So, out fishing today and had a couple of intermittent faults.
Setup is a Yamaha F70 on a Stabicraft, round digital dials for speed and RPM.
Twin batteries set up by me with a voltage regulator, used last season without any issues.
Today, theengine blankly refused to start on 3 occasions. Nothing, nada, zilch. Switching to dual battery combined and all was ok. Thereafter no issues except when the speed dial displayed 'ERR' and nothing else....engine still running ok.
Only thing I noted was the battery voltage on the start battery was lowish on startup and only rose to around 14.4V after a minute or so of running.

Stuffed battery? I run dual identical batteries, both high CCA as I figure the current draw of a sounder and VHF is minimal, and is outweighed by having identical batteries to dual up in an emergency.

Since the batterys are the same try swapping them so the start battery is the other one and see if you have the same problem. If problem goes away i would get the battery tested.

How old are the batteries - 3-4 years can be a typical life you may get longer if this age could be worthwhile getting load tested by an auto electrician?  Sounds like it is being charged,  If one is tired would replace both for peace of mind and rotate batteries every few years to even out usage if one is favoured over the other?

As above for long life, maintaining charge is important.
 check connections as suggested
If going to swap them around.. may as well take them out and get tested...
 but careful who/ where what they test with... far too often the pre setting are not set correct.. either by ill training or to give false  failure/ failing results.

intial sign of a poor battery  / connections is when fire up the engine the gps turns off due to voltage drop.

Could be high resistance somewhere.

This thread is quite a few months old now....