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m : microhydro@yahoogroups.com 27 October 2011 • 1:53AM -0400

[microhydro] BATTERY CHARGING
by Nando

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For the past few weeks I have received a large number of request for
assistant in reference to battery charging via solar, wind , microhydro and
other means.

To be able to charge a battery, the charging source has to produce a voltage
that is higher than the battery may have and One needs to pay attention at
the charged battery voltage that is going to be higher than the nominal
battery voltage.

Nominal voltage is the basic rated voltage of a battery bank, like for a
Lead Acid battery that each cell has a nominal voltage of 2.00 volts , so a
12 Volts battery has 6 cells -- but this voltage is not the charged voltage.

For a 12 Volts Lead Acid battery the 100 % charged value runs to 13.8 Volts
that we called Float Voltage.

In the charging process, the battery in this case needs to be charged to a
bit higher like 14.1 or more this depends on the chemistry of this lead
acid. This  voltage represents the Bulk charging ( deep plate charging) --  
battery temperature does affect the battery voltage level.

Lead Acid to have the longest possible life should not have its 100 % charge
removed by the load, it should be limited to 50 % or the equivalent of the
battery voltage being 12 Volts -- discharging below 12 volts the battery
starts to have non-reversal sulfation problems.

The charging current of a battery should be not more than 20 % of the
battery capacity , i.e. a 20 amphour battery should have a maximum charging
current of 4 amps.

A generator  attached to a wind mill can not charge to  such current level
it the voltage is not higher than the battery voltage plus the internal
resistance of the generator times the charging current, in this example if
the generator has an internal resistance of 2 ohms, then the generator needs
to produce a minimum of 14.1 volts plus 4 amps * 2  ohms = 8 volts for a
total of 22l1 Volts ( in addition we need to take in consideration the wire
resistive losses plus the losses of any additional circuitry that may be in
series).

This is just  a basic information for  those that may need it and to reduce
the direct email traffic I receive.

Nando


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