[RE-wrenches] single string of batts: must the interconnects be inverter size cables?
Warren Lauzon
windsun at wind-sun.com
Sat Feb 20 11:36:19 PST 2010
We usually figure that (for example) in a 2-string bank, that one conductor could carry 2/3 of the current if the batteries are unbalanced for some reason. In other words, rather than cutting the amperage rating in half, we cut it by about 1/3. For 3 banks would cut it by 1/2, etc.
Probably not ideal in theory, as in a worst case scenario you could have one totally failed bank, but on the other hand selling the customer on a full array of expensive 4/0 jumpers is not always an easy thing to do. In fact if someone is using that much battery, they should probably be using a 24 or 48 volt system.
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Northern Arizona Wind & Sun - Electricity From The Sun Since 1979
Solar Discussion Forum: http://www.wind-sun.com/ForumVB/
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----- Original Message -----
From: Allan Sindelar
To: RE-wrenches
Sent: Friday, February 19, 2010 10:03 PM
Subject: Re: [RE-wrenches] single string of batts: must the interconnects be inverter size cables?
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?
<![endif]-->
Allan Sindelar
Allan at 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 <mick at abrahamsolar.com> 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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