Monday I pulled a dead bank out of a machinery shed west of Gin Gin. It was twenty-four 2V flooded cells making up a 48V system, about four years old. A decent set should give you ten years, and these were stuffed. The owner had bought them second-hand off a bloke who was “going lithium,” and he’d topped them up with tap water because the distilled ran out.
That’s not even the worst one this year. It’s just the freshest.
Most battery banks I replace didn’t fail on their own. Something killed them, and usually it’s the way they were set up or charged. So this is mostly about that. If you’re after a spec sheet, every supplier’s website has one.
What the battery’s actually doing
Your panels make power when the sun’s up. You use most of yours when it’s not. The battery sits in the middle and soaks it up during the day so you can run the fridge, lights and pump at night, and it does that every single day.
That’s deep cycling. You draw it down a fair way, fill it back up, and repeat it three hundred-odd times a year for years.
A car battery can’t do that and was never meant to. It’s built to dump a heap of current for a few seconds to crank a motor, then the alternator fills it straight back up. It hardly ever gets drained. Run one down to half every night and it’s done in months. People still do it, mostly because there’s a spare one sitting in the shed. I get it. It still won’t last a winter.
“Dual purpose” marine batteries are a halfway house. Good in a tinnie. Not great on a cabin running a 12V fridge around the clock.
Lead-acid isn’t dead, but it’s fussy
I still put flooded lead-acid in now and then. It suits someone who lives on the property, has a generator that actually starts, and doesn’t mind checking water once a month. A few older blokes on cattle places prefer it because they understand it and can fix it, and fair enough.
The trouble is you only get to use about half of it. Take a 12V 200Ah AGM. On paper that’s 2.4kWh, but you really want to stop at around 1.2kWh if you want it to last. That’s a fridge, some LEDs, the router and phones overnight. If you add a pressure pump that kicks in every time someone flushes, you’re pushing it.
The bigger catch is that lead-acid needs to get properly full, often. I don’t mean 90 per cent, I mean 100, with a good long absorption stage at the end. Leave it sitting part charged for a few weeks and sulphate builds up on the plates. That capacity’s gone, and you won’t get it back.
That’s why weekenders shouldn’t buy it. You turn up Friday night, flatten it, go home Sunday, and the panels might fill it before you’re back. In June they probably won’t. So it sits half charged for a month, every month, all winter. I can usually tell a weekend shack bank just by looking at the specific gravity readings.
Heat’s the other killer. A commonly quoted rule says lead-acid loses about half its life for every 10°C it lives above 25°C. Plenty of batteries sit in a tin shed in January. You do the maths.
Lithium’s better, mostly
When I say lithium for solar, I mean LFP, lithium iron phosphate. Nothing else belongs in a shed or on the side of a house.
It does a lot right. You can use 80 to 90 per cent of it day in, day out. It doesn’t care about sitting half full, so it’s the obvious pick for weekenders. There’s no water, no gassing, and it’s about a third of the weight. Good LFP is rated somewhere in the 3,000 to 6,000 cycle range. It costs more upfront, but over its life it usually works out cheaper per kWh, and you’re not lugging 60kg cells around every six years.
What it gets wrong, or what people get wrong with it:
- Cold. Most LFP won’t take a charge below about 0°C unless it’s got built-in heating. On the Granite Belt or up around Guyra, ask the question.
- The cheap stuff. I capacity tested a “200Ah” battery off an online store last spring and got just under 140Ah out of it. Another one had a BMS (the little board inside that protects the cells) that tripped every time the microwave started.
- The voltmeter on the wall. LFP voltage barely moves between roughly 20 and 90 per cent. That gauge is close to useless. You need a shunt-type battery monitor that counts amps in and out. Otherwise you’ll think you’re full right up until the lights go out at nine o’clock.
Charge settings kill more batteries than anything
If I had to pick one thing, it’s this. Somebody swaps lead-acid for lithium and leaves the charge controller, the inverter charger and the DC-DC charger all on the old lead-acid profile. Now there’s an equalise cycle hitting the lithium once a month, plus a float voltage that’s too high. A good BMS will usually step in. A cheap one might not, and you won’t find out until it’s too late.
It goes the other way too. I’ve seen lead-acid banks running with no temperature sensor fitted, getting overcharged every summer afternoon and boiling dry.
Every charging device on the system needs to agree with the battery. When I do a service call, that’s the first thing I check, and it’s wrong more often than it’s right.
Off-grid and backup aren’t the same thing
Off-grid means the battery is your power station. Size it for June, not January. Then allow a couple of overcast days and get a generator, preferably auto-start, because you won’t want to walk out to the shed at 2am in the rain.
Count what runs when nobody’s using anything. An off-grid inverter can sit there drawing 30 to 50 watts doing nothing, which is around a kilowatt-hour a day before anyone’s boiled the kettle. A chest freezer in the garage and a pressure pump add more. A family home off-grid generally ends up somewhere between 15 and 30kWh of usable storage. It depends heavily on whether you heat, cook and pump with electricity.
On the grid it’s a different job. The battery’s mainly there to push your daytime solar into the evening. A lot of people buy one because of blackouts and assume it’ll keep the lights on. A standard grid-tie setup switches off in a blackout on purpose, so it doesn’t feed power into lines a linesman’s working on. If you want backup, you need a backup-capable inverter and a separate backup circuit. That usually covers the fridge, lights and a few power points. It won’t run the ducted air or the oven. Get it written on the quote.
How people end up paying twice
Mostly it comes down to sizing the battery off a summer bill. Or hanging a 20kWh bank off a 3kW array that can’t fill it from May to September. Or bolting new batteries onto an old bank. Or buying 12V for a whole house when it should’ve been 48V. Run a 3,000W inverter on 12V and you’re pulling over 250 amps, which means cable like your wrist and heat you don’t want in a shed.
There’s also where it goes. Home batteries have to meet AS/NZS 5139, and that rules out some spots near bedrooms and exits. The garage wall you’d picked might be off the table.
Read the warranty properly too. Most guarantee 60 to 70 per cent of capacity after a set number of years or a set amount of energy through it, whichever comes first. If you cycle hard, the throughput cap gets you before the years do.
Check whether the federal battery discount or any state scheme applies to you, because it can knock a decent chunk off a grid-connected install. You’ll need an accredited installer for that anyway.
Before you ring anyone for a quote, write down what actually runs after dark in winter. Include the beer fridge, the bore pump and the freezer full of last year’s beef. If an installer gives you a battery size without asking about any of that, get another quote.
