[RE-wrenches] single string of batts: must the interconnects beinverter size cables?

[R Ray Walters]

These are grid tie with battery backup systems. All inspected. 
Personally, I think that we should have OCP on each series string, but that's a whole nother can o' worms.
Most sealed batteries (and golf cart batteries) are fairly current limited. You need at least 2 parallel strings to even get close to meeting the inverters surge requirements. (especially in 24 v systems)
Main cables and breaker are sized as always, (4/0 with a 250 A breaker) but parallel string jumpers are reduced to 2/0 as they are carrying 1/2 to 1/3 of main current.
When doing HUPs, they have the current capacity to be run in a single string without limiting the system.

All I'm saying is that every part of the system needs to be sized adequately, batteries, series jumpers, main cables, breaker, etc. 
If you have 4/0 cables on a single string of T105s, you need to go back to the drawing board, IMHO.
Just try getting an Outback 3524 to surge to its 5000 watt rating on 4 golf cart batteries; you'll hit low voltage annoyingly fast.
I'm seeing a lot of undersized battery banks on grid tie systems. Your minimum battery bank HAS to be able to meet or exceed the inverter surge ability, otherwise, just sell them a battery-less system.

One last point, if there IS a short circuit at the battery bank, a smaller cable will actually act as a current limiter, and keep things from running away as fast till the battery blows. The added resistance is in effect buying you some extra time to clear the fault. (i.e., the new guy's wrench!) I've seen the lead at the connections start melting, way before the cable gets cooked, so Exeltech's idea of using 2 cables in parallel to increase the surface area contact between the battery and copper is a better idea than 7" long 4/0 cables.
Somebody take an Infared picture of a battery bank under full load, you'll see the contact surfaces and any bad / loose connections getting hottest, not the cable.

R. Walters
ray at solarray.com
Solar Engineer




On Feb 22, 2010, at 10:15 AM, Tump wrote:

>  
>  
> Is solar ray doing installed in Mobil applications or is this something that is going to be in a home & inspected ?
> -----Original Message-----
> From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Mick Abraham
> Sent: Monday, February 22, 2010 10:47 AM
> To: RE-wrenches
> Subject: Re: [RE-wrenches] single string of batts: must the interconnects beinverter size cables?
> 
> ...that last sentence of Ray's...what a fine additional factor to bring up: "Inherent current limitation because the battery just can't do more." I'm going back to my charts!
> 
> Mick Abraham, Proprietor
> www.abrahamsolar.com
> 
> Voice: 970-731-4675
> 
> 
> On Sun, Feb 21, 2010 at 10:19 PM, R Ray Walters <ray at solarray.com> wrote:
>  
> ...Some of the sealed battery manus (B&B for instance) give max current output vs time charts, and that not only gives you something for cable sizing, but also is an eye opener, when expecting a single string of two 12 volt batteries to run a 4 Kw inverter. A battery that can only produce 250 amps for less than 5 sec, doesn't deserve a cable sized for 24 hr continuous at that output.
> 
> R. Walters
> ray at solarray.com
> Solar Engineer
> 
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