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Two Typical Hybrid Cycling Tests: Results from the Hybrid Test Bench

Michael M.D. Ross
RER Renewable Energy Research

Full Text Report
Link to CETC-Varennes

Acknowledgements:

Research conducted on behalf of the Photovoltaics and Hybrid Systems Program at the CETC-Varennes (Natural Resources Canada) with partial funding from the Panel on Energy Research and Development (PERD).

Citation:

Ross, Michael M. D. Two Typical Hybrid Cycling Tests: Results from the Hybrid Test Bench. Report to CETC-Varennes (Natural Resources Canada). Montreal, Qc: RER Renewable Energy Research, 2004.

Summary:

This document reports on two test sequences executed on the hybrid system test bench of the Photovoltaic and Hybrid Systems Group at the CANMET Energy Technology Centre—Varennes. These tests involved cycling the battery under genset power; in future tests, a photovoltaic array will be included. The first test discussed here repeatedly executed a cycle of partial discharge followed by bulk and absorb charging; the second test was different only in that absorb charging was eliminated from the cycle.

During the first four cycles of the second test, only 81% to 95% of the charge withdrawn from the battery is returned on recharge. As a consequence, the charge that can be withdrawn prior to the battery reaching the end-of-discharge threshold decreases by 50% over five cycles. While temperature appears to influence the battery behaviour, there is clearly some other mechanism at play here, perhaps stratification of the electrolyte or the accumulation of lead sulfate on the surface of the negative plates. This is a significant observation because it lends credibility to the contention that absorbed glass mat batteries—like flooded batteries but unlike gelled electrolyte batteries—can not be cycled between partial states-of-charge, may indicate significant stratification in these absorbed glass mat batteries, implies that regular absorb charging may be required in hybrid PV systems, and may go some way toward explaining the abbreviated charge-discharge cycles occurring in some hybrid systems.

The results of the second test were inconclusive, however, and this test should be rerun. In addition, if further tests are to be conducted next summer, some changes to the hybrid test bench should be made. These changes would help control the battery temperature more accurately and thus reduce the confounding influence this variable.