Choosing the Best Batteries: Page 4 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

A battery’s storage capacity—the amount of electrical energy it can hold—is typically expressed in ampere-hours (amp-hours, or AH) at a certain discharge rate. One AH represents a flow of electric current of 1 amp for 1 hour. A battery is like a bucket—the larger your “bucket” is, the more AH it can hold. Hence, the larger the AH value of a battery, given a particular discharge rate, the more storage it offers.

Often there’s a choice of selecting a battery with either higher voltage and lower AH, or lower voltage and higher AH. How do you know which is most appropriate for your application? In general, limit the number of battery series strings in parallel to three or less (two are better, and one is ideal). This reduces imbalances introduced by having multiple paths for the current to follow and extra electrical resistance created by paralleled battery cables. In applications where more AH are needed, buy lower-voltage, higher AH batteries so that several low-voltage batteries can be wired in series and the number of paralleled battery strings can be minimized.

The denoted AH capacity of a given battery depends on the rate at which it is being discharged and the amount of time it takes to discharge it. Large industrial batteries, i.e. for forklifts, are often rated at the “6-hour” rate, indicating a high current discharge rate, which brings the battery to its terminal voltage (often at 80% DOD) in 6 hours, about the length of a forklift’s working shift. For RE systems, a 20-hour rate is typically used, because that is closely aligned with the more modest discharge rates that bring the battery to a terminal voltage (again, often at 80% DOD) over 20 hours—more closely approximating daily home use before recharging.

For converting 6-hour rates to an RE system’s more common 20-hour rate, multiply by 1.24. Using this calculation, a 100 AH, 6-hour rating offers 124 AH at the 20-hour rate.

Bulk Charge Set Point Voltage. When charging batteries, the goal is to put as much current as possible into the battery as efficiently as possible. But charging a battery too quickly can cause heat to build up in the battery, as well as excessive gassing, and can shorten the battery’s life. To keep from harming the battery during charging, charge controllers used in RE systems limit the charge rate based on the batteries’ voltage. As the cell voltage increases, the charge rate (the number of amps allowed in) is reduced to prevent overcharging.

The initial phase when all available current is allowed into the battery is referred to as the “bulk” charge phase. Once the battery has reached its initial bulk-charge voltage, the charge controller will hold the voltage there for a programmed period of time (often 2 hours)—the “absorption” charge phase. This is done to assure full charging throughout the many cells of the battery. Note that the set points listed in this guide are per cell, so you will need to multiply it by the number of series-connected cells to determine the appropriate battery charge set points. For example, if you were to use four batteries (6 V each, wired in series for a 24 V configuration) and the bulk charge set point voltage range is 2.4 to 2.49 V for your battery’s cells, the ideal battery bank bulk-charge voltage set point would be between 28.8 and 29.88 V (3 cells per battery x 4 batteries x 2.4 to 2.49 V).

Float-Charge Set Point Voltage. After the absorption period, the charge controller ramps down the charging current to achieve the “float” phase, which is a lower voltage that greatly reduces the batteries’ gassing while still keeping the battery full. To continue the example, the float-charge set point voltage range is 2.20 to 2.23 V for each cell. With 12 cells total, the ideal battery bank float-charge voltage set point for this particular battery bank would be between 26.4 and 26.76 V.

Both AGM and gel-cell batteries will not tolerate voltages that are as high as FLAs. The charge controller’s bulk and float set points must be programmed appropriately to avoid damaging these batteries.

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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