Lead Acid Batteries fall into two categories:
- Those that need to deliver short bursts of energy (primarily automotive batteries and so sometimes called ‘cranking batteries’)
- Those that need to discharge slowly over a long period
It is the second type which is used in many Emergency Lighting and Alarm Systems where they are kept fully charged while the circuits are connected to the mains, but when this fails the battery must provide constant power over a long period of time.
(Video of What makes a Deep Cycle Battery Different with Transcript)
There is an important structural difference in that deep cycle batteries need thicker plates. Using a cranking battery in place of a deep cycle version causes the plates to buckle and the unit to fail. Thicker plates also stop the build up of corrosion that can occur during the charging and discharging process.
Despite these requirements there have been numerous cases where some manufacturers have labelled cranking batteries as deep cycle. The main motivator is that lead, one of the primary materials in any lead acid battery, is expensive. Thus selling units with thinner plates makes them more price competitive even though it may not be up to the task.
Legislation on labeling is not comprehensive and difficult to enforce as manufacturers who produce cranking batteries but label them deep cycle can claim there was simply an “error in production” rather than it being any purposeful action. But even to create this situation requires a competitor to open and inspect the battery and then report their findings to the correct authority.
To ensure your unit is capable of deep cycling only purchase a reputable brand. This is why BatteryGuy stocks Power-Sonic, a trusted and leading manufacturer in the field of sealed lead acid batteries.
If you have seen an alternative model offered at a much lower price there is a strong possibility that you are looking at a cranking battery mislabeled for deep cycling use.
For more detail see also: Which deep cycle Ah battery?