Busting the Myth of 6 Volt Batteries

We have all heard this myth: 6V batteries are better than 12V batteries. Is there any truth to this? Where does the myth come from? What is the evidence? why do you care?

Foreword

Let’s just skip right to the facts here. 2V, 6V, 8V, 12V, it doesn’t make a difference. There are dozens of more important factors at play when you are choosing batteries. The terminal voltage of each cell in your bank is, for the most part, completely irrelevant.

How do I know?

I am an electrical engineer and a battery technician. I have used, diagnosed, replaced, installed, and interfaced with thousands of batteries during my career. I have serviced battery banks made of thousands of 2V cells in the telecom industry, maintained fleets of batteries in fire apparatus, and built custom battery banks for hundreds of mobile applications. I have worked with nearly every battery technology available in any configuration you can imagine. I have performed logged scientific long-term analysis of charge and discharge rates, real capacity tests, self-discharge tests, temperature variation tests, of many different battery chemistries and constructions. I design, sell, and build battery systems that when measured in their intended installation, show the results predicted in the project plan.

Battery Myths

Here are some of the common misconceptions which have no basis in reality:

  1. Two six volt batteries in series is “better” than one twelve volt battery
  2. “Deep Cycle” is a technical term that means something quantifiable
  3. “‘Golf Cart’ batteries last longer”
  4. Six volt batteries have thicker plates which make them last longer

Battery Facts

  1. Lead Acid batteries are made up of battery cells. Each cell has a potential of 2.1 Volts. Three cell batteries contain three cells in series to form a 6.3V battery. Six cell batteries contain six cells in series to form a 12.6V battery.
  2. There are thousands of battery constructions available from hundreds of manufacturers in three cell and six cell configurations with a vast spectrum of performance characteristics
  3. There is absolutely nothing inherently different in the construction of the cells, plates, materials, or otherwise between three cell (6.3V) or six cell (12.6V) batteries

So where did the six volt Battery myth originate?

In the late 1950’s, and throughout the 1960’s, golf carts gained widespread acceptance in North America. E-Z-Go, Cushman, Club Car, Taylor-Dunn, Harley-Davidson, and Yamaha Golf Car all started producing golf carts in this era, most of them electric. These carts required a different type of battery than what was commonly available in the day. Most batteries back then were designed for starting engines, and then were charged “floated” the rest of the time. This means they needed to deliver very high “short circuit current”, for very short duration. Golf carts, on the other hand, needed to deliver a very steady, reasonably high current, for as long a period of time as possible. Then they would be charged overnight, and this cycle would repeat over and over again, for as many days as possible.

The golf cart companies worked with battery manufacturers to design batteries made specifically for their application. They designed batteries with higher purity lead plates made of thicker material that resists sulfation. Through trial, error, measurement, and corrections, they came up with chemistries and constructions that were optimized for their usage case. Then, they bought millions of them.

In the 1980’s and 1990’s, RVs became popular, and in many cases the owners depended on batteries to keep their lights on while camping out without power sources. Many of the RV’s were equipped with typical 12V batteries better designed for starting engines than sustaining loads for an extended period of time. Some of the RV folks were also Golfers, and along the way someone made the link that maybe batteries designed for golf carts would work well for RVs.

They were right – and the economics worked in their favor. Some companies were marketing “Deep Cycle” or “Marine” batteries, but they were expensive. 6V golf cart batteries were cheap – simply because of the economies of scale, and they suited the application. Thus the trend began – and people started saying “use two 6V golf cart batteries instead of one 12V battery – it’s better”. What they were really saying is “use batteries designed for repeated discharges down to 80% depth of discharge – it’s better suited for your application”. Thus a Myth is born.

What’s different now?

What’s different? Everything! Cycle life and depth of discharge have simply become more important battery characteristics than raw short circuit current. The market has spoken and the manufacturers have listened. Material science has progressed, purity has improved, and battery designs have become increasingly targeted towards specific applications. Just as golf cart batteries were designed specifically for golf carts, now we have batteries specifically designed for UPS systems, on-grid and off-grid Solar Systems, Hybrid Systems, and just about every configuration you can imagine. These batteries and the chemistries and designs they are based on are targeted to various charge and discharge rates, depth of discharge, cycle life, and environmental situations.

What should I care about then?

To answer this question, we need to clarify what the application is. Most of our customers are buying batteries for inverters and for powering DC loads such as lighting and fans. When you’re buying batteries for a mobile energy application, all you really want is to fit the maximum Watt-hours of energy storage into the space you have available with the least amount of weight and cost. Here is what really matters:

  • How many total watt-hours can I fit in the space I have available
  • What is the cost in dollars per kWh (Watt-hours)
  • What is the gravimetric energy density (how many Watt-hours per Kilogram of weight)
  • What is the volumetric energy density (Watt-hours per litre of space
  • How deeply can I discharge the batteries (What Depth of Discharge %)
  • How many cycles will I get before the battery is trash?
  • How good is the warranty? (who are you buying from and how well do they support their customers?)
  • What is the intended environment of the battery? (Vibration, mounting orientation, temperature, etc)

How do I choose Batteries?

Every application is different, and there are so many battery types, configurations, manufacturers, and other variables to consider. A battery systems expert is a necessity to analyze the various factors and recommend the best solution.

Here’s what a battery expert can (and should) do for you:

  • Identify your usage case (kWh usage)
  • Quantify your investment period
  • Measure your raw power requirements
  • Measure your available space
  • Identify your charging capability
  • Compare your available technologies
  • Calculate your cost per kwH over your investment period

Only then can an educated recommendation be made as to what exact battery technology, chemistry, and configuration will be the best choice for your specific application.

Why are AGM Batteries better?

AGM (Absorbed Glass Mat) is a lead acid battery technology in which the lead plates are separated by woven fiberglass mats. The fiberglass mats absorb the electrolyte, as opposed to the liquid electrolyte, which flows freely in FLA (Flooded Lead Acid) batteries. The absorbent glass mats suspend the electrolyte.

All AGM batteries are of the VRLA type (Valve Regulated Lead Acid), which means they are maintenance-free. The valve is a pressure release valve and remains closed under normal operating conditions. The only time the valve will open is if the battery is over-charged, which causes gas to build up pressure inside the casing. The pressure is caused by the process of electrolysis, which decomposes the water inside into its hydrogen and oxygen gas components. The gases have a higher volume than the water they used to be, and thus the pressure inside the sealed container increases.

AGM batteries have many benefits over traditional FLA (Flooded Lead Acid) batteries. These benefits are a result of the battery’s construction and are therefore independent of the size of the battery. Foremost among the benefits is their ability to be deeply discharged down to 80% DOD and recharged while sacrificing a relatively low numbers of total life cycles. Another important benefit of AGM batteries is their low self-discharge rate. AGM batteries typically self-discharge at around 2-3% per year while FLA batteries self-discharge rate at up to 5% per month. Both of these benefits translate to a longer battery life.
Other benefits of AGM batteries include:

  • More environmentally safe and easier to ship, because they eliminate the risk of battery acid spills or leaks
  • Require less maintenance than FLA batteries because they do not need to be topped off with electrolyte
  • Faster charge times due to lower internal resistance
  • Better resistance to vibration due to the immobilized electrolyte
  • More versatile in terms of mounting position due to the immobilized electrolyte
  • Very low self-discharge during storage or inoperation

AGM batteries are most useful in applications where continual, consistent operation is desired and, at times, critical. At Epic Energy, we employ AGM batteries in many of our energy systems. Whether that system is a part of a fire and rescue vehicle or a food truck, it is of utmost importance that the battery system be as reliable as possible.

Conclusion

Whether you have a 12 Volt system comprised of 2 x 6V batteries in series, or a 24V system made from 2 x 12V batteries in series, or a 48V system made of 8 x 6V batteries in series, they are all made up of 2V cells. The only difference is the number of inter-cell connections that are internal and external of the battery cases. All lead-acid batteries are made of 2V cells. So if you hear someone recommending that you run two 6V batteries instead of a single 12V battery, you can confidently tell them “it’s the same thing!”

External connections should be minimized whenever possible, because they are prone to corrosion, loosening, human error, are exposed to the elements, and they’re expensive! When considering what battery you might need for a certain application, there are far more important aspects to consider than the terminal voltage. Once you consider all those other aspects, choose a battery with a terminal voltage that makes sense for your application.

12 Volt Batteries

2 responses to “Busting the Myth of 6 Volt Batteries”

    • Good Question!

      I can answer it in three parts. First, why do highway trucks need 24V?

      You’re correct that many trucks have 24V systems instead of 12V systems. This comes down to current. As you double the voltage to a particular load, you half the amperage required to power that load (Because of Ohm’s law, power = Voltage * current). The size of wire you need to supply power to the load depends on the amperage and the length of wire, not the voltage. The result is that, generally speaking, if you double the voltage to a load, you can half its wire size. Copper is expensive, and highway trucks have many high-power loads. There comes a point where it makes sense to increase the voltage used on a vehicle in order to reduce the wire sizes required throughout the chassis. Whenever you see a vehicle with large inductive loads such as pumps, heaters, solenoids, cooling systems, etc – chances are it will employ a 24V system. In fact we do the same thing in our UPS systems. If you look through our projects, you will see some operating from 12V battery backs, some from 24V packs, and some from 48V packs. The bigger the system in terms of Watts, the higher voltage we use. High Power 12V systems reach a point where the losses from the copper conductors themselves become a significant source of inefficiency. Also, wires larger than #4/0 can be challenging to work with, as they take up lots of physical space, they have larger bend radii, and they require large tooling for crimping and terminating. A typical #4/0 AWG circuit can handle about 4000 Watts at 12V, 8000 Watts at 24V, and 16,000 Watts at 48V.

      Why do these trucks use multiple 12V batteries instead of a single 24V battery?

      This comes down to economies of scale, as well as maintenance. There is a much smaller market for 24V batteries than there are for 12V batteries in a given case size. By pairing up multiple 12V batteries, an OEM can put together their own 24V battery pack easily. For the battery manufacturers, each SKU they add increases their overhead, so it’s simply cheaper to make more 12V batteries. 24V batteries would be made up of 12 x 2.1V cells internally connected in series. If any of the cells develop a short-circuit or other issue, the whole battery would require replacement. By splitting the bank in two, half the battery pack can be serviced if the other half is still in good condition.

      One more thing…

      Sometimes, even if you have a 24V system, there is still need for a 12V power. For instance, how will the driver plug in a 12V phone charger, or install a 12V navigation system? To accommodate these situations, the battery pack can be “center tapped”. This means that a load can be connected between one of the 12V batteries and ground, and supply a 12V circuit. While this is sometimes done, it must be done with care to ensure the two batteries in the bank stay at an equal level of charge. Battery charge equalizers are available for this purpose. In general, center tapping a battery bank should be avoided.

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