Lowrance HDS 10 flickers off and on
If you only have room for one battery, it needs to be the largest you can fit, with the highest CCA you can find. The higher the Cold Crank Amps, the lower the impedance of the battery, and the less it will sag when a starter/pump load is placed on it.
If you have room, adding a second parallel battery will also likely cure the problem. In extreme cases, a diode isolator can be added between the batteries; this will absolutely cure the issue, and allow you to use a small, cheap 12V gel battery as the second battery for sonar only.
I discovered that the hard way. HDS units suck an awfull-lot of juice.
Also,having the locator power off from anywhere except the unit itself is supposedly bad. Says so in the owners manual.
My starting battery powers my running lights and livewell pumpss and bilge pump and interior lights. I have a deep cycle battery for the locators (HDS 7 and HDS 5) Two more deep cycle batteries for the troller.