Basic Guidelines for Sizing Marine Batteries | Rolls Battery
Basic Guidelines for Sizing Marine Batteries
Basic Guidelines for Sizing Marine Batteries
Example of "Light Cycling" would be a house battery for a sailboat 30 feet and under with lights only and limited night sailing.
Example of "Deep Cycling" would be the same as above, but with the addition of an electric refrigerator,auto helm, etc.
A simple guideline to determine sizing of house batteries
The best method is to determine the requirements of the boat. Below only serves as a rough guide and would be considered as the minimum requirements of the vessel.
| Length Of Boat | Capacity Of House Batteries |
| 28 Feet | 136 Amp Hours |
| 29 Feet | 160 Amp Hours |
| 30 Feet | 180 Amp Hours |
| 31 Feet | 202 Amp Hours |
| 32 Feet | 224 Amp Hours |
| 33 Feet | 248 Amp Hours |
| 34 Feet | 272 Amp Hours |
| 35 Feet | 298 Amp Hours |
| 36 Feet | 324 Amp Hours |
| 37 Feet | 352 Amp Hours |
| 38 Feet | 380 Amp Hours |
| 39 Feet | 410 Amp Hours |
| 40 Feet | 440 Amp Hours |
| 41 Feet | 472 Amp Hours |
| 42 Feet | 504 Amp Hours |
| 43 Feet | 538 Amp Hours |
| 44 Feet | 572 Amp Hours |
| 45 Feet | 608 Amp Hours |
| 46 Feet | 644 Amp Hours |
| 47 Feet | 682 Amp Hours |
| 48 Feet | 720 Amp Hours |
| 49 Feet | 760 Amp Hours |
| 50 Feet | 800 Amp Hours |
The rules may change with the addition of an Inverter or other electrical equipment. On a 12 volt system divide 5 into the wattage of the Inverter to determine the size batteries required. On a 24 volt system divide by 10.
If the Inverter is to be run for more than a maximum of 15 minutes at close to full capacity, a larger battery bank will be required!
The exception may be if power is being supplied to the batteries from an alternator or other source.
Depending on the size of the boat it may be desirable to break the house batteries up into 2 or more systems for safety reasons. The essentials should be on one bank and the non-essentials on the other bank. Essentials are considered anything to do with safety, such as electronic equipment. Non-essentials are such things as an electrical refrigerator.
On a 12 volt system when 2 batteries are used to obtain the capacity for the house bank the question usually arises as to whether to use two 6 volts in series or two 12 volt in parallel. With two 12 volts in parallel if one battery should fail, the battery can be disconnected and still have a "live" system. With two 6 volts this is not so.
When installing two batteries in parallel the batteries must be:
- New
- The Same Age
- The Same Size
- The Same Make
Connections must be:
- Perfectly Clean
- Tight and Secure
The starting battery or batteries should be isolated from the house batteries, and used only for starting the engine.
Sizing of Starting Batteries
In sizing batteries for a boat, the rule of thumb is to double what you think you need. In a car, we say you need one "cranking" amp for every cubic inch of displacement of the engine. For marine application you should use two cranking amps, not that a marine engine starts harder, but you should have that extra margin of safety!
We may have the "Greatest Battery System", but without the proper charging system expect trouble!