Each plate is based on a lead skeleton, hardened by antimony or calcium. On the skeleton hangs a paste of lead oxide, which stiffens those bones and forms the battery’s active material.

The separators between plates stop neighbours sort-circuiting, and have just the right degree of porosity to let charge-carrying ions move freely around.

• Cranking batteries have many thin plates, a big internal surface area and are built to deliver high current for a short time, which is exactly what a starter motor wants.

They can stand many such shallow discharges but they hate being almost flattened then recharge. The heat involved buckles the thin plates, and the substantial swelling/shrinkage that goes on in the active material tends to flake it off.

Both these lower capacity, quickly and permanently.The message? Don’t use a cranking battery to power an electric fence (or anything else which demands a low, steady load) unless glory days are already over.

• Deep discharge or storage belt batteries (aka deep cycle – or leisure types) have thicker plates. Used on boats, caravans, electric fence units and so on, they are built to supply power to a steady load and to be run almost flat, repeatedly and without damage.

What don’t they like: Short-duration, heavy loads. Put one on engine-start duty and not only will cranking performance be lower, but you’ll also quickly kill it.

• Absorbed glass mat (AGM) batteries have fine fibre mat between the plates. This stops electrolyte swilling about, cuts shedding and stops evaporation so the battery can be effectively sealed.

The result is potentially long life and high vibration tolerance. Plus plates can be rolled into a spiral. As a result internal resistance is particularly low and cranking performance excellent.

Cells are still vented so charging voltage must be regulated. Conventional trickle or boost chargers will damage AGM batteries.

• Keep casings clean and dry. Current flows between battery posts through a layer of damp, corrosion or oil, increasing self-discharge and hence sulphation. Where battery construction allows, keep cells topped up. Exposed plate material permanently sulphates.

• Where a battery must be left in an unused machine for several weeks, disconnect the negative lead so small parasitic loads like a clock don’t slowly promote sulphation. Radios and such may need coding on reconnection, ECUs may lose stored settings.

Failure is usually down to four things, acting alone or together. Old age in use, plate material slowly dissolves into the electrolyte. As this active material reduces, so battery capacity drops and cranking performance goes down.

This may not be spotted in summer, but often shows up as an increasing reluctance to spin an engine as mornings get colder.

All the while, dissolved material falls into the spaces below the cells. When it builds up enough to short-circuit the plates, the battery suddenly gives up the ghost and will crank hardly or at all. Expect around five years’ service from a good quality, well-looked after battery.

Boost charging both cause plates to shed their material, the latter through rapid heating and shape change.Capacity drops, short-circuiting is likely.

Overcharging

The cells overheat, threatening, plate shape and structure; electrolyte boils off, capacity reduces.

Sulphation

Here’s the prime cause of early failure. During every normal discharge, material at the plates’ surface turns to inactive soft lead sulphate. It acts like an insulating blanket, steadily blocking access to the electrolyte and slowing chemical reaction down, and as a result the battery becomes discharged. Recharging usually splits up the soft sulphate, revitalises the electrolyte and the cycle starts again.

Trouble begins when a battery is not fully recharged, is not kept topped-up with electrolyte or not used for several weeks – as with a combine or self –propelled forager that’s laid up through the winter.

The battery naturally self-discharges so sulphation happens. If this is not reversed by thorough recharging, the normal soft sulphate turns into harder crystals which can’t readily be turned back into the soft form.

The hard crystals lock on to the plates, reducing surface area and lowering battery capacity. First a sulphated battery slowly loses the ability to do its job, then won’t hold a charge and finally fails.

In principle it’s easy to avoid permanent sulphation: use lead acid batteries regularly and charge them completely.