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Electricity 101 Question

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Well-known member
Nov 26, 2001
Ok, so I didn't pay attention during class when they talked about electricity and this is the first time i've had this problem, so I need some help... in layman terms please! I fly a C208 (Caravan) and tonight during preflight, I noticed that the battery was dead. I think one of my loaders turned on the courtesy lights in the cargo area without me knowing this morning. Anyway, it is a 24volt, 43 amp/hour lead-acid battery. The airport manager where I fly out of offered to help. I was expecting him to roll an APU next to my plane. Instead, he brought a golf cart with two 12 volt car batteries lined up in series. I had serious doubts this would work because for an auxilary power start I need a minimum of 800 amps, with a max of 1700 amps. I was indicating 25 volts, but not sure how many amps I had available. I followed the procedure per the checklist, but the starter/generator didn't even start to turn. My question is: Is it at all possible to start a PT6A-114 that needs a minimum of 800 amps with two 12 volt car batteries in series? Is there any way to perform an Aux. Power Start without using an APU? Secondly, I am going to have the battery hooked up to a battery charger tommorow morning. There is a mechanic that is going to help me, but is there anything I should know about charging a/c batteries?

Thanks guys/gals, I really appreciate it!
If I remember right on the caravan you need at least 20 volts on the aircraft battery to close a selinoid to get the starter/genarator to turn.
Last edited:
Hi Illini, I am sorry I cannot answer your question since I don t have any experience on the caravan,but I would like to know for what company you fly for,because I maybe interested to apply for a Caravan job and I would like to have any information about the plane and the companies that operate them (salary ,flight schedule,bases etc..).I don t know if you can tell me about your company but any info would be really appreciated! Good luck with your battery.Thank you Juventus.:D
A/C batteries

First of all, since your batteries are lead/acid, there are no special precautions associated with charging them other than those you would take with a car battery. NiCads, on the other hand require special charging circuits to protect their "memory." But since that's not the case, you should have no difficulty if the charging system is equipped for 24 volts. As to why your starter didn't turn when connected to two 12 volt batteries hooked up in series is somewhat of a mystery because series circuits double the voltage (12+12=24) and the amperage remains the same. Amperage is basically the number of electrons passing a fixed point over a given time period and will vary with the ability of a device, such as a starter, to accept them. The minimum amps for starting is usually the amount required to prevent a slow, hot start and the maximum is the amount which will allow for safe starting withou shearing the shaft on the starter due to excessive torque. Most car batteries (at least good ones) provide for 600-800 cold cranking amps but golf cart batteries are usually stronger. Even if they were low amperage, 600 amps or so, your starter should have turned, albeit slowly. And by the way, not to insult you, but the ground power unit is a GPU, not an APU. APU's are installed in the aircraft. Good luck in the future and stay away from airports that don't have GPU's.

There isn't much information to go on here, but there's no reason why the external lead acid batteries in series couldn't crank your engine. If your aircraft battery is capable of performing a battery start (it is), then the external batteries should be able to do the same thing.

The car batteries are typically rated at up to 1,000 cold cranking amps, and often are higher. How you were able to get 25 volts from the two batteries is a bit of a mystery, as your voltage should have read 24 or lower...but I don't think that plays a part in your problem.

The most likely cause of the failure to excite the starter, was an incorrect hook-up for the batteries. Either they were connected in series in reverse, or not grounded properly when attached to the airplane, or attached in reverse.

An alternate possibility is that the ground power receptacle was used to attach the external batteries. In many airplanes, a minimum value must be available from the aircraft battery in order to close the contacts for the ground power relay, in order to admit external power. If the aircraft battery is dead, no power exists to close the external relay. It doesn't matter what you have for voltage in such a case, because it's not going to put power on the airplane.

In such a situation, your only option is to hook the external power directly to the battery. In most cases this is inadvisable; you're far better off removing the batteries and charging them. However, in a pinch, you can stretch jumper cables directly to the terminal attachments on your aircraft battery, and crank the aircraft directly.

If your aircraft battery had an internal short, this could have provided an additional drain which wouldn't allow enough amperage to reach the starter solenoid.

A problem with charging any kind of battery in the airplane is the potential for sparks. Batteries reliease hydrogen, which is flammable, and in a confined space such as the battery compartment, explosive. Also, the initial rush of voltage to a drained battery can cause a voltage spike which can damage equipment hotwired to the aricraft bus, and even current limiters protecting other attached busses or elements.

Charging should never be done in the airplane, if at all possible. the only exception is passive charging via a GPU or other external power (which can be an APU or GPU, by the way), when external power is attached at system voltage, allowing the aircraft batteries to draw a normal charge in a similiar manner as if receiving generator voltage.

With respect to charging batteries, the lead acid and nicad can be charged together; the special requirement for nicad involves deep cycle charging, which is only done periodically. That's typically done by cell, with the battery opened in order to accomplish the full cycle. Nothing you need to worry about in the field, or as a pilot. A far greater danger of charging with a NiCad is the risk of thermal runaway. Once it starts, the battery will keep heating, and will either catch fire, or melt through the bottom of the aircraft. It happens, which is one good reason among many to switch back to lead acid. A wild or unregulated charging source is more likely to induce a thermal runaway (which can also occur naturally as a result of an internal short).

Your mechanic should know everything necessary for charging batteries. When charging, the filler caps should be loosened to allow the battery to more properly vent. Charge in a well ventilated area away from sparks or sources of ignition. Charge away from NiCad batteries, and if you test electrolyte (you should, or the mechanic should), be sure to use a dedicated instrument which is not used for testing NiCad. Don't look into the cells while charging, and wear eye protection when handling the battery, the caps, or the fluid.

If you'll be running external power while the battery is out (you can do this in some aircraft, and cannot in others), be sure to insulate both the battery terminals (particularly the positive, in the case of an aircraft with negative ground; (virtually all aircraft DC systems are negative ground). Consult your DoM or maintenance department for special precautions about putting power on the aircraft, especially with the battery removed. In some cases, circuit breakers must be pulled, etc.

Despite popular wisdom about cleaning batteries using baking soda, don't do it if you love your battery. It's acceptable to do so in the battery compartment if you're inclined, but wash it thoroughly and rise thoroughly; don't leave electrolyte, or baking soda in the compartment. Dry it completely before putting the battery back.

An obvious caution (but worthwhile, anyway); use caution removing and replacing the battery. aircraft batteries are often hard to reach, require manevering out of small spaces in a less than ideal posture, and involve lifting above the waist. All of these things can contribute to a quick back injury or a pulled muscle.

Also note the arrangement of the battery cables. Note the use of safety wire, if appropriate, on fasteners on the battery case. Ensure that when the battery is replaced, the positive terminal or cable makes no contact with any metal surface other than the battery, and that the battery is imobilized. Don't risk a fire. (I saw a battery melt in a Cessna 207 once in this manner).

Don't be too surprised about the automotive batteries. I've used them many times myself, starting turbine equipment, and even large radial engines. When I was operating a LR35, one airport we frequently used had a rack of lead acid car batteries for the external power source, on a small welding cart. It worked just fine.

One final possibility with your troubles may have been the state of charge for the aux batteries themselves. They just may not have been up to the task. It's always a consideration. Remember that you can still get a voltage indication, but not have any significant amperage behind the battery to drive the starter. Have those batteries checked, as well as your aircraft battery. Look for popped circuit breakers, blown current limiters, etc. Check all the possibilities.

Good luck!!
ILLINI said:
Secondly, I am going to have the battery hooked up to a battery charger tommorow morning. There is a mechanic that is going to help me, but is there anything I should know about charging a/c batteries?

Hooking up the charger is no big deal. Typically you would service the battery right before you do it, but if you're not familiar with how to do that you can skip that step. Just make sure that the charger is off when you hook up the leads. There are potentially explosive fumes around the battery that you don't what to ignite. Make sure you have the positive and negative hooked up correctly. Also, it's good practice to leave the battery box open to provide as much ventilation as possible, as it is likely that the battery will give off fumes when charging.

Not to sound to anal, but be careful, the battery contains hydrochloric acid. In the unlikely event that you get some in your eyes, flush them for five minutes with cold water, then seek medical attention immediately.

DOH! I knew that about the APU/GPU!

Wow, thank you all for the great information! It was definetely a learning experience. I wasn't aware that automotive batteries had that much amperage. Well, i'm still not sure why the external start didn't work but the only thing I can think might have happened based on what i've read here is that either the external batteries were also drained and could't provide the necessary amperage, or the battery in my plane was so drained, that it would not close the solenoid to allow for an external start (I'm leaning a little more towards this one). At any rate, I think I need to read up on the electrical system a bit more.

Anyway, thank you all once again and i'll let you know if I find anything out.

On some airplanes you need to have some voltage(17/18v) from the battery to close the solenoid, the gpu will not turn the starter.
Your alternative is to hook up the battery cart to the aircraft battery.

Hope that help
Fly safe
Ahh Haa!

Well, I found out why that golf cart didn't start my plane last night. First of all, my battery was drained down to 19 volts which is short of the 20 volts required for an external start. Secondly, even if I had those 20 volts, it still wouldn't have worked. I thought the golf cart had two 12 volt car batteries in series. I was wrong. It had six 6 volt batteries in series producing 30 volts (for some reason one of them is kicked out... not sure why) and only 600 amps which is short of the minimum of 800 the Caravan needs. So I had the battery taken out, which was very simple (easier to take out than most car batteries!), and put on a charger. Needless to say nothing blew up and it looked like it was charging nicely.

Thanks for all your help. I learned something new today - and it's not even 10am yet!

As a side note, when performing a battery start, especially after a recharge or a drained battery, use caution when turning on the generator, and then engaging any other electrical equipment. The battery will typically be discharged more than normal following that start, and the system will generally experience a higher load; you'll see more amperage on the system for a few moments.

Turning on other electrical equipment can do one of two things; it can fry that equipment, or impose an escessive load on the generator. Be sure to closely monitor your ammeter prior to putting anything on the bus, and adhere to the manufacturer limitations (should go without saying, but it can sneak up on you, if you don't expect it).

Also, be prepared for a shortened battery life. Any time a battery is drained and then left, it shortens the battery life. It's something to keep an eye on, especially if you're going to be in any remote areas on cold days.

Good luck!

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