Keeping Your Batteries Alive

Bulletin # 8


If you read and follow all of the Battery Bulletins on this web-site and you will avoid the vast majority of battery problems. Remember battery failures are rarely the fault of the batteries themselves. Lead-acid batteries are often considered to be the weakest  link in renewable energy systems. But, today's batteries are better than ever, and so too are the devices that regulate and protect them.

Battery bank sizing and your PV array                              

A PV array, if it is the primary energy source, should be sized to produce 30 to 40% more energy than the load requires. This will compensate for battery losses and for less-than-average charging conditions. Fortunately your PV array can be expanded at any time in the future.

Your battery bank should be sized to a capacity of around 5 days of load. Remember that your energy demands will increase over time, so plan to size a larger battery bank than what you will need at first. Why?...

Because after even 1 year of service, you can NOT enlarge a battery bank by adding new batteries to it. A Batteries' voltage response changes with age. A new addition will be the easiest path of resistance for charging, thus causing the older batteries to become lazy and hard sulphate the plates, making the old batteries even harder to charge. This is a sure way to develop bad cells.

Buy high quality batteries                                                   

Good deep-cycle batteries can be expected to last for 7 to 20 years. Cheaper Golf Cart batteries can give you problems in only 3 to 4 years. 
You do get what you pay for! Talk with Tom first and, buy your batteries from The Solar Biz.

Avoid parallel strings                                                          

Having fewer cells will reduce the chance of developing bad cells, and fewer cells reduce maintenance. What is the ideal bank? The ideal battery bank is the simplest, consisting of a single set of series connected cells that are sized for the job. Higher capacity batteries also have thicker plates, giving you more cycles and therefore greater longevity.

Under no circumstances is it advisable to install more than three parallel battery strings, but at three strings you are courting trouble.

Once again, one of the strings will be the easiest path for the charge current thus becoming the strong string. The other two strings will compete for second place and the looser will be the lazy string. The lazy bank will tend to lose its equalization, resulting in accelerated failure of any weak cells. Weak cells will be difficult to detect for at least a while, because they will "steal" from the surrounding cells, and the system will suffer as a whole and will cost you more in the long run.

Eventually the lazy string will prematurely develop bad cells and require the whole string to be pulled out of the bank. No replacement will be possible at this time because the whole bank is now more than a year old. Now the next lazy string will begin to develop bad cells. All you can do is either try to make due with the decreased capacity or replace the whole bank with large series connected battery sized for the job, which is, by the way, what you should have purchased in the first place, and avoided all this trouble.

The goal is to maintain all of the cells at an equal state of charge. Cells that tend to receive less charge are likely to fail prematurely. This can take years off of the effective life of the battery bank. A fraction of an ohm of added resistance in one battery string can reduce the life of the entire string.

Precautions to take when wiring two or more strings of batteries in series parallel.

(1) Connect the two main cables to opposite corners of the battery bank, and maintain symmetry in wire size and lengths. This will attempt to distribute current evenly through the bank, it won't, but every little bit helps.
(2) Arrange batteries to maintain even temperature distribution throughout the bank. Avoid uneven exposure to heat sources. Leave at least 1/2 inch of air space around each battery, to promote even cooling.
(3) Apply an equalization charge at least every 3 to 4 weeks (bring every cell to 100% charge).

Prevent corrosion                                                                 

Corrosion of terminals and cables is an ugly nuisance that causes resistance and potential hazards. Once corrosion gets hold, it is hard to stop. But it is easy to prevent.  Apply Vaseline or a non-hardening sealant to all of the metal parts of the terminals BEFORE ASSEMBLY. Completely coat the battery terminals, the wire lugs, and the nuts and bolts individually. Vaseline will not inhibit electrical contact. Apply a thin coating with your fingers, and it won't look sloppy. If wire is exposed at a terminal lug, it should be sealed airtight, using heat shrink tubing.

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 per year) will remove accumulated moisture (acid spatter) and dust. This will further reduce corrosion, and will prevent stray currents from stealing energy.

Moderate Temperature                                                        

Batteries lose approximately 25% of their capacity at a temperature of 30°F (compared to a baseline of 77°F). At higher temperatures, they deteriorate faster. Thus, it is desirable to protect them from temperature extremes. If no thermally-stable structure is available, consider an earth-sheltered enclosure. Where low temperature 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.

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 power centers, as well as in the back-up chargers in some inverters. To use this feature, order the accessory temperature probe for each charging device, and attach it to any one of the batteries.

Low-voltage disconnects                                                       

Discharging a battery to exhaustion will cause immediate, irreversible loss of capacity and 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. Don't depend on human behavior to prevent over-discharge. It can be caused easily by accident or by an irresponsible user. Again, most inverters have LVD built-in but if there are DC loads on the system, please incorporate an LVD device.

Bring batteries to a full state-of-charge every 3 weeks   

Bring the batteries to a full state-of-charge at least every 3 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 state-of-charge may occur naturally during most of the year, but do not hesitate to run a generator when necessary, to bring the batteries up. Information like this should be posted at the power center. For more details, refer to the instructions for the inverter/charger and for the batteries.

How do you know when a battery is 100% charged?        

The "charged" indicator on most PV charge controllers means only that battery voltage is relatively high. The state-of-charge may be approaching full, but is not necessarily near 100% A voltmeter reading gets you closer, but it is not a certain indicator. It varies to much with current flow, temperature and time, to give a clear indication.

For flooded batteries, a hydrometer in the hands of a professional, is the definitive indicating device, although not a convenient one. Many people make mistakes with a hydrometer. With it, you can measure every cell individually. Obtain one from a battery or automotive supplier. Even the cheapest hydrometer is fine. Rinse it after use, and keep it clean.
An amp-hour meter is the most informative and user-friendly way to monitor State of Charge. For sealed batteries, it is the ONLY definitive method.

The System Monitor                                                              

Many charge controllers have indicator lights and readouts built-in. For a full-scale remote home, consider the addition of a digital monitor, like Trace TM-500  or a Tri-Metric 2020 meter . These devices monitor voltage and current, record amp-hours, and accurately display the state-of-charge 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.

How to Read a Hydrometer                                                  

A hydrometer will help you to determine whether the battery bank is getting fully charged, and whether any individual cells are falling behind. You should be aware that a hydrometer will give you false readings under the following conditions.

(1) After adding water: For pure water to mix throughout the cell, it takes time and some bubbling during finish charge. A hydrometer will show a greatly reduced reading until the fluid mixes.

(2) Low temperature: As battery temperature drops, the fluid becomes more dense. A temperature compensating hydrometer is best. Otherwise, for every 10°F below 70°F, subtract 3.5 points from the reading.

(3) Time lag during recharge: As the battery recharges, the fluid becomes more dense down between the plates. The hydrometer reads the fluid above the plates. You will get a delayed reading until the fluid is mixed by the movement of bubbles during finish charge. The voltage will rise steadily, providing an indication that something is happening.

During discharge, you will get a true hydrometer reading because the fluid becomes less dense and will circulate to the top. Any time a hydrometer indicates a fully charged cell, you KNOW it is fully charged.


BATTERY ACID IS HAZARDOUS. When working around batteries, wear a full face mask or safety glasses. Always keep sterile eye wash close at hand (where you can find it if you can't see, because you have acid in your eyes). Get a rugged plastic bottle to keep with your service tools, and fill it with baking soda and water solution. Use it to neutralize accidental splash or spills and to clean normal acid spatter from battery tops. Finally, don't ever wear your favorite blue jeans! You won't know if you ruined them till you wash them next time.

Just add water                                                                      

Most batteries only require addition of water every 6 to 12 months. 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.  There is no need to fill them more frequently than needed to submerge the plates. Fill them only to the level recommended by the manufacturer, generally about an inch below the top, otherwise they may overflow during finish-charging.


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. If you have questions about batteries call The Solar Biz. We have been in the battery business for thirty-three years.


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