[RE-wrenches] unique battery bank design question

[Jason Szumlanski]

Sorry - I missed the 4 days part of your post. I'm not sure where you
came up with 4 days - sounds kind of arbitrary.

I'd be interested to know if it makes any difference at all if you tap a
12V load only when there is a 48V charging source present (assuming the
charging source current is significantly greater than the load). Assume
you want to run a small 12V fan only when batteries are charging. In
that case, I would guess the impact is far less important, or perhaps
nil. Maybe this is a case where a tap is acceptable. Not sure...

I'm not trying to argue that a tap is a good idea - it's clearly not
ideal. I'm just trying to be devil's advocate and run this issue to
ground. If there is a legitimate reason to pursue this course of action
(cost, simplicity, size, weight, etc,) it's a good idea to talk about
it.

Another option I've been reading up on is a Battery Equalizer/Battery
Balancer. These seem to be commercially available to tap 12V off a 24V
battery, but I am not sure about the efficiency or cost comparison to a
DC-DC converter, or how/if multiple units could be implemented in a 48V
system.

Jason Szumlanski
Fafco Solar

         
-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Hugh
Sent: Thursday, September 01, 2011 5:05 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] unique battery bank design question

At 16:48 -0400 1/9/11, Jason Szumlanski wrote:
>1a x 12V x 24hrs = 288 watt-hours on a 48,000 watt-hour battery. My
math
>shows 0.6% of the nominal capacity. And that assumes there is no
>charging source present.

Wrong again.  Sorry.

You are taking out four times that percentage out of just one quarter 
of the whole battery.  In four days it will amount to 20% of the 
usable capacity (which is only really 500Ah) of one 12V battery. 
Charging the whole battery will not restore balance unless you dump 
an equivalent 98 Ah of charge out of the other 36 volts, making this 
process at best 25% efficient and probably ending in tears.
-- 
Hugh Piggott

Scoraig
http://www.scoraigwind.co.uk