Investors are losing faith in the once high-flying battery storage company Fluence Energy (FLNC). Over the past week, analysts at Barclays and Piper Sandler issued bearish calls on the stock. Shares are at their lowest point in about year after tumbling 66% since June, according to MarketSurge. Fluence was down 1% ahead of Thursday's open. Fluence specializes in making battery…