I have an old Hummingbird Wide 128 fish finder that I used on a Jon boat in the 90's. It still works - I tested it with a big trolling motor battery. I rigged up a mounting system to have it run off my Hobie Float Cat. I bought a 12 volt, 7.5 amp deer feeder battery at Academy to power it but it doesn't power up. I thought the new battery may be dead so I tried hooking it up to the automotive battery charger with it set on Deep Cycle and 2 amps/hr. I didn't get any readings and there were no sparks when I touched the alligator clips to the battery. I took the battery back to Academy. They don't have any testing equipment and their only suggestion was to exchange for a new one which I did. When I hook up the second deer feeder battery to the fish finder, it makes a beep noise which indicates it is powering up but the screen does not come on. I tried hooking it up to the big trolling motor battery and it works fine. Back to the second deer feeder battery and can only get a power up beep but no screen.

I don't have one of those volt meter testor thingies.
The Hummingbird owner manual shows a "power requirement of 10 - 16 VDC."

Does the second deer feeder battery just need a full charge?
Can you charge a deer feeder battery with an automotive battery charger?
Does the fish finder require more than 7.5 amps?

yes try putting it on a charger im sure that's what it is. I use the 2amp setting on a car charger.
then get a battery tender and keep it hooked up when not in use

I tested it again after leaving it on the charger for 2 hours and it works. I am going to leave it on the charger all night and unplug it in the morning. Hopefully the weather will cooperate this weekend so I can test it at Lake Grapevine.

I use a car charger on my deer feeder batteries all the time, but it is a nice charger and has a setting for sealed batteries. You will be fine charging it at low amps like 2. I will hit mine at a 4amp setting if I am in a hurry, but you need to not let the battery get too hot.

-rob