Due to their powerful motors, cordless vacuums (stick or hand-held) drain batteries quickly, typically within 20, and sometimes only 10, minutes. If a vacuum has max mode, it provides more suction power and drains batteries even faster. Most rechargeable batteries don’t like such rapid discharges and deliver less energy under such conditions than their nominal specifications.
Without smart electronics, suction power will decrease when the battery is running low. And, if a charger doesn’t stop charging after the battery is full, leaving the battery on the charger for a long time can damage it.
The best battery for cordless vacuums is NMC, a subtype of Li-ion, because it combines a relatively high specific energy (see below) and tolerance to rapid discharge.
Battery specifications
Manufacturers specify batteries in terms of type, and nominal capacity and voltage. Nominal capacity and voltage are measured for a certain discharge time, usually 5 to 20 hours. However, during the faster discharge typical of cordless vacuums, these values, can be much lower.
Nominal Voltage
Nominal voltage is reported as that which a battery can deliver during the majority of its discharge time. Actual voltage is usually slightly higher than nominal in the beginning of discharge and lover than nominal in the end.
Nominal Capacity
Capacity is a product of current and discharge time, measured in ampere-hours (Ah) and often specified in milliampere-hours (mAh); 1 Ah = 1000 mAh. A capacity of 1000 mAh rated for a discharge time of 20 hours means that the battery can deliver a current of 50 mAh for 20 hours (50mAh * 20 h = 1000 mAh).
Nominal Energy
Both single batteries and battery packs (containing several batteries) can come in different voltages, in which case their capacities cannot be compared. The energy of the battery, measured in Watt-hours, makes comparison easy. Sometimes manufacturers specify energy on the label, or it can be calculated using the formula:
Energy [Watt-hours] = Voltage [Volts] * Capacity [Ampere-hours]
Energy gives a good representation of a battery’s capability.
Specific energy
Specific energy is battery energy divided by battery weight, typically measured in Watt-hours / kg. Batteries with higher specific energy are lighter than those with lower values. It is particularly important for cordless vacuum cleaners because you have to carry the batteries all the time while cleaning.
Battery types (chemistry)
Li-ion
Li-ion batteries have cell voltage 3.6V and the best specific energy values among all the battery types used in cordless vacuum cleaners nowadays. Until recently, the main obstacle to using them in all home appliances was their relatively high price. However, during recent years their manufacturing cost decreased significantly, more than 80% from 2010 to 2017, taking price concerns off the table.
Unlike other battery types, Li-ions can degrade over time even when not being used. So, keeping your vacuum with a Li-ion battery inside in storage for years without usage is not the greatest idea.
Although manufacturers often label their batteries simply as Li-ion, there are several chemical subtypes with various characteristics that may impact on vacuum performance.
Lithium Iron Phosphate(LiFePO4 or LFP)
Lithium iron phosphate batteries have specific energy of about 100 Wh/kg, which is the lowest value among Li-ions and comparable with Ni-MH. Their advantage is very good performance under fast discharge. They can easily deliver all the stored energy in 5 minutes, which is expected of high-performance vacuums. They can last about 1500 charge cycles.
Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2 or NMC)
Lithium nickel manganese cobalt oxide batteries have high specific energy (about 200 Wh/kg) and can be discharged in 30 minutes or less without significant loss of energy. That makes them the best choice for cordless vacuums. Expected lifetime is 1500 charge cycles.
Lithium Cobalt Oxide (Li-cobalt, LCO or LiCoO2)
Lithium cobalt oxide is the most common Li-ion battery-type for laptops and smartphones because of its very high specific energy (about 220 Wh/kg) but a poor choice for cordless vacuums because they do not tolerate discharge times of less than an hour, degrading more quickly than other types. Estimated lifetime, 700 cycles.
Nickel–metal hydride (Ni-MH)
Nickel–metal hydride batteries have cell voltage of 1.2V and approximately half the specific energy of most Li-ions, or about 100 Wh/kg, making them heavier and less convenient to carry. They lose energy significantly under fast discharge, which makes them a poor choice for cordless vacuums. However, some manufacturers use them. Typically, they last about 500 cycles.
Nickel–cadmium (Ni–Cd)
Nickel–cadmium batteries have cell voltage 1.2V and the lowest specific energy among all types used in vacuums–about 50 Wh/kg. They are nearly twice as heavy as Ni-MH making them much harder to carry. Ni–Cd batteries are rarely used in vacuums nowadays because of their weight, and because their rivals, Ni-MH and Li-ion batteries became cheaper than they had been.
However, Ni-Cd batteries have some advantages. First, they do very well under fast discharge. This is exactly what is needed for appliances like vacuum cleaners. To boot, they are very durable. They can last 2000 cycles under rough conditions and don’t mind sitting fully discharged for a long time. But they don’t like overcharging. Their price per Watt-hour is comparable to that of Ni-MH, but taking into account their higher durability, Ni-Cd are still more cost-effective.
Cordless vacuum power and runtime on battery
Cordless vacuum cleaners have limited runtimes. Usually 10-20 minutes. This is due to the limited energy of the battery and high motor power of the vacuum. We can either choose a powerful motor and sacrifice runtime or elect to have more continuous runtime with a less powerful motor. The formula is simple:
Runtime [hours] = Energy [Watt hours] / Power [Watt]
10 minutes is somewhat enough for quick everyday cleaning. Even with such a short lifetime, the motor will have less power than in corded vacuums. Batteries perform worse under such a high load and deliver less voltage, capacity, and as a result, energy than under a lower load which discharges them fully in an hour or more. The depth of this effect differs among battery types making battery choice more important and complicated.
Normal and max modes
Sometimes there are two modes, for example Normal mode and high (or max) mode. In the high mode, a vacuum cleans better, but the battery dies more quickly. The tricky part is that manufacturers usually specify runtime in normal mode and motor power in max mode. While these numbers are useful, they are not directly related. If you clean in max mode you will get less runtime (usually half as much) and if you clean in normal mode you will get the maximum runtime but lower suction power.
Motorized brush roll
If a vacuum is equipped with a motorized roller brush, there can be another trick in the specifications. The brush roll motor can usually be switched on or off. Using the motorized roller brush can help cleaning carpets greatly, but it is powered by its own motor, which uses battery power and therefore lowers runtime in comparison to with the motorized brush turned off. In order to raise advertised numbers, manufacturers usually specify runtime with the motorized brush disabled.
Advertised runtimes are typically something like “up to 40 minutes.” For maximum cleaning ability, it might be significantly lower.
On-off switch or tumbler
Effective cleaning time on battery can be lower than vacuum runtime in cases when the vacuum is on but the nozzle is somewhere in the air between cleaning spots. Vacuums drain some battery energy even when not vacuuming. To use battery energy solely for cleaning, some models have a trigger that turns the vacuum on only when you press it and off once released. This can add some cleaning time in comparison to a switch with “On” and “Off” positions.
Suction loss when the battery is running low
Battery voltage normally decreases during discharge. When the battery is almost empty, the voltage drops significantly. Normally, motor power will decrease along with suction power. This is sad, because although the battery still has some energy, it can’t be used for effective cleaning. Some vacuums are equipped with smart electronics that keep motor power at the same level regardless of moment-to-moment battery voltage. In such vacuums, when the battery is fully discharged, the vacuum shuts down to prevent battery damage due to over-discharge. This way you can use all the energy stored in the battery and avoid its degrading at the same time.
Cordless vacuum batteries and power evaluation
As discussed earlier, to understand what to expect from a battery, we have to know its energy in watt-hours. However, manufacturers rarely specify battery energy. We can calculate energy knowing voltage and capacity, but often they specify only voltage. Moreover, they often compare voltages of different models for advertising purposes. Generally speaking, this comparison is not accurate, because two batteries can have the same voltage but different capacities.
For purposes of discussion, let’s pretend that when manufacturers do such a comparison, they compare only models with the same battery capacity. Although batteries can have literally any capacity, manufacturers usually use battery cells of standardized size and therefore capacity. For Li-ion batteries, the most popular size is 18650. Which means a cylindrical cell 18 millimeters in diameter and 65 millimeters in height. They usually have capacity between 2-3.3 Ah (2000-3300 mAh). In fact, most vacuums use this size, so when a manufacturer provides only voltage, we can tentatively assume it’s a battery pack of 18650’s.
Usually batteries in a pack are connected in series, which increases voltage while keeping capacity the same. For example, a battery pack of Li-ions with voltage 21.6V is made of 6 cells (21.6V / 3.6V per cell = 6 cells). Assuming they are 18650 cells with capacity 3 Ah (the most common capacity) we can estimate the battery energy at 21.6V * 3Ah = 64.8 Wh.
If the runtime is 10 minutes (1/6 of an hour) such a battery can run a motor of about 390 watts (64.8 Wh / 1/6 hour = 388.8 watt) which is equivalent to an approximately 3.2 Amps motor plugged into a 120 volt electrical outlet (388.8 watt / 120 volts = 3.24 amps). Remember that a motorized roller brush also consumes energy, in which case the actual vacuum motor power will be lower.
NiMH batteries in vacuums typically have capacities around 3 Ah, NiCd around 2 Ah, but there are more options.
Removable and non-removable batteries in vacuums
Some vacuums have batteries that can be easily removed (and/or replaced) by a customer; however, it’s not a rule.
Non-removable battery
Of course, it’s not an impossible task to disassemble vacuum and take out the battery even when the manufacturer doesn’t provide such an option. However, this will require some skills and cause loss of warranty. Another disadvantage is that you cannot see what the battery is inside the vacuum and if the vacuum manufacturer doesn’t provide all the battery specifications, you will be unaware of them.
The only benefit of such construction is to reduce vacuum size with a tighter internal design without separate battery compartment.
Removable battery
Everything is much easier when you can remove the battery by following several easy steps. As a result, you can see the battery and know all the specifications given by the battery manufacturer. You can also buy a replacement battery when the original one is worn out. If the battery is removable by design, the replacement batteries are certainly available to buy. In such cases, you can also check replacement battery specifications before buying a vacuum even if the vacuum manufacturer doesn’t provide all the info about the appropriate battery.
Easily removable battery that can be charged outside the vacuum
Some vacuums have batteries that are part of the vacuum’s exterior and can be detached with one click, just like in cordless drills. In such a case, you can charge the battery outside the vacuum. Sometimes, it’s the only option and you must detach the battery to charge it. This gives a possibility to buy another battery and use it while the first one is charging which can come in handy when you have a lot of cleaning and the battery runtime is insufficient.
Charging vacuum batteries
No battery types except Ni-Cd like to stay discharged for a long time. To prolong battery life, it’s advisable to recharge the battery as soon as possible.
Charge time
All battery types except Ni-Cd can be fully charged in a minimum of 3 hours (Ni-Cd can be fully charged in just 1 hour). If a vacuum’s manufacturer specifies longer charge time, it’s due to the charger’s constitution. To charge batteries faster it’s necessary to control battery state and precisely detect the moment when it’s fully charged and stop charging to prevent battery damage.
Some models require 10 or even 20 hours to fully charge a battery. Such a slow charging with low current doesn’t require complex electronics for battery monitoring, and overcharging with low current doesn’t harm the battery too much.
Can the battery be left on the charger after being fully charged?
Some chargers stop charging once the battery is fully charged while others continue charging indefinitely. Usually fast chargers, which fully charge the battery in 3-4 hours, do stop charging and you don’t have to worry that the battery can be overcharged. You can leave the vacuum or battery on the charger indefinitely and it will be ready to use when you need it, which is very convenient.
However, the models that fully charge the battery in 10-20 hours usually aren’t so smart and continue charging after the battery is fully charged. In such a case, it’s advised to wait for the charge-time specified by the manufacturer and then turn off the charger. This is less comfortable because you not only have to watch the time and switch off the charger, but also, if you accidentally leave a fully-charged battery for a long time it can lose some energy due to self-discharge.
Batteries in robotic vacuums
Robotic vacuums have much longer runtimes, usually 1.5-2 hours. Even the models that have max mode can run about an hour in that mode. This suits all the battery types and makes battery choice easier. Lithium cobalt oxide batteries are the best fit and Ni-Mh is an option. The weight of the battery is not critical for robots because they carry themselves!