Your lead-acid battery is failing early because load shedding hands it the one duty cycle lead-acid chemistry handles worst: partial-state-of-charge cycling. The 300 to 500 cycles a lead-acid datasheet quotes at 50 percent depth of discharge assumes a full, uninterrupted recharge after every discharge. A Stage 4 to Stage 6 schedule never gives you that. It gives you a short discharge, a recharge window that closes before the battery is full, and then another discharge stacked on top of the deficit. Run that loop for a few hundred days and the bank dies long before its cycle counter says it should. It is not a manufacturing fault. It is a mismatch between the chemistry and the way the grid behaves here.
I was standing in my own garage at twenty to nine on a Stage 4 night, headlamp on, DC clamp meter around the positive cable, when that stopped being a theory for me. The bank was fourteen months old. The meter showed the charger pushing maybe a quarter of the current the datasheet claimed the battery would accept at that voltage, and the casing was warm to the touch. Not hot. Just warm, the way a battery gets when it has spent two hours being asked for something it cannot give. I killed the mains again to see what would happen at the next outage, and the voltage dropped away in a straight line instead of the slow curve I remembered from the first winter. Fourteen months, on a bank that should have had another four years in it.
What I want to walk you through is not "lithium is better." That sentence is useless. I want to show you which numbers on a spec sheet actually predict how a battery behaves in a South African garage, and which ones were measured under conditions load shedding never provides.
The recharge window, not the discharge, is what kills it
Everyone worries about how deep the battery goes during an outage. That matters, but it is the second problem. The first is what happens in the two hours after the power comes back.
A lead-acid battery does not charge at a constant rate. It takes the bulk of its charge quickly, then the last stretch tapers off. Getting from empty-ish to roughly 80 percent can happen inside an hour. Getting from 80 percent to full can take three or four hours, because the battery's acceptance of current collapses as it approaches full. That last part is not optional. It is where the chemistry actually finishes the reaction, and it is exactly the part a load shedding schedule steals.
So the battery spends its life bouncing around the 80 to 90 percent band. Every time it does that, a little lead sulphate that should have been converted back into active material stays put and hardens. Once is nothing. Twice a day for a year is a bank that measures fine at rest and falls over under load, which is the classic "it says 12.6 volts but the fridge trips the inverter" complaint.
Cycle life numbers are measured under lab conditions
Cycle life is simply the number of charge and discharge cycles a battery delivers before usable capacity drops to a set percentage of its original rating, usually 80 percent. The figure is real. The conditions behind it are the problem.
The standard test discharges to a fixed depth, recharges fully, and holds temperature steady around 25 degrees Celsius. Every cycle is complete. Nothing is left on the table. In a garage in Gauteng in February, none of that holds.
Two things follow. First, a lead-acid bank rated at 300 to 500 cycles at 50 percent depth of discharge will not see 300 cycles in a house that cycles it twice a day. Second, and more useful, some manufacturers publish a separate partial-state-of-charge cycle life figure. It is always lower, usually dramatically lower, and it is the number that describes your actual life. If you are comparing batteries and nobody can produce that figure, you are comparing a lab result against a household reality.
A lithium iron phosphate battery typically carries a rating somewhere between 3 000 and 6 000 cycles at 80 percent depth of discharge. Two things about that number matter more than its size. It is quoted at a deeper discharge than the lead-acid figure, so it is not an apples-to-apples comparison at face value. And lithium is far less damaged by sitting at partial charge, because it is not relying on a reversible chemical conversion that leaves residue behind when the conversion is interrupted.
Depth of discharge is where lead-acid quietly loses
Depth of discharge, or DoD, is the share of the battery's capacity you have used. Pull 50 amp-hours out of a 100 amp-hour battery and you have gone to 50 percent DoD. Higher DoD costs cycle life, and it costs lead-acid far more than it costs lithium.
This is where a lot of South African installations go wrong before the first outage even happens. The bank gets sized on the average load rather than the Stage 6 load. Four hours at 300 watts is 1.2 kilowatt-hours. On a small lead-acid bank that is not 50 percent, that is 70 or 80 percent, and it happens on every cycle, not occasionally. The datasheet promised 400 cycles at 50 percent. The house is asking for something closer to 60 cycles at 80 percent. Nobody lied. The system was designed around a number the schedule does not respect.
If I am looking at a lead-acid proposal for a house that sees Stage 4 or worse, the first thing I check is what depth of discharge the bank will hit on the worst day of the week, not the average day.
Charge and discharge rates explain the rest
C-rate is how fast a battery charges or discharges relative to its capacity. A 100 amp-hour battery discharged at 10 amps is running at 0.1C. At 100 amps it is 1C. The number says nothing about quality on its own. It says what the battery is willing to do, and how quickly.
For a typical house, discharge C-rate is almost never the limiting factor. Fridges, lights, a TV, a router, a fibre box: the steady draw is gentle. The battery is loafing. Discharge rate is not what kills the bank in a load shedding house.
Charge rate is the opposite. Lead-acid charge acceptance tapers hard as it fills, so even a generous inverter charger cannot force current into a battery that has stopped accepting it. You can have 5 kilowatts of charging capability and still watch the last 15 percent crawl. Lithium holds a high acceptance rate much further up the curve, so a two-hour window that leaves a lead-acid bank at 85 percent can take a lithium bank to full and hold it there.
That single difference, more than cycle count, is why the two chemistries behave so differently on the same schedule. It is not that lead-acid is slow to charge from empty. It is that it is slow to finish, and load shedding never lets it finish.
Heat, and the honest exceptions
Temperature is the multiplier nobody accounts for. High temperatures accelerate degradation in both chemistries, but lead-acid is punished harder. The rule of thumb I work with is that every 8 to 10 degrees above 25 Celsius roughly halves a lead-acid battery's service life. A north-facing garage that hits 38 degrees in January is not a 400-cycle environment. It might be a 200-cycle environment before load shedding even gets involved.
Lithium is not immune either. It ages faster when hot, and its management system will often refuse to charge it at low temperatures, which matters on a Highveld winter morning near zero. That is a real tradeoff, not a marketing point.
There are cases where lead-acid still makes sense and I will say so. A house with one short outage a day, a charger with six clear hours to finish the job, and a bank sized so it never goes below 30 percent depth of discharge will get reasonable service out of lead-acid. It is the multi-stage, twice-a-day, short-window schedule that destroys it. If your schedule looks like that, no amount of careful sizing rescues the chemistry.
What I look at now when someone shows me a battery
I read past the headline cycle count. I want the partial-state-of-charge cycle life if it exists, the depth of discharge that figure is quoted at, the maximum charge rate expressed as a C-rate, and whether the temperature derating has been applied or quietly ignored. On lithium, I want to know the battery management system is included as part of the package, because without it the cells are unprotected and the cycle life claim means nothing. On lead-acid, ventilation and electrolyte level checks are not optional extras, they are part of owning the thing, and I want to see that the enclosure and the room actually allow for both.
Then I do the only comparison that survives contact with a real house. Take the cycle life, multiply it by the usable depth of discharge, and you get how much energy the battery will move across its whole life. A lead-acid bank at 400 cycles and 50 percent depth of discharge moves about 200 times its nameplate capacity. A lithium bank at 4 000 cycles and 80 percent moves about 3 200 times its nameplate. That gap is roughly a factor of sixteen, and it is why the upfront difference, whatever it is on the day, gets eaten long before the second battery is installed. I am not going to tell you which one is cheaper. I am telling you the multiplier is the thing doing the work.
The bank in my garage did come out, eventually, and I kept one of the blocks on a shelf as a reminder. It is a perfectly good-looking battery. It holds a surface charge beautifully. It just cannot do the last 20 percent of a charge inside the two hours load shedding gives it, and neither can any other lead-acid block in the same situation.
