Electric Bike Batteries Explained

The short version

Watt-hours is the number that matters. Volts times amp-hours gives you watt-hours, and watt-hours is the only figure that lets you compare a 36V bike against a 52V one honestly.

A 48V 15Ah pack and a 52V 14Ah pack sound different and are nearly identical: 720Wh against 728Wh. Manufacturers know which of the two numbers flatters them, and quote accordingly.

Volts, amp-hours, watt-hours

Voltage is roughly how hard the pack can push. Higher voltage systems deliver the same power at lower current, which means less heat and less loss in the wiring. 36V is entry level, 48V is the mainstream, 52V is the enthusiast choice.

Amp-hours is how much charge the pack holds, but only at its own voltage. Comparing amp-hours across bikes of different voltages is meaningless.

Watt-hours is the total energy stored, and it is the honest one. Multiply volts by amp-hours:

Turning watt-hours into miles

A rider on moderate assist typically draws 10 to 20 watt-hours per mile. Divide the pack size by your figure and you have a realistic range, which will usually be well short of the marketing claim.

A 720Wh battery at 15Wh per mile is 48 miles. The same bike advertised at 65 miles is not lying — it is quoting the lowest assist setting, a light rider and flat ground.

What pushes consumption up: hills, headwinds, cold weather, low tyre pressure, a heavier rider, cargo, stop-start riding, and throttle use instead of pedal assist. Cold is the one people forget. Below freezing, expect to lose 20 to 30% of usable capacity until the pack warms up.

Chemistry: what e-bikes actually use

Almost every e-bike pack is lithium-ion NMC, not the LiFePO4 you find in a good power station. NMC is far lighter for the same energy, which matters enormously on a bike and not at all on something that sits in a cupboard.

The trade is lifespan. NMC is typically rated for 500 to 800 cycles to 80% capacity, where LiFePO4 manages several thousand. A few brands now offer LiFePO4 e-bike packs; they last longer and weigh noticeably more. Our LiFePO4 vs lithium-ion guide covers the difference in full.

Making a pack last

The habits that matter, in order of how much difference they make:

Avoid both extremes

Charging to 100% and sitting there is hard on a lithium pack, and so is running it to zero. If your charger or app supports an 80% or 90% limit, use it for daily riding and charge full only before a long trip.

Store it at about half charge

For anything over a few weeks, leave the pack near 50% and top it up every couple of months. A battery stored full ages faster; one stored empty can drop below the voltage the protection circuit will accept, which kills it outright.

Keep it out of the cold

Never charge a pack below freezing — that is what causes lithium plating and permanent damage. Bring it indoors, let it reach room temperature, then charge.

Use the charger it came with

Fast chargers and generic replacements run packs hotter than the cells were specified for. The time saved is not worth it.

Replacement and certification

A replacement pack typically runs $300 to $700, which is a real part of the cost of ownership. Before buying a bike, check the manufacturer still sells the pack — some cheaper brands orphan a model after a couple of years and leave owners with a bike they cannot re-battery.

Buy the manufacturer’s own pack. Third-party batteries are where most e-bike fires start, and several US cities now require UL 2271 certification for the battery and UL 2849 for the bike before you may charge or store it indoors.