Posted with permission of Home Power magazine, from issue #59. http://www.homepower.com

Batteries: How to Keep Them Alive for Years & Years... Windy Dankoff )1999 Windy Dankoff

Lead-acid batteries are often considered to be the weak link in renewable energy systems. However, todays renewable energy batteries are better than ever, and so are the devices that regulate and protect them. Battery failures are rarely the fault of the batteries themselves! Follow these guidelines to avoid most battery problems.

Size a Battery Bank Generously The battery bank is the foundation of the power system. Dont skimp here! A good working minimum size can be based on your estimate of seven days of energy storage. Keep in mind that after one year of service, it is not advisable to enlarge a battery bank by adding new batteries to it. Doing this will cause mismatch problems and stray currents between the newer and the older sets. Instead, anticipate your growing energy needs by sizing the set properly from the start, perhaps to twice the minimum (seven day) size. Avoid Multiple Strings

The ideal battery bank is the simplest, consisting of a single series of cells that are sized for the job. Higher capacity batteries tend to have thicker plates, and therefore greater longevity. Having fewer cells will reduce both maintenance and the chance of randomly occurring defects. For example, suppose you require a 700 amp-hour battery bank. You can approximate that by using three parallel strings of golf cart batteries (220 AH), or two strings of the larger L-16 style batteries (350 AH), or a single string of larger industrial batteries. It is advisable to have fewer than three parallel battery strings if possible. Battery banks with multiple strings are more likely to develop cells with unequal states of charge. Weak cells will be difficult to detect because they will steal from the surrounding cells. The system will suffer as a whole and will cost you more in the long run.

Prevent Corrosion With flooded batteries, corrosion of terminals and cables is an ugly nuisance that causes resistance and potential hazards. Once corrosion takes hold, it is hard to stop. The good news is that its easy to prevent! Apply a non-hardening sealant to all of the metal parts of the terminals before assembly. Completely coat the battery terminals, wire lugs, nuts, and bolts individually. A sealant applied after assembly will not reach all around every junction. Voids will remain, acid spatter will enter, and corrosion will begin. Special compounds are sold to protect terminals, but you can get perfectly good results using common petroleum jelly (Vaseline). It will not inhibit electrical contact. Apply a thin coating with your fingers, and it wont look sloppy. If wire is exposed at a terminal lug, it should be sealed airtight, using either adhesive-lined heat-shrink tubing or submersible rubber splice tape. You can seal an end of stranded wire by warming it gently and dipping it in the petroleum jelly, which will liquify and wick into the wire. It also helps to put the batteries over a floor drain, or in a space without a floor, so that they can be rinsed with water easily. Washing the battery tops about twice a year will remove accumulated moisture (acid spatter) and dust. This will further reduce corrosion, and will prevent stray currents from stealing energy. Batteries that we have protected by these measures show very little corrosion, even after ten years without terminal cleaning.

Moderate the Temperature Batteries lose approximately 25 percent of their capacity at a temperature of 300 F (-10 C), compared to a baseline of 770 F (250 C). At higher temperatures they deteriorate faster. So its good to protect them from temperature extremes. If no thermally stable structure is available, consider an earth sheltered enclosure. Where low temperatures cannot be avoided, get a larger battery bank to make up for the loss of capacity in the winter. Avoid direct radiant heat sources that will cause some batteries to get warmer than others. Use Temperature Compensation

When batteries are cold, they require an increase in the charge voltage limit in order to reach full charge. When they are warm, they require a reduction in the voltage limit in order to prevent overcharge. Temperature compensation is a feature in many charge controllers and in the chargers of some inverters. To use this feature, order the accessory temperature probe for each charging device, and attach it to any one of the batteries.

Use Low-Voltage Disconnects Discharging a battery to exhaustion will cause irreversible loss of capacity and reduced life expectancy. Your system should employ low voltage disconnect (LVD) in the load circuits. Most inverters have this feature, and so do many charge controllers and power centers. Buy High-Quality Batteries

You get what you pay for! Good deep-cycle batteries can be expected to last for five to fifteen years, and sometimes more. Cheap batteries can give you trouble in half that time. Buy industrial quality deep cycle batteries from a reputable renewable energy supplier. Charge Completely

Bring batteries to a full state of charge (SOC) at least every three weeks. This reduces internal corrosion and degradation, and helps to insure equalization, so that any weaker cells do not fall continually farther behind. A full SOC may occur naturally during most of the year, but dont hesitate to run a generator when necessary, to bring the batteries up. For more details, refer to the instructions for your inverter/charger and batteries. You may wish to post clear instructions about charging requirements and methods at your power center. When Is a Battery Full?

The charged indicator on most PV charge controllers means only that battery voltage is relatively high. The SOC may be approaching full, but is not necessarily near 100 percent. A voltmeter reading gets you closer, but it is not a foolproof indicator. Voltage varies too much with current flow, temperature, and time to give a clear indication. For flooded batteries, a hydrometer is the definitive indicating device, although not a convenient one. You can measure every cell individually with this tool. Get a good hydrometer from a battery or automotive supplier. Rinse it after use, and keep it clean. An amp-hour meter is the most informative and user-friendly way to monitor SOC. For sealed batteries, it is the only definitive method.

Install a System Monitor Would you drive a car with no instrument panel? Metering is not just bells and whistlesit is necessary to help you keep tabs on the system. Many charge controllers have indicator lights and readouts built in. For a full-scale remote home, consider the addition of a digital monitor, like the Trace TM-500, Tri-Metric, E-Meter, or Omni-Meter. These devices monitor voltage and current, record amp-hours, and accurately display the SOC of the battery bank. They also record more detailed information that can be useful for troubleshooting. The monitor may be mounted in another room or building, for handy viewing.

Just Add Water Note: This applies only to flooded batteries, not to sealed batteries. The plates of every cell in your battery bank must be submerged at all times. Never add any fluid to a battery except distilled water, deionized water, or very clean rainwater collected in plastic containers. Most batteries need water every six to twelve months. Dont fill them more frequently than needed to submerge the plates. Fill them only to the level recommended by the manufacturer, generally about an inch (25 mm) below the top; otherwise they may overflow during finish-charging.

Not a Struggle Batteries are the heart of your power system. They may demand your attention occasionally, but your relationship with them need not be a struggle. With a proper installation, a little understanding, and some simple maintenance, your batteries will live a long and healthy life.

Access Author: Windy Dankoff, Dankoff Solar Products, Inc., 2810 Industrial Rd., Santa Fe, NM 87505 505-473-3800  Fax: 505-473-3830 pumps@danksolar.com

-- Don Kulha (dkulha@vom.com), August 18, 1999

Answers

Don, what's the generally accepted percentage of current drain from your battery bank: no more than 50% before recharging to full, 80%, or is it 20% then recharge to get the longest life span?

I have a 24v bank of L16s. So I have 700 amp/hours at 24v or 16800 watts available less inefficiencies. What should I limit my current draw to on a bank this size? 20% or 3360 watts daily? or as much as 80% at nearly 13kw?

Thanks, Brett.

-- brett45 (brett45@bigfoot.com), August 19, 1999.

The right term for how much we'll pull out of the batts before recharging them is "depth of discharge" (DOD).

The answer to this is "minimize it". Batteries are rated (you might have to contact the batt engineers to get the data) for a certain amount of charge/discharge cycles to a particular depth of discharge. I don't have any charts handy but for a given battery it may go like:

DOD 10% 1000 cycles DOD 20% 800 cycles DOD 30% 650 cycles.....etc.,etc.,etc. (ala Yul Brenner...)

Minimize depth of discharge to maximize life with any battery. Lead acid batts have no "memory effect" so there is absolutely no benefit to discharging them farther than you have to. Understand too that the discharge/charge cycle is not 100% efficient....more like 85%-90% for a new batt (they get progressively worse and the more the batt is discharged the worse it gets). If you have solar try to use most of the power you need when the sun shines and the power comes directly from the PV's so as not to incur the battery inefficencies. I try to limit my batts to a 20% discharge (L-16 batts).

Energetically....DCK

-- Don Kulha (dkulha@vom.com), August 19, 1999.