I’m going to go out on a limb and say the batteries you use on your bike will, more than any other factor, determine how the bike will perform. The batteries determine how far you ride, how much acceleration you have and the weight of the bike. They’re also likely the biggest single expense.
The ideal battery for a bike is compact, since we don’t have a lot of room. We want a battery with a lot of capacity, so we have good range, and we need something that can discharge (for acceleration) and recharge fast. We want as little weight as possible, since weight sucks horsepower and affects handling and range. We also need a battery that’s safe, and won’t spill.
Here are the ratings we need to look at.
Ah, or amp-hours, shows the capacity of the battery. It’s based on a number of amps discharging over a period of time, and the more amp-hours, the longer the battery can power your ride.
C-rate. This is the charge/discharge rate, which tells you how much the battery can dump to your motor, and how fast. It’s based on the internal resistance of the battery, and is a function of the battery design and construction.
Other stuff- Weight and size. Cost. Construction. Voltage. Other things to make the system run. (BMS)
The real choices for batteries for bikes comes down to only a few options, and frankly, there aren’t any silver bullets here. Trust me, there’s nothing out there that hasn’t been thought of, or tried, and there are some really good reasons why the vast majority of builders have settled on what they use. Here’s the list.
Really, there are a few choices in lead batteries that are safe and good, and the old flooded acid batteries, and even the sealed flooded batteries aren’t among them. Basically, if you can hear acid sloshing around when you shake the battery, you want to put it back on the shelf, it’s just too risky to have liquid sulfuric acid potentially running all over your bike and your body. This narrows it down to the AGM type, which are sealed, and the acid is absorbed in a fiberglass matt. There are a few variations on the theme, but any of them need to be “deep cycle”- batteries that can withstand repeated cycles of nearly full discharge and recharge. If they are deep cycle, they’ll say it.
These are for wheelchairs and scooters, they’re typically kind of small, and they’re probably the cheapest option for a bike. They’re pretty hard to find locally, although a few distributors who allow walk-in customers do stock them, so you’re going to pay shipping.
These are certainly a good option for getting the bike up and running for short money, but don’t expect to get either distance from them or fast discharge. So, what we have here are small, deep cycle AGM batteries with fairly low Ah ratings (usually from 7.5 up to 35) that have fairly low C ratings.
Odyssey “Dry Cell” Batteries
Odyssey claims they’re using dry cells, in fact, it’s a fairly typical AGM design with glass mats and lead plates. The Odysseys have very low internal resistance, however, which give them a relatively high C rating. They also are fairly high-priced, certainly at the top end of the AGM price scale.
I’m calling these orbital batteries for lack of a better term. Exide and Optima batteries are still AGM batteries, sealed, but they’re a different construction from the standard lead plate array. They’re put together with spiral wraps of lead, essentially, which gives you more surface area. As a result, they have a good C rating, and they’re on the higher end of the cost scale. Here’ s a pretty complete story on them at Popular Mechanics.
They are still going to be heavy- the basic physics of the lead/acid equation is pretty much a given there’s not much you can do about it, but the advantages they give you over a standard plate array make them a popular option.
Lithium batteries have a much higher Ah rating for their weight and size than lead. There are several brands- A123, Headway, Valence,Sky Energy (now CALB) and Thundersky are the most popular variations on lithium technology, but they’re also more money than lead. Lithium cells are smaller, at around 3.2v each, so you have to gang them together to get your capacity. When you have a gang of individual cells, you then have to make sure each cell is charged at the same amount and rate, and discharges likewise. Enter the BMS.
The lithium Battery Management System, or BMS, is a device that monitors and regulates each individual cell. It’s basic function is to protect the cells from the stresses of overcharging or too-rapid discharge. If the cell is not charging enough, it feeds it more charge, if it’s overcharging it shuts the charge off. It’s a simple concept, but when you multiply it out over a lot of individual cells it gets to be quite an electronic handful. At this stage in the game, the BMS is the hardest nut to crack in the system.
From what I’ve seen, the Thundersky lithium batteries are probably the widest used. They sell for around $50 for one 40Ah 3.2V pack, without the BMS. Let’s say, for the sake of argument, a BMS is going to cost you $300, you’re at around $1500 for 72V, 40Ah, and 80 lbs. (And a C rating of around 3, I think- pretty respectable).
Let’s compare that to the Optimas. A pretty typical price is around $150 for the D51 packs rated at 38Ah. We get six of them to get to 72V, and we don’t need a BMS, so the total is $900, and the weight is 156 lbs.
Almost half the money, but twice the weight. Added weight will cut down your range- it’s simply more work to push.
See how this works? Like anything else, if you want to dance, you have to pay the piper.
The tricky part of this is understanding how the weight difference will affect your performance, and what you want, and what you can live with. There, you’re pretty much taking a guess, unless you’re an experienced builder, because so much depends on the overall picture- the type of bike, the type of riding you do, what you want and need, all those factors, even your physical weight. Knowing exactly how the weight difference of lead is going to affect your overall riding satisfaction is something only you can decide, and usually after you’ve made the build. You can, of course, talk to people, look at builds, and now, even look at commercially available products to see what works and what people claim for numbers. Ultimately, though, you have to see for yourself.
Add to this the fact that battery technology is, thankfully, moving at a pretty fast clip, and you get to my personal strategy (OK, I stole this from a comment Brian Wisman of Brammo made…) of making the bike “battery agnostic”, to allow you to adapt easily to new types and configurations of batteries in the future. You really can simply build a bike and, once the motor and controller are there, just plug the required voltage into the controller. The key is designing a way of holding the batteries that’s versatile- set it up for what you want to use now, and make it so you can change later.