# Do these batteries match these BMS chips?



## e-ghia (Mar 12, 2008)

Trying out the idea of building my own battery packs and finding a management system for all the packs.

I am new to all this so bare with me:

These batteries (or something similar)
*Tenergy Li-Ion 18650 3.7V 2600mAh*

And a management system like this:
Multi-Cell Li-ion Battery Pack OCP/Analog Front End

Atleast I think that is a complete management system...

I am thinking of having a couple thousand of these little batteries and would like a management system setup so I can monitor each cell individually.

What are your thoughts?


----------



## Madmac (Mar 14, 2008)

As the term 'Analogue Front End' says these are devices that interface to a small number of individual cells and allow a low cost micro to monitor them.

Monitoring every cell in a large pack of 18650 cells is not viable. The approach is to monitor each parallel group in the series connected string.

In a commercial system like Tesla the total number of cells is divided into a number of groups ( or cakes) each arranged to output the full DC rail voltage.
Each cake would have 7 or 8 cells connected in parallel and then 94 of these connected in series to give a DC rail of around 340 Volts. Each cake has a BMS that does the usual monitoring / charge balancing. 

The other lower cost approach is to wire the required number of cells for the capacity in parallel ( 40 for approx 100Ah) and then in series packs. This is a much lower cost solution as only one BMS channel is needed per parallel string.

The advantage of the multiple cake approach is that parts of the battery pack can be isolated if a fault occurs. Only part of the capacity is lost. It would be possible that the fault could be hardly noticed by the customer until the car is serviced. With the low cost approach a fault means you grind to a halt. 

I have just finished the hardware design for a system using 18650 cells. The design is based on 25.2 volt packs (7 cells in series) with parallel cells of up to 60 (the number can be selected to give desired capacity). This limit is down to maximum of 150AmpHr. Each pack has a BMS circuit using the Intersil parts you linked to along with a Silicon Labs micro controller and optical isolated interface. Charge balancing is implemented using the flying capacitor method with optomos switches. A 10 way connector will link the 14 modules together and to the master controller.

Current state is hardware working in a test pack of 10 cells in parallel x 7. First firmware is running and controlling charging. Next stage is to redo the PCB layout and decide on the final mechanical shape of the pack. I will post the circuits and PCB layout when done (4/6 weeks).

Madmac


----------



## xrotaryguy (Jul 26, 2007)

If you don't mind me asking, what type of car are you building/converting? I took a tertiary look at these batteries for my needs, and I would need to purchase $30,000 worth of these batteries to beet my needs. You said that you would buy a "couple thousand" of these batteries. Are you building a smaller EV?


----------



## e-ghia (Mar 12, 2008)

Madmac,

Your response gave me lots of new information that I will have a friend look at this afternoon and help me relate the points you made to my specific project. Lots of good knowledge, thanks!!

xrotaryguy,

That many?!? Wow, I guess I don't understand about what the battery specs actually mean just yet. And I thought I was beginning to make sense of all this! 

My project is a VW Karmann Ghia and the final expectations are similar to the Tesla. I want a sub-8 second 0-60 time, range of +120 miles, and enough reserve power to allow uphill driving, passing, and merging into traffic.

I am still at the beginning stages with my learning. Once I understand what the numbers mean I should be able to assemble my own components which affect me specifically rather than a ready-made product that I must fit my project around.

And thanks for the heads up that my figuring was wayyy off!

Phillip


----------



## Madmac (Mar 14, 2008)

xrotaryguy said:


> If you don't mind me asking, what type of car are you building/converting? I took a tertiary look at these batteries for my needs, and I would need to purchase $30,000 worth of these batteries to beet my needs. You said that you would buy a "couple thousand" of these batteries. Are you building a smaller EV?



Taking the example of the Tesla again, that has just under 7000 cells of 2.2 Ahr 18650's. They can be purchased direct from China ( a known brand with PTC and interrupter) for around $2 each in that quantity, total of $14K. 

Tesla will get a good price .... their annual buy will be in the order of 14 million cells.

Madmac


----------



## e-ghia (Mar 12, 2008)

Madmac,

That's what I was thinking with the battery link I posted. If I bought 2000, for example, I could probably get them for about $4 each. So that would be $8000 to state the obvious.

A pack or cake of eight cells in parallel would follow Madmac's example. 94 of these cakes would provide around 340 volts and that's only using 752 cells. The remaining 1248 cells (2000-752) would dictate my range. Correct?

If the above thinking is correct, xrotaryguy, where does the $30k in cells come in? I am probably over simplifying things, would you mind explaining your calculations? 

Thanks, guys!

Phillip


----------



## Madmac (Mar 14, 2008)

E-gia, I think you slightly misunderstand the battery configuration. The term cake has been used to mean one collection of cells, for example 8 cells in parallel and 94 of these in series for a total of 752 cells. This cake can be considered a single battery which is 347.8 volts at 21.6 Amphour.

To increase the range you wire multiple cakes in parallel. Four in parallel would use 3008 cells and give you the same 347.8 volts but now at a capacity of 86.4 Amphour. (nine in parallel would give a system with almost the same capacity as the Tesla unit)

Each cake needs to have a BMS system to monitor the 94 sets of batteries, a total of four BMS's. If one cake has a problem the others would still work.

The low cost approach would (taking the example of 3008 cells above) wire 32 cells in parallel and then 94 of these. The end result can be considered a single battery of the same specification (347.8 volts at 86.4 Amphour). A single BMS would be needed.

18650 cells vary in capacity form 1.2 to 2.7 Amphour. Dividing cost by capacity the 2.2 Amphour gives the highest return. To get the most capacity out of a system then use the 2.7 Amphour versions.

On a safety note the construction of cells vary. Some offer no protection, others a PTC or solid state fuse that opens circuit if current or temperature gets to high and resets if things return to normal. Finally the best makes also have a mechanism to disconnect the cell permanently if the internal pressure rises.

The advantage of using large numbers of small cells are:

> If a single cell has problems it will make almost no difference to the battery pack.

>As these cells are increasing in use, manufacturing volumes are growing very fast and they will show the fastest price drop.

> Easy to configure capacity to fit the space available.

> Capacity v weight is better than most large single capacity single cells.

Madmac


----------



## e-ghia (Mar 12, 2008)

Thanks for taking the time to explain it to me, Madmac. Using your numbers the four cakes of batteries alone without any management system is around $12k. Bump up to nine, which would mostly likely end up bring me my desired results, and now you're looking at $27k. Ouch...

I didn't want to spend that on the whole project. <sigh> Time for more research.

So is the problem causing the inflated price the cost of the cells I chose or is it because I am trying to use tiny individual cells rather than the large lithium cells?

Phillip


----------



## Madmac (Mar 14, 2008)

Using 2.2 Amp hour 18650 cells and around 6000 of them will give you the parameters you want but maybe not at the same time. If you drive sensibly you will get the range.

Buying cells direct from China you can get them for around $2 each, at the moment. That means a pack will cost around $12k plus shipping and building into a pack. Battery management system is not expensive in parts, around $180, but to buy in is quite pricey.

Madmac


----------



## e-ghia (Mar 12, 2008)

Madmac said:


> ...you can get them for around $2 each, at the moment. That means a pack will cost around $12k plus shipping and building into a pack. Battery management system is not expensive in parts, around $180, but to buy in is quite pricey.


That's the dollar amount I was thinking I would spend on batteries. Whew! I got scared for a minute there. 

You said it would give me the parameters I want but not at the same time. So I can either have range or acceleration or torque to pass/climb. What is limiting me here this time? The number of cakes again as in reducing the number from nine cakes to six?


----------



## Madmac (Mar 14, 2008)

Acceleration and high speed uses more energy. The losses are not linear with speed. So to increase range drive at 60mph. Drive at top speed when you want to show off or overtake.

Batteries are not yet at the state were they are an alternative to an IC engine. They can perform to a very usable limit. They cost more up front but have very low running costs. With fuel at current rates it probably does not work out as money saving.

Madmac


----------



## e-ghia (Mar 12, 2008)

Madmac said:


> With fuel at current rates it probably does not work out as money saving.


That is my intent, to knock a dent in the $430/month gas bill on top of oil changes, maintenance items which wear more quickly, and uncertainty of rising fuel costs.

I will continue to research and may find a happy median which will save me money in the long run but still be enjoy able to drive in traffic.

Phillip


----------

