Volts, Amps and Watt Hours; Battery Numbers Explained

E-bike batteries can be confusing; you might see a battery described as 36 volt or 10 amp hour or 500 watt-hours. Which figures should you be looking at and what do the differing battery measurements mean for the performance of an e-bike? Here EBR demystifies the subject.

Definitions – And What They Mean for eBikes

When you are considering what battery numbers do mean it helps to have a bit of background knowledge on the three basic units of measurement you commonly see in battery descriptions.

Volts (V)

Battery voltage can be thought of as the electrical ‘pressure’ available within the battery to push electrons around the bike’s motor system, so powering it. A common analogy is to water pressure – the ability of water pressure to push a fluid around a system being likened to the ability of a battery’s voltage to push current through a system.

On e-bikes with hub motors increasing the voltage can be used to increase the assisted speed of the e-bike and, to a degree, increase hill climbing ability. Motors are made to run within a particular speed range and high-speed motors run on high voltages. Most ready to run e-bikes from large manufacturers (ie not ‘kit’ conversions) are either 36V or 48V systems as these voltages match the power and speed requirements of legally rated e-bikes (whose motor power and electrically assisted speed limits are defined by national or state law, depending on where you live). Barring a radical and unlikely shakeup in e-bike laws across the world e-bike battery voltages have largely settled on one of these two voltages and this isn’t likely to change any time soon.

Don’t get too hung up on battery voltages. As we’ve said already most systems are 36V or 48V and there are many other factors that affect the performance of an e-bike other than voltage. E-bikes from major manufacturers with 36V or 48V batteries will have other parts of the system like the motor and motor controller chosen to match the voltage. Clearly there are other factors at play in producing a powerful e-bike aside of voltage; for example industry leader Bosch has settled on 36V batteries and produces some of the top-performing e-bikes there are.

Really voltage comes more into play if you are a DIY builder and want to coax more performance out of a hub motor system, one way of doing this being to use a higher voltage battery as stated. This is really a separate topic in itself and certainly only for the very experienced. Never be tempted to try and increase the performance of your own ready-built e-bike by upping the voltage – this could lead to damage to the system as well as voiding the warranty and being potentially dangerous.

Voltage Detail

Ebike batteries are in fact made up of cells joined together – these cells are mass-produced and used for all sorts of purposes including to make electric car batteries. They have a standard voltage of between 3.6 and 3.7 V and by joining them together in the correct way you determine the voltage.

We won’t dive too much into the detail of how an e-bike battery is constructed here, suffice to say that the lithium-ion batteries used on the vast majority of today’s e-bikes are made up individual cells joined together, and exactly how they are joined up determines the voltage. By increasing the number of cells joined together you increase the voltage and / or the capacity. So using 10 lithium ion cells that are rated at 3.6V, joined together so that the battery voltages stack up (technically called joining the cells ‘in series’), means a battery voltage of 36V. Clusters of 10 cells in series can then be connected together in a different way (‘in parallel’) to increase the amount of battery capacity available whilst keeping the voltage the same.

Slightly confusingly, voltage is not a fixed value and varies depending on the load put on the battery and its state of charge. In fact, a typical 36V battery will show around 42V when full and not in use. The voltage dips when current is drawn from it and the more current has been drawn and the lower the capacity left in the battery the lower the voltage at rest too. A 36V battery is usually programmed to give up supplying power at around 30V as running it anything lower than this voltage risks permanent damage to the battery through over-discharge.

Amps (A) and Amp Hours (Ah)

Amps is a measure of the current any electrical device – obviously in our case an e-bike motor – is drawing. Pressing hard on the pedals or using the full throttle on an e-bike will mean many more amps are drawn from the battery to make the motor work harder. On e-bikes this is in the low tens of amps at peak power. If voltage is likened to water pressure then amperage is likened to the flow of water through a pipe. As a large bore pipe can carry more water so a larger wire can carry a bigger current. This isn’t the whole picture though, as higher water pressure (higher voltage) means a quicker flow through the pipe (more electricity passing along a wire). So a high voltage system with large wiring is capable of delivering more power than a smaller voltage system with thin wiring. In practice using a higher voltage system means a system more suitable for thin wiring and smaller electronics as the risk of overheating the system is much less. For the same reason older, lower voltage systems that have fallen out of use on e-bikes in fact use thicker wiring and bulkier electronics to overcome the inherent resistance to current in lower voltage systems.

But this is all rather theoretical. A measurement you are more likely to see on a battery spec is the amp-hour rating of a battery (you may also see batteries rated in amps but what they are usually referring to is amp-hours). It is a measure of capacity – in theory a 10Ah battery can provide 1 amp of power for ten hours or 10amps for 1 hour (though in the real world batteries come with all sorts of performance limits).

So, you might suppose that a 20Ah rated battery had twice the capacity (and twice the range under the same conditions) as a 10Ah rated battery. This is true only for batteries of the same voltage and so amps and amp-hours are not really that helpful to e-bike buyers. E-bike buyers wanting to compare the capacity of batteries (and large capacity usually means a long-range before recharging is needed) really need to look at watts and more specifically watt-hours.

Watts (W) and Watt Hours (Wh)

Electrical power (and much other power including human power) can be measured in watts. Watts (W) on their own are usually applied to e-bike motors; a typical e-bike motor is rated at 250W or 300W though this is really an average (and even then it doesn’t really tell us much about the true power of the motor as there is no agreed standard for power output of motors). Peak power for motors, reached briefly on the steepest of hills for example, can be several times the manufacturers rating.

Watts can be calculated by multiplying volts and amps, so a 36V rated motor drawing 10 amps is actually consuming 360W of power (fairly modest in today’s e-bike performance terms).

But for battery figures what matters above all are Watt-hour (Wh) ratings – a measurement of battery capacity. A good analogy might be the size of a gas tank on a motor vehicle. This is really what most potential e-bike purchasers are interested in – how big is the battery and so how far will the e-bike go on a single charge. Wh measures the energy in a battery and is found by multiplying the Ah rating and voltage rating. A 48V battery rated 10Ah has a capacity of 480Wh. Again in reality you will see slightly different figures bandied about depending on exactly how the manufacturers have calculated their battery capacity, but Wh is the best comparison of battery ‘size’ and energy capacity when comparing battery capacity between different e-bikes with different batteries.

Examples of Wh Batteries

E-bike batteries on commercially produced systems (ie not DIY / hobby kits) start at about 150Wh and go above 1200Wh. Why the huge range? Surely all e-bikers just want the biggest battery? Well, no, for several reasons . Lithium-ion batteries are expensive and expensive to replace (all of them eventually wear out though it may take several years) and the bigger the battery the more expensive it is. They add weight, so, if handling the bike easily when not riding is important, the biggest battery may not be the right choice. You may simply not need the extra range of a larger battery if your riding needs are more modest; you are in effect wasting money and adding unnecessary weight by buying too big a battery.

Small, lightweight batteries with modest Wh ratings are often used on folders where keeping weight down and having a portable folded e-bike is of premium importance. A good example is the Cytronex Brompton system as shown above, where the battery resembles a bottle (it also houses the control electronics so, in this case, is more than just a battery).

For the majority of e-bikes though, 300Wh to 600Wh is the standard. Though larger capacity batteries are certainly available they tend to be the exception rather than the norm. Of course, you can always carry a spare battery and some manufacturers have gone a step further by designing dual battery systems that manufacturers can fit their e-bikes, so you don’t even have to stop and change the battery. One very large capacity dual battery system comes from Bosch. Such a large and heavy system might be useful on an e-cargobike or (and) speed pedelec (higher speed models use more power than lower speed ones).

Do note that how Wh equate to range is affected by huge number of factors, some dependent on the rider (eg weight, riding style etc) and some on weather and terrain. Bosch’s range ‘calculator’ is a useful tool for letting you estimate the range from a given size of battery – but even then note it is at best only a reasonable estimate.

As always, it is case of trade-offs and finding the right Wh capacity battery for your specific use of your e-bike.

For a final recap of all the above check out this nice 5 minute summary video on the subject of battery numbers:

Stay tuned for more e-bike news and reviews and thanks for reading!

-Rich