Diagnosing Battery Problems Richard Perez If a battery is like a bucket, then a sick battery is like a bucket with holes in it. Batteries and buckets share some common characteristics. When they are full they will hold no more. When they are empty, they will deliver no more. Both batteries and buckets can leak. In the case of a bucket, we get a puddle on the floor. In the case of a battery we get a diminished capacity to store electric power. Here's how to find out if your battery stores as much energy as it used to. Different Batteries, Similar Diagnostics This article deals with diagnosing capacity loss in both acid and alkaline electrochemical cells. Lead-acid cells and alkaline cells have radically different chemical reactions. The causes and cures of lost cell electrical capacity are different between the two types of cells. However, this method of diagnosing which cells are sick, and how badly they are affected, is just the same for both lead-acids, nicads, and nickel-iron cells. This diagnostic technique uses electrical measurements to identify which cells have lost capacity and quantifies the cell's loss in capacity. If you don't know the battery basics of batteries such as capacity, voltage, and suchlike, please read the article entitled Battery Basics in Home Power #27, page 30. Don't Panic Most users who suspect that their battery has lost capacity, in fact, have nothing to worry about. This perception, which commonly occurs to PV users during the winter months, is untrue. A battery is like a bucket. If you don't put it in, then you can't get it out. Most folks who think that their battery has lost capacity, really just need to give it a full and utter recharging. The Diagnostic Procedure This procedure is simply recharging and discharging the battery (or better yet each cell individually) and making measurements of its performance. Just fill it up until you are really sure it is full and measure how much you can get out of it. There is a big difference between running this diagnostic procedure on a series connected string of cells (a battery) and running the procedure on a single cell. The capacity of a series string of cells (a battery) is limited to the capacity of the weakest cell. Consider this example. A 12 Volt lead-acid battery is composed of six series connected 100 Ampere-hour cells. One of the six cells has lost capacity and now holds 50 Ampere-hours. The entire battery will have a capacity of 50 Ampere-hours, eventhough five of the six cells still contain 100 Ampere-hours. If this procedure is applied to a battery (a series connected string of cells), then it will yield the capacity of the weakest cell in the string. If the procedure is applied to each cell, then you will know exactly which cells are good and which are not. Before beginning this procedure give each cell in the battery a name or number. Get a clipboard and get ready to record all the measurements you will make. If you have a battery Ampere-hour meter (like the Cruising Equip. models), then you already have an instrument that will be of great aid in diagnosing lost capacity. You will need an accurate voltmeter like the Fluke, Beckman, or even a Radio Shack digital multimeter (DMM). If you are making your own power and operating a battery, you should have a DMM. If you don't have one, then now is the time to buy one because you are going to need it. First Really Fill Up the Bucket Recharge the battery or cell. Since most perceived capacity loss is actually chronic undercharging, first examine your charging system. Number one on the hit parade is any regulator or charge controller. If the voltage cutoff or regulation point of the regulator is set too low, then the battery is not being fully recharged. Check your regulators. Set the regulator for 1.6 VDC for each series connected cell in an alkaline system. This means 16 VDC for a ten series cell nicad or nickel-iron battery. In lead-acid systems, set the regulator at 2.66 VDC per series connected lead-acid cell. This means 16 VDC for six lead-acid cells in series. This information applies PV, wind, and hydro systems using either series or shunt regulators. This level of voltage regulation is higher than the system normally operates. We are setting the regulation points higher to make absolutely sure that the cells are being fully and equally recharged. We accomplish this by giving all cells a controlled overcharge. This insures that each and every cell is totally recharged. How do we know when the battery or cell is fully recharged? Just look within the cell. Is it gassing furiously? Is the electrolyte alive with millions of bubbles bursting on its surface? Sustained, heavy gassing is a sure sign that the cell is as fully recharged as it is going to get. Measure the voltage each cell that makes up the series connected pack. Write this data down next to the cell's number on the clipboard. While still undergoing recharging, the voltage difference between the highest and lowest cell should be no more than 0.1 VDC. If continual recharging does not bring the cells within this voltage spread, then the lowest cells have problems and should be singled our for treatment. If you have an Ampere-hour meter, then overcharge the battery to between 125% to 150% of its rated capacity. This equalizing recharge is a radical procedure. The cells will gas violently and will require distilled water replacement AFTER the equalizing charge. The cells will get warm to the touch. This is therapy for lead-acid cells and not recommend as a steady recharging diet. We are performing this equalizing charge because the battery is suspected of having lost capacity. In my experience, lead-acid cells (except sealed cells) love a regular equalizing charge, so do it every five deep cycles or every three months. Alkaline cells generally require no equalization if the regulator is set at 1.6 VDC per series cell. We operate our nicads at 1.61 VDC regulation per cell and their voltages seldom diverge. If you are regulating your alkalines lower than 1.6 VDC per cell to accommodate your inverter or DC loads, then equalize your nicad or nickel-iron cells every six months. Charging each cell individually is tedious, but insures that each cell is totally full while not unnecessarily overcharging the other cells. We have used single ARCO M52 PV laminates to recharge single cells. The M52 (better know as one quarter of a QuadLam) produces over six Amperes at 2 to 3 VDC. This power is ideal for recharging a single lead-acid or alkaline cell. Use the same methods to determine cell fullness as with a battery. Look for violent gassing, high voltage (1.6+ for alkalines and 2.6+ for a lead-acid cell), and 125% to 150% overcharge if you use an Ampere-hour meter. Then Really Empty the Bucket The next step in the procedure is to discharge the battery or cell and measure number of Ampere-hours that it delivers. This is simple to determine if you have an Ampere-hour meter. Discharge cutoff voltage for a lead-acid cell is 1.8 VDC. Discharge cutoff voltage for an alkaline cell is 1.1 VDC. Discharge the battery until it reaches 1.8 VDC per cell for lead-acid cells or 1.1 VDC per cell for alkaline cells. Then read the number of Ampere-hours removed from the battery on the Ampere-hour meter. Manufacturer's Ampere-hour ratings are usually based on a discharge cutoff voltage of 1.75 VDC for lead-acids and 1.0 for alkaline cells. I use a higher discharge cutoff voltage because it better represents our battery usage in home power systems. If you don't have an Ampere-hour meter, then discharge the battery or cell at a constant rate, measure this discharge rate with an ammeter, and multiply this rate by the number of hours it takes the battery (or cell) to reach the discharge cutoff voltage. This results in the number of Ampere-hours removed from the battery Ð its capacity. A Fluke 87 DMM, in record mode, measuring discharge current through a shunt is an excellent way of measuring Ampere-hour capacity. Simply multiply the average current reading of the DMM by the duration of the discharge in hours. When the battery reaches discharge cutoff voltage, measure the voltage of each individual cell, while still under discharge, and record the data. Any weak cells will have much lower voltage by several tenths, or more, of a volt less than the other cells at this point. These are weak cells which are limiting the performance of the remainder of the cells in the battery. Lead-acid cells don't like being fully discharged. This is an experiment to diagnose suspected capacity loss, not a regular or recommended way of cycling lead-acid cells. Perform this procedure on your lead-acid battery only if you strongly suspect capacity loss. Never do this to your lead-acid battery for idle curiosity. Nicads however enjoy a good deep cycle, and occasional use of this test will do the alkaline cells no harm. Emptying Single Cells Discharging single cells can be a problem because it is hard to find a suitable load. We use a coil of 14 gauge insulated copper wire as a load resistor. Consult a copper wire table and figure out a proper length of 14 gauge or smaller wire. Size this wire so that its resistance allows a C/10 rate of discharge for the cell. Wire of 14 gauge or smaller will get warm, but we've never come close to melting its insulation. It makes a cheap, handy, and user programmable load resistor. Discharging a single cells give rock solid data. Instead of a weak cell skewing the data for an entire string, we have real data from each cell. This method allows grouping weak cells into a single series string where they will do the least harm and be most effective. What's next? After doing at least one of these procedures, you will have a fair idea if your battery is delivering its rated capacity. Chances are that you will find that the equalizing charge totally refilled the battery for the first time in months. I often do this procedure several times and find that the repeated equalization charges bring back some of the lost capacity (especially in alkaline cells). If you discovered a weak cell or cells, then tune in next issue when we'll run information about rejuvenating weak lead-acid or alkaline cells. If you haven't discovered any weak cells, then great! You have still accomplished several things. You have investigated your charging system and made sure that the regulators are properly set. You have equalized the battery. And maybe most importantly, you now have recorded data on the performance of your cells. Next winter, during the deep dark days when those niggling capacity loss feelings come again, you will have solid info about your battery. This info and the experience you gained by using this procedure will make it far easier to determine if all is well with your battery. Access Author: Richard Perez, c/o Home Power, POB 130, Hornbrook, CA 96044 ¥ 916-475-3179