48 V, 20 Ah, and the number that is not in the title

Two numbers, and the missing one

Every e-bike battery is sold as two numbers. 48 V 20 Ah. 36 V 13 Ah. They are in the product title, they are the first thing in the listing, and between them they tell you roughly how far the thing will go. Multiply them and you get watt hours, and watt hours are range.

The number that is almost never in the title is how much current the pack will actually give up at once. And that is the number that decides whether the pack works with your motor at all.

The board inside is the part you are buying

Inside every one of these packs is a protection board, the BMS, and its job is to cut the power if anything goes outside what the cells can take. Too hot, too empty, too much current. It is the reason a lithium pack on a bike is a sensible idea rather than a fire.

It also means the pack has a hard ceiling on current that has nothing to do with its capacity. A 20 Ah pack with a 30 A board and a 20 Ah pack with a 15 A board are the same two numbers in the title and they are not the same product.

Now put a controller on it. A 22 A controller asking a 15 A board for everything it has, on a hill, in a headwind, with a full load, will trip it. And the way it presents itself is not a warning light. It is the bike losing all drive at the exact moment you were leaning on it, which is a genuinely unpleasant thing to have happen in traffic.

And while we are here, the charger

Two packs can both say 48 V on the label and want different chargers, because 48 V is a nominal figure rather than a charging figure. A 48 V lithium ion pack is charged to about 54.6 V. A 48 V lithium iron phosphate pack is charged to about 58.4 V. Same label, different chemistry, and the charger for one is wrong for the other in both directions.

Undercharging is merely disappointing, so you get a pack that never quite fills and range that never quite matches the arithmetic. Overcharging is the other thing, and it is the failure mode that ends up on the news.

So the three things I would check on any pack listing, in order: what the board is rated to in amps, what chemistry it is, and what the charger in the box actually outputs. None of those are usually in the title, and all three are usually somewhere in the description if you read to the bottom.