Group 78 Batteries - Dimensions, Features, and Recommendations

BCI Group 78 batteries is a group of car and light truck batteries that are also used in marine and light industrial applications as starting and as general, dual-purpose batteries.

BCI Group 78 batteries are often used in 'Stop-and-Go' automotive applications, requiring the batteries to provide large currents very often and to be recharged quickly - such applications require tough and durable, high-quality batteries.

Updated: October 27, 2023.

Group 78 Batteries - Dimensions and Specifications

BCI Group 78 batteries are medium-sized lead-acid batteries, with practically all modern batteries being made as Sealed Lead-Acid (SLA) Absorbent Glass Mat (AGM) batteries.

Gel-cell, wet/flooded, and lithium iron phosphate (LiFePO₄) group 78 batteries are not as common.

The physical dimensions of the group 78 batteries are very similar to several other battery groups, allowing these batteries to be used almost interchangeably. Here is the list of several such groups with their maximum dimensions (L x W x H):

• Group 78: 10.25 x 7.0625 x 7.3125 inches (26 x 17.9 x 18.6 cm),
• Group 34: 10.25 x 6.8125 x 7.875 inches (26 x 17.3 x 20 cm),
• Group 24: 10.25 x 6.8125 x 8.875 inches (26 x 17.3 x 22.5).

As one can see, group 78 is somewhat wider than groups 34 and 24 but also somewhat lower.

Group 34, Group 34/78, and Group 78 Difference

Another very important difference between these battery groups is that group 78 batteries feature front 3/8" terminal posts (positive to the left, negative to the right), and group 34 on top of the battery.

Since groups 34 and 78 are so similar in dimensions, many manufacturers offer batteries labeled as '34/78' - these batteries feature both top SAE terminals (like group 34 batteries have) and side 3/8" terminals (like group 78 batteries have).

Also, such batteries often have the letters 'DT' (Dual Terminals) as part of their labels in order to distinguish them from 'common' group 34 and group 78 batteries.

Note: just in case, whenever changing the battery (not just group 34, 34/78, or 78), always check the size of the battery compartment and the actual battery size that You intend to buy - battery compartments generally have some spare room to accommodate the batteries that are somewhat larger, but better safe than sorry.

The following comparison chart lists some of the best and most popular group 78 and group 34/78 batteries with some of their features and specifications:

Note: Amazon affiliate links ('Model' column) open in the new windows, feel free to check them for the most up-to-date offers and prices.

On average, group 34/78 and group 78 batteries feature nominal (20h) capacity in the 50-70 Ah range, a nominal voltage of 12 volts, 700-900 Cold Cranking Amps (CCA), 850 - 1050 Marine Cranking Amps (MCA), and 100-135 minutes of Reserve Capacity (RC).

Also, they weigh between 38 and 51 pounds (~17 - 23 kg) - since many of them come with built-in handles, they can be carried to shorter distances by a single adult.

Starting group 34/78 and group 78 AGM batteries are optimized for providing a short but strong burst of energy and are generally lighter than dual-purpose batteries. Starting batteries should be used only as cranking batteries and for powering light loads while the engine is turned off - alarm/security systems and similar.

Dual-purpose group 34/78 and group 78 AGM batteries feature thicker plates and are thus heavier. But, they are also able to provide strong currents for short periods, and they also tolerate deep discharge cycle applications, much better than starting batteries.

People having cars and trucks with devices that must operate when the main engine is turned off should go for dual-purpose batteries. Such devices include various pumps, winches, compressors, multimedia devices, chargers, inverters, etc.

However, note that even the dual-purpose batteries last longer when they are not discharged too deep - when discharged down to 50% DoD, some models tolerate 700-900 charging/discharging cycles easily - Odyssey and NorthStar models, for example.

The prices of group 34/78 and group 78 batteries vary depending on their capacity, warranty, discharge characteristics, and other important features.

Best Group 78 Batteries

NorthStar and Odyssey are premium batteries that often cost more than other batteries, but what you pay is what you get - a tough, durable, and reliable battery that can withstand a lot of use and even some abuse (just don't do it, unless really unavoidable).

Optima batteries, with their spiral-wound cells, are relatively light and vibration-resistant; they offer excellent performances considering their weight and price.

Other battery brands (ACDelco, Delphi, etc.) manufacture good batteries that come at acceptable prices and, when found on sale, can be a great money-saver.

How to Charge Group 34/78 and Group 78 Batteries?

When the battery is connected to the car's electric system, it is being charged and monitored by the very same car's electric system - when the car's engine is ON, the alternator produces electricity, and the part of the electricity is used for recharging the battery.

If there is an issue with the alternator and/or battery, the car's electric system will notify the user - most modern cars will display an error message of some kind (code or detailed text, car-dependent).

However, batteries that are used in industrial applications should be charged using smart AGM battery chargers that can analyze the battery and adjust the charging process to the battery's condition - desulphation and cell-equalization modes are very important for prolonging the operating time of such batteries.

For batteries in the 60-70 Ah capacity range, advanced ~10 Amps AGM battery chargers are recommended.

Stronger chargers may recharge the batteries faster, but not all group 78 batteries can accept such charging current.

Group 78 800 CCA and 1000 CCA Battery

As one can see in the group 78 battery comparison chart, there are several models that are able to provide at least 800 CCA Amps, including Northstar batteries, some of the Odyssey batteries, and RedTop Optima batteries.

Note: RedTop Optima group 78 batteries are almost as strong as best Northstar and Odyssey batteries, but they are also significantly lighter thanks to their wound/spiral cells and the fact that RedTop batteries are pure starting batteries and Northstar and Odyssey batteries are dual-purpose batteries.

However, if we try to find group 78 batteries capable of 1000 CCA Amps, we can't find it in the chart.

But, there are several models that are able to provide 1000+ MCA Amps - again, Northstar, Odyssey, and RedTop Optima batteries.

Long Story Short: Group 78 and 34/78 batteries are common car/truck and light industry batteries, often available at local hardware shops and online stores.

Their performances do come at a certain price, but a good battery is almost never appreciated until badly needed.

When choosing the right model, be sure to choose according to your own needs and requirements.