DIY Electric Car Forums banner

1 - 8 of 8 Posts

·
Registered
Joined
·
661 Posts
Discussion Starter · #1 ·
My master plan for a cheap EV are coming along. I've come to grips with the fact that you to get the range that I'm looking for at a price I can afford, that I'm going to have to work with a hybrid solution. While poking around trying to see what other solutions folks have generated I came across this gem:

http://www.motherearthnews.com/Gree...-01/An-Amazing-75-MPG-Hybrid-Electic-Car.aspx

The numbers just don't seem to add up.

2100 lbs vehicle with a direct drive 400 amp aircraft starter for an engine. Powered by a 48V battery back coupled with a [email protected] genset powered by a 5 HP lawn mower engine. Claims to get 75 MPG with regen braking.

It sounds too good to be true. They throw out numbers like these:

. . because of the fact that the motor has a low draw at cruising speeds: only .23 amps at 1,800 RPM. Since the Briggs & Stratton engine turns at a fixed rate and can generate 100 amps at about 28-1/2 volts, normal driving presents no problem. Hot or traveling in a very mountainous area-could, however, tax the car's charging system . . . but even these demands don't pose much of a problem, because the batteries can be brought from a 1/4 charge (the effective "dead" state, with a built-in safety factor) to a full charge in only 15 minutes.
Crusing @ 6W? Recharge in 15 minutes? 50+ MPH at 1800 RPM?

I just don't like the smell of what I'm sniffing ! :confused:

So what exactly am I missing? Or should be looking to lift an old lawn mower engine immediately?

ga2500ev
 

·
Registered
Joined
·
1,562 Posts
dinosaur stuff I KNOW: I think a typo but at 23 amps he is making about 3 HP, only enough to move a go-cart.

yuppa you CAN recharge "dead" batteries in 15 minutes, but they are gonna get REALLY HOT, and life expectency will be very short. I doubt the motor generator can run at 200% duty cycle that long.

Mother Earth news: HMMM many and extensive articles of questionable data usually not provable by my experimentation.

the numbers don't seem to add up because they dont add up.

OTOH, he could be only going about 1/3 mile per hour.

MY $.02 your mileage can and will vary.
 

·
Registered
Joined
·
183 Posts
My master plan for a cheap EV are coming along. I've come to grips with the fact that you to get the range that I'm looking for at a price I can afford, that I'm going to have to work with a hybrid solution. While poking around trying to see what other solutions folks have generated I came across this gem:

http://www.motherearthnews.com/Gree...-01/An-Amazing-75-MPG-Hybrid-Electic-Car.aspx

The numbers just don't seem to add up.

2100 lbs vehicle with a direct drive 400 amp aircraft starter for an engine. Powered by a 48V battery back coupled with a [email protected] genset powered by a 5 HP lawn mower engine. Claims to get 75 MPG with regen braking.

It sounds too good to be true. They throw out numbers like these:



Crusing @ 6W? Recharge in 15 minutes? 50+ MPH at 1800 RPM?

I just don't like the smell of what I'm sniffing ! :confused:

So what exactly am I missing? Or should be looking to lift an old lawn mower engine immediately?

ga2500ev
http://www.motherearthnews.com/Green-Transportation/1980-09-01/Mothers-Own-Hybrid-Car.aspx

I found a follow up to that article on the same site when they tried to build their own and test it out.
The 23 amps is completely wrong.
I think the recharge in 15 minutes thing was the guy reading the surface charge on the batteries, if you actually tried to discharge them the voltage would drop back down instantly.
They built their own that got 85 mpg, and could go 45 mph, 50-55 downhill. When the engine/generator only (dead batteries) they could still go 45mph and get 60 mpg.
The original that claimed 90 mph was also speculated to be downhill.

Nowhere in any of the articles do the mention the depth of discharge put on the batteries.
 

·
Registered
Joined
·
661 Posts
Discussion Starter · #5 · (Edited)
http://www.motherearthnews.com/Green-Transportation/1980-09-01/Mothers-Own-Hybrid-Car.aspx

I found a follow up to that article on the same site when they tried to build their own and test it out.
The 23 amps is completely wrong.
I think the recharge in 15 minutes thing was the guy reading the surface charge on the batteries, if you actually tried to discharge them the voltage would drop back down instantly.
They built their own that got 85 mpg, and could go 45 mph, 50-55 downhill. When the engine/generator only (dead batteries) they could still go 45mph and get 60 mpg.
The original that claimed 90 mph was also speculated to be downhill.

Nowhere in any of the articles do the mention the depth of discharge put on the batteries.
Thanks for the followup. I'm reading the article now...

Now those numbers make more sense.

What I'm trying to figure out is the necessity of having the genset power the car all on its own. Let me throw out a hypothetical configuration:

Say for the sake of argument you build a 96V EV system using 8 12V 125Ah batteries for a vehicle that averages 400 Wh/mile. You'll get 24 miles of range on a vehicle.

To hybridize it you couple it with a 5Kw genset and charger like this one:

http://www.winnfreenet.com/shop/cat...d=188&osCsid=c3f368205dcac48ebe6bdbbad1f8de69

A lovely price of $99 used plus shipping to get a generator head that'll produce 5004 W (120VAC @ 41.7A) when driven by a 7 HP or higher engine (5004W/746W/HP = 6.7 HP)

I'm missing a magic number here in my estimate which I'd be glad to have someone fill in: How to backcompute the average amperage @ 96V for a vehicle that's consuming 400 Wh/mile? But I'm going to operate on the presumption that's more than 53A, which is the maximum power the genset can produce @ 96V.

The bottom line is that the genset cannot produce enough power alone to drive the traction motor at the amperage it needs. Say for the sake of argument that the traction motor needs 75A to motor down the road. The genset can prove 50A while the batteries provide the additional 25A. This is helpful because since the battery amperage is lowered, more power can be extracted from the pack due to Peukert.

Since the load on the batteries has been lowered from 75A to 25A, the pack should now provide more than 3 times the range.

What I like about the system is that it's a true series hybrid. If you know that you're only going on a short run of 8 miles, you can run it all electric and leave the genset off. However if you know that you're going to be running for 60 miles during the day, fire up the genset and get extended range in hybrid mode.

Have I missed something big in this equation?

I'm trying to figure out cost effective ways to get decent range on a primarily electric vehicle. I can't afford a lithium pack and range over 50 or so miles isn't effective without using literally a ton of lead. A 10 HP diesel engine and genset weighs in at 120 pounds and it doesn't have to be run all the time.

Looks like a winner to me.

ga2500ev
 

·
Registered
Joined
·
183 Posts
400 Wh / mile * 45 miles/hour = 18 kWatts.
Watts / volts = Amps
400 Wh/mile * 45 miles/hour * 1/96 v = 187.5 Amps. But it would be higher because voltage will be less than 96 as your discharge them.

So with 25% of the power needed coming from the generator, your range only goes up 25-33% So the 25 mile range is now 31-33 miles.
The generator needs to come closer to matching the current needed to move the vehicle or it doesn't seem as worth it.
 

·
Premium Member
Joined
·
882 Posts
You also need to note the inverting losses from an AC generator to the controller/batteries. It would be much more efficient to get a DC generator made, but the voltage would have to be higher then the batteries or somehow be matched to the batteries to avoid weird stuff going on. It would function the same as an alternator charging a 12V battery running the accessories, except at a much higher voltage.

The other thing to note is whenever you stop you have time to 'catch up' on charging the batteries, while a series hybrid might not make sense for continuous highway driving it might be possible for longer ranges with traffic lights and lower speed driving or doing 60 miles and stopping for 15 minutes
 

·
Registered
Joined
·
183 Posts
Ahh, I forgot about being able to recharge while coasting/slowing/stopped. If you do a good amount of stop and go, it might boost the range by more than the 25-33 I estimated. I knew it was to easy to say 25% less current from the batteries = 25% more range.
I think a 10kW generator would be a better choice. However if you go with an ac generator you need to come up with a way to convert it to dc, and it needs to be able to handle the current the generator can make.
 
1 - 8 of 8 Posts
Top