Sailing Advantage!
How to Choose the best
Sailboat Marine Battery
Looking for the best marine batteries for your electric outboard motor? After the HOME page introduction and the first page on MOTORS, this is the BATTERIES page, the second of three pages in our step-by-step discussion of how to choose and install the best electric outboard motor for your sailboat.We present the basics of marine battery requirements and wiring, and provide comparisons, and essential information to help you choose the best marine battery for your electric outboard motor.
We strive to provide accurate information and only recommend products that meet our exacting standards and have a reputation for quality, reliability, and applicability, but please take time to read customer reviews and seller-published specifications. Throughout this site, product links are provided for suggested equipment and clicking on the product image or name-link gives you an opportunity to read customer reviews, ratings, and pricing so you can decide what is best for you*. This site is focused on electric outboard motors for sailboats up to 27ft, and up to 5,000 lb. displacement.
Electrical Considerations
What Type and How Many Batteries?
Electric outboard motors run on 12, 24, 36, or 48 volts. That means you will need one, two, three, or four 12-volt deep cycle marine batteries, depending on the motor's voltage requirements. Deep cycle batteries are designed to provide continuous current for longer periods of time, as compared to "starter" batteries that provide higher current for short periods of time. High quality, sealed, deep cycle marine battery types are either GEL or AGM design and, since they have different charging requirements, you should never mix these types of batteries; choose one and stick with it. We recommend sealed AGM deep cycle marine batteries for safe, long life performance. Sealed AGM batteries can endure the unusual and sometimes unpredictable angles and operating positions experienced in a sailboat. Also note that these batteries typically weigh at least 60 lbs each. That's something to consider when choosing a motor.
Let talk about battery voltage and current (amperes). Batteries are rated in volts and Ah (ampere hours or Amp-Hours). A device attached to your 12-volt battery that uses one amp continuously for one hour is said to use one Ah (Amp-Hour). A battery rated at 75Ah can run that device for 75 hours (75 hours at 1 amp per hour). The same battery can power a motor that consumes 75 amps for just one hour. Batteries connected in parallel will increase available current without increasing voltage, so if you connect two such batteries in parallel with each other, you will double the available Ah, running time, without changing the battery voltage: it is still 12 volts but now has 150Ah (two times 75).
Generally, you can expect electric outboard motors to draw about 1 amp for each pound of thrust. So, plan for a motor rated at 55 lb/thrust to draw 55 amps and should run for about two hours on a battery rated at 110 Ah.
If your motor requires 24 volts you will need to connect two 12-volt batteries in series. Connecting batteries in series increases available voltage incrementally but will not increase available current (Ah). If your motor requires 36 volts you will, of course, connect three 12-volt batteries in series. Once again increasing voltage but not changing the available current. Each individual battery that is connected with another, either in series or parallel, should be exactly the same voltage and current rating, otherwise you risk battery damage or reduced battery life due to overcharging, undercharging, or incorrect rate of charge.
Batteries are also classified by size, usually displayed by "Group". The numerical classification standard refers to the BCI Group Number. Common sizes are Group 24, Group 27, and Group 31. Knowing the battery "Group" number will help you choose battery mounting hardware like battery boxes or brackets.
Connecting Batteries
How to connect batteries in parallel or in series, and with charger:
Note: The charger can remain connected to all batteries, but your boat systems battery should be isolated, kept separated from your electric outboard motor batteries. The boat systems battery should never be "run down" like your motor batteries might. Even when you no longer have electrical power for the motor, you should still have battery power for navigation lights, radio, bilge pump, and cabin lights. This is fundamental.
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DEEP CYCLE BATTERIES
12-volt AGM
| Ah | L-W-H | Group Size | Wt. (lb) | find on Amazon* Make/Model |
|---|---|---|---|---|
| 85 | 10.2 x 6.6 x 8.3 inches | 24 | 58 | Vmaxtanks MR107 |
| 100 | 12.17 x 6.61 x 8.3 inches | 30H | 64 | Mightymax ML100-12 |
| 100 | 12.9 x 8.7 x 6.8 inches | 31 | 64 | WindyNation BattaMax ML12-80 |
| 100 | 12.17 x 6.61 x 9.16 inches | 27 | 64 | Universal UB121000-45978 |
| 110 | 13.2 x 6.8 x 9.6 inches | 30H | 74 | Adventure Power 110AH Group 30H Battery |
Battery Charger: Mount the battery charger close to the batteries as well. With the charger in place and already connected to the batteries, you just need to plug into "shore power" to recharge your batteries. Get a good quality multi-stage, multiple bank marine battery charger designed to charge AGM batteries, or GEL cell type, which ever type you are using for your power. These battery types require different charger types to match so make sure you choose a charger compatible with your chosen battery type. Get a charger with as many "banks" as you have batteries. In simple terms, a "bank" is a battery, so, if your motor requires 24 volts (two 12-volt batteries) plus you have another 12-volt battery for boat systems (lights, radio, bilge pump, etc.), then you should get a "3-bank" charger. In this chart, the "Amps/bank" indicates how many amps the charger can provide to each individual bank, or battery, at the same time. Here are some good choices:
Battery Chargers
| Banks | Volts | Amps /bank | Weight (lb) | find on Amazon* Make/Model |
|---|---|---|---|---|
| 2 | 12 | 4 | 6 | NOCO Genius GENM2 |
| 3 | 12 | 4 | 9 | NOCO Genius GENM3 |
| 2 | 12 | 5 | 11 | Minn Kota MK 210D |
| 3 | 12 | 6 | 8 | ProMariner ProSport 20 Plus Gen3 |
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Battery Mounting
Safe and Secure
Batteries should be mounted securely to the boat so that they cannot tip over or come loose due to sudden strong boat movement. Yes, imagine your boat healing over to 90% or even knocked down; always prepare for the worst. Mount your batteries securely. After recovering from a severe knock-down, you will want your systems to be functional. That means lights, radio, bilge pumps, and auxiliary motor. Mount your batteries with easy access in mind and as close as possible to the motor. Battery boxes and mounts are available for easy and reliable mounting. You can build your own mounting but just be sure it keeps those heavy batteries securely in place regardless of boat angle and sudden forces.Battery Boxes & Trays
| Batt's | O.D. L-W-H |
Group size |
Wt. (lb) |
find on Amazon* Make/Model |
|---|---|---|---|---|
| 1 | 12.5x 10.5x 8.5 | 24 | 1.6 | Camco 55362 Standard Battery Box |
| 1 | 16.9x 9.63x 10.9 | 27, 27F, 27M | 2.5 | Attwood Power Guard 27 Battery Box |
| 1 | 16.9x 9.8x 11.5 | 27 | 4 | NOCO HM327BKS Snap-Top Battery Box |
| 1 | 16x 12.6x 9.8 | 27, 30, 31 | 3.4 | Camco 55373 Large Battery Box |
| 1 | 14x 8x 1.3 | 24-31 | 1 | NOCO BT31S Group 24-31 HD Battery Tray |
| 2 | 14x 14x 6 | 27x2 | 1 | T&H Marine DBH-27P-DP Dual Group 27 Tray |
Always use marine quality hardware, wire, and connectors. Marine quality hardware is made of metals and alloys that are specifically formulated to be corrosion-resistant to withstand the harsh humid environment and provide long life service with little or no degradation. Wire connectors should be tinned copper and the wires should be tinned, stranded, AWG sized (not the smaller SAE gage), and marine UL-listed for quality.
Batteries Page Review
Ok, you've chosen an electric outboard motor that delivers your desired "pounds of thrust" and you know the power requirements in volts and current (amperes). You calculated how many batteries you need for the required volts and battery ampere-hours (Ah). And get a charger capable of charging the motor batteries as well as your "boat systems" battery with the same charger all at the same time. Be sure to keep your batteries charged and ready when needed.Your battery power system is critical to your electric motor so be sure to get enough power (amps and voltage) to take you where and when you need it. A sailboat motor is a convenience to be used when necessary to avoid dangers or to easily leave and return to the dock.
For your consideration:
1.) Batteries are critical to your electric propulsion system. Be sure to buy batteries with enough amp/hours for your needs, and just like motor power, more amps is almost always better.
2.) Your batteries should be marine rated and sealed for safety.
3.) Batteries are heavy and should be mounted securely and capable of withstanding a knock-down or even a complete roll-over. Be prepared for the worst circumstances!
4.) Improperly installed batteries and wiring can be dangerous. Hire an experienced and licensed marine electrician if you have any doubts as to your own ability.