2008 ford explorer check charging system on will not start.

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When it starts it runs fine. It may start for 3 or 4 days. Then it will not start and the check charging system message is displayed. Some times will start in a few minutes or coule be couple of hours up to the next day. Had battery checked and it is fully charged and ok. Checked fuses and relay and ok. No funny noises. There is a faint click when turn key to start.