[ausev] AusEV Digest, Vol 31, Issue 15

[Marc Kohler]

Another way of looking at Trojan batteries:

T105:  The name comes from 105 minutes of run time at 75A.

So 105 min / 60 min x 75A = 131.25 Ah (very close to Brian's calculation
with Puekert included)


Marc Kohler

-----Original Message-----
From: ausev-bounces at austinev.org [mailto:ausev-bounces at austinev.org] On
Behalf Of Brian Lasseter
Sent: Wednesday, August 20, 2008 9:56 AM
To: sleeper02_14_06 at yahoo.com; AustinEV News Announcements and General
Discussion
Subject: Re: [ausev] AusEV Digest, Vol 31, Issue 15

On Tue, Aug 19, 2008 at 7:22 PM, Joby Wieser <sleeper02_14_06 at yahoo.com>
wrote:
>
> I have another charger question. To fully charge 8 6volt t105 battries, in
8 hours how many amps must the charger be capeable of?

>From http://www.trojan-battery.com/Products/T-1056V.aspx the Trojan
T105 batteries have 225Ah of charge at best...

   8 * 6v * 225Ah = 10.8 kwh of energy

To figure out how much time it would take a charger to give you
10.8kwh of power, then divide by the charger voltage and the time you
want to charge in:

   10.8kwh / 48v / 8hr = 28.13A

So... The simple answer is that you would need a 28A charger to fully
charge your pack if it was totally and completely discharged...

--

However, thank's to Peukert's Law, you would likely destroy your
batteries and/or get tired of slowly discharging your batteries long
before you were able to pull 10.8kwh out of that battery pack.  See
http://en.wikipedia.org/wiki/Peukert%27s_law and
http://www.smartgauge.co.uk/calcs/peukert.xls for more information.

While leaving out the gory Peukert calculations that you can read
about at the links, the T105 battery has a Peukert exponent of 1.24...
Which means that if you discharge the pack at 100A you only have
134.1Ah of charge, and if you discharge the pack at 200A you only have
113.4Ah of charge.  (In lieu of actually doing Peukert calculations,
dividing the battery energy rating in half is a good conservative rule
of thumb... 225Ah / 2 = 112.5Ah... which is fairly close to our
calculated values)

  8 * 6v * 134.1Ah = 6.4 kwh of usable energy (Thanks Peukert!)

With only 6.4kwh of power, then again divide by the charger voltage
and the time you want to charge in:

   6.4kwh / 48v / 8hr = 16.76A

That means you could get by with a much more reasonable 17A charger.
If you use a 25A charger, then you could charge your pack in a more
reasonable 5.3 hours.  (These simplistic calculations are only for
bulk charging... you would need more time to float and equalize your
batteries to keep them happy on most occasions.)


-- 
TTFN,
Brian "Lasso" Lasseter

"No Sane man will dance." -Cicero (106-43 B.C.)
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