Lead acid battery

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Lead acid batteries are the lowest cost per capacity rechargeable batteries, and are widely used as vehicle and power backup batteries. DIY applications include alarm power supplies, UPSes, emergency Lighting, offgrid power generation, and other power backup applications.


Lead acids consist of lead plates in sulphuric acid, housed in a plastic container. The result is a very heavy battery for its size, but also the lowest cost per capacity of any rechargeable type.

Historic batteries also used glass, rubber or celluloid cases.


Lead acid batteries are widely available from 2Ah to around 100Ah in 6v, 12v and 24v batteries and 2v cells. Specialists can supply almost any voltage and capacity required.

Starting battery

These are widely used in cars & trucks, and are optimised for high engine cranking current. They are not very tolerant of deep discharge, and repeated discharge causes premature failure. The batteries spill acid if tipped.

Car batteries are not suitable for repeated deep discharge cycles. Used this way they have short lives. These have alternating plates closely spaced, and are optimised for high current delivery.

Car batteries are not suited to offgrid power use. Many have tried and they don't last well, if discharged to 10.8v as usual. Its possible to use them by substantially increasing the capacity, and stopping discharge at well above the usual voltage, but deep discharge batteries can be expected to last much longer.

Deep cycle

Some lead acid batteries are intended for deep discharge cycles. These use bulk plates with all the anode on one side and cathode on the other. Leisure batteries are deep cycle type.

Deep discharge batteries still don't like deep discharge. The further they're discharged, the less cycles they last for. Discharge to 20-50% is often used as a means to get longer battery life.



Sealed Lead Acid batteries are spillproof, and don't have to be kept upright at all times. The liquid electyrolyte is held in place using gel or glass fibre mats.

While this is a boon for some apps, they do have downsides. They can only be slow charged at 13.6v. Charging them on a charger designed for non-SLA batteries soon kills them. When they eventually dry out - which they often do - topping up is impossible. They have shorter life expectancy than a properly maintained unsealed battery - though longer life than an unmaintained unsealed battery.

In some there's also a catalyst to recombine the H2 and O2 produced during overcharge, but there's a significant cost premium for this. This helps the cells resist drying out caused by non-ideal charging.

Buggy batteries

Small electric vehicles such as golf carts & mobility buggies require batteries with 3 properties:

  • deep cycle tolerant
  • spillproof
  • vibration tolerant
  • certification for mobility use
  • certification for aircraft carriage is often required too, if the user goes abroad

These properties increase the price of the batteries by around 50%. Use of cheaper types that aren't deep cycle and vibration tolerant causes premature failure.



Fully charged cell terminal voltage is 2.2v, giving over 13v for a nominally 12v battery. This voltage declines in use, and discharge should be stopped at 10.8v, giving an average of 12v. Discharge to lower voltage reduces battery life.


Lead acid cells are generally capable of delivering huge currents, and thus can be used for starting vehicles and generators, spot welding, heavy power backup and so on. A typical car battery will deliver well over 100A.


Capacity is described in amphours (Ah). 20Ah means a battery can supply 1A for 20 hours. This battery could also supply approx 2A for 10 hours. Faster discharge reduces capacity to some degree. So if our example battery were discharged at 10A, it would fall short of 2 hours run time. Rated amphour capacity is measured for a 20hr discharge. Rated capacity describes output, not input.

If a battery is discharged at high current, it becomes flat, then can recover to an extent, allowing further discharge.


The energy a battery can supply is its capacity multipled by terminal voltage. Thus a 12v 30Ah battery could supply 12v x 30Ah = 360 watt hours, or 0.36 kWh.

Energy efficiency

Energy Efficiency of lead acids is around 75%. This means 100 watt hours in will yield around 75 watt hours out. Energy efficiency is maximised by using slow charge and slow discharge, higher current in either direction reduces energy efficiency further.

Life expectancy

Life depends on how long the battery spends flat. Lead acids don't survive well when left flat, and this should never be done if possible.

Life also depends on how deeply the battery is discharged. A battery discharged 50% lasts longer than one discharged 80%, which lasts much longer than one discharged 95%.

Life also depends on maintenance. Batteries need an occasional top up with deionised water, and less often acid needs to be added to restore electrolyte strength. Lead acids are often used without any maintenance, but last rather longer when attention is paid to their basic needs.


Batteries not in use should be recharged monthly. If not in use for a long time the battery should be drained, rinsed quickly and dried. Dry they should store indefinitely. Too much rinsing can reduce plate life after refilling.



Lead acid cells are typically capable of delivering huge currents, and the resultant fire risk is very real. In any application a fuse should be fitted close to the battery to prevent fire in case of a short.

Insulating covers over the terminals eliminate the risk of a dropped tool getting red hot.


Most lead acids spill acid if tipped. Goggles should be worn if moving open topped cells due to the likelihood of splash. Acid can burn skin and eyeballs, and awareness of the safety issues is required when handling battery acid. Baking soda and water can be kept to hand in case of spill. A garden hose is more effective with large battery spills.


During charging, lead acids give off hydrogen and oxygen gases. Normally this explosive mixture mixes with surrounding air rendering it safe. However in some unusual situations the gases can build up and cause a small explosion. A little ventilation should always be provided to prevent any chance of this occurring.


Batteries grossly overcharged can reach boiling point, and emit clouds of sulphuric acid laden steam. This requires excess current rather than just excess charge time, several 10s of amps for a typical car battery. Its not a fun situation to be in, and I speak from experience. Most applications don't include charging apparatus capable of delivering enough power to do this, but where they do and the battery lives indoors, a safeguard should be present to prevent gross overcharge. A fuse or thermal breaker on the charger can be effective.


Lead acids have the highest weight per electrical capacity of all rechargeable batteries, and this is the main limit on their use.

Large lead acid cells can cause back injury on lifting, or broken foot bones if dropped. Very heavy cells can be kept at floor level during moving, using plywood on castors to move them, or the cells can be constructed in situ. Very large purchased cells may be moved with a hand forklift truck.


Charge rate is usually expressed as the ratio of charge current to capacity. For example C/20 is the current in Amps that's 1/20th the battery's Ah capacity. For a 100Ah battery that would be 5A.

Crude chargers can be used at rates of C/20 or slower. The faster charging is desired, the more control the charger needs to exert to avoid battery damage.

Smart chargers enable faster charging without risk to the battery. But some of them are in reality rather dumb, and refuse to charge a completely flat battery.

Lead acids are charged using a constant voltage.

  • Unsealed 12v batteries may be charged at up to 14.4v. Some cars do use higher charge rates, much shortening battery life.
  • SLAs should not be charged at above 13.6v, so max charge speed is slower.

See Also