On 21.05.19 00:31, Chris Albertson wrote:
On Mon, May 20, 2019 at 7:55 PM Erik Christiansen <dva...@internode.on.net>
wrote:
Switching normal loads is back to less than 100 A for less than 5 kW, but
still hard on contacts. (The contacts on the starter solenoid for an old
Caterpillar D6 are blocks of copper with a mm of silver on the surface,
to take the several hundred amps at 24v. IIUC, silver oxide conducts.)
This is why no one builds 24 or 48 volt DC power system for solar or backup
batteries.
Excuse me, but before making wildly inaccurate pronouncements, it might
be useful to make a study of what is on the market. Very many inverter
manufacturers offer 48v inverters, and most manufacturers of household
batteries sell 48v units - some sell nothing else. Take a detailed look,
please.
https://www.solarquotes.com.au/battery-storage/comparison-table/
https://www.solarquotes.com.au/battery-storage/hybrid-inverter-comparison/
...
You were talking abou the cost of a high amp switch. That si NOTHING
compared to the cost of the inverter you would need. You would need
racks of MOSFETs. No one does that.
How very true - and the cost of a house is much greater than the cost of
a roofing tile.
Parallelling several MOSFETs to handle high current is common practice.
As I mentioned upthread, they self-share the current, and with e.g. 7
milliohms of resistance in each, there is modest dissipation in one, and
one ninth of that if you parallel three.
You all know the formula for power loss right? i^2R. That's "i squared R"
because the current is squeared you REALLY want to make it small.
not more than about 40 amps for a residence.
Indeed, that's why it's not necessary to parallel very many 7 milliohm
MOSFETs to handle 100 A. One on a heatsink dissipates 70 W, and four in
parallel dissipate 4.4W each, for 0.35% power loss.
That a high current at 48v is run only between the battery and inverter,
in hundreds of thousands of households around the world, has already
been explained. Yes, a PV system may run 600v DC or more to the PV
inverter. A battery inverter is a different animal, generally taking 48v
DC, and often putting out only 3 to 4 kW, as that's all some battery
banks will deliver. Many systems have both inverters, but now hybrid
inverters are coming onto the market, merging the two functions.
Also no one is every going to dichage a batter at the 100 amp rate.
you design the system so it cycles only one per day.
In the real world, that is being done daily all over the planet. Well,
the half of it in darkness at any moment, if we are to be precise.
No, not all night, but for brief multiple loads like microwaves and
electric kettles, the battery bank and inverter are designed to produce
5 kW, while pulling over 100 A from the battery. The Redflow ZnBr
battery is warranted to do that for 30 min, before dropping back to 3 kW
allowed, i.e. over 60 A.
Inverter cabling and switching is designed to handle that, you'll find.
Even with lead acid
batteries you can place a 10 amp fuse oneach one of them. Then you
KNOW the maximum current on the switch is 10 amps.
If you wire 20 lead acid batteries in series you can have 1KW of power at
only 4 amps.
if the switch is rated at 10A you are good.
Please feel free to build your installation to your own design¹. I'm just
updating you on what is on the market, and how electrical engineers,
including this one, tend to build 'em currently.
The only circumstance in which I'd use lead-acid batteries, is as a
consumable to fill the gap until an adequate battery technology became
available or more cost effective. Even the more expensive "deep cycle"
LA batteries have quite limited lives at 50% DoD, and as outlined
upthread, they're expensive if discharged much less.
Erik
¹ I know of one battery inverter which will take 120 Vdc input, but none
at 240 Vdc.
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