The battery pack is probably the most expensive component of your ebike, so its best you understand what you are paying for before making a big battery or ebike purchase.
But if you are a newbie, selecting a battery for your new ebike build can be an overwhelming endeavor with all these weird numbers to consider. This guide will attempt to make it easier. Consider Volts, Amps, Amp hours (Ah), and finally Watt hours (Wh) when picking your battery. This article will break them all down. With some simple math you can keep yourself from being ripped off when making a purchase from some slick snake-oil selling ebike vendor. Never have the need to ask how fast or how far it will go again. By using some simple math, you will have an accurate idea of the expected range and power expectancy of any pack.
Volume and Weight
The first one is easy one to quantify because we all have been taught volume and weight since grade school. The girl in the picture holds a 20ah 52 volt battery pack which holds a kilowatt of energy (1000 watt hours) and weighs around 12 pounds. A pack holding that much juice 10 years ago would be made of lead acid and would be heavier than a car battery and we would need a different and much bigger girl to even lift it up.
The most precious commodity we have as ebike builders is space, and…none of us want to carry a lot of weight when we ride because we want our bike to feel like a bike and not some heavy motor bike. How large and how heavy a pack should be is one of your biggest considerations. As technology progresses, ebike packs that can take you many miles are being compressed into smaller sizes and lighter weights, so that now in 2015? even a back-pack battery can take you for a long ride. Look and feel the weight of a battery pack, and decide now what size you can put up with…because its not just the high cost of quality lithium you need to consider…size and weight are also big hurdles to care about.
Lets say you got an existing ebike you are trying to revamp with a new 18650-cell battery pack, or you just bought a cheap China ebike kit off eBay, and you’re looking for a battery for it…the first question you need to know is…what is the voltage of your controller and motor?
Picking the wrong voltage battery is the only thing that could do serious damage to your ebike motor and controller if you pick the wrong one. Get the voltage right and you can’t go wrong with the rest.
A 36-volt controller and motor in general requires a 36-volt pack. All controllers allow a range of voltages before they pop so it’s highly probable you could go to a 52-volt battery even though your controller is rated for 48 volts. By overvolting your 48V controller and motor with a 52V battery you get a nice performance gain with a low chance of frying your controller. Upping the voltage of your battery slightly is the easiest way to get a performance gain on an existing electric bike set up.
So once you have the voltage selected, now you have to make one of your biggest decisions…how big do you want your battery to be? How much range do you want? And remember the more range, the larger and heavier the battery.
The 18650 pack above is made up of Pansonic PF cells and is the Tesla S battery pack. It is 85kWh , about 170 tiimes as large as a typical eibke pack. Today ebike builders learn from the technology Tesla is pioneering to make safe and large pack.
The Amp-hour rating is how much energy capacity is in your pack. But the true energy capacity you also need to consider voltage. For example the pack above is for a Tesla and is made up from cells from the the same Panasonic company that Luna Cycles uses. The more Amp hours, the further you will go. But that gigantic pack is only a little more than 200 Amp-hours. Does that mean its only 10x as big as a 20-Ah pack? No. You also need to consider voltage and the above pack is 375 volts. (enough volts to fry chickens). So right here is where it gets a little tricky….you can’t compare pack energy capacity with just the Amp-hours.
Watt hours –
The ebike above is the Juiced Rider which is one of my favorite commercial electric bikes, because it really does have a huge battery pack. I know because I can do the Ebike math. You can buy the Juiced Rider with up to 48V / 32-Ah which is a whopping 1536 Watt hours (1.5 kWh) which (thanks to math) I know this bike could actually have a very real 100-mile range.
However its hard to identify such a spectacular ebike without ebike math because the industry is full of gross exaggerators, and its hard to believe when someone says his ebike he is selling you will take 100 miles on a charge…but the top of the line Juiced Rider is one that really can.
One of the “gotchas” ebike sellers can get you with is to sell you on a high Amp-hour pack, but not tell you its low voltage. For example a 36 volt 10-Ah pack does not have near the range of a 52V / 10-Ah pack. To do a comparison of ranges of two different packs, they must be the same voltage, in which case all you would need is the Amp-hour rating to compare the difference. But…if the two packs are two different voltages then you must calculate Watt-hours.
Watt hours are calculated by multiplying the volts by the Amp-hours. So a 36 volt 10-Ah pack has 360 watt hours and a 52V 10-Ah pack has 520 watt hours. Once you get to 1000 watt hours, its called one-kilowatt, and that’s how electric car packs are measured. Most ebike packs do not get too far over a kilowatt because now, just the pack is going to weigh over 20 pounds and you start leaving bicycle range. Once you know how to calculate watt hours you can fairly well know what you are paying for in a battery pack. Now we get to the fun part…deciding on how much power we want!
This is the power rating of the pack…how much power it will put out. If you multiply the amps by the volts you will get the wattage your battery is capable of. For example, a 30-amp 52 volt pack is capable of 1500 watts plus. In general you probably want to choose a higher amperage battery over a lower amp battery. You can’t burn out your controller with a high amperage battery because your low amp controller will just suck whatever amps out of it that it needs. If you get a battery rated for low amps…your controller might not get all the amps it wants, which will just mean you will go slower so no harm is done. But if you want maximum speed and power that your set-up is capable of, you want a battery that is rated for at least as high amps as your controller is rated for. If you want to be future proof get an even higher amp-capable battery pack than what your controller is rated for, in case you upgrade the controller, or some day build (or buy) a more powerful ebike.
Tools for Measuring the above numbers
There are several great measuring devices to keep everyone honest when it comes to getting the amp hours you paid for etc. The $30 Watt Meter is a very inexpensive way to measure amp hours in a pack and so is the $135 Cycle Analyst. These devices measure the energy as its depleting from the pack and act like a very accurate fuel guage which tells you exactly when your bike will run out of juice and then you can easily calculate how efficient you and your bike are. Why do no ebike manufacturers provide these convenient gadgets on their bike? They do not want you to have an accurate Amp-hour guage…guess why?
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