It's very easy to test a battery. Take it out of the pack and try to charge it. 
It will rise in voltage or not. You BMS boards probably have a minimum voltage 
to operate and cell fell below that voltage and does not imply the cells is 
bad. After you charge it then hook up to a know load and measure the capacity 
of the cell.

Sent from my iPhone

> On Aug 3, 2017, at 3:34 PM, mnevans via EV <> wrote:
> I'm new to this discussion list, so I ask your patience: my understanding of
> EV systems and Li-ion batteries is limited.
> The question is: replace a bad battery in a pack of 19, or make the pack 2
> batteries smaller and suffer an unknown loss of voltage and range?
> Background:
> I think I have a bad battery in one of 19 batteries that is in the second
> bank of a 4 bank x 19 CALB CA40FI battery pack.  The battery pack is used in
> a 10KW battery that is part of a Plug-In Supply conversion of a 2004 Prius
> to a plug-in Prius.  Robb Protheroe installed the system in late 2014, and
> it has seen about 10K miles, maybe 60% of that in all electric mode, about
> 900 partial or full charge cycles.  
> The evidence that the battery is bad is an incomplete loop circuit.  I found
> that the battery management board atop the suspect battery was not powered,
> and moving that BMS board to another battery adjacent powered it.    The
> suspect battery was at 2.67V.  The others adjacent are at 3.0V.  Originally
> they were all balanced at 3.26-3.27V. 
> Robb Protheroe at Plug-In Supply suggested replacing the battery with a new
> one.  Unfortunately CALB no longer makes this 40Ah battery, but the Sinopoly
> or Winston 40Ah prismatic cells are nearly the same dimensions and
> specifications.  It looks like I can get a replacement for about USD$60 
> including shipping to me in Maryland USA, using (a US
> supplier wanted $42 shipping for a $55 cell).  
> The other option is to remove the bad battery and the one adjacent, and
> carry on with a lower voltage pack.  Robb is unsure whether the range loss
> would be proportional (7%, I can live with that), or more, due to "voltage
> sag" and age of cells.  He suggested I ask you, so here I am.
> Questions:
> 1. Would you recommend replacing or removing batteries from my pack?
> 2. If removing, what is your guess on loss of range due to lower pack V,
> voltage sag?
> 3. If replacing, do you expect the new cell to assume the lifetime of the
> rest of the batteries in the pack, as others have written on this listserv?
> So far I am thinking to try removing, see how the range is affected by
> testing on a known route that is longer than the original hybrid range with
> this pack installed.  If unacceptably worse, add back the replacement cell. 
> If not, carry on with the slightly smaller battery pack.  But I would be
> grateful for the wisdom of the list.
> Thank you in advance for your help,
> Mike
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