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When the cabin fridge stops humming at dusk, the deep cycle battery sizes in the bank are often the reason: the bank holds too little energy, or the battery never fit the tray in the first place. Size covers the capacity the battery holds and the physical dimensions of the case. This guide explains what the figures on the listing mean and how to match them to the space and the loads.
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Deep Cycle Battery Sizes: What the Figures Mean
A deep cycle battery size covers the electrical size and the physical size. The electrical size is the voltage and the amp-hour capacity, and the physical size is the length, width and height of the case. A 12V 35Ah battery such as the Mighty Max ML35-12 carries 12 volts and 35 amp-hours, and the listing gives its dimensions as 7.5 x 7.75 x 5 inches.
Group size is the label the industry gives to the physical size, and the listing should state it next to the dimensions. When the listing gives no group size, the dimensions are the figure to match: measure the tray the battery sits in and compare the length, width and height with the listing. A battery that fits the tray but not the terminals, or the terminals but not the tray, still does not fit.
Read the Voltage and Amp-Hours
Voltage states the system the battery fits, and amp-hours state how much charge it holds. A 12V battery fits a 12V system, and a higher amp-hour figure holds more charge at the same voltage. The Mighty Max ML35-12 is a 12V 35Ah sealed lead acid battery, and its listing says it replaces 33Ah, 34Ah and 36Ah batteries.
A deep cycle battery is built to discharge and recharge over many cycles, which is what solar storage, trolling motors and RV house loads need. That is the difference from a starting battery, which gives a short burst to start an engine, and from a tool battery pack such as the Milwaukee M18 REDLITHIUM Forge HD12.0, which measures 7.76 x 7.28 x 3.5 inches and is built for power tools. Match the battery type to the job, then match the voltage and amp-hours to the system.
Measure the Tray Before You Order
Measure the tray or box the battery sits in: length, width and height, and the clearance above the terminals for the cables and the hold-down. Compare those figures with the dimensions in the listing, and leave room for the cables to bend without strain. A battery that is too tall for the box will not close, and a battery that is too wide will not sit flat.
Listings can disagree with themselves, so check the figure twice. The Mighty Max ML35-12 lists dimensions of 7.5 x 7.75 x 5 inches in its spec table, while the bullets give a different set of figures, so check both figures before ordering. Measure the tray, then match the listing figure that fits the tray, and ask the seller when the figures do not agree.
Match the Battery to the Loads
List the loads the battery must run, then match the amp-hours to that total. A 12V 35Ah battery such as the Mighty Max ML35-12 suits a small solar setup, a trolling motor, a mobility chair or a golf caddy, which the listing names as uses. For larger loads, look for a larger amp-hour figure on the listing, and check that the voltage still matches the system.
The listing should state the amp-hours, the voltage and the dimensions together. When a figure is missing, the battery is hard to match: ask the seller for the missing figure rather than guessing it from a similar listing. A battery that matches on capacity but not on dimensions still needs a tray that fits it.
The listing for the Mighty Max ML35-12 names solar generators, trolling motors, mobility chairs, golf caddies and riding mowers as uses. Those uses share a need for steady power over a long stretch rather than a short burst. Match the battery to the use the listing names, and check the voltage and the dimensions against the tray and the system.
Sealed, AGM and Flooded: What the Listing Says
The Mighty Max ML35-12 is a sealed lead acid battery, and its listing also calls it a deep cycle AGM solar battery. The listing calls it maintenance free, which suits a battery that sits in a solar setup with little attention. Match the charger to the battery type: see the flooded vs AGM battery charger guide and the lead acid vs lithium battery charger guide before choosing a charger.
FAQ: power away from the grid
What does 12V 35Ah mean on a deep cycle battery?
12V is the voltage and 35Ah is the amp-hour capacity. The Mighty Max ML35-12 is a 12V 35Ah sealed lead acid battery, and its listing says it replaces 33Ah, 34Ah and 36Ah batteries.
What is battery group size?
Group size is the label for the physical size of the battery case. When the listing gives no group size, match the length, width and height in the listing to the tray the battery sits in.
How do I measure for a deep cycle battery?
Measure the length, width and height of the tray or box, and the clearance above the terminals. Compare those figures with the dimensions in the listing before ordering.
Can a 35Ah battery replace a 33Ah battery?
The Mighty Max ML35-12 listing says it replaces 33Ah, 34Ah and 36Ah batteries. Match the voltage and the dimensions to the tray before ordering.
Are tool batteries the same as deep cycle batteries?
No. A tool battery pack such as the Milwaukee M18 REDLITHIUM Forge HD12.0, which measures 7.76 x 7.28 x 3.5 inches, is built for power tools. A deep cycle battery such as the Mighty Max ML35-12 is built for solar storage, trolling motors and RV loads.
Related solar guides
See every batteries guide and all deep cycle and RV batteries guides.
Also see the battery charger types guide and the best portable power stations for home backup.
