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Lead Acid Battery Basics

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Lead Acid BatteryLead acid batteries are the oldest type of rechargeable battery. They are the easiest battery to maintain and typically are the lowest costing making them the idea battery bank for home solar/wind systems.

This page is going to talk mainly about 12 volt flooded lead acid, but it applies to all you will just need to multiply or divide the voltages. 12 volt lead batteries are made up of 6 cells, each cell voltage is about 2.15v. A 24 volt battery would have 12 cells at 2.15v.

What is a full charge and what is a full discharge

A fully charged lead acid battery will be at 12.6v at 70ᵒ F, this is the sitting open circuit voltage, meaning that it has not been charged or had a load in at least several hours. If the battery is charging the voltage will float higher and if there is a load on the battery the voltage will float lower. To get an accurate reading of the battery it must be disconnected and left to sit for about a day. This also applies if you are using a hydrometer to measure the specific gravity.
If you need to get a quick reading while charging, simply disconnect the battery and wait a few seconds and if the battery is still above 13.2v then you can assume it is near a full charge.

How to Charge

Charging a lead acid battery is easy, just give it more than 2.15v or high per cell, the higher the voltage the faster the charge. The charging voltage must always be higher than then batter voltage, voltage is a force, so you need to have a higher force coming from your charger to pushed the energy into the battery.

Typical charging voltages for a 12 volt battery are between 12.9v and 14.4v, these voltage will completely charge the battery without overcharging or damaging the battery. If the battery is not near being completely charge you can charge at a much higher rate without causing any damage.

Many modern chargers use multiple charging stages to allow the battery charge quickly, efficiently, and completely without damaging the battery. These stages are typically broken down in following three stages:

Bulk Charge – The first stage where most of the battery is charged to around 80 or 90%. A high voltage is applied to the battery; there is no typical range for doing a bulk charge, but typically between from 10.5v to 16v. The charger would wait until the battery rose to a predetermined voltage typically between 12.6v and 13.5v.

Absorption Charge – The second stage is where the battery is ‘topped off’. A constant voltage is applied generally between 14.1v and 15v. Current will gradually tapers off as internal resistance increases during charging. The charger will wait until the battery voltage rose to a predetermined voltage typically around 14v.

Float Charge – The final stage of these chargers are to ‘float’ the battery at a voltage that will keep it full charged and ready for use, it will also help to prolong the life of the battery. After the absorption charge is complete the voltage will be dropped to around 12.8v to 13.8v. This is sometimes referred to a trickle charge.

Many of the modern chargers use PWM (pulse width modulation) that pulses the power to battery only a percent of the time proportional to the charging voltage several hundred or thousands of times a second. This accomplishes the same thing as regulating the voltage.

Temperature offset table for float (standby) charge for FLOODED Lead Acid Batteries:

Battery Temperature

Charge Voltage per cell

Charge Voltage for 12V Battery

Gassing voltage

-30 °C (-22°F)




-20 °C (-4 °F)




-10 °C (14 °F)




0 °C (32 °F)




10 °C (50 °F)




20 °C (68 °F)




30 °C(86 °F)




40 °C (104 °F)




50 °C (122 °F)





Battery Capacity

Coming soon.

Last Updated on Sunday, 26 February 2012 16:03  

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