Who would want a cell phone you couldn’t use when the battery goes below 50%? Or a car you have to fill with gas every time the tank hits half-empty? I wouldn't even buy a cup of coffee if the barista said I could only drink half. So why do people buy so-called deep-cycling lead acid batteries that only let them use half of the battery's capacity? I want a battery that lets me use all of its amp-hours.
Is that too much to ask?
What is deep cycling?
To understand why lead acid batteries are limited, we should first explain what we mean by “deep-cycling application”: any use case where the battery system is sized to be charged up to its upper voltage limits (100% state of charge) and then discharged all the way down to its lower useful limits (0% state of charge). Applications with constrained initial capital expenditure, limited installation space, or widely varying use cases will often require the battery to be discharged all the way down to its limits, again and again.
A remote cabin that relies on a small solar array and a few lead acid batteries would be a “deep-cycle” application. The same would be true of a residential installation of batteries in Germany being used to maximize PV self-consumption, or a remote industrial site in the outback of Australia that relies on renewables and solar for power. In all of these cases, the system owners would be best served by being able to run the batteries down to 0% state of charge, squeezing every single drop of energy out of the battery for their need.
Deep cycling limits of lead acid batteries
The reason why maintenance-free, deep-cycle batteries are so desirable is because, in theory, they offer the best combination of attributes: the ability to use all of the energy in the product and without the need for routine (and often annoying) maintenance.
Lead acid batteries that advertise themselves as “deep-cycling” often stretch the definition of the term. Many lead acid companies recommend that the user not discharge more than 80% of the battery’s energy. Sometimes it’s only 50%. This means that a battery rated at 100 Ah would deliver only 50 Ah. And even at these limits, these batteries are often rated for just hundreds of cycles, meaning that in just a few years I’m going to be paying for another 100 Ah (and only getting to use half of them).
Lead acid battery maintenance
On top of this limited capacity, owners of lead acid batteries need to continually keep the battery boost-charged (to avoid sulfation from sitting at a partial state of charge), check electrolyte levels, refill flooded batteries with deionized water (which I don’t just have lying around my garage), keep the terminals clean, check for loose electrical connections, and ensure the batteries aren’t exposed to elevated temperatures, among other things. All that makes for a fairly labor-intensive system, even for the most “maintenance-free” lead acid batteries.
Despite their annoyances, lead acid batteries used to be the only cost-effective choice for consumers needing deep-cycle batteries. Their drawbacks--oversized systems, the nuisances of ongoing maintenance, and premature cycle failures--were just a way of life for customers.
Now there are newer battery technologies on the market that promise the dream of true maintenance-free, deep-cycle performance, such as saltwater batteries, lithium ion, nickel iron, and flow batteries.