In this video we’ll look at what makes a deep cycle lead acid battery different from a standard lead acid battery.
If you’ve ever been looking to buy a battery for your caravan, motorhome, boat or needed replacements for your emergency light or UPS unit people have probably recommended a ‘deep cycle’ battery.
In some cases you may have been struck by how similar they look to standard lead acid batteries not marked ‘deep cycle’ … except for the price tag. So what makes a deep cycle battery different?
Well first, what do we mean by ‘deep cycle’? This is the industry term for batteries which are good at discharging slowly to low states of charge and then recharging again.
Each discharge and recharge is known as ‘a cycle’ or a ‘charge cycle’. Even if you only half discharge a battery before recharging this is still a cycle. But the more you discharge before recharging the deeper the cycle.
Standard Lead Acid batteries are designed to provide high amounts of energy for a short period, say several seconds to start an engine. Deep cycle batteries are designed to provide lower levels of energy over a longer period, usually several hours.
So what gives a lead acid deep cycle battery the ability to, well, deep cycle and justify its price tag?
The key to understanding this lies inside the battery. In our video ‘How a Flooded Lead Acid Battery is Made’ we covered the basics of lead acid battery construction. If you missed it here’s a quick recap.
Lead based Anode and Cathode plates are manufactured as grids to maximise their surface area. They are coated in an active paste and arranged in pairs but kept physically apart by a separator. The pairs are bundled and placed in compartments to form cells. In this example we’ve got a battery with 6 cells. The cells are connected together and then an electrolyte is added to allow chemical processes to take place between the plates and this will create electricity.
OK lets hold it there and go back to those plates.
The plates are lead based but lead is a soft metal.
When placed in electrolyte and connected to an appliance the chemical reactions that take place push and pull at the plates.
The more discharged the battery becomes the more likely they are to buckle and lose their shape.
Small buckling causes the active paste to fall off reducing how effective the plates can be at giving and taking a charge, reducing the battery’s performance and overall lifespan.
This is why a vehicle starter battery can deep cycle but if its done regularly the battery won’t last for long.
Sometimes the buckling can be too much and a plate rips through the separator making contact with the next one. This shorts out the cell and essentially kills the battery.
To get round these issues manufacturers have come up with a number of techniques.
The most basic is just to make the plates thicker so they are less likely to buckle but thicker plates means more lead and as lead is one of the most expensive parts in the manufacturing process it also pushes up the price of the final product.
There are other options. In the 1970s a company called Enersys came up with the idea of rolling the plates into spirals which makes it harder for the lead to buckle. In fact it is so effective that plates can be made from near pure lead. These batteries are often obvious because the casing looks like cylinders. They come in all shapes and sizes and are sometimes labelled ‘Cyclon’ or ‘Pure Lead’
But the added process of coiling comes at a cost.
Another alternative is to make the plates differently. Some designs for plates are made up of tiny tubes with the active paste placed inside. This way the paste can’t flake off and the circular design of the tubes helps the plates resist buckling.
Its a complex design which means, you guessed it, more expensive.
But we are not out of options yet. While some scientists reasoned that changing the plates could help other scientists were thinking that if the plates were the problem perhaps the separator was the solution. Instead of being a thin, weak barrier why not make it stronger so it holds the paste to the plates and helps keep the plates in shape?
They developed a tough glass matt material that could be soaked in electrolyte which meant the electrolyte and separator essentially became one.
The glass matt helps the plates maintain their shape and goes some way to stopping the active paste from flaking off the plates.
This type of lead acid battery is known as Absorbed Glass Mat (or AGM for short)
But glass mat technology is an expensive process so like all other deep cycle solutions it pushes up the retail price.
Absorbent Glass Mat seemed like the ultimate solution for deep cycling because even if it was expensive it was the most effective cost versus performance design so far.
But it isn’t quite perfect for everyone.
When an AGM battery is subject to constant jarring and jolting the Glass Mat can grind against the plates and actually rub the active paste away reducing its performance and reducing its lifespan.
In the early 1980s researchers decided to see if they could work without the glass mat and instead use a silicon gel electrolyte which could be injected between the plates where it would harden into a gel like state
The Gel lead acid battery was born. A battery that could deep cycle and do so even under the most extreme conditions.
But costly materials and manufacturing processes make this the most expensive of all lead acid deep cycle batteries.
So we’ve seen why deep cycle lead acid batteries need to be different inside even if they often look similar to standard lead acid batteries on the outside and we’ve seen the technological breakthroughs that have taken place.
Thicker plates, Coiled Plates, Tubular plates, Absorbed Glass Matt separators
They all, in one way or another, either require more materials to be used or more complex manufacturing processes and this is why deep cycle lead acid batteries always cost more than their standard lead acid counterparts.
So now you know what makes a lead acid deep cycle battery different from standard lead acid products. When a retailer shows you the higher price tag you know what to ask.
What type of plates are used (thickened grid or tubular)?
Is it flooded lead acid, AGM or Gel technology?
These questions are important because the lead acid market is poorly regulated and some distributors actually label standard lead acid batteries as deep cycle and then claim they have the ‘best price’
You won’t notice the difference for a while because all lead acid batteries can deep cycle but the mislabelled battery will die much sooner than a real deep cycle unit and now you know why.
To avoid this popular con stick to well known and established brands and if the price sounds too good to be true, it probably is.
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