[ausev] Individual battery meters vs. single battery pack meter..?

[Brian Lasseter]

On Tue, Jan 13, 2009 at 6:38 PM, Vern Graner <vern at txis.com> wrote:
> So far, I understand what they display. :)

That's a good start.


> What I don't understand is what you can *do* about any discrepancies you
> see in the display?

Well... I'll use myself as an example.  Lead acid batteries are always
made of 2V cells.  http://en.wikipedia.org/wiki/Lead-acid_battery  My
8V batteries have 4 cells per battery, regular 12V batteries have 6
cells per battery.  I have 18 batteries, but really 72 individual
cells powering my electric car that I want to take care of.

So... lead acid battery cells need 2.50V per cell to charge, they are
nominally full (after sitting for a while after charging) at 2.15V per
cell, they are empty at around 1.5V per cell, and you could damage
them if they go below 1.2V per cell.

Knowing those numbers I know that my 72 battery cells which form a
144V battery pack charge at 180VDC, they are full when they are
sitting around and show 155VDC, they are empty when they show 110VDC,
I really should not push the car to a voltage below 110VDC if I can
help it, and I should never, ever, ever push the voltage below 90VDC
(because the voltage of the cells could reverse).

Now... if my car seems to be hitting 110VDC prematurely... I might
presume that I have a battery going bad... but how could I find that
out?  Knowing the whole pack is at 110VDC only tells me that the
average cell voltage is 1.5VDC.  Some cells could be 1.8VDC (which is
fine) but some other cells could be 1.2VDC (which is very bad).  So...
to diagnose this I could run the car for a bit, and then stop and run
around the car real quick with a volt meter taking individual battery
measurements to see if any of the batteries are at (or below) 4.8VDC
(4x 1.2VDC).

If I had a Paktrakr (or something like it to monitor individual
battery voltages) I could see a single standout battery while I am
driving the car... and I could take action when I get home.  Also... I
would likely notice one standout battery long before it becomes a
problem, and long before I actually damage the battery.


> For example, if the multiple battery meter shows me that battery #3 is
> .6 of a volt lower than battery #4 and #5, and battery #1 is .8 of a
> volt higher than battery #2, what do I need to do about that? Is this
> really more of an "fyi" type thing?
>
> Also, if I do see discrepancies in voltage between the batteries, is
> there a way to "fix" that or, do I just watch the pretty lights till one
> battery is reading something like 2 volts below all the others and then
> buy a new battery to replace that bad one?
>
> Or, do I use the info from the display to know to target a specific
> battery in the pack for an individual charging session thereby giving
> this "lagging" battery an extra shot of juice to keep it alive longer..?


Ideally you would have a battery monitoring system that would monitor
all cells at the same time... but for practical reasons you can only
monitor each battery at the same time.

Basically... you want your battery cells to all have the same voltage,
if any one or two are getting out of sync, you could try charging
those batteries individually to try to help them.  How far out of sync
is bad depends on your batteries... but certainly you want to make
sure that each and every battery is mostly even and that no single
battery cell gets below 1.5V per cell  (6VDC for an 8V battery, and
9VDC for a 12V battery).


> I'm just curious how such a meter is used and how essential/useful it is.

It is pretty useful.  I would be able to use it to see when individual
batteries are low on water rather than checking the water level of all
72 battery cells in my case.  (Low battery water increases resistance
and decreases voltage.)   I would have gotten one had I known about
them before I bought my Xantrex Link 10.

As is, now that my current battery meter system is installed,
installing another one would involve more dash work than I feel like
doing to my perfectly working electric car.   And by now I've
determined empirically that my batteries all lose water at exactly the
same rate.  Or so close to exactly the same rate than I can't tell the
difference.


FYI... my batteries with the water miser caps seem to lose about a
quart of water per battery per month.  That's 4 gallons of distilled
water for the whole battery pack per month.  I'd hate to see what they
would lose without the water miser caps.


-- 
TTFN,
Brian "Lasso" Lasseter

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