What is the lifespan of a deep cycle battery?
Battery lifespan is not a set number of cycles or years. It varies according to multiple factors, such as average discharge, proper charge routine, temperature, and quality of maintenance. In optimal conditions, the lifespan can be maximised to make the most of your investment.
The key factors determining battery lifetime are:
- Average Depth of Discharge (DoD)
- Frequency of discharge (cycles)
- Average temperature
- Typical discharge and charge routine
- Storage state and conditions
- Amount of appropriate monitoring and maintenance conducted
If you regularly discharge your battery deeply (beyond 50%), store it at a high temperature (>30C), discharge it very rapidly (~C/5 rate), or undercharge/overcharge it, lifespan and capacity will decrease. Similarly, if you are unable to provide your battery with proper storage and maintenance it will fail prematurely.
Treat your batteries like a newborn child: think for them, care for them, ease them through their times of suffering, and you will be rewarded with a fulfilling and satisfying relationship. Treat them wrong, and they’ll cause you a load of problems.
Cycles, Depth of Discharge and a day in the life of a battery
The longevity of a battery is often referred to in cycles. A cycle most accurately defines the working life of a battery, allowing it to be given a rating. A 'cycle' refers to one complete discharge and recharge, for example, a 24 hour period of battery use. Of course, a 24 hour day could see several cycles or a cycle may last multiple days (eg. when there is no sunlight to recharge you batteries).
It is assumed that the battery starts at ~100% charge, however, the level of discharge can obviously vary due to the daily requirements. It might only be discharged by 20%, maybe 50%, or potentially 90% (un-wise!). This figure is referred to as the depth of discharge (DoD). A battery 'cycle rating' (amount of cycles it is capable of) will depend on the typical DoD. The rating assumes that this is the typical depth of discharge, for every cycle, over the lifetime of a battery and does not include exceptional discharges.
For example, for battery X:
At a typical 20% DoD per cycle, 6000 cycles are expected
At 50% DoD, 3200 cycles are expected
At 90% DoD, 1100 cycles are expected
Discharging a battery over 60% DoD is not prudent, and over 80% should be avoided except if completely necessary. The further you discharge, the more you reduce the lifespan of your battery.
100% discharge in one cycle is the equivalent of 50% discharge over two cycles and 10% discharge over 10 cycles. A battery can only take a certain amount of energy throughput, therefore cycling down to 100% removes potential energy further down the line. To compound this, the effect is not linear and more 'cycle damage' is done with the deeper discharges. As each cycle occurs, a chemical reaction takes place which becomes slightly less potent at each next cycle. The reason for this is age, and a build up of the lead sulphate by-product from each reaction leaving a coating on the plates in the battery - this is known as sulphation.
To maximise the lifetime of a battery, typical discharge should be restricted to no further than 50% DoD. This represents the best trade-off between lifespan and cost.
You can extend battery life by storing it at a moderate temperature. Sustained heat kills batteries and will reduce their battery life if not controlled. Over 25C, each 8C rise in battery temperature cuts a sealed lead-acid battery life in half. Once a battery has been damaged by heat, the lost capacity cannot be recovered. Most battery specifications are at an operating temperature of 25-27C. In order to improve battery life, ideally a battery would be stored between 10C and 20C, with a sacrifice to performance (capacity).
High temperatures speed up chemical processes, however they may also initiate unwanted and damaging chemical reactions. Similarly, they may also lead to loss of electrolyte, which is especially damaging in sealed batteries.
At extremely low temperatures (below freezing), a battery will freeze when it is past a certain state of discharge. As the sulphuric acid is removed from the electrolyte, the remaining water will freeze, preventing discharge. A fully charged lead-acid battery will not freeze until -62C. However, at 60% DoD, a battery will freeze at -20C, and at 100% DoD, -5C. Naturally, allowing a battery to freeze will heavily damage a battery. If it is going to be operated at a freezing temperature, it must not be discharged to a point where it will freeze.
Design Life & Calendar Fade
If a battery is cycled, discharged, charged, stored, and maintained properly, it will live to its design life and maybe beyond. This could mean a true deep-cycle lead-acid battery lasting 20 years. Longevity of this sort would require optimal conditions and maintenance. Even if these were achieved, the capacity of the battery would still diminish as time goes on due to the natural ageing and degradation of the battery components. Proper maintenance can reduce but not prevent this degradation.