Allen,
You're correct about current dividing between two or more paralleled sets of
batteries, and your resulting ability to use reduced conductor size in each
set. As long as you ensure the conductor ampacity for each set of
batteries is sized according to the current they will conduct .. you're good to
go.
There are generally three primary points to consider:
1) Total current flowing in each circuit;
2) The ampacity of the conductors involved, and;
3) Voltage drop.
This then raises a question. Do you size the battery conductors to meet
the maximum continuous current flow .. or the peak surge current?
The answer is, you can do either one, depending on your goal - and that of your
customer. Conductors of appropriate gauge for maximum continuous current
flow will handle the typical 2X surge current that [for example] would be
required to start a submersible well pump, and will do so without
overheating. Surge currents typically exist for periods of time ranging from
much
less than a second to 2 or 3 seconds at most. After that, if the surge
current still exists, overcurrent devices open or inverters shut down --
whichever happens first.
If you size for maximum continuous current flow .. you economize on conductor
costs. Size for surge current, and your customer pays more, but their
system experiences lower voltage drop across the conductors and connections
during the
surge. It's all a matter of which aspect of the system design is more
important to your customer. As we know, sometimes a few tenths of a
volt one way or the other (at the inverter) can make the difference between
starting a high-surge load -- or not.
As a related comment, when I'm designing and building battery systems that
require large conductors (for instance 4/0), I'll use paralleled pairs of 1/0
from one battery (or cell) to the next. Two paralleled 1/0 are equivalent to
one 4/0, but there are advantages to the paralleled conductors. I place one
cable lug on each side of a battery post, then install and torque the
hardware. This doubles the effective
total surface area of the lugs in contact with the battery posts, reducing the
high-current voltage drop at each of these points in the system. This approach
increases the total
conductor surface area in contact with the cable lugs, and with a lug on either
side of the battery post, also provides something stronger than a standard flat
washer to
torque against. Admittedly a more expensive way to go, but I feel it's
worth it.
Dan
--- On Fri, 2/19/10, Allan Sindelar <al...@positiveenergysolar.com>
wrote:
From: Allan Sindelar <al...@positiveenergysolar.com>
Subject: Re: [RE-wrenches] single string of batts: must the interconnects be
inverter size cables?
To: "RE-wrenches" <re-wrenches@lists.re-wrenches.org>
Date: Friday, February 19, 2010, 11:03 PM
Phil,
Given the scenario that Mick presented, your advice is sound, and I'd do as you
suggest. But by changing the scenario just a bit, it brings up a separate but
related issue.
Mick's scenario describes two 12V batteries in series, or a single string. In
that case all current flows through the one string, so a series interconnect
would need to be sized equal to the battery cables, in this case 4/0. But if
there were two (or more) parallel strings, would the same size requirement
apply? That is, if a battery bank had two strings, in theory each would carry
180 max amps (your example, 8,000/22, shared by 2 strings), which is within the
ampacity of 2/0 interconnects. In reality, this only applies if current is
equally spread among strings. Is it?
It seems to me that a properly wired and torqued set will split current paths
equally until a cell fails, either prematurely or when the set approaches end
of life. If there are three strings, then failure of one cell/string in this
example would still not exceed ampacity on the remaining 2/0 strings.
As with many issues, the goal is to find a balance between safety and
performance versus cost and worst-case accommodation. I will always use 4/0
with one cell string and a 250A breaker, but I have never seen a real-world
problem using 2/0 interconnects, both series and parallel, with 2 or more
strings. Phil, what do you say? Others?
Allan Sindelar
al...@positiveenergysolar.com
NABCEP Certified Photovoltaic Installer
EE98J Journeyman Electrician
Positive Energy, Inc.
3201 Calle Marie
Santa Fe, New Mexico 87507
505 424-1112
www.positiveenergysolar.com
Phil Undercuffler wrote:
The interconnects are part of the circuit.
Typical flexible cables used with batteries are THW rated (75C column), so in
free air 2/0 interconnects would be good for 265 amps, before any temperature
derates. Therefore, you'd be safe from a NEC point of view.
However, if the inverter manufacturer has recommended 4/0 cables, then that
recommendation would apply to all cables in the circuit. Battery based
inverters are asked to surge many times their rated capacity -- a 24V 4kW
inverter surging to 2x capacity (pretty normal circumstances) is going to draw
over 360 amps (8,000 / 22), even if only for a short period of time. You
don't want your interconnects to be the weak link in the system, causing
shutdowns due to undervoltage unnecessarily.
I'd use 4/0.
Phil Undercuffler
Conergy
On
Fri, Feb 19, 2010 at 12:01 PM, Mick Abraham wrote:
Example: 12 two volt battery cells in one 24 volt
string. 4kW inverter/charger has 250 amp DC breaker size and 4/0 cables...as
the inverter folks would want.
So...do the "cell to cell" interconnect cables have to also be 4/0?
In the example above, 2/0 interconnects would suffice from the standpoint of
basic safety...and even from the voltage drop standpoint...right?
Inverter company people may be best qualified to answer this, but any replies
will be appreciated. While I'm at it: do the inverter folks request oversize
cables because they want more capacitance on the DC input...or what?
Jolliness,
Mick Abraham, Proprietor
www.abrahamsolar.com
Voice: 970-731-4675
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