Series Cables for 2 Volt Cells [RE-wrenches]

Robert Warren robertwarren at mail.com
Sun Jun 13 11:17:45 PDT 2004


 

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David,
 I admire your scientific approach to this issue of sizing battery 
cables. Two #6 cables in parallel is the same as one #4 cable. The 
reason they put two negative posts and two positive posts on each cell 
has more to do with optimizing battery construction and electron flow 
beween cells. It is more expensive for the manufacturer to provide 4 
posts per cell than just two, and the little bit they save on cable 
sizing is not significant to them. They do this as this is an industrial 
size battery and can be used in really large systems, such as a 500 KW 
UPS I once had to maintain for the Navy. 
 There is a big difference between 1200 amp hour cells and the old 
L-16's you used to use. The thing is, you still have a small system with 
only a 2,500 or 3,000 watt inverter. Once you start getting up into 
larger systems, say 10KW and higher inverters (like the 5-stack Outback 
system Daryl Thayer just did), then you really have to pay lots of 
attention to series and parallel battery connections.
 I have used Power Batteries for many years in UPS's up to 50 KW.  They 
are good batteries, but for the most part, their real market is UPS's 
(Uninterruptible Power Systems), not so much the off-grid home market. A 
PV system gets a lot heavier use than a UPS, where the batteries are in 
float mode most of the time. The main difference is that a UPS will 
typically only use the top 20% of the battery capacity rather than 50% 
as is more common in home systems (due to the higher low-voltage cutoff 
setting programmed into a critical use UPS)  As they stay up in the 
higher voltages, the amps are lower, and they can get by just fine with 
smaller cables in a UPS than in a home system.
  Ideally, if we want to achieve long battery life, we would like to 
size our systems with a big enough battery bank to only use the top 20% 
capacity like the UPS guys do. But batteries are fairly expensive, and 
this is rarely an option, unless you get a wealthy customer like a 
telecomms application (in which case, I get to size the batteries 
properly).
 The fact that you had even a 6 degree temperature rise on your cables  
shows there was a bit of resistance, even if if didn't seem significant. 
Now, suppose you were loading a larger bank of 1200 amp hour batteries 
with a much larger inverter setup, and then it becomes a problem. Most 
of the batteries I use in the 1,000 A/H and larger cell size come with 
flat bar interconnects, not cables. This is a proper interconnect, I 
say! You want the batteries to last for many years, and even with a 
great installation and gooping up the posts and bars with battery grease 
or vasoline, you will still get some corrosion over the years. Even with 
no corrosion, you still have to provide the lowest possible resistance 
between cells, so that they can charge and discharge equally. Too small 
a series cable and the cells on the ends of the string have to do more 
work, and often die an early death.    
 I think, when it comes to battery cables, always follow manufacturer 
recommendations for the inverter cables, and try to closely match the 
interconnects to maybe just one size smaller than the inverter cables.
This is one area it pays not to scrimp. 
Robert Warren


David Palumbo wrote:
> Wrenchers.
> 
> I recently received (12) HPF85-25P, 1,200 Amp hour, 170 lbs each, 2v 
> cells from Power Battery Co.
> With the cells the manufacturer sent 24 series cables to be used with 
> the installation.
> I initially dismissed these out of hand because they were #6 AWG. Way 
> smaller than the usual cables we use.
> Because there are two series jumpers from cell to cell (there are two
> positive and two negative posts per cell) the #6 are not as undersized 
> as they would be on a single series connection battery. The engineer at 
> Power Battery assured me that they were ample for series jumpers.
> I recalled that in the mid 80's when I put together my first system 
> Peter Talmage (very knowledgeable engineer and solar guy from Maine) had 
> me use #4 cables for the series connections in the battery bank of 
> L-16's I was using.
> 
> I decided to test the 24v bank of 2v cells with the #6 jumpers here in 
> my shop.
> I loaded it up to 2,200 watts and ran the bank down to 20.0 volts when 
> the OutBack inverter shut off. That process took 14 hours. Load 
> resistive heat)
> was steady between 85 to 88 amps. I measured the temperature of the #6 
> series jumpers many times during the discharge and subsequent recharge 
> (at 85A)and found that the cables (10" in length) were, at most, 6 
> degrees F above the ambient air temp of 72F. The FX inverter/charger got 
> up to 115 F at most during the test. The 2/0 cables connecting the PS2DC 
> disconnect box
> and the FX got 12 degrees F higher than ambient and the 4/0 battery to 
> PS box got 2 degrees F above ambient.
> So the question is are we wasting copper by using oversized cables for 
> series connections?
> 
> Dave
> 
> 


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robertwarren at mail.com

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