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Batteries & Chargers Discuss Li-P, Li-Ion, NiMh, Nicad battery technology and the chargers that juice 'em up!

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Old 04-16-2011, 04:00 PM   #1
Aientonni
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Default Transmitter Lipo Voltage Diodes Question

I've bought a Rhino 2650 lipo battery from Hobbyking for my transmitter and have a question about dropping the voltage using diodes. Since other forums indicate it's the safer option to limit stress on the transmitter regulator I've bought the parts to solder my own adapter together.

My assumption is that, since diodes will drop the voltage by about 1.7 volts, I could recharge my lipo roughly at the same voltage as I would with regular Nimhs. Which was around 9.5 volts. Thus, if for whatever reason the battery drains to 8.9 volts and activates the alarm, I would still have a margin of safety to protect my lipo. Is this a safe assumption?

I've read that most people won't let their lipo go below 10.5 volts before charging. Is this with diodes or without?

Yes..this is a basic question but I wan't to be sure. I've bought a "used" Futaba 9C and I want to protect my investment.

Thanks.
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Old 04-16-2011, 10:46 PM   #2
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The voltage drop you get depends on the diode. I was under the impression that most drop about .7 to 1.0 volts - but havent checked the specs in ages and ages so I may be way out of date.

As far as charging - I would never ever consider charging a lipo with the diode in the circuit and I would never consider charging at all unless it was with a proper lipo charger.

Dont even think about using the chargers that came with the tx. They will NOT be safe to use no matter what combination of diodes or resisters etc are in line.

Setting that battery on fire inside the radio - or on your bench - would be much worse than burning up the regulator

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Old 04-16-2011, 11:05 PM   #3
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Hello Larry,

Don't worry, I plan to remove the battery when re-charging. I've been using lipo's in my planes for over a year and have an appropriate charger. My question is more about when will I have to re-charge my lipo when it gets depleted. What is the safe minimum voltage with diodes (2 x 1n4001) and without. I understand if you drain the lipo under a certain voltage you can damage the battery and increase the risk of fire the next time it's charged.

Let's say I run my transmitter with diodes until the voltage drops on the display to 9.0 volts. Would my lipo be safe? I assume that since voltage is dropped by the diodes a total of 1.4 volts the actual voltage without them would be 10.4 volts, leaving me in a safe margin.

This is using a 3s 11.1 lipo battery of course.

Thanks
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Old 04-17-2011, 02:59 AM   #4
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Ah, I misunderstood

IF the diode is actually giving you 1.7 volts drop at very low loads, then you might be safe enough - maybe.

The thing is, the tx amp draw is very low, so that indicated voltage is very close to a no-load voltage.

There is also the issue of the tx volt meter accuracy. Ive read many reports of them reading off by as much as a full volt or more.

I dont like to take my lipos down below 11.1 volts resting voltage. Thats about 95% discharged. You might get away with 11 volts or even 10.9 volts. But thats starting to get risky as far as life expectancy on the lipos. They dont like to be fully discharged.

I would do two things. Use a good DVM thats accurate and measure the voltage of the lipo without the diode hooked up. Then measure the voltage with the diode in the circuit - but still out of the tx. Note those numbers. nThat will tell you for sure what the diode voltage drop is.

THEN, install the lipo and diode in the tx and see what the tx says the voltage is.

I would do this with it fully charged and with it run down to 11.1 volts.

Some meters are off more at one extreme or the other.

That will tell you what the error is at both ends of the charge range and you can make adjustments for the tx reading so you know what the lipo is actually at.

Then I would never let the lipo go below 11.1 volts at an absolute minimum and 11.4 to 11.6 would be even better as far as life expectancy and safety goes.

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Old 04-17-2011, 03:02 AM   #5
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Originally Posted by Aientonni View Post
I've bought a Rhino 2650 lipo battery from Hobbyking for my transmitter and have a question about dropping the voltage using diodes. Since other forums indicate it's the safer option to limit stress on the transmitter regulator I've bought the parts to solder my own adapter together.

My assumption is that, since diodes will drop the voltage by about 1.7 volts, I could recharge my lipo roughly at the same voltage as I would with regular Nimhs. Which was around 9.5 volts. Thus, if for whatever reason the battery drains to 8.9 volts and activates the alarm, I would still have a margin of safety to protect my lipo. Is this a safe assumption?

I've read that most people won't let their lipo go below 10.5 volts before charging. Is this with diodes or without?

Yes..this is a basic question but I wan't to be sure. I've bought a "used" Futaba 9C and I want to protect my investment.

Thanks.
I've done this many times, using a silicon diode to drop voltage in a circuit. Problem is, that voltage drop is not constant, and can and will vary depending on applied load current (And temperature. Some cheap electronic thermometers used to use a silicon diode as the temperature sensor. Highly accurate IC's are now available for this purpose). The typical silicon diode voltage drop will be about a volt DC or so at its rated DC current. But, take a look at a typical 1N4004 silicon diode, its voltage drop will vary from about 0.6 volts DC at 10 milliamperes to that one volt at one ampere. Problem is, at less current than the 10 milliamperes on the specs, that voltage drop could be much less. Especially with diodes rated at high currents of over 10 amps or so. (Nowdays, diodes rated over 500 Amperes are off the shelf items!)

http://search.digikey.com/scripts/Dk...=1N4004FSCT-ND

Take a look at the 1N4004 datasheet, page two, figure two on the Digikey web page above.

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Old 04-17-2011, 03:46 AM   #6
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To be safe.....I charge mine using "Storage" programming. I use a Bantam BC-6 charger. The resulting voltage is about 11 volts. Use balance....the voltage on each cell will be about 3.79 or 3.8 volts. This lesser voltage won't hurt your transmitter.
The biggest danger is forgetting to turn your transmitter off and leaving it on for a really long time....like a week ! You can kiss that lipo goodbye.....it will be all puffed up and may even push the battery compartment lid right off. So you may need to change some of your shop procedures...
When the voltage gets down to 10 volts....put it on the charger. 9 volts (3V/cell) is the lower limit.....don't go below it.

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Old 04-17-2011, 03:49 AM   #7
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Originally Posted by Flash1940 View Post
To be safe.....I charge mine using "Storage" programming. I use a Bantam BC-6 charger. The resulting voltage is about 11 volts. Use balance....the voltage on each cell will be about 3.79 or 3.8 volts. This lesser voltage won't hurt your transmitter.
The biggest danger is forgetting to turn your transmitter off and leaving it on for a really long time....like a week ! You can kiss that lipo goodbye.....it will be all puffed up and may even push the battery compartment lid right off. So you may need to change some of your shop procedures....

Flash
Yeah, old forgetfull here has left his transmitter on, killing the batteries three times now in three years. Those old reliable Nih batteries were finally replaced this flying season, even though they still worked.

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Old 04-17-2011, 07:12 AM   #8
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3 volts per cell is a safe lower limit UNDER LOAD. Thats not a safe resting voltage. Running your pack down that low will kill the lipo in short order.

Becides that, there is virtually no usable capacity below about 3.7 volts per cell. Thats about 95% discharged.

trying to run them at lower voltages in a tx will kill them and you run the risk of the tx shutting down abruptly with little to no warning.

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Old 04-17-2011, 02:09 PM   #9
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A more simple and much safer solution is to use Eneloops in your transmitter. You can charge them with just about any Nixx type charger including the wall wart that came with your transmitter. You can charge them in place, no worry about fire of deep discharge. They hold a charge as well as LiPo's do and cost about the same or cheaper. No pain, much gain when using Eneloops over LiPo.
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Old 04-17-2011, 04:36 PM   #10
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What is the charging time for enloops?
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Old 04-17-2011, 05:08 PM   #11
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I believe its generally best to trickle charge them but they can be charged at 1-2C easily.

Another option that I think beets the enoloops is to use LIFEPO packs. they can be fast charged, last forever and also dont catch fire and also have low self discharge. About the same price as enoloops.

http://www.aircraft-world.com/shopexd.asp?id=3152

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Old 04-17-2011, 05:11 PM   #12
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Some notes from the Hyperion site on the LIFEPO tx packs.

And compared to high-capacity NiMH packs, the Hyperion G3 LifePO4 Radio batteries have these other advantages:
  • More capacity per gram, meaning lighter models in flight, and longer transmitter duration
  • Almost no "self discharge", so your radio is always ready to go even weeks after charging.
  • Superior voltage performance under load, so your servos perform with more consistent torque and speed.
  • Faster charge time (as fast as 15 minutes!)
  • In addition, the LiFe chemistry is not subject to "thermal runaway" fires, which can occur when LiPo are overcharged, making LiFe a much safer alternative.

    Hyperion - as well as other quality digital servos makers - will be introducing "High Voltage" servo types which can handle both LiFePO4 and LiPo voltages. However, unlike LiPo, LiFePO4 can be stored in a fully charged state, and discharged deeper, without damage to the cells. So they are more "rugged" than LiPo, safer, more convenient and a better choice overall.

    In the LiFe pack photos, you can see a Hyperion-style balance connector, and also a red connector with 3.5mm gold plugs, and a "spare" connector set. The red connector allows these packs to be charged at high rates. The "spare" can be attached to your existing Radio plug, or used to make a charge cable.

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Old 04-18-2011, 02:48 AM   #13
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Hmmm... I believe my charger has a setting for "life" batteries already. The main choice for lipo's in my tx was the fast charging times. I like to carry my glider in my car so I can just jump out for a quick blast when the chance comes. Having to charge a Tx battery on a home charger for 15 hours got to be frustrating Especially when your working the sky for a couple of hours then finally hit "that big thermal" only to hear that Tx start beeping.

I'll have to start looking into life batteries. Sounds like a safer solution. Would diodes have to be considered on those also?

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Old 04-18-2011, 02:55 AM   #14
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No diodes needed for the lifepo packs.

Check out the Aircraft World link I posted above. You can order from them or from All E RC in the US or any Hyperion dealer for that matter.

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