The short answer
Three numbers decide it. Voltage must match the controller’s rated input exactly. Continuous current — the battery’s BMS rating — must be at least the controller’s maximum amp draw, ideally with 20–30% headroom. Capacity is set by the range you need: estimate watt-hours as distance × consumption per mile, then add a 20–30% reserve for hills, cold and ageing. A typical 750W 48V kit wants a 48V 15–20Ah pack with a 30A continuous BMS; a 250W commuter kit runs happily on a 36V 10–15Ah pack. Get those three right and the rest — connector, mount, charger — is detail, important detail, but detail.
Start at the controller, not at the battery
The controller is the part that draws power, limits power and protects the motor, and its label tells you almost everything the battery has to satisfy. Find it — usually a metal or plastic box near the motor, under the battery mount or inside the frame — and read two things:
- Rated input voltage or voltage range, for example “Input: 36V–52V DC”. That range is the battery voltage window you must stay inside. Fitting a 52V pack (58.8V fully charged) to a controller rated to 48V nominal is how people destroy controllers.
- Rated and maximum current, for example “15A continuous, 30A peak” or “Current limit 22A”. The battery must be able to supply that continuously without its BMS tripping.
If the label is worn or missing, treat the kit as unspecified and look up the kit model before ordering a battery. Buying a pack that matches a guess is the most expensive way to buy.
Step 1: voltage and series count
Voltage determines how fast the motor can turn and what the controller will accept. Packs are built from series groups of 3.6–3.7V nominal cells, so each nominal voltage maps to a fixed series count — and each series count implies a specific full-charge voltage you should check against your controller rating:
| Nominal | Series count | Full charge | Typical use |
|---|---|---|---|
| 36V | 10S | 42.0V | 250–500W commuter kits |
| 48V | 13S | 54.6V | The mainstream: 500–1500W kits |
| 52V | 14S | 58.8V | Higher-speed builds, 1000–1500W |
| 60V | 16S | 67.2V | Performance hub builds |
| 72V | 20S | 84.0V | High-power and enduro builds |
A battery and a motor must also agree on chemistry conventions. A “48V” LiFePO4 pack charges to roughly 58V rather than 54.6V, which matters if your controller sits at the top of its rated range. Our voltage and configuration guide covers how series counts, chemistry and nominal labels relate, and the series and parallel design article explains how the S and P counts are decided in the factory.
Step 2: current — the specification people skip
Voltage gets the attention; current causes the disappointment. A battery that cannot deliver the controller’s amps will sag, heat up and cut out exactly when you need power — on the climb. Match the pack’s continuous BMS rating to the controller’s current limit:
| Kit power | Typical controller draw | Minimum battery continuous rating |
|---|---|---|
| 250W | 7–10A | 10–15A |
| 500–750W | 15–22A | 20–30A |
| 1000–1500W | 25–35A | 30–40A (peak 50A+) |
| 2000–3000W | 40–60A | 50A continuous, 80–100A peak |
| 5000W+ | 80–150A | High-discharge cells, 100A+ continuous |
The physics is simple arithmetic: watts = volts × amps. If a 48V controller is limited to 22A, the electrical ceiling is about 1,050W regardless of what the motor is labelled. Two numbers on a battery listing decide whether you reach that ceiling: the continuous current rating and the cell discharge rating behind it. Advertised figures are frequently optimistic on cheap packs, so buy where the cell model and BMS specification are published — our article on maximum discharge current explains how to read them, and why a pack rated 30A continuous but built from low-discharge cells will still sag.
Do not over-buy current either. A 100A-capable pack on a 22A controller is heavier and more expensive for no benefit. Pick the rating your controller needs, with headroom for cold weather and ageing, and stop there.
Step 3: capacity and real range
Capacity is watt-hours: voltage × amp-hours. Range estimates start from consumption per mile, which varies enormously with assist level, weight and terrain:
- Pedal assist, moderate effort: roughly 15–20Wh per mile.
- Throttle-only or heavy assist: 30–40Wh per mile and up.
- Hilly terrain: multiply by about 1.3; steep climbs and cargo use multiply by 1.6.
- Cold weather: subtract 20–30% of usable capacity, and expect more voltage sag under load. Our cold weather guide covers what winter does to range.
Worked example: 25 miles round trip, mostly pedal assist on rolling roads. 25 × 17.5Wh = 437Wh, add 25% reserve = about 550Wh. At 48V that is roughly 11.5Ah, so a 48V 15Ah pack (720Wh) gives comfortable margin without being heavy. Typical pairings that kit builders use:
| Build | Sensible pack | Notes |
|---|---|---|
| 250–500W commuter | 36V 10–15Ah | Light, cheap, 15–30 mile range |
| 750W hub, mixed use | 48V 15–20Ah | The default choice for most conversions |
| 1000W mid-drive | 48V 17.5–20Ah, 30A+ BMS | Needs high-discharge cells to avoid sag |
| 1500W+ performance | 52V 20Ah or 72V 20–30Ah | Match controller current first, then size |
| Cargo / long distance | 48V 20Ah+ or dual battery | Weight and mounting become the limit |
Step 4: the parts that decide whether it fits
- Connector. Match the controller’s plug exactly. For high-current builds, XT60, XT90 and similar connectors are the standard because they carry current without heating; small block connectors are a bottleneck on 30A systems. Never adapt by twisting wires together — solder or crimp properly, with the pack isolated.
- Mounting and space. Measure the frame triangle, the rack, or the down tube before ordering. A pack that does not fit is a refund argument, not a battery.
- Charger. Buy the charger matched to the pack: correct voltage, correct connector, correct charge current. Never use a charger of a different voltage because the plug happens to fit. Our charging guide covers what normal charging looks like and how the 20–80% habit extends pack life.
- Communication (or the lack of it). Open kit systems need only power, and most kit builders prefer it that way. If your controller or display expects data from the pack, confirm the protocol before buying — see BMS communication protocols. Closed branded drive systems, covered in our aftermarket compatibility article, are a different problem entirely.
Building it safely
Conversion kits push packs harder than factory bikes do, so the safety margin belongs in the battery. In the US market, a battery sold as part of an e-bike electrical system is expected to carry UL 2849, with the pack itself tested to UL 2271; UL Solutions publishes what the UL 2849 evaluation covers, including overcharge, temperature, vibration, shock and ingress testing. UL Standards & Engagement explains the wider e-mobility standards picture, including which tests a pack has to survive.
Practical rules for anyone assembling their own setup:
- Use a pack with a real BMS: over-charge, over-discharge, over-current, short-circuit and cell balancing protection, with a temperature sensor.
- Never parallel two packs of different age, capacity or chemistry, and never connect two packs to one controller without a proper dual-battery management device.
- Charge where a failure is survivable — not under a stairwell, not beside the front door, not overnight in a bedroom. A swollen pack goes out of service immediately.
- Keep the charger matched and replace it if the pack is replaced with a different voltage.
- If you buy a kit-plus-battery bundle, ask for the pack specification in writing. Unspecified bundles are how people end up with a “48V 20Ah” pack that delivers 12A and dies on the first hill.
Shipping matters too: any lithium pack sold across borders needs UN 38.3 test documentation and compliant packaging, which is why the shipping rules article is worth reading before you order from overseas.
For kit makers and shops buying in volume
If you assemble kits or run a conversion workshop, the battery is the part customers judge you on, and the part with the widest quality spread. Four things make a supplier relationship work:
- Specify in numbers, not adjectives. Voltage, series count, capacity, continuous and peak current, connector, case dimensions, charge rate. Everything else is marketing.
- Ask for the cell model and the BMS part number, then verify that the two match the current rating you were quoted.
- Ask for documented testing — UN 38.3 reports as a minimum, and UL 2271 or UL 2849 test evidence if you resell into the US.
- Standardise. Running two or three pack specifications across your whole kit range simplifies spares, chargers and warranty, and gives you real buying power. Our 48V wholesale guide covers how to structure those orders, and the custom pack overview explains what a factory needs from you to quote a bespoke case and dock.
FAQ
Can I use a 52V battery on a 48V conversion kit?
Only if the controller is explicitly rated for it. A 52V pack charges to 58.8V, which exceeds the rating of many 48V controllers. Check the controller label for its maximum input voltage before ordering.
How many amp-hours do I need for a 1000W kit?
Current rating matters more than amp-hours. A 1000W kit typically draws 25–35A at peak, so the pack needs a 30A continuous BMS minimum with high-discharge cells. Capacity-wise, 48V 17.5–20Ah is the common pairing.
Why does my kit cut out when I accelerate?
Almost always the BMS current limit or voltage sag. The pack cannot supply the controller’s draw, so protection trips or voltage collapses below the low-voltage cutoff. A pack with a higher continuous rating fixes it.
Do conversion kit batteries need to be certified?
For personal use, the requirement depends on your jurisdiction; for resale, certification to UL 2271/UL 2849 (US) or the applicable regional standard is effectively mandatory. Certified packs also use better protection electronics, which matters more on a high-power conversion than on a low-power commuter.
Can I run two batteries on one kit?
Yes, with a proper dual-battery combiner or by switching between packs, never by wiring two packs directly in parallel unless they are identical, same age and same state of charge. Done badly, parallel packs push current into each other.
Need packs built to a kit specification?
We build e-bike packs for conversion kit brands and distributors in 36V, 48V, 52V and higher, with published cell models, real BMS current ratings, matched connectors and UN 38.3 documentation — and we will quote to your controller’s numbers rather than to a label. Send your spec through the custom battery inquiry form or talk to us on WhatsApp.