Whether you need a separate battery bank for your trolling motor electronics is a common question — specifically whether running fish finders and other electronics off the trolling motor battery causes problems. Here's the practical answer.
The One-Battery Setup: When It Works
Running your fish finder from the same battery as the trolling motor works in one scenario: small motors (12V, under 50 lb thrust) on small boats where current draw is modest and the battery is large enough to handle both loads comfortably. A 100Ah AGM running a 12V trolling motor at 30–40% throttle and a 5-inch fish finder draws approximately 10–15A total. That's 6–8 hours of usable run time — adequate for most day trips.
When You Need Separate Batteries
24V or 36V trolling motor systems: These systems use two or three 12V batteries in series. Your fish finder runs on 12V — you tap off one battery in the bank, which draws that battery down faster than the others and unbalances the series bank. The correct solution is a dedicated 12V electronics battery separate from the trolling motor bank.
Heavy electronics loads: Live sonar systems (LiveScope, MEGA Live) draw an additional 2–3A on top of the head unit. If you're running a head unit + live sonar + chartplotter + AIS + VHF radio, your electronics draw may be 5–10A continuously. A separate electronics battery isolates this load from the motor and prevents voltage sag from the motor from corrupting sonar returns.
Long days (8+ hours): Battery management becomes important on full-day tournaments or extended fishing trips. A dedicated electronics battery that you can monitor independently is simpler to manage than trying to estimate remaining capacity across a combined load.
Battery Isolator vs ACR
If you want both batteries to charge from the same source (alternator or charger), use a battery isolator (diode-based) or an ACR (Automatic Charging Relay) that connects both banks for charging but isolates them during use. The ACR is the cleaner marine solution — it automatically connects the banks when charging and separates them when charging stops, so a fully discharged trolling motor bank can't drain the electronics bank.
Connects multiple battery banks for simultaneous charging from one source (alternator or charger) while automatically isolating them during use. The standard solution for dual-battery marine setups.