Prior to Seegrid, Weiss worked for Brooks Automation as the chief technology officer. Weiss came to Brooks after the strategic merger of Brooks with PRI Automation. At PRI, he served as chief technology officer and vice president of strategy/technology, overseeing growth from $20M to $450M in annual revenues between 1994 and 2002. Before these roles at PRI and Brooks, he was president and founder of ProgramMation Inc., as well as co-founder and chief engineer of United States Robots.
Weiss is the co-author of Industrial Robotics: Technology, Programming and Applications. He holds 20 patents and earned several academic and industry awards, including being named to Industry Week's Top 50 R&D Professionals.
Scotty, I need more power!
One of the most overlooked aspects of operating automated vehicles is their batteries. We operate and maintain our vehicles with great care, but we really don't pay attention to the batteries until something goes wrong. In fact, wed on't even suspect a problem with the batteries when we see performance issues because we really don't understand their operation. In this post I will review the operation of batteries and what to do to keep them working at their peak. In subsequent posts I will provide more details on battery care.
We are so familiar with batteries in our everyday lives we think we know what to expect of them and how they work. Which one of us hasn't tried to coax a little extra run time out of a flashlight or cell phone? The same thing actually happens with manually operated forklifts and industrial trucks. Drivers are interested in getting as much work done during their shift as possible, even if that means operating their forklift when the battery is run down. Rather than go to the charging station to swap batteries, they will drive the truck even if the vehicle is "struggling". Something we see often is a "wild west" at battery charging stations. Drivers will limp in with their underperforming vehicles and cruise the charging lane looking for a battery that is most fully charged. Then they will swap out for the fresh battery, even if it is not fully charged or equalized. Of course this shortens the life of the batteries, but no one will notice, because all the trucks still run as normal-the drivers are compensating for, and contributing to, the (bad) battery performance.
So what happens if you are running with weak or damaged batteries? Well, first and foremost, productivity is affected. Flexible AGVs will declare errors, or worse, refuse to run. When batteries are weak, the voltage drops. This means that to do the same work, motors and wiring must carry more current. This results in higher heat, affecting the life of the motor wiring, the bearings, the drive and control circuits, and the batteries themselves. Heat is the enemy of electrical devices and batteries.
Happily, batteries are just like people. If you don't let them get too hot, or too cold, or too hungry, and you let them get a good night's sleep, they will perform at their peak every day.
What is a battery?
A battery is a collection of cells. Each cell contains a mixture of chemicals and materials that are able to store a charge, deliver a charge, or to accept a charge. Cells of similar materials and size are capable of storing the same amount of charged energy. Typically, a battery is composed of similar cells wired in series. There are many types of batteries (classified by their chemistry or materials) like the disposables you use in a flashlight, and the rechargeables you use in your cell phone or your AGV. Disposables are called primary cells, and the two most common types are carbon-zinc and alkalines. Rechargeable batteries (cells) are called secondary cells, and the common ones are nickel-cadmium, nickel metal-hydride, lithium-ion and lead acid. Lead acid batteries are the type we use in forklifts, AGVs, and as starter batteries in our cars. The battery (electro voltaic cell) was invented by Alessandro Volta (get it?) in 1800, and one of the earliest improvements on his invention was the rechargeable lead acid battery in 1859 by Gaston PlantÌ©. What I will talk about from here on out, is the rechargeable lead acid battery we use in AGVs and forklifts.
How is a battery constructed, and how does it work?
A cell is composed of a jar (the housing of the cell), typically made of plastic, housing the positive and negative plates, which are immersed in acid.
The plates are made of porous lead, and the acid is a sulfuric acid solution called the electrolyte. The plates are separated from each other, and supported to allow the acid solution to surround them. Each jar has two terminals, one connected to the positive plates, and one connected to the negative plates. There is also a vent cap, which is used for maintaining the acid solution level in the jar, and to allow any gas pressure to escape.
When charging or discharging, a chemical reaction between the lead and the acid takes place. During charging, the anode (positive plate) converts from lead sulfate to lead peroxide, the cathode changes from lead sulfate to lead, and the sulfuric acid concentration in the acid is increased. When discharging, the reverse reaction occurs, giving up electrons, which flow through the wires powering the truck. This chemical reaction is illustrated above.
In the case of a lead-acid cell, the voltage of a fully charged cell is about 2.041 Volts.
The capacity of a cell is a measure of how much energy the cell can provide over time. This is usually measured in "Ampere hours" or A-hrs. If you have 5 A-hrs of capacity, you could theoretically run something at a 1 Amp rate for 5 hours, or at a 5 Amp rate for 1 hour. The capacity of a cell is determined by the area of the plates that are in contact with the electrolyte (the battery acid) available for storing charge onto. If the plates are smaller, the capacity is lower. If the plates are damaged the capacity is lower. If the electrolyte is not fully covering the plates, the capacity is lower. Remembering that a battery is actually made up from a set of cells wired in series (in the case of a 24V battery, 12 cells), the nominal voltage of the battery is the number of cells*2, but the capacity of the battery is the capacity of the weakest cell. As soon as the weakest cell is depleted, the battery voltage will drop below an acceptable level.
The reported capacity of a cell is determined by the manufacture of the cell. In fact, the actual energy storage capacity of a cell is not precisely known. Think of the gas gauge on your car. Assuming the volume of the fuel tank is well known, the needle on the gauge only approximately tells you what percentage of fuel is left in the tank. It doesn't really tell you how many gallons are left, for example if there is sludge on the bottom, or your prankster neighbor dropped rocks into the tank. It only tells you what percentage of what the tank can hold is available. It certainly doesn't tell you how far you can go, just how much of the tank is left, more or less.
Well, unfortunately, measuring the remaining capacity of a cell is extremely difficult. The capacity of a battery is even harder to measure, as each cell is slightly different. There are two ways to measure the approximate state of charge: check the specific gravity (density) of the electrolyte, or check the voltage of the cell. Checking the electrolyte requires opening the cell and removing a sample of the fluid to measure. Clearly this is not practical while the vehicle is running, so the typical approach is to measure the voltage. Now it gets really depressing. The voltage of a cell is a function of the cells remaining capacity AND its temperature, the rate at which the charge is being consumed and its specific construction.
Below is a chart that shows the voltage of a cell at different discharge rates and at different states of charge. Notice that the lines are curved, not straight, telling us two things. You can't measure the exact remaining capacity from the voltage, and at some point, the voltage of the cell drops off very quickly.
The current that the vehicle is using depends on the acceleration rate and the payload. At rest the vehicle may only be drawing about 5 or 10 Amps. Under full load and during acceleration we can see currents as high as 300-400 Amps. So just measuring the voltage doesn't really tell us how much capacity is left. As a result of these complications, the battery capacity displayed on the vehicle is only an estimate, and more importantly it is an estimate assuming the battery is operating as designed.
Photo courtsey of batteryuniveristy.com
Measuring the capacity or remaining charge of the whole battery is even harder, since the battery is made of 12 cells. If even one cell is performing badly, the whole battery looks defective, or appears to have less capacity than expected.
Captain, I'm givin' ya' all she's got
So how do we get the performance we want from the batteries we have and how do we keep on getting it? As we have seen, the performance of the battery is dependent on a number of things that affect its capacity. Ultimately it comes down to making sure that the plates stay clean and fully covered by electrolyte, and that the batteries are charged properly. Remember, as the batteries discharge, lead sulfate builds up on the plates, and if not properly charged, the lead sulfate remains on the plates, preventing them from performing properly. As the batteries are used, some of the electrolyte is consumed, or evaporates out. If the electrolyte falls below the top of the plates, they will not be able to be fully charged. The two most important aspects of battery maintenance are proper charging, and maintaining the electrolyte levels. Another important maintenance function is to keep the battery clean. Spills and dirt can conduct charge from the battery posts to the case of the battery or to other cells. These shorts can reduce capacity and are potentially dangerous.
Properly charging a battery requires three important steps: full charging, trickle charging, and resting. Not allowing the charge to finish, and not allowing the battery to cool down will result in some of the cells not being fully charged, meaning the cells will have less capacity available after each charge. Eventually, one of the cells will be so much worse that the others, the battery will underperform, and the vehicle battery gauge will over report the remaining charge at rest, and under report it during hard use.
Here are the most important things to do to keep your batteries, and your AGVs working at maximum performance.
1) Keep the electrolyte in the cells at the proper level.
2) Keep the top and outside of the battery clean.
3) Check the battery for damage and leaks.
4) Check all the battery connections for wear and damage.
5) Use the right type and size of charger for the battery.
6) Charge batteries fully every day.
7) Make sure to equalize the batteries at least once a month.
8) Allow the batteries to rest after each charge cycle
9) Maintain records on battery use and charging. Watch for reductions in capacity over time.