I have a red light on my minnkota 210d it says connection problem. Ive checked connections
Do you have a voltmeter? Most "charger problems" end up being battery problems. Check the voltage of the problem battery and report results here.
Your charger has a fuse in each battery lead of the charger wiring. If either fuse is blown, you'll get a connection problem warning. Did you check those fuses? Swap both with the ones from another set of charger leads to see if the problem moves to the other battery. If the same battery still has the problem, continue below.
If the fuse swap doesn't change things, remove the charger leads from two batteries and swap them. Turn the charger back on and note whether the connection problem warning moved also.
If the connection problem warning did not move, you have a faulty charger output (assuming the fuse was confirmed to be good).
If the connection problem warning moved to the other charger output, you have a bad battery that is not accepting a charge. Batteries that are severely discharged (or about to fail) have a voltage so low that the charger will not recognize that the battery is "there" even though it is connected. Your charger will give a warning in this case.
If you have deeply discharged a battery that won't charge with a smart charger, you can sometimes get it back by first charging it for a few hours with an old-school manual battery charger. THEN, reconnect the smart charger and try charging it. Regardless, you may be about to need a battery if this is happening. Consider getting that battery checked at an auto parts store. I think you have a bad battery, and not a charger problem.