A Group 27 deep cycle battery is a lead-acid battery designed to discharge slowly over time and recharge repeatedly, sized to fit standard battery trays in RVs, boats, and off-grid solar systems
The "Group 27" designation refers to physical dimensions set by the Battery Council International — approximately 13 inches long, 6.8 inches wide, and 8.9 inches tall. This size sits between the smaller Group 24 and the larger Group 31, making it a middle-ground choice for vehicles and equipment that need sustained power but don't have room for the largest batteries.
Deep cycle batteries differ fundamentally from starter batteries. A starter battery (like those in cars) delivers a large burst of current for a few seconds to turn an engine over, then the alternator when ready recharges it. A deep cycle battery is built to discharge 50 to 80 percent of its capacity over hours or days, then recharge, and repeat this cycle hundreds of times. The internal plates are thicker and the chemistry is different, which is why you cannot straightforward swap one for the other.
Group 27 deep cycle batteries typically hold between 85 and 110 amp-hours, depending on the manufacturer and chemistry. They weigh 55 to 70 pounds and cost between $150 and $400 new, again depending on quality and type. They are common in travel trailers, sailboats, cabin power systems, and backup power setups because they fit existing battery boxes and deliver enough capacity for moderate loads without requiring multiple batteries wired together.
Key Takeaways
- Group 27 is a standardized size (13 by 6.8 by 8.9 inches) that fits most RV, marine, and off-grid battery compartments without modification.
- Deep cycle batteries are designed to discharge slowly and recharge repeatedly, unlike starter batteries which deliver one large burst and then stop.
- Capacity ranges from 85 to 110 amp-hours depending on the battery type and brand, which determines how long it will power your equipment.
- Lead-acid, lithium, and AGM (absorbed glass mat) chemistries are all available in Group 27 size, each with different costs, lifespan, and maintenance needs.
- Depth of discharge — how much of the battery's capacity you actually use before recharging — directly affects how many years the battery will last.
Lead-acid, AGM, and lithium: the three main types
Lead-acid (flooded) batteries are the oldest and cheapest option. They contain liquid electrolyte and require periodic watering to replace fluid lost to evaporation. They must be stored upright and ventilated because they release hydrogen gas during charging. A quality Group 27 lead-acid deep cycle battery costs $150 to $250 and lasts 3 to 5 years if you do not discharge it below 50 percent regularly. If you do, lifespan drops sharply.
AGM (absorbed glass mat) batteries seal the electrolyte in fiberglass mats, so they need no watering and can sit in any orientation. They are safer around enclosed spaces because they release far less gas. They cost $250 to $350 and last 4 to 7 years. AGM batteries tolerate deeper discharges better than flooded lead-acid and charge faster, but they are more sensitive to overcharging and require a charger designed for AGM chemistry.
Lithium iron phosphate (LiFePO4) batteries are the newest option in Group 27 size. They weigh half as much as lead-acid, last 10 to 15 years, tolerate 80 to 90 percent depth of discharge without damage, and charge in a fraction of the time. A Group 27 lithium battery costs $800 to $1,500. The high upfront cost is offset by longevity and performance, but you need a lithium-compatible charger and a battery management system (usually built in) to prevent damage.
Amp-hours, depth of discharge, and how long your battery will actually last
A Group 27 battery rated at 100 amp-hours means it can theoretically deliver 100 amps for one hour, or 10 amps for ten hours, before the voltage drops below usable levels. In practice, the usable capacity depends on how deeply you discharge it before recharging.
Lead-acid batteries should not go below 50 percent depth of discharge (meaning you use only 50 amp-hours of that 100 amp-hour battery before recharging). If you regularly discharge a lead-acid Group 27 to 80 percent, you will get 2 to 3 years of life instead of 4 to 5. AGM batteries tolerate 60 to 70 percent depth of discharge regularly. Lithium batteries tolerate 80 to 90 percent and still last 10+ years.
This is why the same physical battery can have very different real-world lifespan depending on how you use it. A Group 27 lead-acid battery in an RV that is used heavily every weekend and recharged to full each time will last longer than the same battery in a cabin that sits unused for months and then is discharged deeply when someone finally visits. The pattern of discharge and recharge matters as much as the total depth.
Charging and charger compatibility
A Group 27 deep cycle battery cannot be safely recharged with a standard car battery charger. Car chargers are designed for starter batteries and charge too fast or too aggressively for deep cycle chemistry, which shortens lifespan and can damage the battery.
Lead-acid deep cycle batteries need a charger rated for deep cycle lead-acid. AGM batteries need an AGM-specific charger or a multi-chemistry charger with an AGM setting. Lithium batteries require a charger with lithium mode or a charger specifically designed for LiFePO4. Using the wrong charger is one of the most common reasons a Group 27 battery fails prematurely.
If your RV or boat has a built-in charger, check the manual to confirm it is set for the battery type you are installing. Many older RVs have chargers designed only for lead-acid. If you upgrade to AGM or lithium, you may need to replace the charger as well, which adds to the total cost. Some newer multi-chemistry chargers can be reprogrammed via a switch or menu, so check before assuming replacement is necessary.
Wiring multiple Group 27 batteries for more capacity
One Group 27 battery may not provide enough capacity for your needs. If you need more amp-hours, you can wire two or more Group 27 batteries together. Wiring them in parallel (positive to positive, negative to negative) adds amp-hours while keeping voltage the same — two 100 amp-hour batteries in parallel give you 200 amp-hours at the same voltage. Wiring them in series (positive of one to negative of the other) adds voltage while keeping amp-hours the same.
Most RVs and boats use parallel wiring because the electrical system is designed for 12 volts. Wiring in series would create 24 volts, which requires different equipment. When you wire batteries in parallel, they must be the same chemistry, age, and capacity, or the newer or larger battery will discharge into the older or smaller one, shortening overall lifespan.
Battery cables must be heavy gauge (typically 2/0 or 4/0 AWG for Group 27) and as short as possible to minimize voltage drop. Undersized cables waste power and generate heat, which damages the battery and the charger. If you are not experienced with battery wiring, have a marine electrician or RV technician do the installation. Improper wiring is a common cause of premature battery failure and can create a fire hazard.
Storage, maintenance, and when to replace
A Group 27 battery that is not in use should be stored in a cool, dry place and charged every 3 to 6 months to prevent sulfation — a chemical buildup that permanently reduces capacity. Lead-acid batteries should be stored fully charged. Lithium batteries should be stored at 50 percent charge if they will sit for more than a month.
Lead-acid batteries require periodic watering if they are flooded type. Check the water level monthly during heavy use and top up with distilled water if the plates are exposed. Do not overfill — water should just cover the plates. AGM and lithium batteries require no watering. If a lead-acid battery is stored in a cold climate, bring it indoors or use a battery heater before charging, because charging a cold battery can damage it.
Replace a Group 27 battery when it no longer holds a charge, when the voltage drops below 12 volts at rest, or when it can no longer power your equipment for the expected time. A battery that is 5 years old and failing is normal for lead-acid; a battery that is 3 years old and failing suggests a charger problem or deep discharge abuse. Before replacing the battery, test the charger and the electrical system to make sure the problem is not elsewhere.
Cost comparison and total cost of ownership
| Battery Type | Initial Cost | Typical Lifespan | Cost Per Year | Best For |
|---|---|---|---|---|
| Lead-acid (flooded) | $150–$250 | 3–5 years | $30–$83 | Budget-conscious, occasional use |
| AGM | $250–$350 | 4–7 years | $36–$87 | Sealed, maintenance-free, moderate use |
| Lithium (LiFePO4) | $800–$1,500 | 10–15 years | $53–$150 | Heavy use, long-term investment, weight-sensitive |
The cheapest battery upfront is lead-acid, but if you use your RV or boat frequently, the cost per year of ownership is similar across all three types because lead-acid fails sooner. Lithium has the highest upfront cost but the lowest cost per year if you keep the vehicle or system for 10+ years. For occasional use or a second battery in a backup system, lead-acid or AGM makes financial sense. For full-time RV living or a critical off-grid solar system, lithium pays for itself over time.
When comparing total cost, also factor in replacement labor, charger upgrades, and the cost of being stranded if a battery fails unexpectedly. A lithium battery is less likely to fail suddenly, while a lead-acid battery can lose capacity gradually or fail without warning. Some owners budget for a replacement lead-acid battery every 4 years, while lithium owners may go 12+ years without replacement.
Frequently Asked Questions
Can I use a Group 27 battery as a starter battery in my truck?
Technically yes, but it is not recommended. A deep cycle battery has thicker plates and lower cold cranking amps (CCA) than a starter battery of the same size. It will start your truck, but it will not start it reliably in cold weather, and using it as a starter battery will shorten its lifespan because starter use is not what it is designed for. Use a starter battery for starting and a deep cycle battery for auxiliary power.
How do I know if my Group 27 battery is dead or just discharged?
A fully discharged battery will read around 10 to 11 volts with a multimeter. A dead battery will read 0 volts or will not hold a charge after you try to recharge it. If your battery reads 10 volts or higher and you charge it for 24 hours with the correct charger and it still will not hold power, the battery is likely dead and needs replacement. If it reads below 10 volts, try charging it — it may recover.
What is the difference between a Group 27 and a Group 31 battery?
A Group 31 is physically larger (about 13 by 6.8 by 10.2 inches) and holds 20 to 30 more amp-hours than a Group 27. Group 31 batteries cost more and weigh more, but they fit in larger battery boxes and are common in larger RVs and marine applications. If your equipment has a Group 27 battery box, a Group 31 will not fit without modification.
Do I need a battery monitor or charge controller with my Group 27 battery?
A battery monitor (which displays voltage and amp-hours remaining) is helpful but not required. A charge controller is required if you are charging from solar panels or a wind turbine — it prevents overcharging and damage. If you are charging from an alternator or shore power, the charger itself regulates the charge, but a monitor still helps you avoid over-discharging.
Can I mix old and new Group 27 batteries if I wire them in parallel?
No. A new battery will discharge into an old one, shortening the lifespan of both. If you need more capacity, replace all batteries at the same time. If you have one old battery and one new battery, use the old one until it fails, then replace it, rather than wiring them together.