Batteries are possibly the most costly part of the Solar solution, you need them, so you have no choice but to buy them..
What no one tells you about is the term "Duty Cycle". Generally when you buy a printer it will say "duty cycle 10 000 pages" which generally means you can print 10 000 pages a month and the printer will last 5 years.
Batteries also have a duty cycle, however, the information is slightly misleading. The salesperson may tell you the duty cycle of the battery is 2000, that is fantastic but in real terms it does not mean anything until you add in how depleted the battery is at the end of each day.
What the salesperson does not tell you is that the 2000 duty cycle is at 20% depletion. See the graph below of a Gel Battery Cycle ability.
You will see that at 100% depletion you only get about 300 charge cycles, that means the battery is good for a year. (If its charged every day)
In Part 1 of this blog I eluded to the 50% rule, applying that to the example battery below we see that it's good for 1000 charges. That means your battery is good for 3 years.
Considering the Gel batteries cost around R 2400.00 each it means you are adding a cost of
R 2400 x 4= R9600.00 per 3 years
If we were to use only 20% of the charge then the battery will last approximately 6 years.
This brings me back to my initial statement, the system needs to be designed to use Solar in the day, if solar is not available it must switch to Grid and if the grid fails it needs to go to battery. In other words you actually don't want to use the battery at all unless the solar and grid are not available.
I did the exercise of asking our "slant eyed friends" about Batteries, they can supply me with a 250 AH 12 volt battery for about 230 USD, this is roughly twice the amperage for the same price as we pay for 100 AH here.
Tripping is not a place in China...
One critical item needed is a way to isolate the 100 odd amps flowing through the circuit. Actually "best practise" would be to to have each solar panel on a fuse, then connected to a combiner and the combined current must be fed via a circuit breaker to the Inverter.
So, what you would need is a Solar Distribution board, typically any wall mount one will work, they have the DIN rail solution in most so it makes it easy to slip trip/fuse switches in and out.
Looking around in SA I found a DC fuse holder that fits onto a DIN rail in a DB.The Cost of a fuse module is about 37 ZAR, the fuses cost about 49 ZAR each. (figure the fuse costs more than the module?)
I managed to get DIN fuse modules + Fuses from China for 1.97 USD.
The most important fuse/trip switch is the one that fits between the battery and the inverter, seeing that the battery amps are around 100 amps this needs to be in place to prevent some serious damage should a short circuit occur.
I called around but no electrical supplier in Cape Town could supply me with a 100 amp DC trip switch. The one place I called said they could get but it was expensive, 4000 ZAR.
I called a few solar places and they suggested 100 Amp fuses, these came in at a cost of 1100 ZAR each.
Sustainable.co.za have Hagar trip switches but they do not state specifically AC or DC but they do not have a 100 amp one in any event.
The best I can find is a 100 Amp fuse and holder for 400 ZAR which is better than the other solar place that wanted 1100 ZAR.
I looked to the East once more and found a 100 Amp DC trip switch for 31 USD. Problem solved.
Finally to isolate the Collective solar array from the Inverter we need a trip switch that can handle the sum of current from all panels, my collective current is 40 amps so I went looking for a 40 amp DC trip switch.
Once again here in SA I found a few that did not specifically say DC so I avoided them, they incidentally came in at about 140 ZAR.
I looked East again and from the same supplier as before I got a 40 amp DC trip switch for 2.79 USD.
I decided to complete the "Tripping" by buying a box for the trips/fuses and from the East I ordered a 8 Way DIN rail Distribution Board for 14.6 USD.
So below is my complete shopping list, air freighted to SA is a little pricy but 7 days later I will have my goods.
The same shopping list here in south africa using equivalents of Fuses where I can't get trip switches:
Taking into account that locally I can only get Fuses I think the purchase from the East is far better than buying the local equivalents.
Tying it together with Cable
Finally we need to string all this together, most of the local solar companies will want to supply you with welding cable to make the connections.
Looking at the manual the connection to the battery for a 5 KVA system needs to be 8 AWG or 4 mm in diameter or if you really want to be safe then 4 AWG or 6mm in diameter. Some solar companies are recommending 8 mm in diameter cable AWG 2 others suggest 14 mm diameter cable.
The issue with all this is that the terminating blocks on the Inverter don't allow for cable thicker than 8mm so there is no point in going any thicker.
Possibly the only thick cable needs to be the one from the battery to the inverter, these connect via lugs and can be soldered on. The limiting factor here is the connection terminal on the fuse holder or trip switch, it may only be 8mm.
In my example system I will run 2.5 mm cable from each Solar panel to the fuse, the recommendation for a maximum of 10 amps is AWG 12 which is 2 mm in diameter.
From the individual solar panel fuses I will run a Bus Bar to the 40 Amp trip switch.
The connection from the 40 Amp trip switch will be to the inverter, according to AWG this needs to be AWG 6 or 4 mm in diameter, I will just use a 8mm cable as its about the thickest that will fit into the terminating blocks.
Below are the AWG references i used for cable sizes:
Now I sit back and wait for my trip switches and fuses to arrive, in the meanwhile I will work on the solar array configuration and installation..
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