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Best On-Board Marine Battery Charger: How Many Banks and Amps You Need
Short answer: The best on-board marine charger has one bank for every battery (or battery pack) you need to charge, a charging profile that matches each battery's chemistry, and enough amps to recharge overnight. For a classic 36V trolling setup of three 12V batteries plus a starting battery, that means a 4-bank charger. For a single 36V lithium pack plus a starting battery, it means a 12V & 36V 2-bank charger. Every Pro-Guide on-board charger is IP68 rated and delivers 10A per bank (20A on one model's 12V bank). All of them charge AGM and lithium batteries, and every model except the 12/16V also charges flooded batteries.
An on-board charger lives in your boat, so you plug in one cord at the dock or garage and every battery comes back to full. Getting the right one comes down to three questions: how many banks, which voltages and how many amps.
Key Takeaways
- One bank per 12V battery, including batteries wired in series for a 24V or 36V trolling motor.
- A single 24V, 36V or 16V lithium pack needs a bank built for that voltage. A 12V bank won't charge it.
- Charge time is roughly amp-hours used divided by amps per bank. At 10A, 100Ah takes about 10 hours from empty.
- Match the profile to the chemistry: flooded, AGM and lithium each charge at different voltages.
- All Pro-Guide on-board chargers are IP68 and carry a 24-month free replacement warranty (48 months total).
How Many Banks Do You Need?
A bank is one independent charging output. Count the batteries you need to charge:
- Each 12V battery gets its own bank. A 36V trolling motor bank built from three 12V batteries needs three banks, even though the batteries are wired in series.
- Add a bank for the starting battery if you want it charged from shore power too.
- Add a bank for a separate electronics battery if you run one.
- A single high-voltage lithium pack needs one bank at its voltage. One 36VM60 needs one 36V lithium bank.
Pro-Guide On-Board Chargers Compared
| Charger | Banks and output | Battery types | Weight |
| 12V 10A 1-Bank | 1 x 12V, 10A | Flooded, AGM, lithium | 9.5 lb |
| 12V 10A 2-Bank | 2 x 12V, 10A each | Flooded, AGM, lithium | 10.4 lb |
| 12V 10A 3-Bank | 3 x 12V, 10A each | Flooded, AGM, lithium | 15.9 lb |
| 12V 10A 4-Bank | 4 x 12V, 10A each | Flooded, AGM, lithium | 18.7 lb |
| 12V 10A 5-Bank | 5 x 12V, 10A each | Flooded, AGM, lithium | 24.3 lb |
| 12V & 24V 10A 2-Bank | 1 x 12V + 1 x 24V, 10A each | 12V flooded, AGM, lithium; 24V lithium | 14.3 lb |
| 12V & 36V 10A 2-Bank | 1 x 12V + 1 x 36V, 10A each | 12V flooded, AGM, lithium; 36V lithium | 17.9 lb |
| 12V 20A & 36V 10A 2-Bank | 1 x 12V at 20A + 1 x 36V at 10A | 12V flooded, AGM, lithium; 36V lithium | 24.3 lb |
| 12/16V 10A 1-Bank | 1 x 12V or 16V, 10A | 12V AGM, lithium; 16V lithium | 9.9 lb |
Best Charger for Your Setup
| Your boat | Best charger |
| 12V trolling motor + starting battery | 12V 10A 2-Bank |
| 24V trolling (two 12V batteries) + starting battery | 12V 10A 3-Bank |
| 36V trolling (three 12V batteries) + starting battery | 12V 10A 4-Bank |
| 36V trolling (three 12V) + starting + electronics battery | 12V 10A 5-Bank |
| One 24V lithium pack (24VM90 / 24VM115) + 12V starting or dual purpose battery | 12V & 24V 10A 2-Bank |
| One 36V lithium pack (36VM60 / 36VM75) + 12V starting battery | 12V & 36V 10A 2-Bank |
| One 36V lithium pack + large 12V lithium dual purpose (31M180-DP / 31M235-DP) | 12V 20A & 36V 10A 2-Bank |
| 16V lithium electronics battery | 12/16V 10A 1-Bank |
Pro-Guide also offers matched battery-and-charger bundles in its marine power kits. For wiring a trolling bank with built-in charging leads, see the 24 Volt Series Trolling Motor Connection Kit and 12 Volt Trolling Motor Kit.
How Many Amps? Charge-Time Math
A quick estimate: hours to recharge ≈ amp-hours used ÷ charger amps per bank. Real charge time runs a little longer, because chargers taper the current as the battery nears full.
| Battery | Capacity | Bank | Approx. time from empty |
| 24M100 (12V lithium) | 100Ah | 12V, 10A | About 10 hours |
| 36VM60 (36V lithium) | 60Ah | 36V, 10A | About 6 hours |
| 36VM75 (36V lithium) | 75Ah | 36V, 10A | About 7.5 hours |
| 24VM90 (24V lithium) | 90Ah | 24V, 10A | About 9 hours |
| 31M180-DP (12V lithium) | 180Ah | 12V, 20A | About 9 hours |
You rarely charge from completely empty. After a typical day that uses half the battery, these times roughly halve. For a lead-acid battery that you've discharged about halfway, a 10A bank usually brings it back overnight.
Charging Profiles: Lithium, AGM and Flooded
Each chemistry needs a different charge voltage. Pro-Guide on-board chargers use these ranges:
| Battery | Charge voltage |
| 12V flooded lead-acid | 14.1 to 14.3V |
| 12V AGM | 14.3 to 14.7V |
| 12V lithium | 14.4 to 14.7V |
| 16V lithium | 18 to 18.3V |
| 24V lithium | 28.8 to 29.2V |
| 36V lithium | 43 to 44V |
Set each bank to match the battery it charges. A lead-acid profile is not the right match for lithium, and a 12V bank can't charge a 16V, 24V or 36V lithium battery. The 24V and 36V banks on Pro-Guide's dual-voltage chargers are built for lithium packs. If you charge a 24V or 36V lead-acid bank, charge each 12V battery on its own bank.
Installation Tips
- Mount it in a dry, ventilated spot close to the batteries, and keep the DC leads short. IP68 protects the charger, not the AC cord connection.
- Use a proper AC inlet. A marine AC receptacle makes plugging in easy and keeps the cord protected.
- Fuse the DC leads near each battery, as the charger's manual specifies.
- Label the banks so they stay matched to the right batteries after service.
Frequently Asked Questions
What is the best on-board marine battery charger?
The one that matches your setup: one bank per 12V battery, or a dedicated 24V or 36V bank for a single lithium pack, with the right profile for each chemistry. For a 36V trolling motor on three 12V batteries plus a starting battery, the Pro-Guide 12V 10A 4-Bank is the best fit.
How many banks do I need on my marine charger?
One for each 12V battery you want to charge, including each battery in a series trolling bank, plus one for your starting battery. A single 24V or 36V lithium pack needs one bank at its voltage.
Can an on-board charger charge lithium batteries?
Yes, if it has a lithium profile. Every Pro-Guide on-board charger charges 12V lithium. The dual-voltage models also charge 24V or 36V lithium packs, and the 12/16V model charges 16V lithium.
How long does an on-board charger take to charge a trolling motor battery?
Divide the amp-hours used by the charger's amps per bank. A 100Ah battery on a 10A bank takes about 10 hours from empty, and about 5 hours from half.
Can I charge a 36V trolling motor bank with a 3-bank charger?
Yes. If the bank is three 12V batteries in series, connect one bank to each battery. Add a fourth bank if you also want to charge your starting battery.
Are Pro-Guide on-board chargers waterproof?
They are IP68 rated. Mount them in a dry, ventilated location and protect the AC connection.
Shop On-Board Chargers
Count your batteries, note their voltages and chemistries, and choose the charger with a bank for each one.
Not sure which one fits your boat? Call our team at 1-800-827-3232 and we'll match a charger to your batteries.
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