Large Motors Class
Lesson 2: Big Batteries
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Introduction: Big Batteries

Picture of Big Batteries

When dealing with large electric motors, you will almost assuredly be powering them with batteries. It is important to understand a little bit about selecting the right battery before we moved forward. Not all batteries are created equal, and systems such as these requires a special type of battery to really do it right. Additionally, working with large batteries requires special considerations that you may not encounter with smaller power sources.

There are three characteristics to look for in a large battery.

The first and most important thing to look for in a battery is to find one that is deep cycle. This means that it is designed to be deeply discharged and recharged repeatedly. Most automotive (car or motorcycle) batteries are considered "starter" batteries and are designed to briefly discharge a short high current burst, but otherwise remain charged. Using an automotive battery for constantly powering a motor is going to quickly destroy the battery as it is repeatedly drained and recharged.

The next thing you want to look for is a battery that is sealed. This is important because it means that you can tip the battery sideways or mount it upside down without worrying about it leaking caustic materials.

Finally, you would want to try to find a battery that is gel cell. Many large sealed batteries will be gel cell, but do not necessarily take this for granted. There are other battery chemistries out there such as SLA (sealed lead acid) and LiFePo4 (lithium iron phosphate) that require different charging and safety handling considerations.

Nevertheless, the benefit of a gel cell battery is that the acid has been absorbed into a silica gel so it doesn't corrode, off-gas, leak chemicals, or require maintenance. It is also more ruggedly built to absorb shocks, and less likely to leak dramatically if it were to accidentally get punctured. In this way, a gel cell battery is relatively "safer" than other battery types.

Once you have found a deep cycle, sealed, gel cell battery, the next order of business is figuring out how large of battery you need. Batteries are rated in Ampere Hours (Ah). This rating is the maximum number of amperes (or "amps," for short) that your battery can provide in one hour.

Let's say that your battery can provide 100Ah and the motor you are running requires 5 amps. You would be able to run that motor for approximately 20 hours off of that battery. In this case, it would be drawing 5 amps per hour.

On the other hand, let's say you have the same motor, but now only have a 20Ah battery. In this case, you would be able to run the motor for approximately 4 hours before the battery was dead.

You can calculate this by dividing the Ah rating of the battery by the number of amps required by your circuit. The resulting number is how many hours your circuit can theoretically run for off of the battery.

Of course, even if a motor is rated for 5A, it will likely be drawing significantly less under most normal operating conditions (assuming it is not constantly running at or near its stall current). Thus a motor rated for 5A might actually run for longer than 4 hours off a 20Ah battery. Also, it is worth noting that as the battery discharges, the voltage drops, and efficiency decreases, causing the motor to potentially slow down.

Due to the fact that batteries can discharge a lot of current very quickly, it is important to get wires capable of handling both large amounts of current and heat passing through it.

When working with hobby electronics, a typical wire gauge that you may work with is in the 16 to 24 gauge range. When working with large motors, you are working in the 00 to 14 gauge range. For the 300 amp Alltrax motor controller that we are going to use in the next lesson, we use 4 AWG welding cable. This wire is recommended by the retailer for 300 - 400 amp controllers. Since the motor we are controlling has a maximum current of 98 amps, this should give us plenty of headway.

Determining wire size is a little bit tricky and is dependent upon current transmission, temperature, distance and the cable's manufacturing specs. Welding cable (aside from being flexible) is designed to handle high current and high heat over distance. This makes 4 AWG welding cable potentially more capable of handling high loads than other types of 4 AWG cable. It is important to check the rating of the wire you are buying, and when in doubt, get higher rated wire. It never hurts to overshoot.

When dealing with such thick cable, you can't just wire it like you would when working with 22 AWG hookup wire.

To begin, you need a pair of heavy duty diagonal cutting pliers to cut it to length. This may even require 'chomping' down on the wire two or three times to get all the way through.

Once the cable is cut length, the next order of business is to connect it to something. It is far too thick to be effectively soldered to anything. Instead, this requires using wire lug ring terminals as a form of mechanical attachment.

Using a razor blade, carefully cut around the circumference of the insulation about 1" (or so) from one of the edges. Once the insulation has been fully cut, carefully pull it off of the inner copper core. Repeat this process for the other end of the cable.

Attaching the ring terminals to the cable simply requires a hammer and a really hard surface such as an anvil, concrete floor, or hard metal surface. I have neither an anvil nor concrete floor here, so I am using a thick block of aluminum as my striking surface.

To make the attachment, I simply slide the ring terminal onto my cable, lay it flat on my striking surface, and bash it with a hammer until the ring terminal is securely "grabbing" onto the cable. When you tug on it with a reasonable force (think monkey force - not gorilla), it shouldn't come free.

Everything mentioned thus far is in relation to a single 12V battery. However, most serious motors require 24V to 72V power for peak efficiency.

In order to get more than 12V, we simply need to wire identical batteries in series. This means that we connect the positive terminal from one battery to ground on the next. The remaining free positive terminal becomes the 24V terminal. The remaining ground terminal is simply just ground.

It is important that the batteries be identical or strange things might start happening where the bigger one recharges the smaller, and/or they discharge at different rates and one corrupts. It is even potentially unsafe to connect together two power sources dramatically different in size, and could result in fire.

Assuming all of the batteries are of the same make and model, each additional battery wired in series increases the voltage by 12V. For instance, if you have two batteries wired together, and then connect a third, you have just increased the value of the power source from 24V to 36V. In this way, it is easy to achieve a high voltage and high current power supply.

When multiple batteries are wired in series in this manner it is considered a battery bank.

Charging a large battery requires a certain set of considerations such as the chemistry of the battery, and the rate at which it charges. Batteries with different chemistries need to be charged using different charging methods.

Most high end battery chargers can accommodate multiple different types of battery types. Some need to be manually set to match your battery, and others can even automatically detect the type of battery being charged.

The rate at which a battery charges is determined by the output current of the charger. For instance, if you have a 1A charger and a 20Ah battery, it will take 20 hours to charge. On the other hand, if you have a 20A charger, and 200Ah deep cycle battery, it will take approximately 10 hours to charge it up from full discharge.

Charging battery banks with two or more batteries requires a balancing charger suitable for the number of batteries that you are charging. For instance, to charge two batteries you need a dual bank battery charger rated for 12/24V. Additionally, you need to confirm that the charger is suited for charging the battery chemistry of your particular batteries.

Now that we have power requirements out of the way, let's move on to the next lesson where we will power up and control the speed of a motor.


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