Re: Battery charging with intermittant power source

On Fri, 09 Jan 2009 21:10:18 -0800, Scott Ronald <scottm361@xxxxxxxxx> wrote:

rebel wrote:
On Fri, 09 Jan 2009 22:21:55 +0100, Jan <Nospam@xxxxxxxxxxx> wrote:

rebel wrote:

On Thu, 08 Jan 2009 19:41:18 -0800, Scott Ronald <scottm361@xxxxxxxxx>
wrote:

Hi I am wondering about the best way to charge lead acid batteries from
a source that cannot be relied on for constant power such as wind, or
solar. So far I am looking at 2 ways to do this:

Cycle charging
For this method I need a constant current, until the charge voltage is
reached, then maintain the charge voltage for a time, or until the
current drops to a minimum, after which I drop to a float voltage.
The difficulty with this is that the charge cycle can be interrupted at
any time, and might be restarted when the battery is partly or fully
charged.

Floating
For this I would set the current limit to the maximum battery charging
limit plus the battery's output current, and set the regulation voltage
to the lead acid float voltage. From the reading I have done on this
topic I believe that this will shorten the service life of the battery.
(a) Attend to what Ed said.

(b) Are you thinking of flooded cells, SLA/VRLA or ... ?

(c) Why do you think your "cycle charging" approach is compromised by the
intermittent source?
Lead acid batteries are best charged with a constant voltage source.
Charge current depends on the state of the charge of the battery.
It works similar in a car.

Constant voltage source and a discharged battery can mean quite huge intial
charge current. In ALL types there needs to be SOME mechanism for limiting
current into the battery. Whether this is intrinsic in the charger (which it
often is) defines the need to provide a specific limiting mechanism.

For SLA types, simple CV charging is NOT the regime of choice. A worthwhile
read on care and feeding of SLA's can be found in Unitrode (now TI) app note
U-104 (search for SLUA115.pdf).

I'm still waiting to see the O/P's responses to (b) and (c) above.
(b) I am dealing with SLA at the moment, but I am interested in learning
about VRLA as well.

(c) according to this site: http://www.batteryuniversity.com/partone-13.htm
The stage 2: "constant voltage" is required for a fixed time (assuming
you are charging a battery from a discharged state) for optimal battery
life. I cannot guarantee that 1. the battery is discharged when the
regulator powers up and 2. That the power will be on for the fixed time.
Using a fixed time is not practical, unless another termination
condition (such as temperature, or current) is used.


Did you read the Unitrode AppNote? The regime used by that controller chip
(UC3906) can be restarted at will regardless of the SOC of the SLA. The only
requirement is that the charger can deliver *enough* current to trigger the
changes to the charge mode.

To provide a few more details, I am working with a 3.6kW power supply
with both programmable voltage regulation and programmable current
limiting. Obviously the 3.6kW number is not guaranteed, depending on
wind or solar conditions, so the current limit may not even come into
effect, and the supply may not be able to reach the desired voltage
either. I am looking for the method that will cause the least damage to
the batteries using this type of supply.

Scott

.

Relevant Pages

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