Choosing the Best Batteries: Page 3 of 5

2009 Battery Specifications Guide
Beginner

Inside this Article

Most battery manufacturers have a full line of products.
Most battery manufacturers have a full line of products in different amp-hour capacities and voltages.
A classic Trojan L-16H flooded lead-acid battery
A classic Trojan L-16H flooded lead-acid battery—420 AH at 6 V.
Golf cart batteries
Golf cart batteries are inexpensive but shorter-lived than industrial batteries. They can be good “starter” batteries for people new to battery use and maintenance.
Surrette industrial battery
Some large, industrial batteries, like this Surrette model, come in single 2 V cells.
A Hup industrial battery from Northwest Energy Storage
This Hup industrial battery from Northwest Energy Storage demonstrates how single cells are contained in protective steel cases.
Bogart Engineering Tri‑Metric AH meter
An AH meter like the Bogart Engineering Tri‑Metric is an important tool for monitoring battery state-of-charge.
A single 2 V FLA cell, in a protective steel case.
A single 2 V FLA cell, in a protective steel case.
A Sealed FullRiver AGM Battery
A sealed FullRiver, similar in dimensions and capacity to a flooded L-16.
Concorde’s Sun‑Xtender AGM battery
Concorde’s popular Sun‑Xtender battery is about the same size as an automotive starting battery.
Sealed batteries installed on their sides
Sealed batteries can be installed on their sides to limit the amount of space required.
Most battery manufacturers have a full line of products.
A classic Trojan L-16H flooded lead-acid battery
Golf cart batteries
Surrette industrial battery
A Hup industrial battery from Northwest Energy Storage
Bogart Engineering Tri‑Metric AH meter
A single 2 V FLA cell, in a protective steel case.
A Sealed FullRiver AGM Battery
Concorde’s Sun‑Xtender AGM battery
Sealed batteries installed on their sides

Battery type. Flooded lead-acid (FLA) batteries are the most common type used in RE systems, particularly off grid. They are the least expensive per capacity and, if well maintained, can have a relatively long life span. However, they require the most maintenance. Distilled water needs to be added to the cells on a regular basis, depending upon how often and how deeply the bank is cycled, and upon battery charging regimens.

Valve-regulated lead-acid batteries (VLRA, a.k.a. sealed batteries).  Two general types of VRLA batteries are available for RE systems—absorbed glass mat and gel cells. Absorbed glass mat (AGM) lead-acid batteries are similar in chemistry to FLA cells. In their construction, glass mats, placed between the lead plates (anodes and cathodes), allow the electrolyte to be suspended close to the plates’ active material. These sealed batteries offer the advantage of not needing to be watered and greatly reduced gassing during charge cycles. This type of construction—adding glass mats, sealing the cells, and constructing the plates to operate with less electrolyte—increases cost while potentially shortening life span.

Gel cells use a “gel”-type electrolyte—with a silica additive that causes the liquid to stiffen. Gel-cell batteries are also sealed, which means no water to add—less maintenance and less gassing. However, because lost electrolyte cannot be replaced, they also have a shorter life. They are typically more expensive than FLA or AGM batteries.

Because AGMs can’t be watered, they have to be charged more lightly to avoid using up the finite amount of electrolyte they contain. Gel cells also aren’t watered but need to be charged even more lightly to avoid drying out the cell, which will kill it. 

So why would you ever choose shorter-lived, more expensive batteries like AGM or gel cells? The reasons vary, but often portability, poor battery area ventilation, and maintenance are factors. AGM and gel cell batteries have no liquid electrolyte to spill, so they can be a good choice for mobile systems. And because they hardly gas, they can work well in places where adequate ventilation for FLA batteries isn’t possible. Because they are freeze-resistant, they may be a good choice in applications where extreme cold is a factor.

AGM batteries are often the best choice for grid-tied applications with battery backup, since they are designed for float or standby applications. Because low-capacity battery banks are typical in backup applications, both decreased cycle life and increased cost can be offset by the fact that these batteries are rarely cycled. Plus, users with grid-tied systems are usually less inclined to pay attention to the battery maintenance, since they are also unaccustomed to “maintaining” their grid power. Finally, VRLA batteries will outlast FLA batteries that are not maintained properly (i.e. not watered regularly). If batteries are to be deeply cycled (50% to 80% DOD), gel-cell batteries may offer a longer life (more overall cycles) than AGMs.

Nominal Battery Voltage. Lead-acid batteries are built from individual cells with a “nominal” voltage of 2 V. Battery packs for RE systems are made up of combinations of cells to achieve nominal battery bank voltages of 12, 24, or 48. When designing small systems (loads less than 1,000 WH per day), 12 VDC is often selected as a nominal battery bank voltage if that system is not projected to grow. So a system for a hunting cabin that isn’t going to become a vacation home will keep battery costs down by having this low-voltage design.

For systems with heavier load profiles, larger (and more electrically efficient) battery voltages of 24 and 48 are commonly used. With commercial deep-cycle batteries (like golf cart and L16), the basic unit is often a 6 V battery made up of three, 2 V cells. In the medium-to-large systems, these 6 V units are typically combined in series (four for a 24 V string; eight for a 48 V string). To get greater AH capacity at that voltage, additional strings are then paralleled or higher-capacity batteries are selected.

Amp-Hour Capacity. The sizing of the battery bank depends on the storage capacity required, the maximum discharge rate at any time, the maximum charge rate, and the temperatures at which the batteries will operate.

Comments (3)

Thrush's picture

Have you heard that lead acid batteries must be kept within 20% of full charge for longevity? Have you heard that Nickel Iron batteries are not significantly damaged by freezing, full discharge, and are tolerant of over charge? Redundancy on a remote homestead is desirable. Natural progression of a system is inevitable and in our case with better batteries, more water pumping, refrigeration, additional solar panels, more summer cooling, misting, etc I discovered this advantage which is not listed in any literature I've seen. It is natural with increased consumption that a 24 V system would parallel a 12V system, and maybe in the future even move to a 48V system. But the important part is that voltage bracketing of lead acids with alkaline cells (they have a wider voltage range) works really well. As a ham radio operator, maintaining the 13.5 volts for radios is much easier. As a reminder, don't strike an arc about any cell that is gassing or one might explode a cell. :-)

Thrush's picture

If you have a 12V lead acid bank of batteries, in another room or area you can place a 24V Nickel Iron bank to 'piggy back' and maintain the quality of the lead acid bank. Just tap off from the 24V bank what you need to keep the lead acids at 12.6 V. When the Nickel Iron bank reaches a full charge and the lead acids are also charged add a diversion load like a well pump or hot water heater element to th 24v bank. Having both 12v and 24v inverters add redundancy. When running short of power on a cloudy day add in extra panels and throw all the power into the 12v bank :-)

Michael Welch's picture

Hi Thrush. I guess I do not understand why one would want to do this. It sounds expensive and a hassle. There's a lot to be said for keeping things simple and straightforward.

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