what did I do wrong? wiring electronics

Last fall I added a 597 HD to my bow and wired all the way back to the main cranking battery. I also run a 598 HD SI in the rear by the tiller and it runs from the cranking battery. Well, I started getting low voltage readings on the H’bird and alarms from the mercury about low voltage. I replaced the battery with a dual purpose Group 34M AGM with 880 CCA and 134 RC. I still got low voltage readings from the units and motor. So I decided I would just wire the bow mount to a small 12V 8Ah battery that you would use on your flasher in the winter to solve the problem. This worked fine, but not ideal.
Fast forward to Thursday, got out on pool 4, was in a rush and I don’t think crank battery was fully charged. Drove around for a bit, fished for about 3 hours, went to start up motor to head back and engine won’t turn over but all over things work. Got home, fully charged battery, went out again today and even turned off unit in the back when fishing on the bow and still had the motor alarm go off at the end of 4 hours of fishing
Is it just me or should this battery be able to handle a lot more? Did I wire something wrong or forget to do something? Not an electrician but I’d like to think I’m competent. How many electronic accessories should this battery be able to handle?

I got stranded once. It will never happen to me again. I run 4 batteries in my boat. 2 dedicated to my terrova. one dedicated to my main motor and a fourth for all of my accessories. all 4 are on a bank charger when the boat is not on the water. I have never had the issue again. Some will say the extra battery adds weight but I don’t fish tournaments and speed is not a priority. If you isolate your starting battery nothing can rob the juice and your motor will never fail you again.

I question the connections to the battery. I had nearly the exact same as you described with a Mariner 40 mag in my Alaskan running a Bottomline 310XT and a Mach1 paper graph (I know…I’m old) Ended up being a poor connection at the battery terminal. Cleaned everything and removed the corrosion from the brass connectors and problem solved. Hopefully its that simple for you

You need to make sure that all the wires that you put on the battery are clean. The first wires that you put on the battery should be your battery cables for your engine, follow that with the rest of your wires. If you have wing nuts on your battery take them off and throw them away. Replace them regular nuts and tighten them. None of the wires should move or wiggle. If the problem still continues you may want to have the charging system of your engine looked at. It may not be charging at all or it may be overcharging enough to ruin the batteries.

First thing I would check is the battery connections make sure there tight and clean. The next thing I would do is have the battery load tested to make sure it is good. If the battery checks out then I would make sure that the engine is properly charging the battery when running and that it doesn’t have a big draw on the battery causing the battery to go dead.

Thanks guys. I took off the ring terminals and buffed them up, they were a little dirty, but not aweful. The motor terminals were already on the bottom. Then I replaced the wing nuts with regular ones and tightened them real good. Give it a try tonight and see what happens.

If I can ask, how did you pick the group 34 battery? I don’t know what brand you bought, but these 34 specs are less than the 24’s. I’d think about getting a better battery with higher capacities.

I’ve included a chart from Interstate as a comparison for reserve capacity numbers.

HRG