DIY Electric Car Forums banner

1 - 20 of 23 Posts

·
Registered
Joined
·
4,302 Posts
Discussion Starter #1
Here's a question that makes my head hurt.

Suppose you have a battery that is rated at 10ah at 12 volts (perhaps a motorcycle or garden tractor battery). The terminals on that battery are probably more than sufficient to handle that load.

Now suppose you hook 100 of them in series for 1,200 volts, and hook up a 10 amp load (12,000 watts).

Why don't the terminals fry? Or, do they?
 

·
Registered
Joined
·
1,308 Posts
Here's a question that makes my head hurt.

Suppose you have a battery that is rated at 10ah at 12 volts (perhaps a motorcycle or garden tractor battery). The terminals on that battery are probably more than sufficient to handle that load.

Now suppose you hook 100 of them in series for 1,200 volts, and hook up a 10 amp load (12,000 watts).

Why don't the terminals fry? Or, do they?
current in a series circuit is all the same, each terminal only passes the 10A load. The only problem with higher voltage is the potential for it to arc, so you need wiring etc. that can handle it.

Besides, a high power car battery can put out 1000-2000A and with good terminals they don't fry either.

But many people try to design a higher voltage pack for their EV's to keep the power up and current down, if you stayed at 72v the current required to make decent power would be huge (not to mention the size wiring, and controller etc.)
 

·
Registered
Joined
·
370 Posts
I'm no expert, but the way I understand wire sizing, the thing that matters, the thing that melts wires, is the amperage not the voltage, so long as your insulation is up to containing it and not arching. It's the reason why you can use a high voltage pack and use smaller wire.

It's the same thing with power lines. They use huge voltages and relatively low amps so they can minimize the size/weight of wire they have to string along the poles.
 

·
Registered
Joined
·
1,858 Posts
Phantom they won't fry because the only voltage they see is their own. There won't be a problem unless somehow they connect to another location in the string. 1200V would product some spunky performance. Voltage is what forces the amps to go. No voltage, no current flow. Voltage is like pressure in liquids. No pressure, no flow.

I'm upping the voltage on my motor from 144 to about 167 when I upgrade to lithium. That will make a big difference in torque.
 

·
Registered
Joined
·
2,222 Posts
I'm upping the voltage on my motor from 144 to about 167 when I upgrade to lithium. That will make a big difference in torque.
What? Are you upping the amps too? If you don't up the motor current limit you won't increase your torque. You will keep the same torque to a higher rpm by just increasing voltage.
 

·
Registered
Joined
·
1,148 Posts
What? Are you upping the amps too? If you don't up the motor current limit you won't increase your torque. You will keep the same torque to a higher rpm by just increasing voltage.
You're forgetting that series DC motors (and others) are limited in the amount of current that they can draw (and hence torque they can produce) by the back EMF. By increasing the pack voltage, the back EMF at higher speeds can still be be less than the applied voltage, so the motor can be made to draw current that it would not at a lower pack voltage.

[ Edit: but only at higher motor speeds, and only up to the current limit of the controller. ]
 

·
Registered
Joined
·
2,222 Posts
You're forgetting that series DC motors (and others) are limited in the amount of current that they can draw (and hence torque they can produce) by the back EMF. By increasing the pack voltage, the back EMF at higher speeds can still be be less than the applied voltage, so the motor can be made to draw current that it would not at a lower pack voltage.

[ Edit: but only at higher motor speeds, and only up to the current limit of the controller. ]
No, I'm not forgetting and from your reply it seems you know why. Until the controller comes out of current limit it is setting the maximum motor current. A higher voltage will allow maximum torque to a higher rpm, but not a higher peak torque. You have max torque, set by controller motor current limit, starting at zero rpm and extending up to the point where back EMF (instead of the controller) limits current.
 

·
Registered
Joined
·
1,148 Posts
No, I'm not forgetting and from your reply it seems you know why.
Ok, I wasn't clear, and sorry to appear to jump on you.

I'm just saying that increasing the pack voltage could well make the vehicle feel "torqier", but as you say only at the upper end of the speed range. But it could make the difference between whimpy overtaking and decent.
 

·
Registered
Joined
·
1,858 Posts
What? Are you upping the amps too? If you don't up the motor current limit you won't increase your torque. You will keep the same torque to a higher rpm by just increasing voltage.
When I nail it now, it takes a while to get the amps up sometimes. Voltage will hopefully move things along a little faster.
 

·
Registered
Joined
·
2,222 Posts
It certainly can improve passing performance. It looks like ElectriCar will be pushing about a 14% power increase. Since series wound electric motors tend to have rather low rpm peak power point this should be quite noticeable on the freeway.
 

·
Registered
Joined
·
2,222 Posts
I'm looking for more low end torque. The truck will hit 85 already, maybe more. When I did that speed the one time I tested it, I had a terminal melting down as I drove so I was losing power there. So I can already go as fast as the speedometer shows. :D
It won't make more torque, what you are seeing is the controller limiting current. The Lithium pack should cut the weight and that will improve acceleration.

At low motor rpm the controller is limiting current to 500 amps. At some low point on the rpm curve the motor will draw 500 amps with only 40 volts applied. That is 20 kW applied to the motor so about 20 kW has to be supplied by the battery pack. That might be 150 amps at 134 volts from the battery pack. So you see 150 amps but the motor is getting the full 500 amps. This is the ramping up current you see. It will actually ramp up a little slower with the higher voltage but the motor is seeing the full 500 amps until you see 500 amps on your ammeter.
 

·
Administrator
Joined
·
6,060 Posts
Hi Electricar

When I nail it now, it takes a while to get the amps up sometimes.

That sounds to me like your controller's ramp up setting - you should consult your manual or ask at the forum
 

·
Registered
Joined
·
1,858 Posts
I hadn't thought it might be the controller CL or acceleration rate setting. I don't look at the motor amps, only the pack. Heck I was really hoping for a little more oopmh at low end. The ultimate goal for the pack change out is mileage, about a hundred if I'm lucky. Power was something I figured would just result from that. It won't be completed until likely March or so though.
 

·
Registered
Joined
·
4,302 Posts
Discussion Starter #16
You are joking right?
I try to restrict my joking to the Chit Chat section, unless it's aimed at myself - per the TOS. :)

Although I've seen many times that people put batteries in series, intuitively it just seems like there would be some point at which you needed bigger wires - even if only to over come the total series resistance of the wire.

I've studied a lot about aircraft systems over the years, but we never have more than two batteries in a circuit (e.g. 2x12v to get 24 volt). Thus, I have never considered the concept of literally dozens or even hundreds of batteries in series before joining this forum.

Part of the reason I'm asking this is because I have an idea to manufacture Edison cells using tricks employed by other manufacturing verticals - specifically, use metal foil instead of plates and wind them like the "spiral wound" batteries as a trick to increase the surface area and make them smaller. There may or may not be patents on this idea, although since capacitors do the same thing I hardly think they are enforceable just because it's a battery. But, if you have to increase the current capacity of the wires significantly the idea falls down because the "terminals" will be simply an attachment to one end of the foil strip, and might melt the foil in high current situations. The other concern I have is that if I use an insulating layer too thin then either the electrolyte won't circulate well or it will become a giant capacitor and likely sizzle, perhaps spectacularly (and perhaps painfully).

Anyway, if I make cells that are much smaller (the smallest Edison cells for sale are quite large) then logically I will need to use a lot more of them to get decent current. I want to target two banks of cells at 380 volts, or about 250 cells per bank, to make best use of a Solitron. That's a lot of cells to assemble per vehicle, even if you make a simple assembly line. Target size would be approximately the size of a 32 oz drink bottle, available in quantity very inexpensively, and materials price would be around $10/cell. It might look something like this:



In the 70's, apparently some major car maker made an Edison cell car with 200km range and highway speeds - then junked the project. They must have done SOMETHING to the cells, and my first hunch is surface area. Nickel and mild steel (iron) foils are rather inexpensive, and so I should be able to construct some test cells fairly inexpensively.

The materials should be really inexpensive. If it all works, I may sell kits to homebuilders who either can't afford LiIon, or who simply want bragging rights to say they built their own batteries that will last forever.

Well anyway, I'm not likely to get rich doing it it's more for the fun of experimentation. I'm sure if I ever tried to sell finished cells I'd get a dozen harassing lawsuits from all the regular suspects. :cool:
 

·
Administrator
Joined
·
6,060 Posts
Hi Phantom

Sounds like a good idea - don't worry about patents - 20 years there are thousands of concepts now in public domain - could be worth a search - what it is for after all

In the 70's, apparently some major car maker made an Edison cell car with 200km range and highway speeds - then junked the project

Did they shoot all of the engineers? - the guys who worked on that are now mostly retired - there is nothing more likely to make an engineer talk than an instruction to shut up!

There IS lost technology - mostly because engineers are crap at writing their reports - the immediate problem gets fixed and the data/technique used to fix it gets lost

Be interesting to see how you get on
 

·
Registered
Joined
·
1,773 Posts
Although I've seen many times that people put batteries in series, intuitively it just seems like there would be some point at which you needed bigger wires - even if only to over come the total series resistance of the wire.
Wire? Why on earth would you use wire on inner-cell connections? All those compression connectors with their high resistance just waiting too fail and start a fire.

Why not take a page from utility companies like electric and telephone and use lead plated copper inner-cell straps? Basically they are small buss bars with holes drilled in the to allow for 1/2 to 1/2 bolts to connect to the battery post.. We require the connection resistance to be less than 10 micro-ohms as tested with a DLRO.

This is not a good example of what I am talking about, just the only picture I have handy right now, but it will give you the idea.



This is a rather small 1600 AH battery @ 24 volts that only uses single bars with one-hole attachment to the battery post. Larger FLA batteries use a large tab with two or more holes, and double bars (one on each side of the post). This allows for much more secure and even lower inner-cell resistance on 4000 AH 2 volt cells. With two or more bolts you can get 5 micro-ohms and less, plus with two or more bolts greater strength and far less chance of the bars rotating loosening the lock nuts and washers.
 

·
Registered
Joined
·
2,222 Posts
Wire? Why on earth would you use wire on inner-cell connections? All those compression connectors with their high resistance just waiting too fail and start a fire.
I can tell you why, I don't know if it's a good enough reason or not. The vibrations from being in a vehicle and thermal cycling from use may break the seals at the cell terminals. Wire or braid connections remove that stress from the terminals. Sometimes bars with a bend in them are used because the bend (like a V in the middle) can reduce the stress on the posts.

I've seen flat bus bars, bent bus bars, braid and wire terminals used on battery packs. They all seemed to work out fine -- well, I didn't see any of them catch fire. :D
 

·
Registered
Joined
·
1,773 Posts
The vibrations from being in a vehicle and thermal cycling from use may break the seals at the cell terminals. Wire or braid connections remove that stress from the terminals.
Well the vibration and racking is certainly a concern, and i am no expert with EV's. But I do know from experience with Golf Carts and battery plants the compression terminal connectors, even worse mechanical pressure connectors, are a high failure rate component.

Vibration is the same regardless if it is a wire or term bar from what I can picture in my mind. Buy using term bars you can use two or more bolts and locking hardware and nuts to counter it, not too mention much higher torque values with through bolts and nuts.

I am just thinking out-loud and passing my two-cents worth.
 
1 - 20 of 23 Posts
Top