The battery is the heart of any conversion: it is usually the most expensive component, the heaviest, the one with the biggest impact on your range, and the only one that can genuinely catch fire if you buy badly. Get it right and your conversion is a joy; get it wrong and you are either stranded short of home or living with a fire risk in your hallway. Here is how to choose well.

Wh, V and Ah: the numbers that matter

Three figures define a pack, and they relate by one simple formula: watt-hours (Wh) = volts (V) × amp-hours (Ah).

  • Volts (V) sets the system voltage and must match your kit, commonly 36 V, 48 V or 52 V. A higher-voltage system can deliver power more efficiently at the same current.
  • Amp-hours (Ah) measures charge capacity at that voltage.
  • Watt-hours (Wh) is the figure that actually predicts range, because it captures total stored energy. A 48 V, 14 Ah pack holds 672 Wh; a 36 V, 10 Ah pack holds just 360 Wh.

When comparing batteries, always compare watt-hours, not amp-hours, because a 10 Ah pack at 52 V stores far more energy than a 10 Ah pack at 36 V.

Sizing the pack for your range

Start from how far you actually ride between charges, not the longest ride you can imagine. Mixed-terrain riding with moderate assist typically uses somewhere around 7–20 Wh per kilometre. Heavier riders, strong headwinds, lots of climbing, cold weather and heavy throttle use push you toward the top of that range; a light rider cruising on the flat with light pedal-assist sits near the bottom.

Multiply your real distance by an honest Wh/km estimate, then add roughly 20–30% headroom so you never fully drain the pack (which also prolongs its life). For example, a 25 km commute at 15 Wh/km needs about 375 Wh of actual use, so a 480–500 Wh pack gives comfortable margin. Common conversion batteries land in the 360–720 Wh range. For a deeper treatment of distance estimates, see e-bike conversion range explained.

Form factors: where the pack lives

Downtube packs

Mounted to the bottle bosses on the downtube, these keep weight low and central for the best handling and look the most integrated. They need adequate frame clearance and the right mounting points.

Rear-rack packs

Bolted onto a rear carrier, these are the easiest to fit and remove and suit bikes without bottle mounts. The trade-off is a higher, rearward centre of gravity, which can make a loaded bike feel slightly top-heavy.

Frame-bag and seatpost packs

Soft frame-bag packs and seatpost-mounted batteries suit bikes with no convenient hard mounting points and can be discreet, though capacity is often smaller and you must protect the cells from impact and water.

Cell quality: 18650 vs 21700

Lithium packs are built from many small cylindrical cells wired together. The two common formats are 18650 (18 mm × 65 mm) and the larger 21700 (21 mm × 70 mm). The 21700 cells generally offer higher capacity and can simplify pack construction, while 18650 cells remain extremely common, well-proven and often cheaper.

Far more important than the format is who made the cells. Genuine cells from reputable manufacturers behave predictably and safely; unbranded or counterfeit cells may not hold their rated capacity and carry a real safety risk. A pack built from quality cells with clean welds and a proper enclosure is worth paying for.

The BMS: the pack's brain

Every good pack includes a battery management system (BMS), a small circuit board that balances the cells, prevents overcharge and over-discharge, limits current, and shuts things down if a cell overheats or a short occurs. A quality BMS is the single biggest factor in whether a pack is safe and long-lived. Cheap packs sometimes skimp here, which is exactly where fire risk creeps in. Never buy a pack that cannot tell you it has a real, appropriately rated BMS.

Chargers and charging habits

Use the charger supplied or specified for your pack's voltage and chemistry, never a mismatched one. Charging to 80–90% rather than 100% when you do not need full range noticeably extends pack life, as does avoiding leaving the pack fully drained for long periods. A slower charger is gentler than a fast one if longevity matters to you.

Safety, fire risk and storage

Lithium battery fires are rare with quality packs but serious, so treat the battery with respect:

  • Charge on a hard, non-flammable surface, away from exhausts, sofas, doorways and anything that could ignite, and ideally while you are home and awake.
  • Never charge or use a pack that is swollen, dented, dropped, water-damaged or behaving oddly.
  • Store at a moderate state of charge (around half) in a cool, dry place, not in freezing or baking conditions.
  • Buy from reputable sellers with genuine cells and a real BMS; the savings on a suspiciously cheap pack are not worth the risk.

Matching voltage to your kit

This is the rule people most often break: the battery voltage must match the controller and motor kit. A 48 V controller expects a 48 V pack; feeding it 36 V leaves you underpowered, and feeding it too much can damage the electronics. Confirm your kit's nominal voltage before buying, and remember the kit and battery must agree on the connector type too. If you bought a complete kit, the simplest path is a pack sold for that exact voltage. The motor and controller choices that drive this decision are covered in hub motor vs mid-drive.

Continuous and peak current: matching the controller

Voltage is not the only electrical figure that has to agree between pack and kit. The battery also has to supply enough current for the controller to draw at full power, and that current capability comes from both the cells and the BMS. If a controller is configured to pull, say, 25 A at peak but the BMS is rated to cut off at 20 A, the pack will trip out under hard acceleration or on a climb, leaving you stranded mid-effort. When you buy a pack for a specific kit, check that its continuous current rating comfortably exceeds what the controller demands, with some margin for hills and headwinds. A pack run repeatedly near its limit also runs hotter and ages faster, so a little headroom buys both reliability and longevity.

How packs age, and how to slow it

Lithium packs wear out gradually rather than failing all at once. Capacity slowly declines with each charge cycle and with calendar age, so a pack that delivered its full range when new will give a little less after a few hundred cycles. This is normal, and a quality pack ages gracefully over years of use. You can meaningfully slow the decline: avoid routinely charging to a full 100% or draining to empty, keep the pack out of extreme heat (the single harshest thing for lithium cells), and do not store it fully charged or fully flat for long stretches. Riders who treat their pack gently often get far more useful seasons out of it than those who hammer it to empty and leave it on the charger overnight, every night. Thinking of the battery as a long-term component rather than a consumable changes how you treat it from day one.

Cost context

Because the battery is frequently the largest single line item in a build, it is worth budgeting for properly rather than economising on the one component where quality maps directly to safety. The full project breakdown lives in how much it costs to convert a bike to electric.

Next steps

Write down your real per-charge distance, pick an honest Wh/km figure for your weight and terrain, and calculate the watt-hours you need with headroom. Then choose a form factor your frame can carry, insist on genuine cells and a proper BMS, and match the voltage to your kit exactly. If you are still mapping the whole project, return to the complete guide to converting a bike to electric and slot the battery decision in alongside your motor and legal checks.