Deep Cycle Performance

We often come across the assumption that a battery is just a battery and, for some reason, build quality, materials and design of the battery don't make much difference. That's clearly nonsense, in the same way that there is a difference between $2 wine and $600 wine, or between cheap tools and quality tools. 

To illustrate the point, in batteries capable of doing cyclic work (being discharged and recharged), there are plenty of different materials and modes of construction. Many, though, are of AGM construction (absorbed glass mat) where a glass matting is placed between the plates of the battery, and this absorbs the electrolyte (acidic solution) and swells so that the battery is moist rather than full of liquid, it is sealed (with a pressure valve to release gas if needed), has very good vibration resistance (due to the matting helping hold the plates in place), generally has more capability to produce quick bursts of power when needed, can be cycled more deeply without damaging the battery, and should last longer than an equivalent sized wet battery.

Now there is a huge difference between manufacturers, and there are some (many) brands that we simply would never be willing to sell. Even with a single manufacturer, they might create (say) 3 levels of AGM cyclic battery. Typically the cheapest will be one that is designed for standy use (in a burglar alarm for example) where the battery sits on charge for years on end and, in the event of a power cut, is meant to leap into life so that burglars can't just cut mains power to silence an alarm. These batteries don't do many "cycles" or discharges, so don't need to be built to handle the demands of frequent deep cycling. Then there tend to be 2 different levels of (frequent) deep cycle models - one that is rated for general deep cycle use, and a higher level that is rated for frequent and heavy cycling, often called an EV battery - suitable for use in electric vehicle applications such as golf carts, mobilty scooters etc. The difference is that the EV model is (or, at least, should be) built to an even higher specification than the standard deep cyle battery. It will cost more, and it should have longer design life (eg 8 years or more for the better brands). Some manufacturers use the EV in their model name eg HZB-12-100 and HZB-EV12-100.

To illustate the difference, the graph, below courtesy of Lion Batteries, shows a typical comparison between a standard deep cycle battery and an EV battery. You'll see that the depth of discharge makes a huge difference - if you cycle 80% out of a battery you might get 200 cycles before it is significantly impaired, whereas if you only take 20% out each time, you might get 5 times the cycles, but also look at the difference in battery life, with the EV battery lasting around 25% more cycles at a 50% discharge, and around 40% more cycles at a 20% discharge. 

What does this tell us? I'd suggest:

• Buy a battery that is suited for the intended purpose
• If an EV battery costs 10% more, you are likely to get that back several times over in terms of performance and longevity
• It is better to cycle batteries less, recharge more often, and use a bigger bank of batteries so that the % discharge is lower
• I'd add to that to buy respected brands and avoid bargain basement rubbish but, if you've read this far, you'll already know that.