Added battery module, cal factor refresh bug

src/data/general_store.h

@@ -0,0 +1,9 @@
+// Copyright (C) 2026 Hector van der Aa <hector@h3cx.dev>
+// Copyright (C) 2026 Association Exergie <association.exergie@gmail.com>
+// SPDX-License-Identifier: GPL-3.0-or-later
+#pragma once
+
+extern volatile double vbat_global;
+
+void vbat_global_read(double& out);
+void vbat_global_write(const double& in);