Battery Question

Ok....need advice.

Have 3 identical batteries. Group 31 AGM Duracell. 2 in parallel for 12v TM. One identical battery for starting battery. Bought 10/2020. Used a bunch.

Minnkota 315D onboard battery charger.

Plugged in battery charger. 2 charger banks for the 2 TM batteries come on fine. Indicator lights show those two batteries currently charging.

3rd light, the one for starting battery, is red. Never seen red light come on. This battery also has directly connected Depthfinders.

So I got small separate charger and connected it directly. Welp...that one also shows red light, with icon of a male cord plug.

Tried to start motor, just to see....nothing.

I've checked inline fuses. There's a bunch!! But I can't find a bad one.

My guess is bad battery. If so makes me suspect the other two. Not sure why same type of light would come on for two different chargers unless that's the issue.

Now...if that's it, I see no reason to have to buy heavy (and expensive) Group 31 AGM battery as starter. In fact, these are all 3 deep cycle, which I suppose could be part of the issue...
using deep cycle as starting. Probably need one with decent CCA.

Comments requested.

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Sounds like the charger bank for the starting battery may have quit, so I’d put a meter on that lead and see if it’s pushing any current. I once sorted out a weird charging issue after comparing specs on an www.batteryequivalents.com/lr1130-battery-equivalents-and-replacement.html, so digging into small details like voltage drop or a bad connection can point you in the right direction. Also check for corrosion hiding under the heat‑shrink.