[RE-wrenches] Lithium Ion Phosphate for grid back up

[John DeBoever]

Larry,

You are right, the battery bank voltage drives the PWM constant voltage of the controller. The MPPT is a higher frequency DC/DC converter driving the PV module I-V curve to max power point, to which a 300 Hz PWM constant voltage is combined.

John

From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Larry Crutcher, Starlight Solar Power Systems
Sent: Wednesday, August 24, 2011 12:54 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] Lithium Ion Phosphate for grid back up

Hi John,

Current is not reduced "due to" the PWM or the MPPT process ass you described. Current is reduced as a function of the electro-chemical process causing the internal resistance to increase as the cells reach full absorption. The PWM regulation only controls voltage. MPPT is only converting voltage to current before PWM controls voltage.

Larry Crutcher
Starlight Solar Power Systems




On Aug 24, 2011, at 9:36 AM, John DeBoever wrote:


Darryl,

Lead acid battery typically recommended bulk charge is 10-15% of C20 for the flooded lead acid and 20 -25% of C20 for the sealed valve regulated lead acid.  Depending the application and the environment, these rates could be optimized, on a case by case basis. As the battery voltage raise to the absorption voltage (regulation voltage), the current automatically tappers due to the PWM or MPPT regulation.
Off-grid standalone PV systems optimum designs feature 5-days autonomy at 80% DOD at the worst month-average low temperature. Off-grid PV-genset hybrids optimum designs typically feature 2 to 3 days autonomy at 80% DOD same conditions.  The purpose of the large battery bank is to take the best advantage of the solar energy when it is available to store Ah in the battery for a desired power supply system power availability level. So the maximum current available from the PV array, after powering the load, is not wasted and sent into the battery during bulk charge and then tapered in as the voltage raise.

Lithium-ion batteries charge mode have certain benefits in some well-defined applications requiring fast charge with the requirement of a BMS.

John



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