What are common applications for 24V batteries?

All electric vehicles (EVs), including Battery Electric Vehicles (BEVs), contain a conventional, 12V lead-acid battery. It is used as an independent power supply for the keyless entry and alarm system, which need to still function even if the main traction battery is drained, and also to supply legacy 12V equipment, such as the airbag system, seat-belt tensioner, and dashboard displays, where recertifying them with a different supply voltage would be time-consuming and uneconomical.

In ICE (Internal Combustion Engine) vehicles, the lead-acid battery is also used as a starter battery for the engine. In mopeds and motorbikes the battery can be 6V, in most cars it is 12V and for heavy-duty trucks, it is usually 24V. The multiples of 6V are also not a coincidence. In the world of batteries, with many different chemistry types (i.e. – lead-acid, Li-ion, LiFePO4, etc.), the most fundamental unit is a battery cell, which can have a floating, open-circuit (e.g. – native) voltage in the range of 1-4V. Therefore, higher voltages are derived from combining many cells in what is formally known as a battery pack (especially when combined with protection circuitry), but more frequently referred to as simply a battery in common vernacular. Cells are combined in series to achieve the necessary output voltage (and can also be paralleled to increase output current).

For a lead-acid battery, the cell voltage is 2V, so three cells in series will deliver 6V, six cells deliver 12V and 12 cells deliver 24V. Lead-acid batteries for military vehicles and airplanes use 14 cells to deliver the military standard 28V supply. For Lithium-ion batteries, the cell voltage is 2.4 - 3V, so a six-pack Li-Ion battery will deliver the typical battery voltage of 18V for portable electric drills and other workshop equipment.

In general, lead-acid batteries are used in heavy-duty (high current) applications, where the weight is not so critical as the price, and Li-Ion chemistries are used when fast charging and light weight are more important, but battery chemistry (and energy storage in general) can be far more nuanced and “moody” than this oversimplified explanation is letting on so it is recommended to consult a more detailed resource, starting with this [1] RECOM blog.

While the most common voltages in battery-powered applications are 6, 12, and 18V, there is a growing trend in many different application spaces to push bus voltages to higher voltages, such as 24 and 48V. The motivations for this trend in various use cases are detailed in the following section.

Why not continue to use 12V batteries since they are much more common?

It is true that 12V lead-acid batteries are more ubiquitous than 24V or 48V solutions and therefore tend to be cheaper and more available than higher-voltage alternatives. For motor starting, a heavy-duty battery can deliver several hundred amps, but the maximum continuous current is limited to around 100A due to the current-carrying capacity of the wiring loom, which limits the maximum available power to around 1200W.

As shown by the equations below, power is directly proportional to current and voltage, but exponentially proportional to the current in the path of resistance (e.g. – a wire).





When Watt’s Law is combined with Ohm’s Law, the exponential effect of current on power consumption becomes obvious. A wire’s resistance results in power dissipation and voltage drop before arrival at the end load.



There are several conclusions to draw from this:

• Doubling the voltage to deliver the same amount of power will halve the amount of current.
• Halving the current means halving the current-handling capability of the system, therefore reducing the size of conductors to deliver equivalent power.
• Halving the current in the same conductor halves the voltage drop along the path, therefore delivering a higher voltage to the end load (e.g. – increased system efficiency).
• Halving the current allows for the doubling of the conductor length for an equivalent voltage drop.
• Halving the current in the same conductor results in only a quarter of the power losses of the distribution network.

Although the world is transitioning rapidly towards electric vehicles, ICE vehicles will still be in production for at least the next twenty years and therefore on the roads until beyond 2050. During this time, innovation will not stand still. Cars will continue to be tech-heavy with such developments as electrically-adjustable, adaptive suspension for the perfect ride under all road conditions, more sophisticated air conditioning control, replacing mechanical pumps with electrical counterparts and turbo-chargers, and instant engine start-stop systems – all of which are power hungry and would exceed the power delivery capability of the standard 12V battery.

A 48V battery system in both ICE and hybrid EV cars would be able to deliver 5kW of power, while still being classed as a Safe Extra Low Voltage (SELV), meaning that conventional wiring insulation and safety training for mechanics are sufficient to reduce the risk of electric shock (all DC voltages below 60V can be treated as being “safe” in most use cases). Large vehicles and other forms of battery-powered transportation may also contain a very significant amount of wiring contributing to overall weight, though it should be noted significant wire weight can be just as prevalent in combustibles as in electrified vehicles. Sometimes, the mitigation of copper use alone is justification for higher voltages. Combining these weight/cost savings with that achieved by the use of higher-voltage battery packs can move the needle in terms of range (whether it be in terms of fuel or battery life).

These factors all translate into many value propositions in the real-world applications tabled in this article. Whether it be increased power handling, system size reduction, improved energy efficiency, reduced wire sizes, support for longer wire runs, or even reliability factors, higher bus voltages can be highly advantageous.

So where are the common opportunities and applications [particularly] for 24V or 48V batteries?

Electric motors of all sorts are great candidates. Small motors, such as those used in hand tools, fishing boats, golf carts, wheelchairs/scooters tend to be sensitive to overall system size/weight as they are commonly untethered and therefore can consume significant battery power merely supporting the weight of batteries and related power electronics/wiring. On the other end of the scale, industrial motors and motor-driven systems tend to be the largest consumer of any industry’s total energy footprint[2], just to stress how much potential there is for improvement and increased energy efficiency opportunities in this space.

Aside from motors, there is a variety of electrical systems benefiting from higher-voltage batteries. Photovoltaics (PV) is a perfect example of this since solar arrays can be modular, just like the batteries that provide the analogous and appropriate energy storage sized to the array and application. 24 or 48V off-grid solutions can deliver sufficient peak power to power mountain huts or remote weather stations or cellphone masts while having enough battery capacity to supply critical systems for several cloudy days.

The convergence of several of the aforementioned application spaces is becoming more prevalent. Bigger boats will not only benefit from higher battery voltages for traditional marine use but also from the incorporation of PV. The same concept applies to recreational vehicles (RVs), which have seen an incredible surge thanks to COVID-19. Defense and other high-reliability applications using a lot of redundancy and battery power are rife candidates for transitioning 12V-based to 24 or 48V-based systems. RECOM also offers a 48V battery-powered AC inverter for marine applications, generating 3-phase 115VAC with an output power of 1200VA.

Once batteries are used to power sensitive electronics or radio transmitters, then a stable, regulated supply voltage becomes essential. The voltage regulators need to be efficient to maximize the use of stored energy, have a wide-input voltage range to cope with the difference between a fully charged and a fully-discharged battery, and, in many cases, need to have a galvanically-isolated output to avoid ground loop interference or to protect the equipment from load-dump voltage surges or induced voltage transients from events such as lightning strikes or external electromagnetic fields. As a rule, the more exposed the environment, the higher the isolation barrier required.

Wider Input Voltage Range = Wider Application Coverage

RECOM offers a huge range of both low-cost, sub-miniature, non-isolated voltage regulators for board-level power supplies that are particularly suitable for battery-powered equipment with wide-input voltage ranges, very high efficiency, and ultra-low standby power consumption, such as the RPM and RPX series, as well as isolated DC/DC converters with 4:1 input voltage ranges that are suitable for 12 / 24V systems (9 – 36V) or 24 / 48V systems (18 – 75V). If a universal solution is required, the RPA150E series will deliver 150W of regulated, isolated output power over an input voltage range of 9-60VDC, covering 12 / 18 / 24 and 48V battery pack voltages with a single, eighth-brick part. As it is an isolated DC/DC converter with an internal planar transformer, it can also act as a 24V or 48V bus voltage stabilizer, delivering a constant voltage and short-circuit-proof output even if the input voltage is higher, the same, or lower than the output.

Combining increased bus voltages with power electronics rich in features can further expand the number and kinds of applications benefiting from these advancements. Computing applications (both tethered and non-tethered, i.e. – data centers and laptops) have the same uses for the benefits outlined above. For instance, computer servers may run off battery backup so given large, constant loads that can never go down, locating energy storage as close to the load (physically and by voltage level) as possible is highly advantageous for optimizing design as well as mitigating energy OPEX, which tend to drive the overall, total cost of ownership (TCO) in data centers. This is achieved directly at the front end of the system with battery backup units (BBU), providing critical backup power in the form of energy storage made available to the load with zero transition time.

Conclusion

Just as 12V batteries provided a path to 24V solutions, 24V batteries will also enable the migration to 48V applications. In general, any consolidation of cells into higher-voltage battery packs should reduce packaging overhead and only add to the value proposition. As it is now, 24V batteries are right in a sweet spot between 12V and 48V buses.

Similar to the utility grid, just about any large system can benefit from a perpetual push to higher distribution voltages.

References

[1] RECOM, “What is energy storage?” RECOM Blog, Nov 4, 2022, https://recom-power.com/en/rec-n-what-is-energy-storage--233.html
[2] IEA, “Motor-driven system electricity use as a share of electricity use by industry subsector,” IEA, Paris,https://www.iea.org/data-and-statistics/charts/motor-driven-system-electricity-use-as-a-share-of-electricity-use-by-industry-subsector (accessed January 3, 2023).