What does the number of Ah tell you about a battery?

Ah stands for ampere-hour, a unit for the amount of electrical charge a battery can store and deliver. Put simply, the Ah figure tells you how much current a battery can supply for an hour. To make that concrete: a battery rated at 10Ah can in theory deliver 1 ampere for 10 hours. Ah matters most when you have your battery re-celled, because in a cell replacement the voltage (measured in volts) stays the same to prevent damage to the other electrical components.

Charging from the mains

If you charge an e-bike battery with a current of 1 ampere at a mains voltage of 230 volts, the charger uses 230 watts (1 ampere x 230 volts = 230 watts).

Bear in mind that the capacity of the battery, 500 watt-hours (Wh) in this example, is a measure of how much energy the battery can store, not of how fast it charges. The charging speed is set by the current (1 ampere here) and the voltage of the charger.

The time a full charge takes depends on both the capacity of the battery and the current of the charger. In this example, with a completely empty battery, it would take 500 Wh / 230 W = about 2.17 hours to charge the battery to full, assuming the charger delivers a constant 230 watts and there are no losses.

That is a theoretical example. A normal e-bike charger delivers far less power, as the next example shows.

How many watts does an e-bike battery charge at?

When an e-bike battery with a nominal voltage of 36 volts is charged with a 2 ampere charger, the power calculation is slightly different from the earlier example with mains voltage.

Here you use the voltage of the battery (36V) instead of the mains voltage. The formula stays the same: ampere x volt = watt. So for a 2A charger and a 36V e-bike battery the power is calculated as:

2 ampere x 36 volt = 72 watt

This means the charger uses 72 watts to charge the battery. The capacity of the battery, 500Wh, tells you how much energy the battery can store. To work out how long a full charge takes, you divide the total capacity of the battery by the power of the charger:

500 Wh / 72 watt ≈ 6.94 hours

So it would take about 6.94 hours to charge the battery to full with a 2A charger at a nominal voltage of 36V, assuming the battery is completely empty and nothing is lost during charging.

How many watts are lost?

Several kinds of loss occur when you charge an e-bike battery, which means that not all the energy the charger uses ends up stored in the battery. The usual sources of loss are:

1. Heat loss: During charging both the battery and the charger produce heat. That heat is energy that does not go into the battery but is given off to the surroundings instead.

2. Internal resistance of the battery: Every battery has an internal resistance, and that costs energy. The higher the internal resistance, the more energy is lost as heat while the battery charges.

3. Charger inefficiency: No charger is 100% efficient. Part of the energy the charger takes from the socket is used up by its own internal processes and never reaches the battery.

4. Voltage conversion losses: If the charger has to convert the mains voltage to a lower voltage that suits the battery, that conversion causes extra losses.

5. Battery management: Modern batteries usually have a built-in management system that protects them against overcharging, overheating and so on. These systems use a little energy too.

6. Incomplete charging: Sometimes a battery is not charged all the way, so the available capacity is not fully used. The cause can be incomplete charge cycles or the age of the battery, because capacity declines over time.

Taken together, these losses mean that the real time needed to charge a battery to full can be longer than the theoretical calculation suggests. So it is always wise to allow some extra time for a full charge of an e-bike battery.

The terms '10S' and '16S' refer to the configuration of the cells in a lithium-ion or lithium-polymer battery. The 'S' stands for series. The number in front of the 'S' tells you how many cells are connected in series inside the battery. When cells are connected in series their voltages add up, while the capacity stays the same.

1. 10S: A 10S battery has 10 cells connected in series. If each cell has a nominal voltage of 3.6 volts (the usual figure for lithium-ion cells), a 10S battery has a total nominal voltage of 36 volts (3.6 volts x 10).

2. 16S: In the same way, a 16S configuration means that 16 cells are connected in series. With a nominal cell voltage of 3.6 volts, a 16S battery has a total nominal voltage of 57.6 volts (3.6 volts x 16).

In this table the 'Nominal voltage' column shows the total voltage of the battery at an average state of charge, 'Maximum charging voltage' shows the total voltage when the battery is fully charged, and 'Minimum voltage' shows the total voltage at the lowest recommended level of discharge.

Electric bikes (e-bikes) are powered by batteries of different voltages. The voltage of a battery matters because it affects the performance and the range of the e-bike. Below we explain three types of e-bike battery:

1. 48V e-bike battery:

   • A 48V battery generally delivers better performance and more power. That gives a higher top speed and stronger climbing, especially in hilly areas.
   • You often find these batteries on faster e-bikes and electric scooters.
   • The higher voltage lets the motor work more efficiently, which can make the motor last longer.
   • Because they are more powerful, they can also handle heavier loads.
   • They are generally used more often on speed pedelecs.

2. 36V e-bike battery:

   • A 36V battery is a common choice for e-bikes and offers a good balance between performance and cost.
   • It delivers enough power for most everyday riding, such as commuting and leisure rides.
   • These batteries are usually lighter and cheaper than 48V batteries, but they give a lower top speed and less climbing power than a 48V battery.

3. 24V e-bike battery:

   • A 24V battery suits lighter use and is often found on entry-level models and children's e-bikes.
   • Some years ago this was the standard.
   • They are generally cheaper and lighter than 36V and 48V batteries.
   • The range, top speed and climbing power are lower than on e-bikes with a higher voltage.
   • These batteries are a good choice for flat terrain and shorter rides.

The battery in an electric scooter or moped usually has a higher voltage than an e-bike battery. Voltage, expressed in volts (V), is the electrical potential the battery can deliver. Scooter batteries and electric mopeds commonly come with a voltage of 48V, 60V or 72V. These higher voltages are chosen for two reasons:

1. Power:

   • A higher voltage means more electrical power is available to drive the motor. The result is a higher top speed and better acceleration, which matters most on heavier vehicles.

2. Efficiency:

   • A higher voltage is more efficient because less current is needed to deliver a given amount of power. Less energy is lost as heat, so the scooter or moped runs more efficiently. Thinner cables can be used as well.

If you choose more Ah in a cell replacement than your battery had before, you will notice the difference, as you can see in the table below. The figures are worked examples to show the principle. On the road the current draw changes all the time, so real ranges are lower.

The example shows the difference between a 10Ah and a 20Ah battery. The table shows how battery capacity (10Ah versus 20Ah) and current draw (from 2A to 10A) affect the range and the riding hours at an average speed of 25 km/h. A higher battery capacity gives a longer range and more riding hours, while a higher current draw shortens both. A 20Ah battery at a current draw of 2A has a range of 250km, whereas a 10Ah battery at a current draw of 5A reaches only 50km. Range and riding time come down to the balance between current draw and battery capacity.

Lowering the average speed

In the next table the average speed has been lowered to 20 km/h, which reduces the range proportionally compared with the original situation at 25 km/h. The current draw stays the same in this table, so you do not get as far.

Possible causes are more resistance, for example from soft tyres, a motor with more drag or a stronger headwind.

Halving the current draw

If the current draw is halved, you get further. That happens with harder tyres, a tailwind or a downhill stretch, for example.

In this table the current draw has been halved, which increases the riding hours and with them the range.

A bigger battery does not put a heavier load on the motor of a bike. A larger capacity gives you assistance for much longer. It is simply a myth that the motor of an e-bike breaks down or wears faster because of a bigger battery (capacity). Think of a car that has been fitted with a larger fuel tank: as long as you fill it up, you get much further on one tank.

Conclusion

The number of Ah of a battery tells you how much energy, or capacity, is stored in the battery when it is full, and it determines how far you can ride, depending on the conditions. Under the same conditions a larger battery takes you proportionally further: if the capacity goes up by 10%, you can ride 10% further.

Tense about voltage?

Now that we have explained the capacity of the battery, it is worth saying something about its voltage. Voltage is measured in volts, abbreviated to V.

The voltage of an e-bike battery differs from bike to bike. One makes do with 24 volts, another runs on 36V or even 48V. This matters, because the stated battery voltage must always be the same as the system voltage of the bike and of the charger. If the system voltage of the e-bike differs from that of the battery, or the charger differs from the battery, the damage can be beyond repair. We can almost hear you thinking: “When I measure the voltage of my nearly full battery it reads 28V, and yet it is a 24V battery.”

That is right. The battery voltage a manufacturer states is a nominal voltage, which means the average of the maximum and the minimum voltage. Compare an e-bike battery to a balloon. As you blow it up, you notice you have to blow harder and harder: the pressure rises. Let the air out slowly and the pressure drops. You will also notice that less air comes out as the balloon empties than when the pressure was much higher. The same holds for your battery.

Knowing this, you can explain a few typical things about batteries.

1. As a battery runs down, you may feel there is a little less “oomph” in the assistance. Now you know why: there is less pressure, or in battery terms less voltage.

2. To charge a battery you need a slightly higher voltage to “push” the energy in. The fuller the balloon, the harder you have to blow, but blow too hard and the balloon bursts. So always charge a battery with a charger that matches it.

3. Tie off an inflated balloon and after a while you will see it getting smaller. The older the balloon, the faster it loses air. An old e-bike battery has more trouble holding on to stored energy than a new one.

4. Leave the balloon tied off, put it in the freezer, and you will see it shrink further as the temperature drops. When the balloon warms up again, it returns to the size it was before you cooled it. The same applies to an e-bike battery. In the winter cold an e-bike battery can deliver up to 10% less power than at 20°C.

5. Take the ice-cold balloon out of the freezer and condensation forms on it straight away. The same happens to a battery that comes out of the cold into the warmth, or that is charged in the cold. We all know that moisture and electricity are never a good combination.

Moisture inside an e-bike battery can cause a lot of trouble: cells, electrical connections and the electronics can be damaged beyond repair.

6. A balloon that is warm and supple is easier to blow up and less likely to burst than an ice-cold, brittle one. An e-bike battery is best charged at 20°C.

7. A full balloon lying in the blazing sun can burst. A battery that gets really hot in the sun can be damaged too.

Enough hot air. Back to capacity, or Wh.

What is a watt-hour? The abbreviation Wh stands for watt-hour, a measure of an amount of work, or energy. You work out the Wh of a battery quite simply by multiplying the (nominal) voltage by the number of ampere-hours. Taking 24V and 10Ah as an example: 24 x 10 = 240 Wh. Think of your battery as a bathtub: it holds a certain amount of water, and that is what you have to make do with.

A little wiser? Or do you still have questions about the best choice for your battery cell replacement? Our customer service team is happy to help you make the choice that suits you.

Do you have any questions?

Feel free to contact us for advice that fits your situation.