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Bicycle Bob 12-17-2019 10:36 PM

Choosing an RV "tender"
A friend with an RV would like to get an electric car as a tow-along. Chores would include scouting locations and fetching groceries. He is especially interested in using the car battery for RV utilities when needed. He might want to charge it by towing it, and otherwise integrate the systems. It would be nice if the combination gains streamlining by smoothly extending the RV tail.
I am wondering about getting an old Urba-car or similar pre-Tesla EV and possibly updating the battery pack for him. Any recommendations, advice, or leads? TIA.

redpoint5 12-17-2019 11:27 PM

I don't know about older EVs. Something like a used Spark EV would be on my radar. Does your friend have a price range in mind? The Spark can be had around $9k or so and has a 19 kWh battery. Most battery for the price is probably the Chevy Bolt. 60 kWh and some reports of heavy discounting on the 2019 models. 3 year leases will start coming up on the Bolt next year, so we should see a decent used market open up on them.

JSH 12-17-2019 11:45 PM

The only first generation EV I would recommend is a Leaf. I say that because they are the only ones made in real production numbers with replacement battery packs cheap and readily available. My Spark EV was fun but they only made ~6000 of them and a replacement battery is $22,000.

A Bolt is much nicer but much more expensive. The Volt is an option if he wants a PHEV instead of an EV.

I would forget about towing to charge and just plan on plugging into the shore power at RV parks.

redpoint5 12-18-2019 12:05 AM

Well wait just a minute here. Normally an RV has no way to capture excess energy coming down a steep grade. Put someone in the tow vehicle and have them soak up that excess speed by putting it back into the battery.

Bicycle Bob 12-18-2019 05:34 AM

Thanks, guys. He had only priced a Tesla, without going into sticker shock, but he got the RV for $1k and a bit of work. I doubt that there are plug-ins where he is going. He considers it a portable house, not a vacation device. I'm mostly wondering about the 2-seat urban runabouts, hoping to keep the weight down. There won't be anyone available to drive the car, but I could imagine a regenerative brake being set, and possibly some remote instruments.

redpoint5 12-18-2019 10:59 AM

On some EVs like the Bolt, there is a button on the steering wheel that activates regenerative braking.

It would be neat to have a wireless transmitter send the signal to the Bolt, perhaps being tied into when the brake light activates.

There's the Acrimoto trike that would fit the bill as a minimalist EV.

Hersbird 12-18-2019 12:22 PM

I don't know of any EVs that have the ability to use the energy in the battery for anything other than moving the car. Maybe some have a single low amp 110 outlet on a converter. Maybe barely enough amperage to run a small flat screen. I think you would spend more trying to get that working than just buying a couple nice Li-on 12 volt house batteries and a solar setup on the roof. As far as charging while driving again, I think you would spend more trying to reverse engineer applying just enough brake but not too much. Just but a $400 generator if the EV doesn't have one, and run it while driving with an extension cord going back to the normal charging port on the car. The added drag on the RV with the EV trying to regen brake is going to use as much gas as a smaller inverter generator. My 2400 watt Westinghouse will run 6-8 hours on a little more than a gallon loaded and it automatically throttles based on load.

Bicycle Bob 12-18-2019 12:58 PM

There would be considerable hacking involved. I'm hoping to re-use the car and motor, but modernize the battery and set up the controls afresh. The regenerative braking would be most welcome on long downgrades. We have a large generator available, but that would just move the fuel requirement. I think his dream may be to park in the boondocks, and pick up enough solar to run to the store once or twice a month. He won't be fighting traffic or hauling family.
Has anyone had experience with the glorified golf carts sold for urban use?

Around 1990 I heard a guy declaring that he'd buy an electric car, if there was only one available. I pointed out that a gas station 3 blocks from his house had a dozen of those wedge-shaped city cars for sale, ringing their lot. People seem to need to see four seats to think "car!"

redpoint5 12-18-2019 01:47 PM

No hacking needed to get 120v from the traction battery of an EV if you just connect an inverter. As long as the vehicle is "on", it will maintain the 12v battery using the traction battery.

I still like the idea of turning an RV into a hybrid by using regen to brake. It would be easy to implement in something like the Bolt with the steering wheel button for regen since it's a simple open/closed switch. Splice in a relay to those wires using a cheapo RF receiver, and splice in an RF transmitter to the brake switch/light of the RV.

One downside is most EVs including the Bolt don't coast when left in D (which is required to get regen). There would be a constant drag on the RV from the slight regen.

Estimating how discharged the EV should be when starting the trip would be tricky too.

Something I'd love to play with as a proof of concept considering the low cost to implement, if you already have an RV, EV, and associated tow equipment, which I have none.

mpg_numbers_guy 12-18-2019 02:07 PM

Regarding towing behind the RV to charge...

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