You can really go off the deep end trying to investigate batteries. There are many choices, but they are often limited in many ways, so that making the ‘right’ decision is confusing. There are several popular routes for electric bikes.
Note: adding batteries in ‘series’, meaning, connect the + of one battery to the – of another, then use the two unconnected terminals, adds their voltages. Connecting batteries in parallel adds their capacity together. This means connect the – terminals of each battery together and the + terminals, same as jump starting a car.
Discharge rate is how fast a battery is being discharged. This is usually written as a multiplier of ‘C’, the battery’s capacity. So if a 7 Amp hour battery is being discharged at 2C, then 14 Amps is being drawn from the battery.
Cycle life is how many times a battery can be discharged and then charged again.
Calendar life is how fast the batteries degrade over time, regardless of usage.
Life is usually rated in how many cycles or years it takes to drop to 80% of rated capacity.
Capacity is usually rated at the “C20” rate, or how much capacity is available if the battery is discharged over 20 hours. For vehicle applications, this means the rated capacity is usually optimistic, as the batteries get drained much faster. This is explained later in this document.
Depth of discharge or DOD is how much of a battery’s capacity is used before charging. Cycle life tends to drop really quickly after roughly 50% discharge. The Toyota Prius discharges its pack down to something like 40% to increase it’s life.
Here’s a quick rule: as of the summer of 2008, reasonably priced batteries cost $1 per watt-hour for about 700 cycles. I’ll explain this later in the document too.
Lets move on to battery types.
Lead Acid
Sealed Lead Acid batteries (SLA) are lead acid batteries that are sized about the same as motorcycle batteries. They are used in uninterruptable power supplies. They will not spill acid if turned upside down. Instead of being filled with liquid like a car battery, they are usually filled with gel. 12 Volt, 7.5Ah batteries can be had for $16-$20 from Mouser.com and some other sources. Four batteries will power a bicycle. However, they weigh about 20-30 pounds. They are easy to charge (just apply about 14 volts). If you connect the batteries to each other using connectors rather than permanently soldering them together, you can just use a car battery charger to charge each set of parallel cells. Normal car batteries have very poor cycle life and should not be used. Plus they are super heavy. You need deep cycle batteries.
Pros
- Cheap (~$0.35 cents per Watt-hour)
- Easy to create a battery pack and charge it
Cons
- really heavy
- cannot support high discharge rates (max is about 4C), so you need to carry more Amp hours of batteries to deal with acceleration and the extra weight
- low cycle life (100-300 cycles)
Thoughts: appropriate for vehicles that are going to be heavy anyway or for those who want very low initial cost.
Nickel Metal Hydride
Nickel Metal Hydride rechargeable batteries are used in digital cameras and many other consumer goods. 1.2v 2.6Ah Sub C (slightly smaller than C cell) can be purchased for about $3 each. It takes about 30 of these cells to equal one 7.5Ah SLA battery. However, thanks to the weight savings and higher discharge rates that are possible, you won’t need as much spare capacity.
Pros
- Much better performance than lead acid batteries
- Light weight
- Sub-Cs have high discharge rates (10C-20C)
- Safe
- Proven – NiMH cells have powered cars and consumer products for years and they keep on working. Hard to argue with that.
Cons
- Charging in parallel is almost impossible. Each series string must be charged separately
- Building the pack means connecting lots of small batteries together.
- Costs just about as much as lithium, which has better performance
Thoughts: if you want rock solid reliability, probably your best choice. But you’ve got to deal with the packaging headaches.
Lithium Ion Cobalt
Lithium ion batteries are mostly used in laptops, but are also popular for newer RC vehicles. They’re also making it into some next generation hybrid cars. The nice thing about lithium ion cobalt cells is that they are usually rated for around 3.6v– so even though each cell costs between $5 and $10, you only need 1/3 as many as with NiMH to get the same voltage. Capacities are comparable with NiMH. Additionally, Lithium ion batteries allow much higher discharge rates.
Pros
- high energy density (3.6+v per cell)
- high discharge rates
- light weight
Cons
- melting laptop batteries anyone?
- Older cells have a calendar life of 3 years or so
- Cycle life is OK compared to NiMH (700-1400 cycles)
- Expensive up front
- Lithium ion cobalt cells are very sensitive to the conditions under which they are used. Their capacity drops with time. Additionally, if they are abused, the cells will turn into a resistor, heating up with use until they rupture. “Battery management” is required to monitor that the batteries won’t suddenly die. Charging is also more complicated.
Thoughts: unless you get a good deal on these, you might as well go with LiMn or LiFePO4.
Lithium Ion Manganese
Used in many power tool packs. Not as good as A123′s Lithium Iron Phosphate batteries performance wise.
Pros
- Relatively stable – not as good as A123s, but pretty good
- reasonable life – about 700 cycles
- high energy density (3.2v/cell)
- high discharge rate
- light weight
- much more reasonably priced than the A123s (about half the cost up front)
- If you use the stock power tool chargers you won’t void the warranty on the batteries
Cons
- still expensive up front
- The best source is power tool battery packs, and you have to find them and then rip them apart, which is time consuming, or buy connector blocks if you want to maintain the warranty (which also means charging each pack separately, which can get annoying)
Thoughts: Good compromise choice. If Makita and Milwaukee have gone with these, they’re aren’t crap, but the new generation of tool packs is only a few years old, so we’ll see.
Lithium Iron Phosphate
The A123 LiFePo batteries are making quite a big splash. They are expensive when purchased alone, but can be obtained by buying DeWalt 36v power tool battery packs or Ping Battery packs on eBay.
Pros
- Stable – they won’t catch on fire and they’re tolerant of abuse (eletrical and mechanical)
- Long life – A123 claims 1000+ cycles
- high energy density (3.2v/cell)
- very high discharge rate
- light weight
Cons
- expensive up front
- 3.2v is less than 3.7v– so you need more batteries to get the same voltage
- The best source is DeWalt battery packs, and you have to find them and then rip them apart, which is time consuming
Thoughts: If you are after high performance and reliability, go with the A123s.
Other Lithium Ion Chemistries
There are quite a few other chemistries in existance. Use Wikipedia for more informatiom.
Lithium Polymer
These batteries are used mostly in RC airplanes and helicopters because they have very high energy density and discharge rates and are very expensive. If you have the money, you could power a bike with these.
Pros: the best performance you can get
Cons: can catch on fire with abuse. Very expensive!