The basics of ebike batteries
The battery is one of the most expensive parts in an e-bike set up. Unfortunately are the cost for batteries not dropping as expected, mainly because of the high demand from new EVs coming on the market (cars, RVs, buses and even trucks).
Nevertheless you will find some significant price differences between battery packs having almost the same specifications.
Taking about specifications – there are usually two main values: voltage and wattage or ampere hours (Ah). The wattage (Wh) is what tells you how much energy is actually stored in the battery. For example a 36V battery with 16Ah has a wattage of 36*16= 576Wh.
Depending on the kind of motor your ebike uses you don't want to have a too small battery, otherwise the range will be very limited. Offers with batteries having 36V and 10Ah are a gimmick and just good enough to get your shopping done. Since the battery is the cost driver this indicates then also why those bikes appear to be cheaper than other, but actually they aren't.
In terms of the internals most batteries are pretty similar, they consist out of many LiIon cells, typically using the 18650 standard. Those are connected to a BMS (Battery Management System board). The BMS measures the voltage between the different cell packs to monitor potential problems with cells failing. It can also to a limited amount balance the cells during recharging. If the BMS detects a problem it will turn the battery off to protect from further damage. There are complex BMS boards on the market, having BlueTooth or a wired interface, this can help to identify problems at an early stage. But it also drives the cost for such batteries.
The case of a battery is mostly designed to match the purpose for the frame mounting position. There are generic cases and many customized cases on the market. A customized case will be better integrated into the frame and therefore improves the optical appearance of an ebike. Some batteries are build into the down tube, almost invisible.
But customization has it's limits – when trying to get as much capacity as possible these packs can't reach what a generic case can offer. There is also to consider if batteries from even the same company are interoperable. If you think about buying two bikes, but maybe different styles, it would be great if you still could swap batteries around. But often different bike models use also different case styles.
We have decided to use a generic case called “Hailong” - these cases offer different sizes for a capacity range from 13Ah up to 20Ah. As they still use the same holder these batteries can be swapped around between all models we offer.
It will be also interesting to see how the long term availability will be for such specialized battery cases in let's say 5 years... as there are almost every year new variations from the same brands. The Hailong style is widely used and won't vanish short term. This allows us to guarantee supply for the coming years.
The cells we use are from well known brands: Samsung, Panasonic and LG. The 18650 standard is based on the shape and size of the cell: 18mm diameter and 65mm long. The typical cell capacity for ebike use ranges between 3000mA and 3500mA. To get the final capacity many cells will be connected together (parallel) to increase the capacity, i.e. 5 cells to reach about 16Ah. To actually get 36V you have to connect 10 of these parallel cells packs in series, so altogether 50 cells to get a 36V, 16Ah battery pack.
The main difference in quality between different suppliers is how precise are these batteries matched together. Matching means each cell is slightly different due to production variations, the battery manufacturer has to measure each cell to identify similar ones. Only these cells can be mounted in a parallel pack. Small variations between series packs can be mitigated by the BMS.
Cheap Chinese imports often use battery packs from B-production with higher variations, still claiming having i.e. Samsung cells. But these packs won't perform well over a longer time period. Make sure you get at least a two year warranty on the battery. Everything below this indicates that the supplier does not trust in his own product.
Temperature plays a big role
LiIon cells perform well in a temperature range between 5C and 40C, but not so well when it gets cold. So if you plan to use your ebike in Winter, keep the battery inside and warm before you go for a ride.
The life time of a Li battery can be expected with 5 to 6 years – after that the battery will still work but the capacity has probably dropped to less than 70% and the range might be not enough any more for what you want to use the bike for. All cell manufacture state a typical cell cycle life time of about 500-600 full cycles. So there is no big difference if you pay $800 or $1400 for a battery – they both will be worn out after the same time...
If you plan not to use your ebike battery for a longer time period (>3 months), we recommend to discharge the battery to about 50% and store it at a cool place (<20C). Long term studies showed that fully charged Li cells tend to generate tiny amounts of gas at the positive electrode. This increases the cell pressure and internal resistance. In the end it will degrade the cell to a point were it will loose most of it's capacity.
If you intend to use your ebike for more than 5 years you will have to add to your equation the cost for a replacement battery. Ask the retailer for replacement cost. An average 16Ah battery (576Wh) shouldn't be much more than $800. If you pay more than mostly because you support a famous brand name or someone tries to rip you off...