Depth of discharge is an important consideration when purchasing an electrochemical battery to power a device. There are two subtle shades of meaning to this expression:
- The term may refer to the maximum amount of energy we can safely withdraw from a battery without damaging it.
- But it may also relate to the amount of available energy remaining in a battery after a period of use.
Safe Depth of Discharge in Secondary Batteries
Secondary, rechargeable batteries deliver energy, and receive top-ups many times in their useful lives. However, this should only be within a mid-range of their total potential depth of discharge, to preserve their useful lives. In fact, many batteries should positively avoid both over-charging, and deep-discharging too.
This is why many batteries, and battery chargers limit taking cells to extreme levels of state of charge, and state of discharge. The safe range of lithium-iron-phosphate LiFePo4 batteries for example is between 15 % and 85 %, and this greatly increases their life cycle. There may also be a safety factor, in that their capacity is greater than their stated value.
Not All Potential Should Be Used During a Discharge
We accept the principle of not wearing our auto tires beyond the safety marker. In fact, there are safety factors everywhere we go, including urban speed limits. For this good reason many battery manufacturers include a discharge limiter, and a controller to prevent overcharging.
If we exceed the recommended depth of discharge – which is often significantly higher than the cut-off point – we can damage the battery and shorten its useful life. Our takeaway is to recharge batteries safely, well before they go flat.
To recap then, we should not view the usable capacity of a battery as anywhere between the extremes. Instead, we should always keep it somewhere in the sweet spot we mentioned. The small amount of time this takes should surely be far less tiresome than having a battery fail on us.
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