Marine Battery Buying Guide: Starting vs. Deep Cycle, AGM vs. Lithium
Quick answer: Use a starting (cranking) battery for the engine and a deep cycle battery for trolling motors, electronics and house loads. A dual purpose battery works on small single-battery boats. For chemistry, flooded is cheapest, AGM is sealed and maintenance free, and lithium (LiFePO4) is lightest with the most usable power and longest life. Fit the battery to your tray (group size) and your engine's cranking requirement.
Buying the wrong battery is one of the most common and expensive mistakes boaters make. A starting battery used to run a trolling motor will be dead in a season. A deep cycle battery asked to crank a big outboard can leave you stuck at the ramp. This guide covers battery types, chemistry, sizing and the gear that keeps batteries healthy.
Step 1: Know the three battery types
- Starting (cranking): many thin plates that deliver a huge burst of current for a few seconds. Rated in MCA (marine cranking amps) and CCA (cold cranking amps). Not built to be drained deeply.
- Deep cycle: thick plates that deliver steady current for hours and tolerate being discharged and recharged hundreds of times. Rated in amp-hours (Ah) or reserve capacity (minutes).
- Dual purpose: a compromise that can start a modest engine and run some accessories. Good for small boats with one battery, not a substitute for a real house bank.
Step 2: Choose a chemistry
| Chemistry | Usable capacity | Typical life | Weight | Upfront cost |
|---|---|---|---|---|
| Flooded lead acid | about 50% | 3 to 5 years | Heavy | Lowest |
| AGM | about 50% | 4 to 6 years | Heavy | Moderate |
| Lithium (LiFePO4) | 80 to 100% | 2,000 to 5,000+ cycles | About half | Highest |
Flooded batteries need water checks and must be vented. AGM batteries are sealed, spill-proof and handle vibration well, which suits pounding center consoles. Lithium delivers almost all of its rated capacity at steady voltage, charges faster and weighs far less, which is why tournament anglers and cruisers are switching. It does need a compatible charger and, often, upgraded charging from the engine.
Real-world math: a 100 Ah AGM gives you about 50 usable amp-hours. A 100 Ah lithium gives you about 90. That is nearly twice the runtime from the same rating.
Step 3: Size the starting battery
Check your engine manual for the minimum MCA or CCA. Outboards commonly call for somewhere between 500 and 1,000 MCA depending on horsepower. Meet or exceed it. A bigger starting battery will not hurt the engine, and it gives you margin on a cold morning or after a long day of running accessories.
Step 4: Size the deep cycle or house bank
Add up what you run and for how long. Multiply each device's amps by the hours you use it to get amp-hours per day.
- Fishfinder/chartplotter: about 1 to 3 amps
- Livewell pump: about 3 to 5 amps
- Stereo at moderate volume: about 5 to 10 amps
- LED cabin lights: about 0.5 to 1 amp each
- 12V fridge: about 2 to 5 amps averaged
Double the daily total for lead-acid or AGM (since you should only use about half), or add about 20 percent for lithium. Trolling motors run at 12V, 24V or 36V, which means one, two or three 12V batteries in series. Size those batteries to how many hours you fish.
Step 5: Fit and group size
Battery group sizes (24, 27, 31, 4D, 8D) define physical dimensions. Measure your tray and battery box before upgrading. Bigger groups hold more capacity but must fit and be secured. ABYC requires batteries to be strapped down so they cannot shift more than an inch, with terminals covered to prevent shorts.
Step 6: Keep batteries healthy
- Recharge promptly after every trip with an onboard multi-stage charger.
- Separate start and house with a battery switch and an ACR so you never kill your starting battery.
- Monitor state of charge with a shunt-based battery monitor instead of guessing from voltage.
- Keep terminals clean and tight with tinned lugs and a light coat of terminal protectant.
- Do not mix old and new, or different chemistries, in the same bank.
Our picks for your battery system
Pair your batteries with the gear that charges, protects and monitors them, all in stock at Boat Supplies Direct.
Keep them charged
Guest 10A 2-Bank On-Board Charger for start plus house. For lithium or AGM trolling banks, the Dual Pro PS3 Auto 15A 3-Bank Lithium/AGM Charger.
Protect the starting battery
Blue Sea 7650 Add-A-Battery Kit, a battery switch plus automatic charging relay that turns one battery into a proper two-battery system.
Master disconnect
Blue Sea 6006 m-Series Mini ON/OFF Switch.
Know your state of charge
Xantrex LinkLITE Battery Monitor or the Xantrex LinkPRO for two banks.
Lithium house banks
Victron Lynx Smart BMS 500 for Victron Smart Lithium systems.
Cable and terminals
Ancor 4 AWG Battery Cable by the foot and the Ancor Tinned Battery Terminal Combo.
Frequently Asked Questions
What is the difference between a starting and deep cycle battery?
Starting batteries deliver a short, high-current burst to crank an engine. Deep cycle batteries deliver steady power for hours and tolerate being drained and recharged, which is what trolling motors and electronics need.
Can I use one battery for starting and accessories?
On small boats, a dual purpose battery can do both. Larger boats should use a dedicated starting battery and a separate deep cycle house bank, managed by a battery switch and ACR.
Is lithium worth it on a boat?
For trolling motors and house banks, often yes. Lithium offers nearly twice the usable capacity of the same rated AGM, weighs about half, charges faster and lasts many more cycles. It costs more upfront and needs a compatible charger.
What size battery do I need for my outboard?
Check your engine manual for the minimum marine cranking amps (MCA) or cold cranking amps (CCA) and choose a starting battery that meets or exceeds it.
How long do marine batteries last?
Flooded batteries typically last 3 to 5 years and AGM 4 to 6 years with good charging habits. Lithium batteries are rated for thousands of cycles and can last 10 years or more.
Can I mix battery types?
Do not mix chemistries or old and new batteries in the same bank. Separate banks (for example AGM start and lithium house) can work with the right charging equipment.
This guide is general information. Follow your battery and engine manufacturers' recommendations, and consult a qualified marine electrician when upgrading to lithium.
