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Author Topic: LIPOs command respect !  (Read 9228 times)
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anwar
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« on: March 12, 2008, 11:18:00 AM »

Lithium Polymer battery technology has allowed RC flying to take on a whole new dimension in the past few years.  As powerful as they are, they also demand a lot of respect, in terms of care taken while charging, transport, setup and use.
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fernie_vital
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« Reply #1 on: May 30, 2008, 10:57:15 PM »

Lithium Polymer Battery Guide

Lithium polymer batteries offer a practical and powerful solution to the power needs of AEG’s today. Boasting lighter weight, smaller sizes, higher discharge rates, and higher energy density li-po batteries are ideal for a powerful airsoft gun.

While there are risks associated with li-po batteries, they are far from the unstable doomsday devices they have been depicted as in the past. This guide will serve to inform of the potential hazards of li-po batteries as well as emphasize their benefits.

Battery Traits & Qualities
-Why Li-po?
Li-po batteries excel in applications where long battery life and high discharge rates are needed but battery space is severely limited. They will also serve just as well in larger battery spaces and still offer performance benefits. It’s always important to measure your battery space dimensions to determine what size battery will fit your space constraints.

-7.4v or 11.1v?
Li-po battery use in airsoft is fairly limited to 7.4v or 11.1v battery packs. Each li-po cell is 3.7v meaning a 7.4v pack is comprised of two battery cells in series and an 11.1v pack is comprised of three battery cells in series.

A high quality 7.4v pack is more than sufficient to power a 400fps spring with a stock motor. With properly shimmed gears and an upgraded motor (such as a Systema Magnum) a 7.4v pack can easily power a 400fps AEG to 20rps (results will vary between configurations). Depending on your motor a 7.4v pack can outperform a 9.6v sub-c battery, but not by a huge amount. With a high quality pack trigger response will be just as good if not quicker than a 9.6v nimh/nicd.

11.1v packs are used for high-speed applications and pulling high tension springs. They will wear out electrical components faster and can easily power 400fps AEG’s above 20rps. Trigger response will definitely be improved. A side-effect is the "double tap" syndrome where on semi two shots may be fired. Also the piston may be re-caught due to the extra momentum of the motor. A  MOSFET with active braking will prevent this.

Another notation for 7.4v and 11.1v li-po batteries is “2S” and “3S.” The number indicates the number of cells in the pack and the “S” indicates they are wired in series. For those looking for a “nun-chuck” configuration there are some cells wired in series and others wired in parallel. A 7.4v nun-chuck pack would be labeled 2S2P. The “2P” portion of the notation denotes that there are two packs in parallel. There are 4 cells total in this pack. Two cells make up each pack and there are two packs in parallel. Cells in series add their voltages together, cells in parallel add their mAH together.

-Is the mah on li-po's the same as in nimh/nicd's?
While the conventional wisdom for nimh and nicd batteries is 1mah = 1 shot that rule is an understatement for li-po batteries. Li-po batteries are able to deliver around 3 shots per mah and reports of more are common.

-What is a “C” rating and what does it have to do with discharge rates?
The “C” rating of a li-po battery helps determine the max continuous discharge rate of that particular battery. The max continuous discharge rate of a li-po battery can be calculated by multiplying the C rating by the mAH rating. If you have a 2000mAH lipo that is rated at 10C your max continuous discharge rate will be 2000mah x 10C = 20,000mAH or 20amps.

Li-po batteries have an interesting feature called “burst discharge.” Burst discharge is only good for a second or two but during this duration the li-po battery is able to deliver up to twice the normal C rating in current (burst magnitude may vary between packs). Looking at the previous example with the 2000mAH battery with a C rating of 10, a possible burst discharge rate could be double that of the C rating. The burst discharge will be 2000mAH x 20C = 40,000mAH or 40amps. Li-po batteries increase trigger response with the high amount of current on tap. Since electric motors draw the most current during startup, your AEG motor can easily draw more than the 30amps delivered by high quality nimh batteries at startup. The ability to deliver this current with a li-po battery decreases spool up time and improves trigger response.

-What’s a balancing tap, and do I need one?
A balancing tap is a second connection on a li-po battery meant to be connected to a balancing module. When connected to a balancing module during charging, each cell is fully charged to 4.2v. Without being balanced it is common for li-po cells to have different voltages. This cell imbalance will lead to decreased li-po performance. When one cell is at 3.8v and the other is at 4.0v charging without a balancer will cause the higher voltage cell to be fully charged to 4.2v, but the lower voltage cell will only be charged to 4.0v reducing capacity.

When charging with a balancing unit all you need to do is connect the balancing tap from the battery to the balancing unit. Do not attach the main leads to the charger as all charging will be done via the balancing tap.

Due to a lack of connector standards in the li-po industry not all manufacturer’s balancing taps are the same. To ensure a compatible connection your balancing unit and battery can be from the same manufacturer. There is also absolutely no downside to mixing battery and balancing unit manufacturers as there are adapters out there. It is advisable to check for the availability of an adapter before you mix brands however.

Li-po “proofing” your AEG
-Do I need upgraded internals?
While li-po batteries often convey the message of extremely high rates of fire and increased amounts of stress on your gearbox, it really doesn’t take much to make your AEG li-po ready. As long as your gearbox is durable enough to run a 400fps spring it will be able to handle a li-po battery. If going for a faster rate of fire it is advisable to make sure the second to last tooth of your piston is shaved down/removed.

-What about my electrical system?
Stock AEG wiring will be able to handle a li-po battery without any problem. It’s highly recommended that you remove your fuse however as that is a major source of electrical resistance in the system impeding the output of the battery. It is also advisable to upgrade to a higher quality electrical system if using stock Classic Army electrical parts.

-What other things do I need to do?
While not critical, it is highly recommended you change you the electrical connectors from the standard tamiya type connectors to something that allows higher current flow such as deans connectors or traxxas connectors.

Li-po Chargers and Balancing Units
-Do I need a special charger?
Li-po batteries require their own specific chargers and are incompatible with regular nimh/nicd chargers.

-Will any li-po charger do?
There are many cheap li-po chargers out there that will run the risk of overcharging your battery and possibly causing it to catch fire. Look for a charger made by a reputable li-po company. Newer budget chargers are available from reliable manufacturers and will do just fine. The old adage that li-po chargers will cost you an arm and a leg isn’t valid anymore and there are many solid chargers available at reasonable prices. Avoid generic looking “wall-wart” type chargers.

-What’s this about a 12v power supply/source?
Many li-po chargers are designed to run off of a 12v power supply. This means that they don’t come with a regular plug for your wall socket. A separate 12v power supply unit that plugs into the wall is required to power the li-po charger.

-What’s a balancing unit and do I need one?
A balancing unit is usually an external module that hooks up inline between your charger and your li-po battery. This ensures each cell is charged to 4.2v for a full battery charge. Cells become imbalanced and end up with different voltages, which can lead to undercharging or over charging.

There are also now integrated balancing units in li-po chargers. This is the most economical way to both charge and balance your li-po battery. As with just chargers, it’s highly recommended to find a unit by a reliable manufacturer. These 2-in-1 combo units are not difficult to find fortunately.

Li-po Hazards
-Will my battery catch fire/explode?
The persisting image of li-po batteries and AEG’s going up in a mass spectacle of flame and light most certainly can happen without the proper care. This can result from over charging your battery, draining your li-po battery too low, or your li-po battery being hit by a significant amount of force so that it deforms/rips apart. The force of impact/tearing of your li-po battery will not cause it to catch fire right away, but only if you attempt to charge/discharge your battery after.

-What is “draining my battery too low?”
Li-po cells can become damaged if the voltage per cell drops below 3.6v. To ensure this doesn’t happen it is advised to acquire a multimeter so that you can test the voltage of your battery pack in between games, ect. Should your li-po battery be over drained it should be discarded.

-What if I drop my li-po battery?
Li-po batteries aren’t immensely fragile and can take a fair amount of abuse from dropping, banging around a battery compartment, ect. Should your battery outer covering tear however your battery runs the risk of releasing hydrogen gas during charging (which leads to flames).

Cheers,
Fernie
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anwar
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« Reply #2 on: May 31, 2008, 09:11:35 AM »

We don't want any copyright issues here, so in the interests of full disclosure, it is noted that the information about LIPOs was taken from this site .
« Last Edit: June 03, 2008, 06:36:55 AM by anwar » Logged

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