Wireles Networking is a practical guide to planning and building low-cost telecommunications infrastructure. See the editorial for more information....

Discharging Characteristics

A 12 Volt lead-acid battery that delivers energy to a consumer provides a voltage depending on it's state of charge. When the battery is 100% charged it has a output voltage of 12.8 Volts which is quickly dropping to 12.6 Volts under load. Given that the battery has to provide constant current the output voltage is now linear, dropping from 12.6 Volt to 11.6 Volts over a long period. Beneath 11.6 Volt the output voltage is dropping down quickly over time. Since the battery provides approximately 95% of it's power within this linear voltage drop, the charging state could be estimated by measuring the voltage under load. The assumption is that the battery is 100% full at 12.6 Volts and has 0% charge at 11.6 Volts. So, when measuring a battery that is currently discharged, the status can be estimated with a digital multimeter. For example a reading of 12.5 Volts corresponds 90% charge, 12.3 Volts corresponds 70% charge, etc.

Lead acid batteries degrade quickly when charging cycles go down to 0% charge. A battery from a truck will lose 50% of it's design capacity within 50 150 cycles if it is fully charged and discharged during each cycle. At 0% charge the battery still has 11 Volt at the terminals under load. Never discharge a 12 Volt lead acid battery beneath this value. It will forfeit a huge amount of storage capacity. Discharging to 0 Volt will utterly ruin it. To avoid this, a low voltage disconnect circuit (LVD) should be used to build a battery powered system. In cycle use it is not advisable to discharge a simple truck battery beneath 70%. Not going beneath 80% will significantly increase it's durability. Thus a 170 Ah truck battery has only a usable capacity of 34 to 51 Ah!

A battery from a car or truck should stay beyond 12.3 Volts in the system. In rare cases it may be allowed to drop down beneath this value -an unexpected long period of bad weather for example. This is tolerable if the battery is fully charged after such an incident. Charging to 100% charge takes quite a while because the charging process slows down when approaching the charging end even if there is plenty of energy from the power source. A weak power source may seldom achieve a full charge and thus wear out batteries quickly. It is recommended to charge aggressively to keep cost of ownership low. A wind/solar charging regulator or automatic battery charger (with advanced charge characteristic) will help save money. Best is IUIacharacteristic, IU characteristic is second choice.

Starter batteries are the cheapest batteries available, but they may not be the best option. There are special solar batteries on the market which are designed for use in solar systems. They allow deeper recharging cycles (down to 50% charge, depending on type) and have a low self-discharge current. The same applies to most sealed lead acid batteries. Sealed lead acid batteries are more expensive but safer to handle.

Truck or car batteries that carry the label maintenance-free should have neglectable low self-discharge current. However, maintenance-free batteries still need maintenance. The level of the electrolyte fluid must be checked frequently, especially in hot climate. If there is loss of electrolyte, distilled water has to be used to fill up the fluid. Neglecting this will ruin the battery.

Charging your batteries too much will destroy them too! The charging current in a battery buffered system must be regulated. Excessive and unlimited charging will destroy the battery. If the voltage in the battery is too high, the water component of the sulfuric acid will be cracked up by electrolysis, causing an atmosphere which contains a concentrated amount of oxygen. Oxygen is very corrosive and will destroy internal connectors.

Last Update: 2007-01-25