My DB board

When I had my inverter installed I decided to put in a whole new DB board. In this post I'll talk about why and how I wired it up.

The main reason for redoing the DB board is that I wanted to be able to switch individual circuits on and off of the inverter.

An easier solution would be to get an automatic changeover switch box and switch the entire supply to the DB board, but I wanted more fine-grained control:

I have a pumped solar geyser which needs a bit of power to pump the water through the collector, so it makes sense to run this off the inverter in summer. Obviously I don't want the geyser element running off the inverter though so in winter it will stay off.

We also have a gas stove with an electric ignitor, so it's nice to be able to turn it on easily.

It gets pretty hot where I live so the aircons can also be run off the inverter. We have a Samsung inverter aircon in the bedroom which only draws a few hundred watts in economy mode, so it's no problem running off battery.

So this is my db board circuit, it's reasonably straight forward. The idea is there are two completely isolated supplies with changeover switches that select where each subcircuit is powered from. It's very important that the inverter and utility supplies can never be connected to eachother.

I have indicator lights on the supplies because I read somewhere that that's a legal requirement on dual supply systems, and it's nice to know when each supply is active.

I then ordered all the stuff, basically copying the breaker ratings from the old board. My old db board used minirail circuit breakers, but they seem to be a South African thing, so you don't get changeover switches in that size, so I got DIN breakers instead.

Can you follow what's going on?

I mounted the new DB board on hinges over the recess from the old board so the wires behind it can be accessed and worked on if necessary.

I then printed out labels, which turned out to be wrong (or at least I need to swap the wires on the lights and plugs changeover). I should get round to printing another label...

As you can see I'm in loadshedding now- only the red light is on... Time to make some coffee :)

Installation

Here's a rundown of the parts involved:

Inverter (Inverter, AC charger, MPPT solar charger): R13000

Batteries (4x100Ah): R6000

Battery cabinet: R1000
- http://www.bidorbuy.co.za/item/178019194/battery_cabinet_105ahx4.html

Battery cabling and lugs: R500
Make sure to get the lugs crimped properly. Don't forget you'll need cables between each battery. Make sure you get the right type of lug for your battery, I'm using 6mm ring lugs, but some batteries use post terminals (eg a car battery)

DB Board, MCBs, Changeover switches, wiring: R3000
I'll try write another blog post about the DB board, but if in doubt get an electrician to do this.

Electrician to wire in DB: R2500
I got wildly varying quotes. This is the cheapest for installation and certificate of compliance.

Total R26K

I bought the stuff and wired up the DB board myself to save a few bucks. It's quite a tedious job.
I just got an electrician in to pull out the old DB and wire in the new one.

*This is an old photo, the breakers on the bottom left are place-holders for a changeover switch

Online vs Offline UPS

There are two different types of UPSes: online mode and offline mode.

Offline mode is the simplest and cheapest: it has a relay switch that supplies your appliances directly from Eskom utility power while it's available, then switches over to battery+inverter power when utility power fails.

The process of switching over takes a bit of time, only a fraction of a second, but in this time your stuff will have a brief power cut. Usually it's not a big deal but it could cause your computer to reset, your microwave timer to reset etc.

A fast changeover could also cause a surge: appliances with motors in them typically store a bit of power as a magnetic field, and release that power later. 

AC power is delivered in waves, where power cycles back and forth, but normally a cheap inverter and the utility won't be synchronised. If your washing machine is spinning, the current it generates once power is cut could be added on top of the power the inverter generates as it kicks in and cause a surge. Newer appliances are protected against this sort of thing though. I'd only worry if I had an ancient old fridge or washing machine.

A safer, more expensive option is an online UPS where the appliances are always run off the inverter, and the inverter takes its power from the utility when available, or from battery when its not. This means power is never interrupted, and everything stays nice and smooth.

Offline UPSes are cheaper because they only need a small charger circuit. They usually take quite a long time to recharge after a power failure.

Higher end inverter units will usually let you run in either mode. The tradeoff with running in online mode is that it uses more standby power because all the power you use needs to be converted from AC to DC back to AC again. My cheapo Axpert aka MPPSolar unit uses about 50W in online mode, which is about R40 a month's worth. Decent inverters might only draw 10 to 20W.

I tend to leave my inverter completely disconnected because I know when the load shedding is coming anyway. It's important to keep the batteries topped up regularly though.

Advanced: Communicating with the inverter

I wanted to be able to make my own gadgets that can communicate with the inverter, so the first step is to understand how the inverter's serial communication protocol works

After googling a bit I found this post, which is the inverter's rosetta stone
http://forums.aeva.asn.au/forums/pip4048ms-inverter_topic4332.html

I used two USB-serial cables and an arduino configured to AND the serial lines together to sniff the traffic between the supplied WatchPower application and the inverter.

Messages to the inverter follow this format:

<command string><CRC1><CRC2><CR>

Messages from the inverter follow this format:
<command string><CRC1><CRC2><CR>

Eg, to query the general status you'd send QPIGS. The CRC for QPIGS is B7 A9, so in hex you'd send 0x51 0x50 0x49 0x47 0x53 0xb7 0xa9 0x0d


The forum above contains info about how to generate the CRC, or get the C# code off my github.

Here's an example of the command messages for quick reference, see my github for the names of the fields:
QPI
(PI30
QSID
(1111111111111111111111
QPIRI
(230.0 21.7 230.0 50.0 21.7 5000 4000 48.0 46.0 42.0 56.4 54.0 0 10 010 1 0 0 6 01 0 0 54.0 0 1
QVFW
(VERFW:00052.30
QVFW2
(VERFW2:00000.00
QPIRI
(230.0 21.7 230.0 50.0 21.7 5000 4000 48.0 46.0 42.0 56.4 54.0 0 10 010 1 0 0 6 01 0 0 54.0 0 1
QMCHGCR
(010 020 030 040 050 060 070 080 090 100 110 120
QMUCHGCR
(002 010 020 030 040 050 060
QFLAG
(ExDabjkuvyz
QDI
(230.0 50.0 0030 42.0 54.0 56.4 46.0 60 0 0 2 0 0 0 0 0 1 1 0 0 1 0 54.0 0 1
QMOD
(S
QPIGS
(240.4 50.1 000.0 00.0 0000 0000 000 435 54.10 001 100 0027 0000 000.0 00.00 00000 00000101 00 00 00000 100

Using this, I wrote a C# app that emulates the inverter so that I can write other apps without having to connect to the inverter. This will let me test the fault codes etc.

github.com/scottwday/InverterEmulator

Let me know if you manage to build anything!

Dodgy Isolator Switch

A word of caution about switches- Just because it says 100 amps on the box doesn't mean it can switch 100 amps.

I bought this isolator switch from an auto spares shop, and it seemed to work well until one day when I was removing the fuse it gave an almighty spark!

On closer inspection it turned out that the isolator had failed in the on position.

I drilled out the rivets holding it together and it together and found that the copper switch bar had welded itself closed. There is only a spring to return the bar and no way to tell (mechanically at least) that the switch had failed.

I replaced the switch with one of the same type because I couldn't find anything else locally, and I still have the fuse as a safety cutoff. It might arc while switching and damage the fuse holder but its a surefire way to disconnect the battery in an emergency.

Video overview of my backup power system

An overview of my home backup power system

Starting up and shutting down the inverter

Lead Acid Battery Life Expectency

Lead acid batteries hate to be discharged.

They especially hate it when you drain them completely. Your battery should come with a datasheet that details the life expectancy vs. the discharge current. This will let you estimate how long your batteries are likely to last based on how much you're going to use them.

Different types of batteries have different lifespans. High cycle batteries may only last a few hundred cycles if you use them with an inverter system.

This is the graph for the batteries I have at home (Stride 100Ah Solar)

So if I drain them completely each time I use them, I can expect 300 cycles (or load-sheds) from them before they give out, defined as when the battery only holds 60% of the original capacity.

I'm drawing roughly 100W per battery for 2 hours at a time, so that's 200Wh, or 20Ah per 12V battery rounded off (There's no point working accurately here because these are rough estimates). The batteries hold 100Ah in total, so I expect to use 20% capacity per load shed.

I estimate that I'll get about 2000 cycles based on 20% usage. Even if I get load-shed every day then the batteries should be good for 5 years.

Even without load-shedding, don't expect low-end batteries to last much past 5 years though.