Stop guessing on battery size. This guide gives you the exact formula and real-world range estimates so you buy the right battery the first time.
Battery sizing is the most common place e-bike converters waste money. Buy too small and you'll have range anxiety. Buy too large and you're carrying unnecessary weight (and spending $100+ more than you need to). The math is simpler than the spec sheets make it look — once you understand three basic concepts, you can size any battery for any build in under five minutes.
This guide covers the three core concepts (watt-hours, amp-hours, and voltage), the formula for calculating range, real-world Wh-per-mile estimates, and the practical sizing recommendations for common use cases. By the end, you'll know exactly which battery capacity you need.
The Three Numbers You Need to Understand →
The Range Formula →
Battery Capacity by Use Case →
48V vs 52V: Does It Matter? →
Quality Matters More Than Capacity →
The Three Numbers You Need to Understand
Every e-bike battery has three key specifications: voltage (V), amp-hours (Ah), and watt-hours (Wh). Most listings only show two of these, but the third is the one that actually matters for range.
Voltage (V): The 'pressure' of electricity. Most modern e-bike batteries are 48V; some performance builds use 52V. Higher voltage means more top speed and torque on BAFANG motors. Voltage must match your motor — a 48V motor needs a 48V (or 52V) battery.
Amp-hours (Ah): The 'capacity' of the battery in raw terms. A 15Ah battery can deliver 15 amps for one hour, or 1 amp for 15 hours, or any combination that multiplies to 15. But Ah alone is misleading because it doesn't account for voltage.
Watt-hours (Wh): The actual energy capacity, calculated as V × Ah. A 48V 15Ah battery is 720Wh. A 36V 20Ah battery is also 720Wh — same total energy, different voltage. Wh is the only number you should use to compare batteries across voltages.
The Range Formula
Range (miles) = Battery Wh ÷ Wh-per-mile consumption
The trick is knowing your Wh-per-mile consumption. This varies based on motor wattage, rider weight, terrain, assist level, and weather. Here are realistic estimates based on real-world testing:
250W motor, level 2 assist (eco): 10-12 Wh/mi
500W motor, level 3 assist (touring): 15-20 Wh/mi
750W motor, level 3 assist: 20-30 Wh/mi
750W motor, full throttle: 30-40 Wh/mi
1000W motor, full throttle: 35-50 Wh/mi
Example calculation: You have a 750W BBS02 and a 48V 15Ah battery (720Wh). You ride in level 3 assist at an average of 25 Wh/mi. Your expected range is 720 ÷ 25 = 28.8 miles.
Cut those estimates by 20% in cold weather (under 40°F), 10% for hilly terrain, and 30% for battery degradation after the first year. So a 28-mile range might become 16 miles on a cold morning with an 18-month-old battery.
Battery Capacity by Use Case
Here are our capacity recommendations based on common use cases:
Casual rider (5-10 miles per ride, flat terrain): 48V 10-13Ah (480-624Wh). The minimum capacity we recommend. Cheapest option that still gives usable range.
Daily commuter (10-20 miles round trip, mixed terrain): 48V 13-15Ah (624-720Wh). The sweet spot for most riders. Handles a daily commute with 30% range headroom.
Long-distance commuter (20-40 miles round trip): 48V 17-20Ah (816-960Wh). Enough for 30-40 mile rides with hills, or 50+ mile flat rides.
Cargo bike (hauling kids or gear): 48V 20Ah+ (960Wh+). Cargo adds 50-100% to Wh-per-mile consumption. Plan for 30-50 Wh/mi loaded.
Touring/long-distance rider: 48V 20-30Ah (960-1440Wh), or carry a second battery. Range anxiety is real on tours — better to have headroom.
Off-road/trail rider: 48V 15-20Ah (720-960Wh). Off-road riding consumes more Wh per mile than paved riding; budget for 25-35 Wh/mi.
48V vs 52V: Does It Matter?
Most BAFANG motors (BBS02, BBSHD) work with both 48V and 52V batteries. The question is whether 52V is worth the premium.
52V advantages: ~8% more top speed (28mph becomes 30-31mph on a BBS02), ~8% more peak torque, slightly better hill climbing. 52V also runs the motor at lower current for the same wattage, which means less heat in the controller.
52V disadvantages: ~$50-100 more expensive for the same Ah capacity, fewer listing options, and the higher voltage can stress older motor controllers (pre-2018 BBS02 units sometimes struggled with 52V; modern units handle it fine).
Our recommendation: For new BBS02/BBSHD installs in 2026, go 52V if it's only $50 more. For budget builds, 48V is perfectly fine — the 8% performance difference is barely noticeable in real-world riding.
For Tongsheng TSDZ2/TSDZ8 motors, stick with 48V. Those motors are specifically designed for 48V and may have reduced lifespan on 52V.
For hub motor kits, match the voltage exactly to what the kit specifies. Don't run a 52V battery on a 48V hub motor — you'll burn out the controller.
Quality Matters More Than Capacity
A quality 48V 13Ah battery will outperform and outlast a cheap 48V 20Ah battery. The difference is in the cells, BMS, and build quality.
Cell quality: Name-brand cells (Samsung, LG, Panasonic, Sony) are consistent and reliable. Generic cells are unpredictable — sometimes great, sometimes dangerous. The HAILONG battery we recommend uses quality cells; UPP has been documented using recycled laptop cells.
BMS quality: The Battery Management System protects against overcharge, over-discharge, over-current, and short circuits. A 30A BMS is the minimum for a 750W motor; 40A for 1000W. Cheap batteries often have undersized BMS that can't handle peak loads, leading to premature failure.
Welding quality: Cells are connected to the busbars by spot-welding nickel strips. Cheap batteries sometimes use solder (which damages cells with heat) or insufficient welds (which fail under vibration). Spot-welded is mandatory.
Packaging: Quality batteries have sturdy cases, proper strain relief on cables, and keyed locks for security. Cheap batteries have flimsy cases and unsecured cables that chafe through.
Our recommendation: buy the HAILONG 48V 15Ah. It's the safest, best-built 48V pack at a fair price, and it pairs perfectly with every motor kit we recommend.