On Fri, Jul 22, 2011 at 10:42:54AM -0000, greenpjs04 wrote:
> Hi Mark,
> Yes, you would have one bank at 200 ah. But, be very careful doing
> that. Make sure both banks are at the same state of charge before
> turning on both switches. If one bank is fully charged and the other
> is discharged, a huge amount of current will flow from the charged
> bank to the other. There is nothing to control how much current flows
> other than the internal resistance of the batteries and wiring. That
> is not a good situation.
You know, that's a common misconception. Fortunately for us all, it's
not true - otherwise, we'd be wrecking batteries constantly, every time
we threw the battery switch from '1' or '2' to 'All'. :) You see,
there's a number of factors that mitigate against that high inrush
current: first off, batteries - at least lead acid batteries, which are
what I'm most familiar with - don't take up a huge amount of current
initially; there's a "buffer efffect" that takes place when you first
start charging, with the batteries being progressively more capable of
taking a higher charge current as they warm up. Second, in order to
charge a battery, the input voltage needs to be ~.2v/cell higher than
the nominal cell voltage (given average temperatures) - i.e., if you
want to start charging a battery that is at 12v, you need to supply
13.2v (nice chart from WindSun here:
http://www.windsun.com/pictures/Charge_voltage.gif). Third, batteries
can take a "surface charge" rather quickly - meaning that their voltage
will rise before they accept any appreciable current - and without a
voltage differential, current just won't flow. (If you postulate a high
initial load, the "high" battery's voltage will also drop under that
load - reducing the differential even further.) Lots of factors working
against this, you see.
I recently ran across an _excellent_ site of a BMS manufacturer - some
British company (I don't recall the name at the moment) - that went over
this in detail; their analysis for the worst case they could come up
with was right around 21 amps for the inrush current, rapidly tapering
off. Not exactly a world-shaking event. :)
--
Ben Okopnik
-=-=-=-=-=-
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