Michael Broderick  – Contributing Author

When was the last time you thought about your aircraft’s main battery? If you are like me, you most likely thought about it when you had to remove it either because it failed or due to required normal maintenance. In either case, you sent it to that mysterious place called the “battery shop.” Ever wonder what happens to the battery in the “battery shop?”

Wonder no more. Today as we venture into the world of battery maintenance, we will find out what shocking things those battery wizards do behind closed doors. Also along the way, we will come across what I refer to as Cocktail Knowledge or CK. CK is that arcane knowledge that you will not find in most technical manuals and is meant to be shared for the purpose of impressing the uninitiated or impressionable. Sharing CK may reap you unimaginable rewards and/or accolades in the right circumstances. You can rest assured that I will always identify this type of information as we come across it.

One more item before we get started. I must give credit where credit is due. I elicited a great deal of assistance from the best of the best in the world of aircraft battery maintenance, and thus they need to be acknowledged. Scott Marvel, president of Marvel Aero International Inc., provided invaluable maintenance material to me for this article. Also a big thank you to Skip Koss, vice-president of marketing for Concorde Battery Corporation for his technical help on this piece.

Let’s get started and discuss the care and feeding of the vented Ni-Cad, vented (flooded) lead-acid, and valve-regulated lead-acid batteries, as these are the batteries you will most likely see in your daily activities. Grab a cup of coffee and we’ll begin today’s session.

Maintenance … Why?

Why do maintenance at all? The simple answer is that we do maintenance to preserve the usefulness and performance of the battery. Like other aircraft components, the battery needs to be inspected regularly to assure it will function and perform as expected when called upon. If we think about it, in all aircraft electrical systems the battery is the last link in the aircraft electrical emergency chain as well as the first link in the electrical system for getting the engines started. 

Who set the standard for the aircraft main battery’s “performance?” (CK alert: The FAA via FAR23.1353 and Technical Standing Order (TSO)-C173.) Cutting through all the regulatory semantics, these documents state that the battery must be able to produce at least 30 minutes of electrical power to those loads essential for continued safe flight and landing. We can see that a failed battery can be a simple inconvenience of missing a flight, or be the cause of a turbine engine hot start, or even as serious as the following from a very distressed pilot: “Ahhh, ATC? Hopefully you can hear this transmission. We are dealing with an electrical failure …”

I see there is a question about using ground power assist for starts to help preserve the battery performance. Yes, that does extend the battery life; however, it also can give a false sense of security. If we don’t do the recommended battery maintenance while continually using ground power assist for starts, we will not notice the battery losing performance, and if we need it during flight and it fails …we just added some excitement to our pilot’s day. In all cases, follow the battery manufacturer’s recommendations regarding maintenance. (You will see me repeat this.)

Battery Maintenance Basics

Let’s begin with some safety tips we should follow around the battery shop: remove your rings, watch and any bracelets. Always wear a face shield, protective apron and gloves that are impervious to electrolyte contact.

As a reminder, Ni-Cad battery and vented lead-acid battery maintenance should not be attempted in the same room. The reason is the chemical differences in the electrolytes between the battery types. The Ni-Cad uses a potassium hydroxide (KOH)/water solution and the lead-acid battery uses sulfuric acid (H2SO4)/water solution. The problem is that if the two chemicals become mixed they will neutralize each other. Therefore intermingling tools, or even servicing the batteries in the same room, gives us a good chance for electrolyte cross-contamination, thereby chemically neutralizing each battery and rendering them useless for service. This potential for cross-contamination is somewhat reduced when servicing Ni-Cad and sealed lead-acid batteries, but all battery shops should keep them separate as well, just to be cautious.

Ni-Cad Batteries

Now that we have the basics, let’s start our discussion with the type of maintenance required for a Ni-Cad battery. On page 10 we have what could be considered a typical Ni-Cad battery for an aircraft application. Next to our assembled battery is a cutaway of an individual Ni-Cad cell. A Ni-Cad has a periodical check, a regular check and a general overhaul.

Periodical check: The periodical check should be performed based upon the flight time, start (discharge)/generator recharge cycle and age of the battery. In other words, this maintenance event is based largely upon your aircraft’s particular flight profile and the manufacturer’s recommendations. A periodical check typically consists of a voltage check and visual inspection of the overall external and internal condition of the battery. Using a volt meter, we are looking for excessive voltage differences (0.25 volts or more) between cells, while visually we are looking for electrolyte residue and bulging battery cells. We are looking at the condition of the temp sensor, as well as corroded inter-cell hardware, and we are verifying that the vents are clear. If all is OK, note the voltage readings on the worksheet, along with the aircraft time, and then carry on with the rest of the aircraft maintenance.

Regular check: Suppose the voltage check from the periodical check shows a great disparity in cell voltages, but the battery looks OK visually. The battery should be removed from the aircraft and put on a constant current top charge at the C/10 rate until all the cells have reached at least 1.55V each, and for the time specified in the maintenance instructions. This is when we adjust the electrolyte in each cell by adding distilled water as required. (CK alert: C/10 means 10 times the rated battery capacity which is stated on the battery data plate.)

During this part of the charge cycle, cell voltages may peak and then slowly start to drop. This is an indication of possible cell internal gas barrier damage. Additionally, no cell should rise above 1.75V. This may indicate a dry cell (very low electrolyte level). This maintenance charging is done with the vent caps loosened or open. As the cells build up pressure they can vent and spray electrolyte. Also, a clogged vent might increase the pressure in the cell, possibly damaging it. With the caps loose or off, it makes it easier to adjust the water level before the end of the top charge cycle while the charge current is still on. This is also the time to check and clean the vents in the caps. As soon as we have “watered” our cells and have completed the top charge, we reinstall the caps because carbon dioxide dissolved from the outside air carbonates the cells and can degrade the battery.

Now that the battery is fully charged and the electrolyte adjusted, it’s time to perform the capacity check to determine if the battery meets the minimum capacity requirements stated in its component maintenance manual. The minimum capacity can vary and is normally from 85 to 100 percent of the nameplate rating. The capacity check consists of a constant current discharge at the one-hour rate (amp/hour rating of the battery) to 1.00V per cell. We record the time the first cell reaches 1.00V. This time must be equal to or greater than 51 minutes for 85-percent minimum capacity, or 60 minutes for 100-percent capacity. If the battery passes the capacity check, it should be allowed to cool back to room temperature and then be recharged in accordance with the component maintenance manual. The recharge consists of a main charge followed by a top charge. If the battery fails the capacity check, then it should be deep cycled as performed during the general overhaul.

General overhaul: If we have found defective cells during the regular check, or if the battery has failed the capacity check, or the overall visual condition of the battery indicates that it is time for some major cleaning and inspection, it’s time for a general overhaul.

First, we do the top charge, then service the water and perform a capacity check at the 1C rate. Then, “clip off” the individual cells with 1-ohm resistors (when they have reached 1.00V during discharge) or shorting clips (when the cell falls to 0.5V). By “clipping off” we are connecting the positive and negative poles of each cell. This stops the discharge event within that particular cell while allowing the other cells to continue discharging. If we do not do this, the cell voltage will reverse polarity, which can permanently damage the cell.

Once all of the cells have reached 0V, they should be allowed to sit for 24 hours and cool down. The reason we want to totally discharge each cell is so that when the battery is charged again, the cells will have a more balanced final capacity. If the battery needs an overall cleaning and hardware change, now is the time to do it.

During this general overhaul is also a good time to replace any defective cells and inspect and test the battery temp monitoring system, replacing it if damage is noted, or if it does not pass an electrical continuity check or over-temp verifying test. After the battery has “rested” for 24 hours, we proceed with the main charge and top charge cycle as previously described. By and large, we can replace the weak cells individually within the Ni-Cad battery, which is a good thing. However, if we need to replace more than five cells in a 19- or 20-cell battery, it’s time to replace the battery. Why? The remaining cells, although acceptable, are “weaker” by comparison, which will mean the new cells will be doing most of the work and will soon join the rest of the group.


Ni-Cads should be stored in a discharged state. That is zero detectable volts. The industry term is “strapped off.” The battery can then be safely stored in a cool, dry environment for extended periods of time. Always follow the manufacturer’s directions.


These batteries are constructed of nickel and cadmium with a potassium hydroxide solution which are all considered hazardous materials (HAZMAT) and must be disposed of using the battery manufacturer’s recommendations as a guide and of course, abiding by any local environmental laws. 

Vented (Flooded) Lead-Acid Battery Maintenance

The lead-acid battery doesn’t have an overhaul event, but we still do a visual inspection and then a measurement of the electrolyte specific gravity, as well as a voltage check.

A visual inspection is carried out by looking for corrosion, a cracked battery case, leaking electrolyte, the electrolyte level and overall condition of the battery. Should the electrolyte level be low (no visible fluid above the battery plates), we add distilled water. Now, depending upon the conditions found at the visual, the age of the battery and duty cycle, we proceed to the specific gravity check. We can clean the connections and surrounding area with a solution of 50 percent baking soda in water. If the battery is dirty and has been installed in the helicopter for about 12 months, the electrolyte level is low, and the helicopter is in the shop for a normal maintenance cycle, well, what the heck — let’s do a specific gravity and voltage check.

Remove the battery from the helicopter, clean it and then charge the battery fully with the shop’s constant current battery charger as per the maintenance manual instructions. Then let the battery sit for about 10-12 hours. Then remove the surface charge by subjecting it to a steady discharge at the 1C rate for about 30 to 40 minutes. Now we are ready for the test, but remember to recharge the battery immediately, as leaving a lead-acid battery in a discharged state will damage the battery. The remaining battery life and overall condition of a lead acid battery can be determined by measuring the specific gravity of the electrolyte.

We can dig up our temperature compensating hydrometer and a digital volt meter. Why a temperature compensating hydrometer? Temperature has an effect on the electrolyte and we need to compensate for it to get an accurate reading. By using the hydrometer, we are measuring the amount of sulfuric acid in the electrolyte. If our reading is low, it means that the chemistry of the electrolyte is off and thus electron production will be lacking, resulting in no power. Where did the sulfur go? It’s resting on the battery plates, and when we recharge the battery the sulfur returns to the electrolyte. The chart on page 13 can be used as reference. The hydrometer readings should not vary more than .05 between cells. The voltage should read about what the readings are in the reference guide. If we get a voltage reading of 21.6 on a charged battery, that typically indicates a shorted cell, and it is time to visit our local battery vendor.


Most of the time these batteries are stored in a “dry” condition with the electrolyte in a separate container. Shelf life is not normally a big issue as long as the batteries are stored properly. When we get the battery we add the electrolyte and follow the manufacturer’s recommendations on preparing the battery for installation.


These batteries contain lead and sulfuric acid, and like their Ni-Cad brethren, must be disposed of in accordance with the manufacturers’ recommendations and local laws and regulations.

Valve-Regulated (VR) Lead-Acid Maintenance

The VR battery is not maintained like your typical flooded-cell lead-acid battery. Following my own advice, I consulted with the major manufacturer of this type of battery. The inspection and maintenance requirements are broken down into scheduled inspections, nonscheduled inspections, troubleshooting, servicing discharged batteries, repair/replacement, storage limitations and disposal. Because of the design of the VR battery, it may be serviced with or next to Ni-Cads because the VR battery is sealed, and thus should prevent cross-contamination of the electrolytes. However, although in theory it should be OK, you won’t see me mixing these guys during a maintenance event in my battery shop. With that being said, let’s discuss how we should maintain the VR lead-acid battery.

Scheduled Inspections

In this section, the manufacturer gives us detailed inspection intervals and capacity tests based upon battery usage. For example, if the battery is used to start turbine engines (including a turbine APU,) the initial capacity check should be performed at 12 months or 600 hours of operation, whichever occurs first.


Subsequent checks are at three-month or 200-hour intervals.

Non-Scheduled Inspections

Perform a capacity check if the turbine engine is starting slowly, or if an abnormally high charging current is required to maintain the battery at bus voltage.

Inspecting the battery

First we charge the battery (using the recommended constant potential type charger) at 28.2V for a 24V battery until the charge current stabilizes for one hour. Then we perform the capacity test. The battery temperature must be stabilized at 59 degrees or higher for at least 24 hours. Discharge the battery at the 1C rate on the label to an end point voltage of 20V for a 24V battery. Record the end point voltage. The battery may be returned to service after a recharge if the ampere-hour capacity (hours of discharge X ampere rate of discharge) is 85 percent (or greater) of the nominal rated capacity. If it is less than this, recharge and try the capacity test again. If it fails a second time, replace the battery.


1. Low or no voltage which may be a partially or fully discharged battery. This condition is corrected by a recharge as previously described.

2. Battery does not hold a charge. The battery is beyond serviceable life and it must be replaced.

3. Battery gets hot while recharging. The battery is beyond serviceable life and it must be replaced.

Servicing Discharged Batteries

This section reiterates the type of charging that must be followed, as well as the recommended charging equipment to be used to accomplish this work.


Only a manufacturer-approved battery shop should perform repairs. It is recommended that the battery be replaced after three years or 1,800 hours of operation, whichever occurs first.

Storage Limitations

The batteries are serviced and charged at the factory prior to shipment. However, if they are going to be stored when they arrive at your facility, they should be stored in a cool, dry place and the open circuit voltage checked periodically. If it falls below 25V (26V is normal for a fully-charged battery) it must be boost charged, which is the same procedure as previously described. Of course, if batteries are allowed to sit for long periods of time, then they must be reconditioned by charging and temperature stabilizing, and then capacity tested.


The VR batteries contain lead and sulfuric acid and must be disposed of in accordance with local laws and regulations.

Had enough for today? Me too. Time to pull pitch and head for home. Hopefully you enjoyed reading this article and I hope you now know a little more about the care and feeding of these phenomenon’s of chemical alchemy.

Thanks again to Koss and Marvel for their assistance. 


MIKE BRODERICK is the vice president of business development at Helicopter Engine Repair Overhaul Services Inc.(H.E.R.O.S. Inc). During the past 30 years he has served as a shop technician, engine shop supervisor, engine program director, director of maintenance, director of operations, and owner of a Rolls-Royce engine overhaul and MD helicopter component overhaul shop. He is a certified A&P and holds a bachelor’s degree in aviation administration. Broderick has been appointed as an FAA representative for the FAA Safety Team (FAAST) and is also a member of the Helicopter Association International (HAI) Technical Committee.