/1024/rheinbahn-versuchs-elektrobus-9056-um-1980-duesseldorf-garath-97609.jpg
http://www.rheinbahn.de/unternehmen/fuhrpark/Fotos/9063.JPG

My mother went to school in these, the trailer is full of batteries.

Do you really think this is a good idea? Modern battery technology is there so trailers and decentralized burning of fossil fuels isn't necessary anymore. If you can't find or afford a car with decent range for your needs then just wait. There will be happening alot in the coming years.
 

jdeadman

Member
Joined
Mar 28, 2017
Messages
963
What we need a better solar per in^2 to charge the cars when in sunlight. for work you drive 1 hour then the car sits for 8 hours then another 1 hour drive. It would be awesome to have a solar system on the car to keep it topped up. But alas that's not realistic. And that tow behind is just funny
 

Korishan

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Joined
Jan 7, 2017
Messages
6,617
The panels are efficient enough for this. I have planned on doing something like this with one of my cars. Not sure if I would be able to do this with the truck as it doesn't have a camper top.
Like you said, the vehicle would sit unused for 8hrs. So there's no reason why the roof couldn't have panels. For regular commuting to/from work, this would be feasible. For doing several hour drives per day, maybe not so much.
 
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