Hi Dominant,

Let me try to explain the issue you are dealing with:

Each cell of a DeWalt pack is 3.3V and 2.3Ah as you quoted.

In a pack, the 10 cells are all connected in series, which

has the effect that the same current goes through each cell.

So, if one cell can deliver 2.3 Amp for 1 hour (2.3 Ah) then

each of the 10 cells can deliver that 2.3 Amp for 1 hour

before they are empty. Since the same current goes through

all of them, they all reach 'empty' at about the same time

after 1 hour of 2.3 Amp, so it is correct that the pack

has the same Ah rating as the single cell.

Only if a pack would have cells in parallel would the Ah

rating increase. For example if it had 2 strings in parallel

then each string could deliver half the current, so it would

last twice as long; or you could draw double the current for

the same time, in both cases the pack would have 4.6 Ah

because two cells parallel have their 2.3 Ah added up.

But then the pack would need 20 cells (2 strings of 10) to

get this double capacity - this is the same thing as having

two complete packs connected in parallel and drawing half

the current from each pack.

Now, what is the advantage if the Ah capacity does not increase

to connect so many cells in series?

The reason is that the power of the pack is defined by the

current multiplied by its voltage: P = I x U

In terms of the DeWalt pack, say that you are want to see

how much power you get when draining a cell and the pack

in 1 hour:

1 cell: Power = 2.3A x 3.3V = 7.59 Watt.

So, after 1 hour the cell has delivered an amount of energy

of 7.59 Wh (Watthour)

The pack: Power = 2.3A x 33V = 75.9W

After 1 hour the pack delivered 75.9 Wh.

So, you see that the power available to drive tools or to

propel a vehicle goes up with either current or with the

voltage. In the DeWalt packs the current capability was

apparently sufficient (the current will be much higher than

in my example, but the Ah can stay the same if the power of

the two stays equal: 23 Amp for 6 minutes is still 2.3 Ah)

If you have 10 cells you can connect them it at least 4

different ways, but you can calculate that each way delivers

the same power and energy:

1. One string of 10 cells, like in DeWalt pack.

The current is the same as for a cell, voltage is 10x as large

2. Two strings of 5 cells (current doubles, voltage is 5x)

3. Five strings of two cells (current 5x, voltage 2x)

4. All ten in parallel (current 10x, voltage same as cell)

The way a pack is connected is a design decision, dependent

on what the motors need that they want to feed with this pack.

In addition, the higher the voltage, to lower the current so

the less stress on wires and contacts, also the losses in

motor controllers tend to be lower at 33V than at 3V while the

controllers are cheaper.

Hope this clarifies,

Cor van de Water

Systems Architect

Proxim Wireless Corporation http://www.proxim.com

Email:

[email protected] Private: http://www.cvandewater.com

Skype: cor_van_de_water IM:

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Tel: +1 408 542 5225 VoIP: +31 20 3987567 FWD# 25925

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-----Original Message-----

From:

[email protected] [mailto:

[email protected]] On Behalf Of Dominant

Sent: Sunday, September 23, 2007 8:12 AM

To: Electric Vehicle Discussion List

Subject: [EVDL] A123 M1 Capacity question

G'Day all! I've been lurking for a while, and just had a question about the a123 M1 cells found in the DeWalt 36V battery packs. This has almost certainly been covered before, but I can't remember the answer.

The a123 website says the M1 cell (found in DeWalt packs) is 3.3V and 2.3AH, yet I've read on web forums and the like that the DeWalt pack is also 2.3Ah. Surely both can't be correct? I was of the understanding that, wether connected in Series or Parallel, capacity was the combined total, ie, 2.3*10=23Ah, for the DeWalt packs, just like the Voltage is 3.3*10=33V (36V). I understand that Voltage is different depending on configuration (33/36V for Serial, 3.3V for Parallel).

Cheers.

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