Battery Charging

There comes a time in most battery owners life when current methods of charging simply are not enough. You must know the feeling, otherwise you wouldn't be here on this page: The sun won't shine, the wind wont blow or you wish to undertake a power intensive activity that is just to much for the setup you have.

What is battery charging?

Simply put it is adding electricity into a battery that is less than fully charged. There are a few basic sorts of rules to assist you in selecting a method of charging and an appropriately sized battery charger.

To get power to flow into a battery the charging source must have a higher voltage than the battery. If you ever check out the voltage from a 12 volt solar panel you will quickly discover that the voltage present on the terminals when the panel is in full sun is around 20 volts or more. The voltage of an alternator as used to charge your car battery can be even higher, up to 90 volts or more on old alternators, and diode limited on newer alternators to around 40. When you connect any of these devices to a battery the output voltage is stabilised by the battery and held down to slightly above battery voltage.

The charging stages:

You will hear a lot about battery chargers, solar regulators etc. and the type of output they have, often referred to as charging stages. A quality modern charging device like a solar regulator or a battery charger will typically offer what is called three stage charging. Let's look at the three stages as are offered by most quality charging devices.

Stage 1: Bulk charging

This is the initial stage and is when you first connect your charger to the battery. Current is "poured" in until the battery terminals reach a certain pre-determined voltage. Usually around 15 volts for a 12 volt flooded lead acid battery. This voltage is called regulation voltage. A good battery charger will determine when bulk charging is finished because as the regulation voltage is maintained and the battery fills, less and less current will be required to maintain this voltage.

Stage 2: Absorption charging

Once the regulation voltage has been reached and the current flow stabilised a timed period should commence at which this voltage is maintained. Usually around 1 hour.

Stage 3: Float charging

After the period of time at absorption has passed the battery is for all intents and purposes "full of charge". At this stage if you were say running a generator you would "switch off" and that would be the end of the process. If your charging source was a solar array via a solar regulator you would want a further stage other than disconnection. When a battery is full it can be maintained full at a lower voltage then the absorption voltage. The advantage of this is that there will be less water loss, the battery will last longer etc. Typically the voltage on a 12 volt battery would be further reduced to around 13.5 volts.

Sometimes a forth stage is added, usually on battery chargers designed for permanent (live) connection to a battery bank. This will be a slightly lower voltage than the float stage.

Some useful information

A battery will only efficiently take a certain amount of current, regardless of state of charge. This is called acceptance current. You can work this out easily for flooded lead acid batteries, it is usually around 10% of the batteries capacity in amp/hours @ C100.

A 100 amp/hour battery will efficiently take a 10 amp charge. Which of course begs the question; what if I have a huge size battery charger connected instead. Typically if you connected say a 50 amp battery charger to a 100 amp/hour battery you would do no harm, what would happen is the terminal voltage would rise very fast to the regulation voltage and the battery charger regulator would taper off the charge rate to something the battery could accept like about 10 amps. The other more common way is to have a battery charger that is less than large enough, again no problems, it will of course just take longer to charge the battery than it would take with an appropriately sized charger.

Charging while consuming

It is common practice to charge a battery bank while you are drawing power from it. This is a time when a large capacity charger on a small battery bank will work highly efficiently. An electrical load on your batteries will always get its supply from the highest voltage source. If you are charging the batteries the highest voltage source will be the battery charger, no matter what form it takes. Power will flow from the charging source to your load using the battery as a buffer.

Finally ...

Charge efficiency is a term you may come across. A modern, good quality lead acid battery in new condition has an efficiency of around 96%. As is ages this efficiency will drop slowly to around 92%. Simply put if we push in 10 amps for one hour into a battery, about 9.2 - 9.6 amps will be stored.

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