Choosing a Battery for a Conversion Kit
The listing sells you amp hours. The spec that decides whether your build works is the discharge rating buried in the description, and plenty of sellers never state it at all.
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The battery is usually the largest single cost in a conversion, the component most often bought wrong, and the only part with a failure mode that can burn a building down. It deserves more attention than the motor and generally gets less, because motors are sold on a number everybody understands and batteries are sold on a number that does not describe what matters.
Two packs can both say 48V 14Ah on the label and behave completely differently on a hill. What separates them is what they can deliver under load, and how honestly the seller has described it.
The BMS rating decides whether the build works
Inside every pack is a battery management system: a small circuit board that monitors cell voltages, balances them during charging, and cuts the output if anything exceeds its limits. One of those limits is continuous discharge current, and it is the specification that determines whether your kit performs as intended.
Your controller has a peak current draw. A 750W mid-drive kit typically asks for something in the region of 25A, and a 1000W kit around 30A, as covered in Bafang BBS02 vs BBSHD. Hub kits vary more widely, commonly 15A to 25A. If the BMS limit sits below what the controller requests, the BMS wins, every time.
The symptom is unmistakable once you know it: the bike is fine on the flat and cuts out at the bottom of hills or under hard acceleration, then recovers a second later. Riders diagnose that as a motor problem or a controller fault and replace the wrong part.
- Hub kit, roughly 500W
- Controller peak commonly 15 to 20A. A 20A BMS is workable, 25A is comfortable.
- Hub kit, roughly 750W
- Controller peak commonly 20 to 25A. Look for 30A continuous.
- BBS02B class mid-drive
- Around 25A peak. A 30A BMS with headroom, not a 25A one running at its ceiling.
- BBSHD class mid-drive
- Around 30A peak. A 40A BMS is the sensible pairing, and a 30A one will disappoint you.
Headroom matters because a BMS running permanently at its rated limit runs hot, and heat is what ages both the board and the cells nearest it. Sizing up one step is cheap insurance.
Sizing capacity to your route
Capacity in amp hours is meaningless across different voltages. Watt hours is the comparable figure, and it is simply voltage multiplied by amp hours. A 48V 14Ah pack is 672 watt hours. A 36V 14Ah pack is 504 watt hours, despite the identical amp hour number. The arithmetic behind this is covered in 36V vs 48V vs 52V batteries.
Consumption for a 750W-class conversion typically falls between 18 and 25 watt hours per mile, depending on assist level, rider weight, terrain, and how much you actually pedal. Use 25 as a planning figure, because planning with the optimistic end of a range is how people end up pushing a heavy bike home.
- Work out your longest regular round trip in miles. Not your average, your longest.
- Multiply by 25 watt hours. A 20 mile round trip is 500 watt hours.
- Add a third. Cold weather, headwinds, hills, and cell ageing all take a bite. That 500 becomes about 665.
- Round up to a pack you can buy. In this case a 48V 14Ah pack at 672 watt hours.
Cadence-sensing kits burn capacity faster than the figures above suggest, because they deliver the selected assist level whenever the cranks turn, whether you are working hard or spinning lightly. That behaviour and its alternative are covered in torque sensor vs cadence sensor. If your kit is cadence sensing, lean towards the larger pack.
Cells, and why the name matters
A pack is an assembly of cylindrical lithium cells, usually 18650 or the larger 21700 format, wired in series to reach the voltage and in parallel to reach the capacity. A 48V pack is typically thirteen cells in series, and the parallel count sets the amp hours.
Cell quality determines three things that show up in use: how much current the pack can deliver without voltage sag, how many charge cycles it lasts, and how it behaves when something goes wrong. Cells from Samsung, LG, Panasonic, and Molicel have published, tested specifications. Unbranded cells are frequently factory rejects or recovered from other packs, sold on a capacity claim nobody verifies.
Voltage sag is the practical difference. Under a 25A draw, a pack built from good cells might drop a couple of volts from resting. A pack built from tired or cheap cells can drop far more, and since power is voltage multiplied by current, the motor makes noticeably less power exactly when you are asking for the most. Two packs with the same label deliver different bikes.
Expect roughly 500 to 800 full charge cycles before a good pack settles at around eighty percent of its original capacity. Cheap packs can lose that much in a single season, which makes them more expensive per mile than the pack you did not buy.
Mounting, connectors, and fusing
Where the pack goes
Mass placement changes how the bike handles more than most people expect, and a 672 watt hour pack weighs somewhere around seven to nine pounds.
| Mount | Handling | Verdict |
|---|---|---|
| Down tube, bottle bosses | Low and central, barely noticeable | First choice |
| Frame triangle bag | Low and central, fits larger packs | Best for big capacity |
| Rear rack | High and far back, bike gets tail-heavy | Acceptable, not ideal |
| Seat tube bosses | Higher than the down tube, still central | Fine if the down tube is taken |
| Strapped to a tube | Moves, chafes paint, can shift under braking | Avoid |
Swipe sideways to see all columns →
Bolt mounts into bosses rather than relying on straps. A pack that shifts while you are riding is a pack whose main connector is being worked loose on every bump.
Connectors and fusing
Kit looms commonly use XT60 or XT90 connectors, or Anderson Powerpoles. Any of them is fine when correctly rated and correctly crimped or soldered. What matters is that the connector is rated comfortably above your continuous current, and that the joint was made properly, because a high-resistance connection under 30A generates real heat.
Fit an inline fuse between the battery and the controller if the pack does not have one. It costs a few dollars and it is the difference between a chafed cable becoming an annoyance and becoming a short circuit across a pack that can deliver hundreds of amps into a dead short.
Charging, certification, and fire risk
Conversion batteries carry more risk than factory ebike packs for a straightforward reason: the pack, charger, controller, and motor were not tested together as a system by anybody.
Two standards come up. UL 2271 covers the battery pack itself, and a good pack may carry it. UL 2849 covers the complete electrical system of an ebike, motor and controller and battery and charger together. A self-assembled bike cannot hold UL 2849, because there is no system to certify. If your building, workplace, or municipality has adopted UL 2849 requirements for storage or charging, a kit build will not satisfy them, and that is worth establishing before you order rather than after.
The habits that actually reduce risk are unglamorous:
- Charge where you can see it, and not in a doorway, stairwell, or the only exit from a bedroom.
- Use the charger that matches the pack. Voltage and connector both. A charger for a 52V pack on a 48V pack overcharges it.
- Do not charge below freezing. Charging a cold lithium cell plates lithium metal onto the anode, which is permanent damage and a genuine hazard. Bring the pack indoors first.
- For daily use, stopping around eighty to ninety percent extends life considerably. Charge to full when you need the range.
- For storage over a month, leave it near half charge in a cool place, not full and not empty.
- Retire damaged packs. A pack that has been crushed, dropped hard, or has a swollen case is finished. Do not open it, and dispose of it through a battery recycler rather than household waste.
The same care applies to any pack, factory or kit, and ebike battery replacement covers the end-of-life side in more detail.
Reading a listing honestly
Signs of a pack worth buying
- Cell manufacturer and model named explicitly, not just "A grade cells"
- BMS continuous discharge rating stated in amps, separate from any peak figure
- Case, mount, and connector type all specified with the pack
- A charger included that matches the pack chemistry and voltage
- A seller who answers a direct question about the BMS rating with a number
Signs to close the tab
- Capacity that is implausible for the case size and price
- No discharge rating anywhere in the description
- Cells described only as "high quality" or by a name with no datasheet
- A price far below comparable packs of the same stated capacity
- Listing photos that show a different connector than the description
Price is the fastest tell. Cells are a commodity with a known cost, so a pack priced far below the field is not a bargain someone found; it is a different pack. A 48V 14Ah build with named cells and an adequate BMS lands somewhere around $400 to $700 depending on the seller and the case, and packs at half that are made of something else.
Once you have the pack sized correctly, the rest of the build is straightforward. How to convert a bike to electric covers the installation order and the commissioning checks, what an ebike controller does explains where the current limit is actually set, and the best ebike conversion kits covers matching a kit to your frame in the first place. If range is the whole point of the project, longest range electric bikes is a useful reality check on what capacity buys.