Regardless of where the electricity you use comes from in your buses' electrical system, It must be stored in batteries in order to be used when you are not connected to the electrical power grid or running a generator. This installment will help you select a battery system that best suits your needs and provides the best value in terms of longevity and cost.
Batteries come in a wide variety of types, some of which are better suited to the service required in domestic use than others. The most common types you might encounter are described as follows:
Automotive Batteries: Engine starting batteries are by far the most familiar to people, but are the least suitable for powering household loads. These batteries are designed to provide high current (up to several hundred amps) for only a few seconds, and then be immediately recharged by the vehicle's alternator. They do not stand up well to the deep discharges that a renewable energy system requires, usually failing after only 50-100 discharge cycles.
Marine/RV Batteries: These cells are an improvement over engine batteries, as they handle deeper discharges and repeated cycling better, lasting 400-600 cycles. They are commonly available as 12 volt, group 24 or 27, with capacities in the 50-65 ampere-hour range. Their smaller size makes them an attractive choice where space is limited.
Traction Batteries: Specifically designed for heavy duty deep-discharge service, these batteries are frequently called "golf cart batteries" and are used in a variety of applications, including cordless floor polishers, lift trucks, electric vehicles for street use, etc. Their thick plates and internal chemistry allow them to be drained repeatedly to an 80% depth of discharge and then fast-charged, with an estimated life expectancy of 500-1,000 cycles or more. They are commonly available in 6 and 8 volt configurations, in capacities of 180-245 ampere-hours. These batteries are currently the best choice for any renewable energy system, given a reasonable budget.
Gel Cells: Also known as sealed batteries or Absorbed Glass Mat (AGM), these cells use an electrolyte which has been compounded to have a heavy, paste-like consistency. There are no vent caps, and no way in which to add to or inspect the electrolyte level. As such, they can be operated in any position, including inverted, with no ill effects. They are generally quite good at deep-discharge, and can take fairly quick recharges. Disadvantages include high cost of purchase and strict charging current and voltage regulation.
Telephone Batteries: A specialized battery of lead-calcium construction. Not very good for deep-discharge, but often available free or at low cost when a PBX system is removed or replaced. Generally these cells have a slightly lower terminal voltage than normal lead acid batteries. The calcium impregnated in the plates helps limit the production of hydrogen during recharging and helps minimize self-discharge during periods of inactivity.
Nickel-Cadmium: Absolute top-notch battery technology! Can not be damaged by over discharge, misuse, freezing, boiling, left lying about dead, or any other abuse. Quite expensive to purchase new, but will be the final battery you will need to buy in your lifetime. Requires 10 cells to make 12 volts, and has a higher fully-charged voltage, which make it incompatible with some charging systems. If you are ever offered some of these used, snap them up, no matter how bad they may look, for they can be reconditioned to as-new at only a small price. Virtually unlimited life cycle, given reconditioning when needed (about every ten years).
Nickel-Iron: Also known as Edison cells. Every bit as good as Nickel-Cadmium. Often used in railroad signaling devices, can be reconditioned easily. Capacity is slightly lower per Kg of weight than NiCad.
Exotic battery technologies:Nickel-metal-hydride, Fuel cells, Lithium-ion, Sodium sulfur, etc. Obviously, if you or I could afford any of these types of batteries, we wouldn't be driving around in converted trucks or buses, would we?
It is considered essential that all of the cells in a battery, or batteries in a multiple battery system are of the same age, type, capacity, and condition and are exposed to similar conditions of charge, discharge, temperature and maintenance. Mixing and matching battery types or adding new batteries to a bank of older cells is a sure way to be disappointed in your power system.