weird car charging system for a weird electric car

[wirednuts]

ok, without getting into detail (because there is none) i have been proposed a problem by a relative of mine

he has a car he made electric. 10x12v batteries wired in SERIES to get 120v dc to a direct drive motor (where he got a 120v dc motor who knows)

so he can charge it with a basic charger off a wall socket (not sure how since he needs 120v dc, but he is certain this part is taken care of)

then he says he wants to haul a trailer with gas engine generator that can put out 120v dc @ 100a or so to charge his 'custom' battery pack.

now, my first thought was "in series? seriously? why not in parallel with a pwm box that can step up the voltage


...... ok you know what? this is ridiculous. i dont know why i get asked these questions. supposedly he is trying something groundbreaking but i think its 120 years late. if anyone wants to add funny comments please do. this is bad

 

[C1]

If it's a labor of love, then let the relative work out the details & do the trial & error learning.

Otherwise: http://www.e-volks.com/

There are all kinds of electric conversion services available which have done the research & worked out the necessary details.

 

[pm]

Series charging should work... Parallel would work too, but would take a long time due to amperage limitations. I'd charge it series too if it was me. The nice thing about lead acid is that you don't need to balance them well... so series charging works fine.

I wouldn't think you'd get a generator that could generate 120V DC - generators always use alternators and they are AC... And 100A... that's 12000kW.. that's an insanely large generator. It would be more than $2500.

You'd want to be very careful - very careful - charging at a fast rate... and 100A is going to be a fast rate unless the cell is over 300Ah... (anything over C/3 is considered a "fast charge" for lead-acid). Since I've never even heard of a 300Ah battery (even 100Ah is considered pretty big), I'm going to assume he's trying to charge at something higher than C/3. That can get tricky. You want to be careful about exceed more than 80% capacity during the constant-current charge stage or you risk excessive out-gassing which could lead to dramatically lower battery longevity (best case), or worst case an explosion and fire.

But as far as the original question, I think series is fine. Just make sure that he makes the charger at least somewhat temperature aware because lead-acid charging depends on the temperature of the batteries... it's not a good idea to just shove 138V into 10 series 12V batteries. You want to adjust the charge voltage based on the temperature of the cells. And you want a constant current/constant voltage dual-charging system if you want maximum capacity.

And, yeah, as C1 says, I'd suggest asking at a dedicated electric car forum. I've built 3 EV's so far, but I'm far from an expert on lead-acid. I just know the basics (my first EV was lead-acid based). I do know first-hand about building a non-temperature aware charger though... I cooked one pack pretty badly because I charged when it was really, really hot.

 

[wirednuts]

Hes in way over his head.... i agree he should learn that fact by failing. Or like you said just buy a premade kit.

 

[VirtualLarry]

Hes in way over his head.... i agree he should learn that fact by failing.

Given the subject matter that we are dealing with here, that sounds unwise and even dangerous.

Just my impression.

 

[FloatingSpots]

Actually, this is about right.
Charge voltage on lead acid should be in the 2.3 - 2.4 volts per cell.
This puts the charge voltage for a 60 cell string (120 vdc) at 138-144.
Equalization voltages would be in the 150 ~153 vdc range.

120 VAC run through a full wave rectifier will result in ~ 169 vdc.
A variac will allow one to adjust charge voltage down into the correct range (on the cheap) without an expensive electronic charger.


Series versus parallel:
Think of voltage equaling torque and current equaling longevity or driving range.
The higher the voltage, the more oomph.
The commercial hybrids / electrics are often around 300 vdc. (However, these often have 3 phase AC drive electronics.)

It is much easier at these voltages to use a PWM drive controller that bucks the battery voltage down to drive voltages. Step up would be much more costly, complex, large, and less efficient. Even the commercial systems that create 3 phase AC drive signals are creating bucked down drive signals (less than the battery voltage).

The easiest systems to put together are the PWM controller, DC motor systems in this voltage range.

Charging: C1 charging on lead acid is an absolute no-no.
C/8 is about as fast as I believe you want to go with deep cycle batteries with any reasonable life span.
If you are hauling a generator behind your vehicle, you are most likely losing any efficiencies that you would be gaining. My opinion would be to run the electric on short hops around the home and use the regular vehicle on longer runs.

 

[Modelworks]

Given the subject matter that we are dealing with here, that sounds unwise and even dangerous.

Just my impression.

DC is generally safe to work with , but when you start to get in the current ranges we are talking about here you better know what you are doing or have a life insurance policy paid in full.

 

[imagoon]

DC is generally safe to work with , but when you start to get in the current ranges we are talking about here you better know what you are doing or have a life insurance policy paid in full.

Arc welding voltages and currents are fun right?

That battery string could vaporize a wrench so yeah be really careful.

 

[pm]

Arc welding voltages and currents are fun right?

That battery string could vaporize a wrench so yeah be really careful.

Yeah, I vaporized a really nice set of wire cutters one time... I was swapping connectors and I just snipped both wires at the same time. Not my finest hour. Being extremely careful around big batteries is a very good idea.

 

[wirednuts]

well thats the other part he doesnt get yet. he wants to put a gas powered 120v/100a charger on a trailer, which means you need a flexible disconnect with at least 10ft of copper to deliver that power to the batteries.. dangerous is just the first problem.....