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Deep-cycle batteries – watts it all about?

Deep-cycle batteries – watts it all about?

Materials handling equipment from Simpro is typically powered by deep-cycle batteries. These are a complex and rapidly-evolving technology, so it’s important to know watt’s watt when you’re diagnosing, repairing or replacing the batteries in a machine.

Okay, no more puns. Here’s watt it’s all about.

About deep-cycle batteries

Deep-cycle batteries are reusable chemical-energy storage devices, designed to provide steady power over an extended period, and to be fully discharged and recharged many times. Although close cousins of the regular car (SLI) battery, they have different characteristics: car batteries are designed to provide a brief burst of intense power and then return to charging. This distinction is important, since using a car battery in equipment designed for deep cycle batteries – or vice versa – can have rather expensive consequences.

There are several types of deep-cycle battery, with different chemistries and internal structures.

Flooded lead-acid batteries

These batteries, also known as wet cells, are based on the lead-acid chemistry discovered in 1859 by Gaston Planté, and are also the oldest form of rechargeable battery. They feature electrodes (lead plates) immersed in electrolyte (a pool of distilled water and sulfuric acid), in an unsealed case. Although cheap and simple, flooded lead-acid batteries are increasingly being phased out in favour of more modern types.

Forklift parked at charging station, showing vents for hydrogen gas generated while charging


  • Inexpensive
  • High surge current
  • Uncomplicated design allows easy troubleshooting and repair


  • Require regular maintenance and topping up with distilled water
  • Unsealed case must be kept upright or the electrolyte will leak
  • Very heavy relative to the amount energy stored (low energy density)
  • Emits explosive hydrogen gas while charging so requires good ventilation
  • Must be shipped as Dangerous Goods
The need to safety vent hydrogen gas while charging flooded lead-acid batteries is the main reason for the vents and grilles found on on older battery-powered equipment.

Sealed AGM batteries

Sealed AGM (Absorbent Glass Mat) batteries use the same chemistry as flooded lead-acid batteries, but rather than a pool of acidic water, the electrolyte is composed of glass fibre mats soaked in sulfuric acid. The case is completely sealed, apart a pressure-release valve. This allows for a compact, maintenance-free battery – ideal for the military applications for which it was originally developed.


  • Lighter and smaller than flooded lead-acid batteries for the same capacity
  • No regular maintenance or topping-up required
  • Low internal resistance allows very fast charging, with reduced energy loss
  • Sealed case allows batteries to be used, stored or transported in any orientation
  • No hydrogen emissions in normal use, reducing ventilation requirements
  • Slow self-discharge rate, suitable for long-term storage
  • Does not need to be shipped as Dangerous Goods
  • Most common type of deep-cycle battery


Recombinant batteries convert hydrogen back into water, rather than releasing it
  • More expensive than flooded lead-acid batteries
  • Charging too quickly or overcharging may damage the battery
  • Relatively difficult to diagnose or repair
  • Very heavy, with low energy density
Sealed AGM batteries are a type of ‘recombinant’ battery, meaning the hydrogen generated by charging is recombined with oxygen internally, forming water, rather than being discharged. If the battery is charged too quickly this process may not keep up, causing excess hydrogen to be released through the safety valve.

Sealed gel batteries

Sealed gel batteries are another type of lead-acid cell, with a semi-solid ‘gel’ electrolyte. Like AGM batteries they have a sealed case, can be used in any orientation without leaking, and don’t normally release hydrogen. However, they can be damaged by incorrect charging. Simpro uses smart digital chargers to protect the gel batteries in our products.


  • Small, robust, extremely reliable batteries, requiring no regular maintenance
  • Sealed case allows batteries to be used, stored or transported in any orientation
  • Can withstand very deep discharges without being damaged
  • No hydrogen emissions in normal use, reducing ventilation requirements
  • Very slow self-discharge rate, as low as 3% per month, suitable for long-term storage
  • Does not need to be shipped as Dangerous Goods
  • More resistant to shock, vibration and temperature variations than other types of lead-acid battery
Simpro deep-cycle sealed gel batteries


  • Typically cost 30% more than equivalent flooded lead-acid batteries
  • Can be permanently damaged by incorrect charging
  • Relatively difficult to diagnose or repair
  • Very heavy, with low energy density
Sealed gel batteries are the most expensive type of lead-acid battery, and generally the most robust. Simpro products are supplied exclusively with this type of battery.

Other types of deep-cycle battery

In addition to those listed above, other deep-cycle batteries are available with different internal chemistries. The best-known of these is lithium-ion – the new lightweight, powerful batteries used in mobile phones and electric cars. Although deep-cycle lithium-ion batteries are available for industrial use, they are rather prone to catching fire, and their cost is generally not yet low enough to compete with lead-acid batteries.

Understanding battery ratings

Even the most durable battery will eventually need to be replaced – and since deep-cycle batteries are expensive, it is helpful to understand the different ways they can be rated and compared. Here are the four most common indicators:


The first number most people look for is Amp-hours (Ah), which expresses the total amount of chemical energy the battery can store and release as electricity. Amp-hours can be simply understood as the length of time the battery will operate for before needing to be recharged.

A multimeter can be used to check the voltage generated by a battery


Another important measurement is voltage (V), which indicates the electromotive force – or ‘power’ – of the current supplied by the battery. Like most electrical systems, Simpro products are designed to operate with a particular voltage, and will be damaged by a more powerful current. It is essential when replacing batteries to ensure the voltage is correct.


Because the capacity of a battery actually varies depending on how it is used, another useful indicator is the C-rating. Putting it simply, the C-rating defines the length of time over which a battery should be discharged, in order to delivery its rated capacity. A 10Ah battery with a 1C rating will deliver 10Ah over 1 hour, but less power if discharged more quickly, and more power if discharged more slowly.

Although the C-rating refers to time in hours, it is an inverted unit – a rating of 2C means 30 minutes, and a rating of 0.2C means 5 hours. This is often simplified by using a division operator to re-invert the unit, so C/2 means 2 hours, and C/20 means 20 hours. Most deep-cycle lead-acid batteries are designed to discharge their rated capacity over 20 hours, which can be expressed as either 0.05C or C/20.

Cycle life

Battery cycle life comparison

Batteries are also sometimes rated by cycle life – the number of times they can be discharged and recharged without losing more than 20% of their capacity. The most common benchmark is IEC 60896, which is based a repeated 100% discharge/recharge deep cycle. Although deep-cycle batteries aren't normally used this way, it is a good indicator of their likely longevity.

Quite often batteries that are otherwise identical have different cycle life ratings, which is useful when comparing batteries from different manufacturers, or selecting a battery for a particular application.

Simpro products are supplied with sealed gel batteries, typically rated 21Ah 12V C/20. They are extremely durable, but if one does fail it should be replaced like-for-like with a battery supplied by Simpro. If a generic battery must be used, make sure it is a sealed gel type with the same Amp-hour, voltage and C-rating as the original. Never mix old and new batteries together, or batteries of different types.

A Quikstak™ with the cover removed, showing the powerpack and two deep-cycle sealed gel batteries

Battery maintenance

Although sealed lead-acid batteries require little maintenance, their lives can be greatly extended by observing some simple rules.

  • Batteries should not be regularly discharged to below 20% capacity, because recharging a very flat battery generates heat and accelerated sulfation, damaging the electrodes. Increasingly, automatic cut-off switches are used prevent excessive discharging.
  • Batteries that are in storage will slowly self-discharge, and should receive an occasional top-up to prevent them from going completely flat – this will cause sulfation and damage the battery. The sealed gel batteries used by Simpro have a very low self-discharge rate of about 3% per month, which means they can be safely stored for up to two years without charging. Other types may need topping-up more often.
  • Lead-acid batteries must be charged correctly. Charging too quickly or overcharging might not only damage the battery; it can cause an acid leak, short-circuit, hydrogen fire, or battery-case explosion. It is always recommended to use a smart digital float charger with automatic current and temperature regulation, like the onboard chargers fitted to Simpro products.
  • Flooded lead-acid batteries require additional checks and maintenance, but they are no longer common in industrial equipment. See this article for more information.

For most modern deep-cycle batteries, that’s all there is to it. Observe these simple rules, and your battery will give you hundreds of cycles, and many years of trouble-free service.

The deep-cycle batteries in a Simpro Dumpmaster™ can lift more than ten tonnes of material on a single charge