# Parallel LFP Arrangements.



## Sunking (Aug 10, 2009)

I am curious why Many EV's parallel lithium batteries in a Ladder Type arrangement. Say 2 cells paralleled, in series with 2 more cells in parallel. As opposed to Pb batteries where you first series your batteries to desired voltage, then in parallel with identical strings to meet required AH capacity.

My only thought is economics. Example a 48 volt battery in two series string would require 32 BMS control boards as opposed to 16 if used in a Ladder configuration. 

Is that it, or are there other reasons?


----------



## major (Apr 4, 2008)

Davide covered some good points on the subject here: http://elithion.com/pdf/BPC2012Elithion.pdf


----------



## dougingraham (Jul 26, 2011)

Sunking said:


> My only thought is economics. Example a 48 volt battery in two series string would require 32 BMS control boards as opposed to 16 if used in a Ladder configuration.
> 
> Is that it, or are there other reasons?


I would call it a safety concern. In the parallel first example if you have a cell that develops a soft short it will discharge its paralleled neighbors. This will result in a pack that has that parallel assembly ruined. If the same thing happens in a series first arrangement then the other strings will over charge every cell in that series string with the shorted cell. And Lithium cells don't like being overcharged. Both cases have issues when you connect a charger up but the series first arrangement has the issue all the time. If a single cell goes open circuit the current in the rest of the cells only in that paralleled block is increased. If it exceeds the cell ratings then only the cells in that one block are affected. You can continue to drive and only ruin that one block of paralleled cells. If you do this with the series first arrangement then all the cells in all the remaining strings will see the increased current and all will suffer from the additional loading.

The economics of not having to have multiples of cell monitors is however a huge one and the only thing you would gain would be the ability to detect exactly which cell has failed instead of just which paralleled block of cells has a failure in it.


----------



## Sunking (Aug 10, 2009)

dougingraham said:


> The economics of not having to have multiples of cell monitors is however a huge one and the only thing you would gain would be the ability to detect exactly which cell has failed instead of just which paralleled block of cells has a failure in it.


Thanks. That would be fairly easy to figure out. A simple heat scanner would spot it.


----------



## Sunking (Aug 10, 2009)

major said:


> Davide covered some good points on the subject here: http://elithion.com/pdf/BPC2012Elithion.pdf


Excellent. Thank you.


----------



## Sunking (Aug 10, 2009)

major said:


> Davide covered some good points on the subject here: http://elithion.com/pdf/BPC2012Elithion.pdf


Major or really anyone can you explain the Short Discharge Time Graph? Th eone near the end with Blue lines for various battery manufactures.


----------



## Tomdb (Jan 28, 2013)

Complete theoretical graph.

Build up as follows, How long would it take to discharge the cell if shorted. 
When shorted the only resistance burning up the energy stored in the battery is the *internal resistance* of the battery.

This is in proportion to the power capability of the battery or otherwise specified as C rating.

Lower short discharge time -> lower resistance -> higher "C" rating

This further on in the discussion is used to indicate what happens when different soc (voltages) batteries are connected and what would happen. 

I would suggest topping all cells before connecting them in parallel, and just to be extra safe make one connection then use a 12volt bulb to balance the other connection before making it permanent.


----------



## Sunking (Aug 10, 2009)

Tomdb said:


> Complete theoretical graph.
> 
> Build up as follows, How long would it take to discharge the cell if shorted.
> When shorted the only resistance burning up the energy stored in the battery is the *internal resistance* of the battery.
> ...


THX, that confirmed my thought it was Ri


----------



## major (Apr 4, 2008)

Tomdb said:


> Complete theoretical graph.
> 
> Build up as follows, How long would it take to discharge the cell if shorted.
> When shorted the only resistance burning up the energy stored in the battery is the *internal resistance* of the battery.
> ...


Thanks Tomdb,

I see it as a means to normalize Ri with regards to C. Sort of a time constant parameter.

And I think actually: 
Lower short discharge time -> lower resistance -> higher "C-rate" rating. The short discharge time or Ri values do not affect the value of C. That remains unchanged.

For example: For a 90Ah cell, C = 90, regardless of the Ri or SDT. Maybe it has a continuous C-rate of 3C. A different version of that 90Ah cell still has C = 90, but has a lower Ri (and SDT) therefore has a higher continuous C-rate, perhaps 5C. 5C > 3C. C-rate ratings.


----------



## Tomdb (Jan 28, 2013)

major said:


> For example: For a 90Ah cell, C = 90, regardless of the Ri or SDT. Maybe it has a continuous C-rate of 3C. A different version of that 90Ah cell still has C = 90, but has a lower Ri (and SDT) therefore has a higher continuous C-rate, perhaps 5C. 5C > 3C. C-rate ratings.


Major,

You are absolutely correct by "C" rating i did indeed meant discharge rate not the energy capacity. I usually use Q or E to dictate capacity to reduce confusion with discharge rating C (dimensionless value).


----------



## major (Apr 4, 2008)

Tomdb said:


> Major,
> 
> You are absolutely correct by "C" rating i did indeed meant discharge rate not the energy capacity. I usually use Q or E to dictate capacity to reduce confusion with discharge rating C (dimensionless value).


Yep, C should stand for *C*onfusion  Is it units (Coulombs)? Temperature (ºC)? Or


----------



## Caps18 (Jun 8, 2008)

How often do these cells go bad though?


----------



## dougingraham (Jul 26, 2011)

Caps18 said:


> How often do these cells go bad though?


This is an excellent question. And the answer is that if you don't abuse them and you got good ones to begin with they don't suddenly go bad. The reliability is probably higher than the reliability of the electronics people try to put on them to protect them.

There are aging issues where they lose about 1.5% of their capacity per year if they just sit. And there are wear issues from charge/discharge cycles. The result of both of these are a reduction in capacity. Neither results in a direct failure shorted or going open circuit.

The biggest abuse issue is over discharging such that a cell reverses polarity. This is pretty much death for a cell. They become resistors when this happens without much other drama. (Bottom balance lessens this to make it almost a non issue.) The other abuse issue to be concerned about is overcharging. At a minimum you see a loss in capacity which makes a future over discharge event more likely and at the worst you see cell failure eventually resulting in a fire that is very difficult to put out. I get asked about cooling because people hear about this and their own experience is that batteries get hot when used. Heat has been shown to increase the aging issues but this is temps of 50C (122F) or greater and I have never managed to get my cells that hot even in the summer on days where the air temp was 38C (100F) and I was doing back to back 1000 amp runs. Difficult to get LiFe prismatic cells hot if you stay within the ratings.


----------

