Battery System Maintenance and Repair


Inside this Article

Battery System Maintenance and Repair
Regular care of your renewable energy system’s batteries will help ensure that they can provide years of high performance.
Add distilled water to the batteries
Add distilled water to the batteries to about 1/4 inch below the bottom of the fill tube. Always wear eye and skin protection.
Corrosion between a battery terminal and the metal frame of a battery rack creates a potential path for current, which could create a ground fault or shock hazard.
Excessively corroded battery connections
Excessively corroded battery connections can have enough resistance to affect a battery’s performance, both when charging and discharging, and result in reducing a battery’s performance and life.
Clean all battery terminals and cable lugs
Take the time to make all battery terminals and cable lugs clean and bright before connecting the cables.
Petroleum jelly can protect terminals against corrosion
Petroleum jelly can protect terminals against corrosion, but also may attract dirt and makes using a wrench on them messy.
Always use insulated tools when working with batteries
Accidental short-circuiting between battery terminals can damage a battery and result in severe injury—always use insulated tools when working with batteries.
A battery meter
A battery meter is crucial to monitoring your battery bank’s electrical state.
Keep good records
Keeping good records is part of good battery maintenance, and can help you identify trends and spot problems if they occur.
Overcharging can result in excess gassing
Overcharging can result in excess gassing and loss of water. Exposed plates will oxidize and the battery will be permanently damaged.
Undercharging can result in electrolyte stratification
Undercharging can result in electrolyte stratification and/or sulfation (shown) and a damaged battery.
Battery System Maintenance and Repair
Add distilled water to the batteries
Excessively corroded battery connections
Clean all battery terminals and cable lugs
Petroleum jelly can protect terminals against corrosion
Always use insulated tools when working with batteries
A battery meter
Keep good records
Overcharging can result in excess gassing
Undercharging can result in electrolyte stratification

Good design and installation practices are essential to a well-functioning inverter–battery system, but that is only a start. This article will review the ongoing battery maintenance that is crucial to a system’s longevity.

Adding Water

For flooded lead-acid (FLA) batteries, it is critical to check the electrolyte level on a regular basis. As batteries charge, some water in the electrolyte is converted to hydrogen and oxygen gas, and released through the vent caps. The electrolyte needs to be replenished to the proper level by adding distilled water.

In cool climates such as Washington State, a system with a moderate C/10 charge rate should be checked every one to two months. In hot climates like Haiti, the electrolyte level needs to be checked at least twice a month.

Recording the date and the amount of water added to the battery allows you to identify usage trends and spot potential problems. Batteries in hotter climates will use more water than ones in cooler climates, as will systems with heavy discharging and higher charging rates. When a battery starts requiring more water, it is typically a sign that it is failing—or that something has dramatically changed in the charging or load profile. Keeping track of the changes in water consumption will help determine when it’s time to replace the battery or investigate other problems.

The correct electrolyte level is usually about 1/4 inch below the vent tube. Adding water to a battery cell should be done only after charging the battery to 100% or after an equalization charge has been completed. However, if the plates inside the battery are close to being exposed (which can cause permanent damage to the battery), add enough water to keep them covered before recharging. However, be careful not to overfill during recharging or equalization. Otherwise, the electrolyte can overflow, making a hazardous mess. Losing electrolyte and replacing it with distilled water results in dilution, effectively reducing the battery’s capacity and, therefore, its performance. Automatic watering systems and recombination caps can reduce the time required for refilling batteries. However, most of them require removal before doing an equalization charge on the batteries.

Cleaning Battery Terminals

Corrosion can occur on and between the cable lugs and the battery terminals, creating higher resistance that impedes the flow of current when charging or discharging. Corrosion can also occur between battery terminals and the metal casing of the battery rack, potentially resulting in ground faults and creating a shock hazard.

If even only one terminal has corrosion, the high resistance on that series-connected string of batteries will result in less charging and discharging. Specifically, the other battery strings will compensate for the loss of total capacity, and be charged and discharged at higher rates. This will create imbalances within the battery bank, reduce system performance, and decrease the life and performance of the entire battery bank.

During the monthly inspection, check all battery connections for corrosion. If there’s a metal rack or enclosure, also check for corrosion between the terminals and the metal. If caught early, it can be removed with a wire brush without much effort. If excess corrosion is found covering the terminal and hardware, shut down the system by removing all charging and discharging connections from the battery, disassemble the battery hardware and cables, and thoroughly clean the terminal, hardware, and cable lug. A baking soda and water mixture can be applied to the corroded areas, which can then be scrubbed with a wire brush. However, be extremely careful that none of the baking soda mixture gets into the battery cells, as it will neutralize some of the electrolyte, reducing the battery’s performance. Finally, rinse all of the baking soda off with clean water and dry with a clean rag.

Before reconnecting the battery cables, wire-brush the terminals until the lead is shiny, tighten the connection, and then cover the terminal and cable with an anticorrosion coating—the best is a spray-on type that dries upon contact. Common petroleum jelly can also be used, but it can attract dust and dirt, and will be a sticky nuisance to deal with when checking the terminals for tightness.

Checking Connections

The bolt and nut connection needs to be tight enough to fully compress the split-type lock washer (which should always be included, along with two flat washers). The bolt should be tight enough that the cable lug cannot be moved on the terminal when pulled. Battery manufacturers will specify torque values for their terminals. Typical torque levels for 1/4-inch hardware is 6 foot-pounds; for 5/16-inch hardware it’s 11 foot-pounds. Be careful when using metal wrenches on the battery as it is very easy to accidentally cause a short circuit to adjacent terminals. Using specially made, insulated tools or, at the very least, wrapping the handle of the wrench with insulating tape, is highly recommended. Some tips for making battery connections are:

  • Always use stainless-steel hardware to connect cables to terminals.
  • Always clean the terminal posts and cable lugs so that they are shiny prior to installing the connections.
  • Always include flat washers to prevent the bolt and nut heads from digging into the soft lead terminals and copper cable lugs (but do not place them between the cable lug and the battery terminal).
  • Always include a split-type lock washer to help keep the bolted connection tight.
  • Always use an anticorrosion coating over cleaned connections, and reapply the coating after retightening or replacing cables.

Recharging & Equalizing the Batteries

One of the most critical factors in maintaining good battery health is regularly charging the battery to a full state of charge. Ideally, this should be done once a week.

No matter the cause—an undersized PV array, running excessive loads, or not having a secondary energy source for extended cloudy periods—undercharging batteries can permanently harm them over time. When batteries are not fully charged, sulfate crystals form on the lead plates’ surfaces. This reduces the contact area between the lead and the electrolyte, decreasing the battery’s capacity. Sulfate crystal buildup can become an irreversible condition that will worsen until the battery is unusable.

An intentional overcharging—called equalization—helps remove the sulfation from the battery’s plates. Equalization charging is a process where the battery is intentionally overcharged to bring weaker cells up to parity with stronger cells and should only be done with FLA-type batteries. During equalization, the battery’s cells will gas vigorously, mixing up the electrolyte and eliminating stratification. Most sophisticated inverter/chargers and PV charge controllers have an equalization function, which allows the charging source to charge the battery for a timed period and achieve a preset, high-voltage setpoint. During equalization, closely monitor the electrolyte’s level and temperature—and be prepared to shut down the charging when the equalization is finished or if the temperature exceeds 125°F (52°C).

During the monthly or quarterly maintenance checks, measure and record the battery’s voltage and each cell’s specific gravity. A SG difference of 0.025 or higher between cells indicates an equalization charge is needed. To perform an equalization charge on an FLA battery:

  • Draw a diagram or label each battery cell with a reference number.
  • Charge the batteries through the bulk and absorb stages before starting the equalization process.
  • Measure and record the voltage and SG of each battery and cell.
  • Choose the cell on each battery string with the lowest SG reading as a pilot cell—these will be the cells tested to determine when equalization is complete.
  • Check the electrolyte level in each cell to make sure the battery plates are covered.
  • Initiate the equalization charge.
  • Check and record SG readings of the pilot cells every half hour.
  • Check electrolyte levels to ensure that they never fall below the top of the plates.
  • Check temperatures of the batteries’ electrolyte every 30 minutes to make sure it never exceeds 125°F (52°C). Stop equalization if this temperature is exceeded.
  • Stop equalization when the SG reading in the pilot cells stays constant for three readings.
  • Water each cell to 1/4 inch below the vent tube and replace the caps.

Not all users will want or be able to check the electrolyte’s SG on a monthly or quarterly basis. A more general method is to set a predetermined routine equalization maintenance on a monthly, quarterly, or biannual basis without checking either voltage or SG. To keep it simple once the equalization process has started, charge time can be set depending on the battery bank and charger’s size, and how often the batteries are charged to 100%. The amount of time typically will range from two to five hours.

Daily Monitoring

If it’s your first time dealing with a battery-inverter system, it will take some time to understand the ebbs and flows of the system’s readings in a “normal” day of charging and discharging, and recognize if something is wrong. Checking the battery’s voltage and state of charge every day can provide a valuable orientation to the system—you’ll begin to understand what is (and isn’t) normal.

Monthly Maintenance Checklist

Using a monthly battery-inverter maintenance checklist can help you stay organized and help you pinpoint recurring issues with your battery system.

  • Battery/Inverter:
  • Clean corrosion from battery terminals and cable ends.
  • Clean tops of batteries.
  • Clean the inverter fan filter.
  • Check electrolyte levels, and add water if necessary.
  • Keep track of water additions—an increased need for water may indicate increased cycling or a failing battery.
  • Check to see if surge (lightning) protectors are intact.
  • Check inverter(s) for error and event indicators.

Battery Temperature Sensor­—Each month, visually inspect the battery temperature sensor (BTS), its cable, and its connection to the inverter:

  • Check the adhesion of the sensor on the battery case.
  • Verify that the sensor is in the proper place on the battery’s side.
  • Verify that the sensor is in an interior location in the pack or enclosure.
  • Check for breaks, nicks, or tears in the cable from the BTS to the point of connection at the inverter or charge controller.
  • Make sure the BTS cable’s inverter connection is not damaged or corroded, and that its protective insulation is still intact.
  • Verify that there’s no dust, dirt, corrosion, or insects in the BTS’s connector at the inverter.
  • Verify that the BTS is connected to the correct inverter.
  • Verify a reasonable battery temperature is displayed, based on ambient or battery temperature.

Common Battery Problems & Solutions

The battery bank has a mixture of strong and weak individual batteries, and is close to the end of its life. Group the strong batteries in one string and group the weak batteries in another string. This will ensure a more even charging and will allow the strong batteries to contribute more when the system is discharging.

One cell or battery has a low voltage and a low specific gravity. Remove that cell or battery and charge it separately using a power supply or battery charger. Another option is to equalize the entire bank, but this may require too much time and can put a lot of stress on the good cells in the battery.

A battery is requiring frequent watering. When there is a failing battery or cell, the “good” cells in the battery string will become overcharged and will often require more frequent watering compared to the rest of the battery. The damaged cells cause the other good cells in the battery to be overcharged. Investigate the entire battery bank to identify which cells are having problems.

One failed cell. If the battery bank is in good overall condition (verified with voltage, specific gravity, and load testing) and is less than two years old, replacing just the failed cell or battery can be a viable option. Be sure to fully charge the new cell or battery before adding it, and check all of the cells frequently to ensure that there is not a problem with the new cell or battery becoming out of balance with the others. In an older battery, a failed cell or battery indicates that the entire battery is at the end of its life and needs to be completely replaced.

Poor crimps on cable lugs or loose connections on battery terminals. Loose connections or poor crimps on cable lugs will cause high resistance. This causes the battery’s voltage to appear higher when recharging, resulting in the charger shutting off before the battery is at a 100% full level. It will also cause the voltage to appear lower when the battery is discharging, resulting in the inverter shutting off earlier than it should when running loads. Check that all the cable lugs are properly crimped and that all the connections are tight.


Carol Weis is a NABCEP-certified PV installer and ISPQ Master PV trainer. She writes curricula and teaches national and international PV classes to technicians and end users. She has worked as a licensed electrician and solar installer in Colorado, and was part of Solar Energy International’s PV technical team for 15 years.

Christopher Freitas is an engineer and project manager for international RE projects. He was a cofounder of OutBack Power Systems and was the director of engineering at Trace Engineering.


Comments (13)

Solveig Madsen's picture

This article is very helpful in terms of explaining how to clean battery terminals but I have a few additional questions. Am I correct in assuming that all charging and discharging needs to be eliminated from the battery bank before disconnecting the battery cables? How do I this? Do I need to disable the inverter or place it on standby? If so, will the inverter settings be saved or will I need to re-set everything (our inverter is Xantrex XW Series Hybrid).


cliffgrimes's picture

Yes, you should completely isolate the battery bank - disconnect it from the PV system and your inverter. You should have disconnect switches to allow you to do this.
I'm not familiar with your specific inverter but all the ones I've seen will keep the settings configured.

Solveig Madsen's picture

Thank you. I'll call Xantrex to double check on the settings.

cliffgrimes's picture

I have a battery bank where one of the batteries appears to have a bad cell. The SG is low and the voltage across the battery is only 4.2v, these are 6v Trojan L16s. The rest of the batteries are in good shape - SG is good and voltage levels are consistent. I am going to replace the bad battery with a new one, fully charged. My question is, after I do that is there anything I should do for the entire battery bank?

Michael Welch's picture

Have you tried a full, 8-hr. equalization on the battery bank as it is? The cell may be weak, but without occasional equalization it will steadily get worse and worse.

If you end up replacing the battery, use the bank a few cycles as a break-in then go ahead and perform a full equalization on the bank. That will bring the older batteries up as close as possible to the level of the new battery. But if your battery has multiple strings, the bank will never be better than the lowest-performing string, even with a new battery within one of the strings.

Marsha Robison's picture

I hear you Eric. But I am in a very harsh environment. Temps range from 0 in the Winter to 109 (sometimes) in the Summer. One day can have ranges of 70 degrees. My battery banks just don't preform as expected, even with diligent maintenance.
Please let me know your expert opinion on the Lithium as an alternative. I have been watching this technology but want to wait till it is perfected. Are there any other options.

Ed Mahoney's picture

I've supplied thousands of systems thoughout the world; many in extreme temperature environments. Your problem is not unique.
In high temperatures batteries will suffer extreme loss of useful life and in cold temperatures their capacity is diminished.
If the air temperature canot be controlled the best method of solving the high & low temperature extremes is to BURY the batteries in a container. Getting the batteries a couple of feet undergroung will minimize the temperature effects. You can reduce the battery's temperature by 25 F. in hot summer months and increase it by 25 F. in cold winter months.
In addition, use a battery temperature compensated charge regulator to optimize the charging voltage.
By doing this your battery will last at least 50% longer and help provide it's maximum capacity.
If you need further assistance please let me know.
Ed Mahoney

eric roberts's picture

Hi Just a point to Jim and Elaine stack? Lithium batteries have no history yet? we do not even know how to dispose of them yet, they can be highly toxic and a fire hazard, but i understand what you mean about, lead acid batteries, which are incidentally 95% recycled now. thanks eric roberts

Jim and Elaine Stack's picture

Use lithium batteries and you never need to water, they last much longer and are non hazardous. It's the 21st century and we have taken lead out of paint and even gasoline so don't use it.
Just mining it kills us. People say it gets recycles but that is a poor excuse since it gets in the air and everyplace. LEAD is one of the worst containments ever used.
Solar City used Tesla Lithium batteries for backup. Tesla is building a new battery factory that will make them 30% better or more in a few years. Invest in the future not the past.

Paul Hancock_2's picture

Great article!

I was recently moving my inverter and inadvertantly the pos and neg battery leads touched momentarily causing some sparking while they were on the ground. I know this is not good for the battery bank (I have 12 surrettee S-530s) but is there any specific way I can test the batteries to see if they sufferred any permanent damage?

Michael Welch's picture

Hi Paul. Just minor quick sparks? Should not have affected the batteries at all.

Paul Hancock_2's picture

Thanks for the reply Micheal. The sparking was just for a second or so but did melt a bit of the copper connector on the end of the cable. Batteries seem to be performing as usual but would you recommend any maintenance or testing I should be doing after this incident?

Thanks again.

Michael Welch's picture

Just go ahead and use your batteries. Rest easy, you have not harmed them.

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