I have been busy over the last few weekends and the weather has been unappealing for climbing around the roof when I have had time so I still don’t have an solar panels installed but all the parts of this system have been cluttering up the house so I decided to mount things on the wall and test out strapping the battery cells together this afternoon.
The straps came in a set of four at Bunnings and the timber end plates are some leftover Ikea solid timber bench. The best advice for LiFePO4 cells is that they should be strapped together because the cells bulge when they are full charged due to the lithium ions that are deposited within the graphite coating of the cathode during charging.
The two solar isolators are overly optimistic Chinese switches that claim they are rated at 32 amps. I’m doing 10 amps per contact so we should be good. The DC breaker is rated for 600 volts DC and 125 amps which is the absolute maximum the inverter is rated for. I hope I can find another enclosure the same for my second inverter. I bought this one when I made the 20 amp Tesla charging adaptor. 
The cells will be tied together with these intercell links. They are 25mm by 3mm so they have a 75mm² cross section. That’s equivalent to 50mm² of coper which can handle around 150 amps with a recommended breaker of 125 amps. That’s around the maximum continuous rating of the cells anyway.
The white tube on top of the cells is Alvania electrical junction grease designed for aluminium connections. You can buy it here. This will all make sense when I join everything up and integrate the Battery Management System.
I found 13 x 250 watt PV panels on Gumtree for $100 each. It turned out they came with mounting rails and most of the clamps. That’s about $500 of cost avoided! The panels all appear in perfect condition. That’s 3,250 watts of panels for 40 cents per watt. By the time I get them on the roof they will owe me nearly 60 cents per watt. That’s about 4 cents per kWh over ten years. Cheap power!
The open circuit voltage of the panel, Voc is 37.6 so three in series is 113 volts which is less than 120.
There is an interesting loophole in the electrical safety rules. You don’t need to be a licensed electrician to install DC equipment that operates below 120 volts. Technically you don’t have to follow all the pesky safety rules around isolators, conduits, fuses and earthing. You know, all the SAFETY rules. I will be following those rules regardless. (You can find the rules online if your Google-foo is good. I don’t want to draw attention to them because hiding Standards behind paywalls is what we do in Australia: put up a fence and charge rent and if I link to the set I found they will probably be taken down.
More to come! Installing and wiring the panels is next.
The goals of my home brew off-grid solar storage system are:
- make my house relatively black-out proof – keep the lights, fridge, hot water and internet running
- run big daytime loads direct from solar – the pool, the air-con
- reduce my effective electricity cost to $0.10 per kWh assuming a 10 year life of the system
- cover the capital expense with two years of savings
I need to generate around 1 megawatthour of electricity each month to do this.
Based on the performance of my current system, in summer I need 5 kilowatts of solar panels, but in winter I need significantly more, around 10 kilowatts.
I have 3 kilowatts on a 2.8 kW grid-tie inverter already and I want that to export like crazy to maximise my return.
A key reason for building an off-grid solar system is to avoid losing my feed-in tariff. That’s going to take some dancing at some point after SAPN look at the NearMap photos and notice I have a bunch of new solar panels.
In the short term I need around 6 kilowatts of extra panels to generate all the electricity my household consumes.
Of course I will need to focus consumption into daylight hours as much as possible because that’s easier than storing it. My largest nighttime consumer is the pool but I can just change the timer. If I have a surplus and my car is home I will charge it. That will be a whole other project 🙂
Strategic advice, pragmatic solutions
bigjsl 