Home › magazine › october 2011 › special features › Some truths about batteries
Some truths about batteries25th of October 2011
Some machine manufacturers may disagree, but the most important component in battery operated cleaning machines is … the battery. Without energy, absolutely nothing happens. But which battery is right for my job, fits my machine and my budget? Stefan Louis of Emrol takes a look at the issues.
There are five important aspects to consider when choosing a battery:
Maintenance: Some batteries require regular maintenance: flooded batteries require topping up with demineralised water every 10 to 20 charge cycles. Who will provide this service? Failing to do so will kill even the best battery in just a few months. The market tells us that in installations where the owner also operates the machines, flooded batteries are the best option. But in most contract cleaning situations machines don’t get the right amount of attention and maintenance-free batteries are by far the best choice. Although flooded batteries are usually less expensive to purchase compared to maintenance-free, the latter usually have the lowest cost of ownership. You must decide which is important to you.
Sizes: Not all models are available in every technology. The space available in the machine may therefore determine the battery type. So check the dimensions first, your choice may be limited.
Frequency and intensity of use: Usually several technologies are available within the same physical size. There’s a big difference between the once-a-week 30-minute run through a garage’s showroom versus a daily deep discharge in the shopping mall. Make sure to pick the technology that can handle the load.
Budget and cost of ownership: Batteries are consumables just like printer cartridges and will not have the same life as the machine they provide power to. A battery vendor can estimate the expected life of a certain battery for your application. Try to match this with the contract term. If you plan to use your machine for a long time, the best quality will give you the best cost
Safety: Flooded batteries have a liquid electrolyte which is often spilled during topping up. Eventually this will end up on the floor you are trying to clean, something you don’t want, especially in environments like healthcare or places where food is present. Maintenance-free batteries are significantly safer in usage and transport because there’s no acid slushing around in the batteries. Even when they get damaged and break there will be no leaking.
Flooded versus maintenance-free
There are two types of lead-acid storage batteries, based on their method of construction. These batteries are either called flooded (aka vented) or maintenance-free (aka sealed). Flooded and maintenance-free batteries also differ in the way they work. All lead-acid batteries produce hydrogen and oxygen gas during charging. These gases are allowed to escape from a flooded cell, however the sealed cell is constructed so that the gases are contained and recombined.
It should be noted that hydrogen gas is explosive in air at only four per cent volume concentration.
Flooded lead-acid batteries. Flooded cells are those where the electrodes/plates are immersed in electrolyte. Since gases created during charging are vented to the atmosphere, distilled water must be added occasionally to bring the electrolyte back to its required level. This type of battery has been built since the 1890’s. Two construction types exist: the positive plate is either flat or tubular (a collection of adjacent tubes forming a thick plate). The tubular plate is stronger than the flat plate, accepts deeper discharges, but is somewhat more difficult to charge.
Advantages of flooded batteries:
•Economic choice: Flooded batteries are easier and therefore less expensive to make.
•Cycle life: Best laboratory life, provided regular and proper maintenance is applied.
Disadvantages of flooded batteries:
Sealed lead-acid batteries. These types of batteries confine the electrolyte, but have a vent or valve to allow gases to escape if internal pressure exceeds a certain threshold. The valve-regulated battery is the most common type of sealed battery. Sealed batteries have been around since the 1950’s; however they have proved themselves to have many advantages over the flooded lead-acid battery in some areas.
Also sealed batteries come in two versions, referred to as AGM or gel.
Let’s take a look at the differences. Gel is short for gelled electrolyte which is used in combination with a plastic separator, whereas AGM (absorbed glass mat/microfibre) retains all of its electrolyte in a spongy separator of glass fibre. Consider AGM and gel as two ways to make a battery maintenance-free without having a direct impact on battery life. The separator influences power performance but the life of the battery mainly depends on the plate. Therefore make sure to select the deep-cycle version of maintenance free batteries for any cleaning application.
Advantages of sealed batteries:
•Sealed batteries contain enough electrolyte to last the lifetime of the battery. This means they never need watering and are considered to be maintenance-free batteries.
•Since the hydrogen in these batteries is absorbed in them, there is no poisonous gas emitted by these batteries. Not only does this mean that these batteries can be recharged almost anywhere without special ventilation, but they also are more environmentally sound.
•They do not spill their electrolytes even when turned over. They also will not corrode like flooded lead-acid batteries so there is less chance of getting acid burns when handling these batteries.
•They have a low discharge rate; from one per cent to three per cent per month is typical
•They are sealed with a special pressure valve that must never be opened.
Disadvantages of sealed batteries:
•Although the price of sealed batteries is somewhat lower than it once was, they are still heavier on the budget compared to flooded lead -acid batteries.
Flooded or sealed?
When choosing between a sealed and a flooded lead acid battery you need to consider where and how you will be using the battery. What kind of ventilation is available for charging and what about the availability of water for flooded batteries to be maintained? In some instances, flooded batteries may well be a better choice due to their cost. However, there are many situations where sealed batteries may not only be a better choice but may be the only possible choice.
Tip: Some manufacturers produce spirally wound cells or use thin plate pure lead. These construction types are also a form of AGM technology and offer a low impedance and thus the capability to be charged much faster and/or make use of opportunity charging. Unfortunately these advantages come at a high cost, and relatively shorter life.
Tip: Battery capacity is often specified with confusing units. Nevertheless, capacity is important because it is in direct relation to the working time you will get from the machine. Look at a battery label and you can find the rating eg, 80Ah/5h and 100Ah/20h. But wait a second, how can one battery be 80Ah and 100Ah at the same time? Well, it’s not.
When discharged slowly (over 20 hours) it will be more efficient in converting the chemically stored energy to electricity and provide 100Ah before being fully discharged. But with a faster discharge (let’s say five hours) that same battery would only provide 80Ah before terminal voltage would collapse. Hence why battery capacity can be different depending on the discharge rate. When comparing battery capacities always make sure to do such using the same units.
•Myth: Sealed batteries can only handle shallow discharges.
•Fact: Only deep cycle batteries can support deep discharges. The recommended maximum depth of discharge is 80 per cent, true for flooded and sealed batteries.
•Myth : Gel has a longer life then AGM. Life is in relation to plate construction and paste formulation, not how you immobilise the electrolyte. AGM offers a lower resistance, resulting in a lower temperature increase and therefore longer life. AGM resists heavy vibration better due to construction
•Fact : AGM batteries are easier to charge. Gel batteries are easily overcharged resulting in gel dry-out. AGM accepts higher charge currents more easily.
•Myth : Flooded batteries provide more capacity than sealed. On the contrary, AGM provide most capacity due to their compressed construction and the absence of a sedimentation area.
•Fact: AGM batteries have a lower resistance, enabling a better performance for starting applications.
•Fact: AGM batteries have a higher energy density. The available volume is used more efficiently.
•Myth: Gel is better suited for deep discharges. Again, it’s the plate that sets the performance. Some manufacturers don’t know how to make a proper deep cycle AGM and give the technology a
•Fact: Gel offers better protection against microshorts. The plastic separator acts as a wall between the plates.
•Fact: AGM works better at low temperatures. Gel power reduces faster when temperature is <0°.
•Fact: Gel has an equal gravity from top to bottom which can extend life.
•Fact: The plates in AGMs are tightly packed and rigidly mounted. They withstand shock and vibration better than any standard battery.
Now that we’ve talked about the four main categories of flooded and sealed batteries, let’s take a look at how to charge them. A charger restores the energy in a battery that was used, similar to your mobile phone's battery. And it has to do such under strict control and precise guidelines to maximise battery life: not too fast, and not too much nor too little. Both overcharge and undercharge will lead to premature battery failure.
Modern battery chargers are programmable current sources. The fact that they are programmable makes them compatible with different battery technologies, but also introduces a weakness: an incorrect setting can cause incorrect charging leading to battery failure. So called high-frequency chargers convert mains power to a voltage compatible with the battery set with the best efficiency technology has to offer. Knowing that every Watt produced causes CO2 emissions and costs money, a high efficiency charger is important. In most cases a high efficiency charger saves you money in the long run.
Batteries may not have the greenest reputation, but can actually be very well recycled. On the condition of course that they are passed on through the proper channels to the recycling installation. As a motivator you can get up to 0.5 euros per kilo of battery weight. For an average battery pack in a cleaning machine that is 20 euros!
Batteries are dangerous and toxic. They can provide an uninterruptible high power short circuit current and explode. They produce gasses and contain aggressive chemicals. Make sure to read and apply the safety instructions that are available from your supplier.
Even though lead acid has been around for 120 years, new chemistries are quickly claiming its market-leading position. Developments like the electric vehicle and renewable energy have pushed for better energy storage solutions. Knowing that you need a metal to store electrochemical energy and with lithium being the smallest metal on earth, it’s the obvious candidate for new technology batteries. Our mobiles and laptops depended on lithium-ion batteries quickly after they emerged. The lithium advantages in a nutshell are: more autonomy, volume/weight saving, longer life, fast charging, operation in extreme temperatures, lowest cost of ownership, ecological.
Large format lithium batteries are now also coming into the market but deep pockets are still required. Machine manufacturers are seriously looking into the technology and agree it will take a few years for prices to come down. But then this technology will be a viable candidate alongside the matured lead-acid chemistry.