E-Bike Battery for a Conversion: Position, Space, Range

The battery decides range, balance and safety of your conversion, and it is usually the most expensive single part of the kit. It sits best on the down tube, if there is room. Otherwise you are left with a rear rack, a frame bag or a handlebar battery. Measure the space before you order.

Where the battery can go

For balance the down tube is the best choice. That is experience, not a law. If your frame cannot take it, a rack battery is perfectly fine. You will only notice the extra weight at the back when you manoeuvre and push the bike.

Measuring the space

Many frame batteries need roughly 35 to 45 cm along the down tube. That is a rough range; the exact length is in the battery's data sheet. Here is how to go about it:

1. Measure the free, straight stretch of the down tube between the bottle-cage bosses or between other fittings such as cable guides.
2. Check the depth: does the battery clear the seat tube without touching the tyre or crank?
3. On frames with a low top tube or in small sizes, length is often the problem.
4. Check lock position and removal. You should be able to take the battery off without tools and without turning the bike around.

If it does not fit, it is not the end of the world. Take a rack battery or a frame bag instead. For a rack you only need to check its load rating.

Cells and capacity

You get capacity in watt-hours from volts times amp-hours. 36 V times 14 Ah is 504 Wh. More Wh means more range, but also more weight and a higher price. See the glossary for more.

Branded cells from Samsung, LG, Panasonic or Molicel generally last longer and are considered safer than no-name cells. The extra cost is usually worth it, because the battery is where cutting corners backfires most often. Ask the dealer which cells are used. If they will not say, there is usually a reason.

Battery, controller and display must share the same voltage. A 48 V battery on a 36 V kit is not an option.

Range: what you actually get

Manufacturer figures assume ideal conditions. In reality you often get only about 70 % of them (GIGA). As a rule of thumb, plan on 8 to 15 Wh per kilometre. A 500 Wh battery then takes you roughly 33 to 60 km. The range is wide because several things play in:

• Terrain and headwind
• Your weight and any luggage
• Assist level and how much you pedal yourself
• Cold, which reduces usable capacity
• Tyre pressure and rolling resistance

When in doubt, plan generously and use the lower end of the range.

Safety

• Never charge unattended, especially not overnight in a hallway or on a flammable surface.
• Use only the matching charger. Third-party chargers with the same plug are a typical source of trouble.
• After a fall, check the battery and its casing. Dents, cracks, smell or heat are warning signs. Do not charge it again then.
• Store the battery dry, and not in direct sun or in a car.

EU battery regulation from 2027

From February 2027, batteries are meant to be replaceable by specialists, and replacement batteries are meant to stay available for five years (bike-x). For you that means spare batteries should get easier to find for new products. Whether this applies equally to conversion kits from outside the EU is unclear. When buying, look for a dealer in the EU who offers replacements.

Disposal

Batteries never belong in household waste (bike-x). Hand them in at collection points, at a specialist shop or at a recycling centre. If a battery is defective or damaged, say so beforehand, as these need separate handling. Tape over the terminals for transport.

What a battery costs and how it fits into the total price is covered under costs. Which motors go with which battery is in motor types.

Does a conversion fit your bike?

How reliable is this?

The statements on range (70 %), the EU battery regulation and disposal come from the linked sources. Position, space requirements, cell brands and the safety tips are experience and assumptions that Fabio is still checking.