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January 2005
Criteria for Selecting a Battery
A lot of decisions go into selecting a battery type, manufacturer and model.
| Criteria |
Description |
| Gravimetric Energy Density |
How much energy does the battery have for its weight? |
| Power |
Can the battery put out a enough electricity when accelerating or going up hills? |
| Maintenance |
How much maintenance is required? How often? |
| Memory |
Does the battery have a memory? Does it have special charging requirements? |
| Battery Management |
Is the battery tolerant to subtle overcharging or undercharging? |
| Safety |
If the battery was in a crash, how toxic/corrosive/flammable are its chemicals? Can it handle overcharging without a safety concern? |
| Initial cost |
How much does the pack cost? |
| Lifetime cost |
How long will the pack last? Is it cheaper to by many packs over the years or one expensive pack? |
Selection of a Battery System
The following spreadsheet contains a number of different batteries and their characteristics:
Battery Spreadsheet
The general evaluation of each battery chemistry is as follows:
- Flooded Lead Acid - inexpensive, heavy, requires maintenance
- AGM PbA - little more expensive, heavy, no maintenance, less capacity than FLA, non-spillable, high amp output
- Gel PbA - heavy, no maintenance, only suitable for lower amp draw systems (like high voltage AC systems)
- NiCd - very expensive, requires maintenance, great value in the long run
- NiMh - quite expensive, minor maintenance (memory effect), good energy density
- LiIon - expensive, no maintenance, great energy density, requires BMS
Selection for the New Beetle Project
Given that the New Beetle conversion project has limited weight available to battery systems, we are actively investigating LiIon technology.
Although expensive and requires a BMS, LiIon will provide the best range for the weight.
A number of companies have developed high voltage/high current LiIon systems which handle overcharging and equalization.
The cost of a LiIon pack and BMS is prohibitive for the first release of the New Beetle project. It is likely that Lead Acid Gel cells will be used for the first couple of years,
until the cost of LiIon comes down.
Update January 2005 After extensive research and consultation, it has been decided to go with 26 60Ah Deka Gel Batteries, model 8G34R.
These gel batteries were selected because:
- Less expensive than AGMs
- If properly maintained will have over 1000 recharge cycles
- Are of a low profile, so will allow easy fit into the New Beetle
- Have been proven to be very successful in the Solectria Force vehicles (Now Azure Dynamics USA)
- Are less susceptible to developing charging imbalances than AGMs
- Over the long run are more cost effective than any other battery
These batteries do have one major drawback: The cannot put out as much current as other batteries (Wet, AGM). However, since they are going into
a high voltage AC system (higher the voltage, reduces the current for the same amount of power), it is expected that the large current demands will
be within the limits of the battery.
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