Batteries are a difficult issue. Battery choice is probably the single most defining choice that determines how your car performs and what it will cost to drive. Estimating the battery replacement cost as it works out in a per mile format is a important part of the battery choice for most people with limited budgets. My 914, which is raced and whose batteries are often abused, comes out close to 50 cents per mile in battery cost while the wind generated electricity for it costs only 3 cents per mile. I suspect that many people would consider my battery cost too high for a commuter car.
In addition to up front costs, there are many other variables to consider when choosing a battery type and configuration, here are a few important ones:
How many cycles will the target battery be expected to last? It is often difficult to get data for estimating this, but it is important to consider how your planned power draw levels and depth of discharge (DOD) will affect life. Sometimes finding someone with experience in a similar application is the only source of information. The answer to this along with your expected normal driving range will allow you to determine your battery replacement cost per mile.
Will your target pack capacity fit in your vehicle? Capacity on lead acid batteries is closely related to weight, so I suggest estimating your target battery pack weight and going from there to see what voltages various options would provide. Or, if a small pack is your target such as with lithium cells, then I suggest you start with your required capacity in kWh and make the weight a less primary concern.
When figuring out how much battery you will need, be sure to consider the “C” Rating of the capacity. A battery will supply much more energy at a 20 hour rate than it will at a 1/2 hour to 1 hour discharge rate more typical of EV’s.
Battery pack voltage has little direct effect on vehicle range. A 1000 lb lead acid battery pack with 144V will have a very similar range to a 288V pack of the same weight. Lower voltages such as 156V and under can be easier to use since safety devices like contactors and fuses are easier to find and controllers cost less, but higher voltage systems can get more power out of our currently available motors. In general, speed costs money, and higher voltage gives higher speed.
Will you be able to end up with an acceptable pack voltage when you have put in your target weight of batteries? This is primarily determined by the weight and voltage of the battery module. In some cases people will run more than just one series string of batteries opting for multiple parallel strings or batteries in “buddy pairs”. You should be forewarned that this makes cell equalization tricky and so may impact battery life.
Do you need maintenance free batteries? They generally cost more per mile, but often give more performance (power in AGMs, life in Gels) as well.
How much power do you need? Will gel or flooded batteries provide enough power or do you need the higher horsepower of AGM (or maybe high power lithium) batteries?
There are thousands of batteries available, most of which are not regularly used in EV’s. In general I suggest finding a battery that someone has used in a similar application for several years with good results, since the sales literature is often misleading or optimistic on a batteries capability. A good place to look for such a person might be on the EV List where there are many people running EV’s and in the EV List Photo Album. You can find links to these on the Cafe Electric links page currently located under “Other Info”.
When comparing battery specifications it is helpful to keep in mind the quote often attributed to Thomas Edison:
“There are liars, there are damn liars, and then there are battery salesmen.”