Part - or most - of the loss in charging and discharging batteries is due to internal resistance. This is converted to heat, which is why batteries get warm when being charged up. The lower the internal resistance, the better. There is a good explanation and demonstration of Internal Resistance here (YouTube)

 Slower charging and discharging rates are more efficient. A battery rated at 180 amp-hours over 6 hours might be rated at 220 AH at the 20-hour rate, and 260 AH at the 48-hour rate. Much of this loss of efficiency is due to higher internal resistance at higher amperage rates - internal resistance is not a constant - kind of like "the more you push, the more it pushes back".

 Typical efficiency in a lead-acid battery is 85-95%, in alkaline and NiCad battery it is about 65%. True deep cycle AGM's (such as Concorde) can approach 98% under optimum conditions, but those conditions are seldom found so you should figure as a general rule about a 10% to 20% total power loss when sizing batteries and battery banks. 

Practically all batteries used in PV and all but the smallest backup systems are Lead-Acid type batteries. Even after over a century of use, they still offer the best price to power ratio. A few systems use NiCad, but we do not recommend them except in cases where extremely cold temperatures (-50 F or less) are common. They are expensive to buy, and very expensive to dispose of due the the hazardous nature of Cadmium. 

We have had almost no direct experience with the NiFe (alkaline) batteries, but from what we have learned from others we do not not recommend them - one major disadvantage is that there is a large voltage difference between the fully charged and discharged state. Another problem is that they are very inefficient - you lose from 30-40% in heat just in charging and discharging them. Many inverters and charge controls have a hard time with them. It appears that the only current source for new cells seems to be from Hungary. In the past they were often used by railroads as backup power, but nearly all have now changed over to newer types. 

An important fact is that ALL of the batteries commonly used in deep cycle applications are Lead-Acid. This includes the standard flooded (wet) batteries, gelled, and AGM. They all use the same chemistry, although the actual construction of the plates etc varies. 

NiCads, Nickel-Iron, and other types are found in a few systems, but are not common due to their expense, environmental hazards, and/or poor efficiency. 

Batteries fall into two main categories: primary (single-use/disposable) and secondary (rechargeable). 

Primary (Disposable) Battery Types

• Alkaline: The most common household battery (sizes like AA, AAA, C, D, and 9V). They use zinc and manganese dioxide to provide a reliable, steady 1.5 volts for everyday items like remotes, toys, and flashlights. 

• Lithium (Primary): Non-rechargeable batteries that offer high energy density and a very long shelf life. They perform well in extreme temperatures and high-drain devices like digital cameras.

• Zinc-Carbon: An older, economical disposable technology best suited for low-drain devices like wall clocks and simple remotes.

• Button Cell (Silver Oxide / Zinc-Air): Tiny, coin-shaped batteries used in watches, calculators, and hearing aids.

Secondary (Rechargeable) Battery Types

• Lithium-ion (Li-ion): The leading rechargeable battery for modern electronics. They power smartphones, laptops, and electric vehicles due to their light weight and high capacity.

• Lithium Polymer (LiPo): A flexible variation of Li-ion built in soft pouches. They are commonly used in radio-controlled drones, RC cars, and slim personal gadgets.

• Nickel-Metal Hydride (NiMH): Eco-friendly rechargeable replacements for standard AA or AAA cells, frequently used in digital cameras and handheld electronics.

• Nickel-Cadmium (NiCd): Older rechargeable cells known for enduring heavy-duty use and rugged conditions, though largely replaced by NiMH and Li-ion.

• Lead-Acid (Flooded, AGM, and Gel): Heavy, large-format rechargeable batteries. Flooded types require maintenance, while AGM (Absorbent Glass Mat) and Gel are sealed. They are widely used for automobiles, backup power (UPS), and solar storage.

Newer technology batteries are the:

Sodium Batteries: Are a rechargeable battery technology using sodium ions, similar to lithium-ion batteries, but with advantages like lower cost, greater safety, and better performance at low temperatures, thanks to the abundance of sodium. While sodium batteries are promising for applications like grid storage, budget electric vehicles, and home energy storage, they currently have lower energy density than lithium-ion batteries, which are superior for high-performance applications like long-range EVs.  

Solid State Batteries: Are a next-generation energy storage technology that replaces the liquid electrolyte in traditional lithium-ion batteries with a solid material. This innovation allows for higher energy density, faster charging, and improved safety due to the non-flammable nature of the solid electrolyte. While still facing hurdles like high manufacturing costs, major companies are testing and developing SSBs for commercial use.

Batteries come in all different sizes. Many have "group" sizes, which is based upon the physical size and terminal placement. It is NOT a measure of battery capacity. Typical BCI codes are group U1, 24, 27, and 31. Industrial batteries are usually designated by a part number such as "FS" for floor sweeper, or "GC" for golf cart. Many batteries follow no particular code, and are just manufacturers part numbers. Other standard size codes are 4D & 8D, large industrial batteries, commonly used in solar electric systems. 

Battery Group Size Chart - The following BCI battery group size chart lists the most common BCI battery sizes.

Motorcycle Battery Size Chart - Most Popular Motorcycle and Powersports Battery Sizes Comparison Chart

Some common battery size codes used are: (ratings are approximate)

 U1                                  34 to 40 Amp hours 

Group 24                         70-85 Amp hours 

Group 27                        85-105 Amp hours 

Group 31                        95-125 Amp hours 

4-D                                180-215 Amp hours 

8-D                                 225-255 Amp hours  

Golf Cart & T-105          180 to 225 Amp hours

 L-16, L16HC etc.          340 to 415 Amp hours  

Lithium battery size codes are dimensional codes where the numbers directly represent the physical width (diameter) and height (length) of the cell in millimeters. 

Cylindrical Cell Codes (5-Digit Numbers)

For standard cylindrical lithium-ion cells, the 5-digit code breaks down as follows: 

• First two digits: Diameter in millimeters (mm).

• Last three digits: Height or length in tenths of a millimeter or whole millimeters depending on standard, but generally scale to length. 

Common cylindrical sizes include:

• 14500: 14 mm wide by 50 mm long (similar size to a standard AA battery). 

• 16340: 16 mm wide by 34 mm long (often used as a rechargeable version of the CR123A camera battery). 

• 18650: 18 mm wide by 65 mm long (the most popular size for laptops, flashlights, and e-bikes).

• 21700: 21 mm wide by 70 mm long (used in high-drain tools and electric vehicles).

• 26650: 26 mm wide by 65 mm long (used in larger tactical flashlights and heavy-duty gear).

Coin and Button Cell Codes (4-Digit Numbers)

Coin-style lithium cells (like non-rechargeable CR batteries) use a 4-digit code: 

• First two digits: Diameter in millimeters.

• Last two digits: Thickness in tenths of a millimeter.

• Example: CR2032 is 20 mm in diameter and 3.2 mm thick. CR2025 is 20 mm in diameter and 2.5 mm thick. 

Rectangular / Pouch Cell Codes (6-Digit Numbers)

Prismatic or pouch lithium polymer cells often use a 6-digit code representing metric dimensions:

• First two digits: Thickness in millimeters (or tenths with a prefix).

• Middle two digits: Width in millimeters.

• Last two digits: Length/height in millimeters.

• Example: 501010 measures roughly 5 mm thick, 10 mm wide, and 10 mm long (common in tiny electronics like wireless earbuds). 

Drop-In 12V Battery Codes (BCI Group Sizes)

A 12V LiFePO4 battery packaged for RVs, boats, or trolling motors, manufacturers use traditional BCI (Battery Council International) group sizes originally designed for lead-acid batteries. These codes match standard physical dimensions rather than cell math: 

• Group 24: Roughly 10.25" L x 6.81" W x 8.88" H.

• Group 27: Roughly 12.06" L x 6.81" W x 8.88" H.

• Group 31: Roughly 13.00" L x 6.72" W x 9.44" H.

• GC2: Roughly 10.38" L x 7.19" W x 10.88" H (common tall golf cart battery size).

LiFePO4 (lithium iron phosphate) batteries typically last 3,000 to 5,000 cycles and provide a calendar lifespan of 10 to 15 years before dropping to 80% of their original capacity. 

Cycle Life and Depth of Discharge (DoD)

A cycle is one complete charge and discharge process. Partial discharges add up to equal full cycles. There is a strong link between how deeply you discharge the battery and how many total cycles it delivers: 

• 100% DoD: ~3,000 cycles

• 80% DoD: ~5,000 cycles

• 50% DoD: 8,000 to 10,000+ cycles

• 20% DoD: Up to 15,000 cycles 

Key Factors Affecting Lifespan

• Temperature: The ideal operating range is 15°C to 25°C (59°F to 77°F). Operating or charging at high temperatures (above 45°C/113°F) accelerates degradation, and charging below freezing (0°C/32°F) can permanently damage the cells. 

• C-Rate (Charge/Discharge Speed): Gentle charge and discharge rates (0.2C to 0.5C) put less thermal and chemical stress on the battery. High, erratic current loads reduce total cycle count. 

• Calendar Aging: Even if left sitting on a shelf, chemical processes cause LiFePO4 batteries to naturally age out after 10 to 15 years. However, a consensus on platforms indicates that with very low cycle frequency and optimal cool storage, calendar life can occasionally stretch to 20 years or more.

• BMS Protection: A built-in Battery Management System prevents overcharging, over-discharging, and voltage imbalances, which is vital for reaching maximum longevity.