Alternative Energy Battery Type 101



Many of the batteries used in alternative-energy ‘off-grid’ solar powered systems are Lead-Acid type batteries. Even after over a century of use, they still offer the best price to power ratio.

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

Here’s more information about some of the different types of batteries, and a request for your input regarding what you may be using yourself…

(Updated and re-posted)

Expected Battery Life

These are some typical expectations for battery life if used in deep cycle service.

Note: ‘Deep Cycle’ is discharging a battery down to apprx 20% (80% discharge).
Note: Battery life will increase if only discharged down to 50% (a common design goal).

Starting battery: 3-12 months
Marine battery: 1-6 years
Golf cart battery: 2-7 years
Gelled deep cycle battery: 2-5 years
AGM deep cycle battery: 4-7 years


Starting batteries are commonly used to start and run engines. Engine starters need a very large starting current for a very short time. Starting batteries have a large number of thin plates for maximum surface area. The plates are composed of a Lead “sponge”, similar in appearance to a very fine foam sponge. This gives a very large surface area, but if deep cycled, this sponge will quickly be consumed and fall to the bottom of the cells. Automotive batteries will generally fail after 30-150 deep cycles if deep cycled, while they may last for thousands of cycles in normal starting use (2-5% discharge).

Deep cycle batteries are designed to be discharged down as much as 80% time after time, and have much thicker plates. The major difference between a true deep cycle battery and others is that the plates are SOLID Lead plates – not sponge. This gives less surface area, thus less “instant” power like starting batteries need. Although these an be cycled down to 20% charge, the best lifespan vs cost method is to keep the average cycle at about 50% discharge.

Golf cart batteries are quite popular for small systems and RV’s. The problem is that “golf cart” refers to a size of battery (commonly called GC-2, or T-105), not the type or construction – so the quality and construction of a golf car battery can vary considerably – ranging from the cheap off brand with thin plates up the true deep cycle brands, such as Crown, Deka, Trojan, etc. In general, you get what you pay for.

Marine batteries are usually a “hybrid”, and fall between the starting and deep-cycle batteries. In the hybrid, the plates may be composed of Lead sponge, but it is coarser and heavier than that used in starting batteries. It is often hard to tell what you are getting in a “marine” battery, but most are a hybrid.

Gelled deep cycle batteries, or “Gel Cells” contain acid that has been “gelled” by the addition of Silica Gel, turning the acid into a solid mass that looks like gooey Jell-O. The advantage of these batteries is that it is impossible to spill acid even if they are broken. However, there are several disadvantages (must be charged at a slower rate and lower voltage to prevent permanent damage, In hot climates, water loss can be enough over 2-4 years to cause premature battery failure).

AGM, or Absorbed Glass Mat deep cycle batteries have all the advantages (and then some) of gelled, with none of the disadvantages, and they can take much more abuse. Since all the acid is contained in the glass mats, they cannot spill, even if broken. This also means that since they are non-hazardous, the shipping costs are lower. In addition, since there is no liquid to freeze and expand, they are practically immune from freezing damage. Nearly all AGM batteries are “recombinant” – what that means is that the Oxygen and Hydrogen recombine INSIDE the battery. AGM’s will cost 2 to 3 times as much as flooded batteries of the same capacity. In many installations, where the batteries are set in an area where you don’t have to worry about fumes or leakage, a standard or industrial deep cycle is a better economic choice.


The lifespan of a deep cycle battery will vary considerably with how it is used, how it is maintained and charged, temperature, and other factors.

Although some batteries are designed to be able to deep cycle down to 20% (80% discharge), a good guideline is to limit discharge to 50% (or less) to prolong its usable life.

If batteries will be kept indoors, either you will need a special designed (and vented to the outside) battery box (hydrogen gas), or you will need AGM batteries (sealed and safe).

I have some experience with the Lead Acid Trojan T105 (6 volt, 200AH) golf cart batteries, and they’ve been a ‘work horse’ on a previously installed solar panel system. I believe they’re one of the best values going – probably due to high volume production for golf carts, etc..

I am currently running with 16 Trojan AGM-31 batteries (12 volt, 110AH each, wired in a 48-volt configuration). Reason being… my battery bank is kept indoors (AGM type required), and I can handle them at 70 pounds each whereas other larger batteries are hundreds of pounds. I chose to keep them indoors because I live in a cold weather climate during the winter. Batteries perform very poorly when they’re cold (e.g. if they were outside in a battery shed). So I ‘bit the bullet’ and paid the higher price for AGM’s, while also choosing not to build a special designed battery room for lead-acid vented batteries.

I’m curious to hear your experiences and opinions, or which batteries that you are using on your alt-energy system…