Smart Car Battery Charge Line?

I've been flat towing my 2009 smart for 2 years with no problems. I follow the instructions in the owners manual.

The car's curb weight is 1,800 lbs (US). Many U.S. states do not require supplemental brakes on less than 2,000 lbs.

From PDF page 192 of the 2009 smart owners manual: http://owners.smartusa.com/My_Manuals.aspx

> Preparing the vehicle: Contact an authorized smart center for information on recommended tow bar equipment and installing a matching on/off switch on the battery terminal.

....

> Turn the battery off using the previously installed battery switch.
> Release the parking brake.

Some RVers run 12V power from the motorhome to power the supplemental brakes. Others have resorted to placing a car battery on the passenger-side floor - just for powering the supplemental brakes - and then they charge the spare battery each night.

click on photo for more images


the Hella 87181 battery disconnect was installed by the original smart center Portland that was run by Kuni Auto Group.

The battery disconnect switch is connected to the negative battery terminal.

for spare battery disconnect keys, google: Hella 87185

The Service Manager at the time would tell you in the strongest of terms that Automatic Disconnect products (like the one offered by Roadmaster) must not be used in a smart car as it can feedback power to the computer. People have done things as simple as entering their smart to retrieve an item (while in tow mode) and accidentally turning the ignition key to the ON position without realizing it. The computer then woke and engaged systems that caused damage beyond repair while being towed.

The computer does not know that the car is being towed. It only understands that it is moving and the engine isn't running. It does not have a 'tow mode' option programmed into it.

For those of you successfully not using any battery disconnect - you are the exception. I will not risk it. I made a large-print copy of the flat towing instructions from the owners manual, laminated it, and follow it religiously.

..Very easy way to find out if a charge line is required, with ANY vehicle you're going to flat tow. Just put a clip-on ammeter across the battery's main ground wire, when hitched up and ready to tow (key in approrpiate position it has to for towing, tranny in neutral, everything off inside, etc).

You don't even have to hitch it up necessarily, just set everything up with it (tranny gearshift, ignition switch, etc) like it would be when towing. Then, like I said, put an ammeter across the battery and see what it reads.

If you read more than 1 amp or so, then you either need to install a charge line, or need to find the source of the draw and put a stop to it (maybe pull a fuse?). I'm not a fan of pulling fuses, and have found that its much easier to just wire in a charge line and forget about it. Thats what I did for our Fusion hybrid.

Also, sometimes a supplemental braking system can draw enough battery power, where a charge line is needed to prevent draining the battery.

However, based on the last poster's comments, it sounds as though you may want to tell your friend to re-consider flat towing the smart car at all. If we could get by with something as small as a smart car, I'd just flat-bed tow it on one of those small, collapsible trailers they make just for smart cars. I think that'd be the best solution.

I have a real life friend in Anacortes that just had the transmission and engine replaced on his smart car. He actually first had the transmission replaced.

In the first instance, the electronic control between the shifter and transmission was faulty and the transmission was damaged while flat towing. The manufacturer replaced the transmission, and said that the vehicle was not designed for flat towing long distances.

The second time, apparently something similar happened, but this time the engine was also damaged (~$12,000). After several months, the manufacturer agreed to fix it and then eventually declined. His insurance company finally picked up the tab. I suspect that they are having a discussion with the manufacturer. It took another couple of months to get a new engine from Germany.

The manufacturer said that he must have hooked the vehicle incorrectly (he said/he said?) that caused the damage, and again said that the vehicle is not designed for flat towing for long distances. Which is apparently not actually specified in the manual?

My friend is not sure is he will ever tow his Smart Car again?