# Forums

## How is the battery power drawn from the car?

If there is a thread for this I apologize but just out of curiosity, lets say there are "X" or should I say "S" amount of single cell batteries and the batteries are standing up in the pack. Is the power drawn:

1)from all the batteries simultaneously from the top of the batteries down or
2)from the battteries in the front of the pack first to batteries towards the back as the front ones get drained or
3)none of the above and some other way.

If it is number 2, seems the batteries in the back may never get used??????

Teoatawki | 13 maggio 2012

When the car is plugged in, but done charging, the controller balances the cells for max battery lifetime.

I picked that tidbit somewhere. But, I am not an EE, nor do I play one on TV.

EcLectric | 13 maggio 2012

I will try to answer from what I know about batteries. Each cell has some lithium ions in it. They 'want' to be in one location in the cell (not necessarily the top or bottom) but near one of the two electrodes. If you allow them (by providing a path for the electrons to flow from one electrode to the other outside the cell) then the lithium ions will migrate to this electrode where they want to be. As they migrate to where they want to be, causing the electrons in the external circuit to flow (through the motor and electronics), the cell is discharging. If they all get there, this is full discharge. If they are all where they don't want to be (at the opposite electrode), the cell is fully charged. It's like a spring that doesn't want to be compressed. When we compress it, we are charging it up, and then it can do work as it returns to the state it naturally wants. How badly the cell wants to return to its fully discharged state is called the voltage of the cell. I'm not sure what the voltage of a single LIon cell is, but most cells are less than 3 volts. Since the motor runs on several hundred volts, we need a way to add up the voltages in the cells. We do this by putting the cells in series, which means attaching the negative electrode of one cell to positive electrode of the next. When we hook up the cells in series, the voltages add, so if the voltage of a single cell is 3 volts and we put 100 of them in series, we get a pair of electrodes at each end of this 'battery' (the negative electrode of battery 1, and the positive electrode of battery 100) that have 300 volts between them (3+3+3... A hundred times). In the case of this 300 volt, 100 cell battery, the electrons actually have to flow through all 100 cells as the battery charges and discharges.

If the battery pack has on the order of 7000 cells, what about the other 6900 cells? Those are hooked up in parallel with the 100 cells we hooked up in series. Parallel means the positive electrode of one cell (or in this case an entire battery) is connected to the positive electrode of the other cell (or battery). In the case of cells hooked up in parallel, the same electrons do not go through both cells. The two batteries in parallel both contribute their electrons to the same external circuit to drive the motor. So an EV battery is a set of batteries (in our example around 70 of them) hooked up in parallel, with each of these batteries consisting of 100 cells ( in our example) which are hooked up in series.

So the short answer is that ideally, every cell in the battery pack is used all the time. One of the amazing things that Tesla Motors has done is to deal with the many challenges presented by such a large battery pack. For example, with 100 cells in series, all it takes is one bad cell, and that entire set of 100 cells is out of commission. The Tesla battery pack has to make sure that no single cell failure will have a noticeable affect on the overall battery pack performance.

Volker.Berlin | 14 maggio 2012

lgagliardi, you may want to read these blog posts. They are from 2006 and 2007, respectively, but the gist of it still applies to the Model S battery. Tesla did not share and likely will not share more detail that that:
http://www.teslamotors.com/blog/most-coddled-automotive-battery-ever

EcLectric | 14 maggio 2012

Volker,

Thanks for the links. I stand corrected. Apparently they reach over 4 volts per cell. Still, the principle is the same - a lot of cells need to be connected in series to provide the required voltage to drive the motor.

TonyF | 14 maggio 2012

@ EcLectric - thanks for the very clear explanation of what is no doubt a complex subject (to non-EE types). Thanks to you, I have a much better understanding of how these batteries work.

@ Volker - how do you keep finding related links like this? You must have a very very good memory. Good links.

Brian H | 15 maggio 2012

TonyF;
It's possible to search this (or any) site with any search engine by including the URL. E.g. [ battery charge site:www.teslamotors.com/forum/forums/ ]

William13 | 15 maggio 2012

Tesla engineers shared more information with a college class of electrical engineers which was posted then deleted at the request of Tesla. I agree that Tesla wants and needs to keep intellectual property private. The main idea is simple. Groups of cells are in parallel for redundancy in cases of failure of individual cells. These cell groups are then in series to achieve requisite voltage around 270 I think. The cells are fused individually and liquid cooled.

To answer your actual question, all the batteries drain simultaneously(equally) but just like in Animal Farm some batteries are more equal than others. This requires the car to sometimes reequalize. We don't know what Tesla will reveal about this on the Model S.

Timo | 16 maggio 2012

I think the voltage is somewhere between 350-400V. Otherwise your explanation sounds correct. Higher voltage allows lighter cables.

stephen.kamichik | 16 maggio 2012

I believe wire gauge depends on the current drawn.

David70 | 16 maggio 2012

But for the same power, higher voltage means lower current.

EdG | 16 maggio 2012

For those who don't know much about electricity: The power drawn at any moment is the product of the voltage and the current. Watts and Kilowatts are common units. Energy is power used over some time. So if you use your 1000 watt hair dryer for an hour, you've used one kilowatt-hour of energy.

The energy lost in wires is due to resistance to current flow. Generally, the thinner the wire, the higher the resistance. If you triple the length of the wire, you triple the energy lost sending the current through the wire. Energy lost is the current times the resistance of the wire.

So, if you want to reduce the thickness of the wires (saving expense and weight) you could raise the voltage while proportionally lowering the current to get the same amount of power sent. You'd just need better insulation on the wires.

Ohms.Law | 16 maggio 2012

@EdG: Good explanation!

EdG | 16 maggio 2012

Wow! A compliment directly from Ohm's Law! Wouldn't have imagined that would happen when I learned it!

EcLectric | 16 maggio 2012

Thanks, William!

Parallel first, then series. I think someone would have to think WAY outside the box to improve very much on what Tesla has done so far in terms of packaging cells together. Envia Systems claims to have a breakthrough in traditional Lion batteries. This would have the same setup, but much better performance characteristics (higher power/energy density, quicker charge, cheaper), but they are still in the R&D stage (several years out from production). Some others are working on 'solid state' batteries, but I believe these are in just 'R' stage.

gagliardilou | 16 maggio 2012

Thanks everyone. Seems that all the batteries are used simultaneously and are recalibrated so to speak to keep them all equal when recharged. Still seems so impressive to move a 4000lb vehicle for 300 miles on battery power. If it works day after day after day for years, it will have to change the course of the automobile industry. I believe it will.

Volker.Berlin | 17 maggio 2012

Still seems so impressive to move a 4000lb vehicle for 300 miles on battery power. If it works day after day after day for years, it will have to change the course of the automobile industry. I believe it will. (lgagliardi)

I agree. This has nothing to do with your argument, but it reminds me: We do not yet have any specifics on the Model S' actual final curb weight. It's one of the few details still missing on the Specs page.