Lipo's and current limiting resistors

Ah, one of my favorite subjects to rant on

1. A 20C rating is the max current that the Lipo can supply safely all day (until it runs out of juice). In a dead short they have deliver much more current.

2. If you must use a current limiting resistor then put in series with ematch and not the altimeter. This way if you overcurrent the outputs you won't affect the voltage feeding the computer and possibly browning it out.

John D and Cris E are just the men to get this kind of info from,, for sure..
They have both given me solid info more times then I can count..
Wayne,,
The batt example you gave ,,
you do have the 3 necessary pieces of information..
A 2s is 7.4 volts,, your batt had a 20 c rating and it's capacity was 1000 mah..
This battery is capable of putting out 20 amps at it's rated voltage...
The 300mah batt is the same,, it is capable of putting out 20 amps at it's rated voltage, which is the same,, they're both 2s 7.4 volt..
The 1000 mah batt can do this for a bit more than 3 times the length of time as it has 1000 milliamps stored in it...

Teddy

Wayco said:

First, let's check my understanding of Lipo's. If I have a 1000 mAh 2s (7.4v) rated at 20c, it could provide 20 amps of current, correct?
Most of our altimeters (RRC-3 and SLCF for me) are rated to 5 amps. If a short occurs on the ematch, the Lipo could easily exceed the max rating on the altimeter.
I know that using a smaller battery, 300 mAh at 20c might save the altimeter, but it's still putting out 6 amps according to my calculations, right?
I mentioned this to Chris Erving (Mr. Eggtimer) and he suggested I put a 1 ohm 10 watt resistor in series between his Wifi Switch and the altimeter. The Eggtimer Wifi switch uses 85 ma, so a larger battery is suggested in the documentation. Minimum of a 300 mAh 2s, and more if connected to an energy hungry device that has to sit on the pad for a while.

My questions are:

1. Are my calculations correct on Lipo current output?

2. Has anyone experienced an altimeter burnt out by a Lipo?

3. Has anyone added a current limiting resistor to their avbay?

Click to expand...

1. Yes and no. John explains it a bit in his post. Using the C as a multiplier, you get 20 amps for the 1000 mah and 6 amps for the 300 mah. This is their safe discharge rate. The larger capacity, translated more mah, the longer the battery will last. With a resistor in series, their discharge will be the same. With a 1 ohm resistor, they will both be limited to 7.4 amps. a 1.5 ohm brings them down to 4.9 amps, which should protect the altimeters just fine.

The batteries will discharge at a much higher rate across a dead short. But will get very hot and swell, leak, explode, catch fire, whatever. Without knowing the internal resistance of the battery or the detailed specs of the battery, I don't know of any way to determine the peak discharge current of a battery across a dead short other than using test equipment. The C rating alone does not tell you the max discharge rate AFAIK. That would be info that comes from the manufacturer most likely, may be on the data sheet, may not.

2. No and I fly with and without resistors.

3. Yes. I have built several bays with resistors. Several ways to do it really and not hard at all. I use 1.4 or 1.5 (not sure what I have ATM) ohm resistors and put them on the negative/return side of the e-match. I have some built right into the boards. On one board I used a terminal block so I could add a resistor if I wanted to fly LiPo or a jumper if I wanted to fly 9volt.

Some folks just use two e-matches in series. So long as they don't both short, it achieves the same thing except you burn an extra match. In a redundant dual deploy setup, you are looking at 8 of the jokers. Most of mine usually ohm at around 1.2 so two of those and your battery is putting out 3 amps. That should still be enough to blow a charge.

Mikey D

Wayco said:

2. Has anyone experienced an altimeter burnt out by a Lipo?

Click to expand...

While it isn't an altimeter, I have heard of the output transistors in the BlackSky Timer2 getting fried. (This used an alkaline 9V battery.)

The same transistors are used in the AltAcc altimeters and those never seem to have problems. The reason is a combination of things. First, the Timer2 leaves the FETs on for a whole second while the altimeter limits it to a tenth of a second. The other is that the timer is typically used for staging and this could result in funky things happening to the igniter after the bridgewire opens.

Like Mikey said, a resistors in line with the match works well to limit the current on the firing circuit. Just get one rated for the current that can add enough resistance to save the altimeter if the ematch shorts. (that being said, I highly recommend putting a meter on any ematch prior to installing it in your rocket. Pretty sure you know this since you've flown more high power high performance flights than years I've been alive )

I was planning to do things that way for my first Elec flight on an 800mAh 2S with a stratologger, but lack of resistors made me choose the two ematch in series option. Nothing toasted yet, and I'm actually really leaning towards using the two in series method for any rocket without a backup altimeter.

2 matches in series increases the necessary voltage,, not the amperage drawn..

2 matches in parallel increases the amperage drawn,, not the necessary voltage..

When the altimeter tells you it sees continuity on a circuit it should,,
and on that circuit you have 2 matches in parallel,,
the altimeter is only telling you it sees continuity on one of the matches not both..
This is the reason on my dual 38mm at the same time air starts I always put the matches in series..
First,, when the altimeter says I see continuity on that circuit,, there must be continuity on both matches..

20c on a lipo is the continuous current ( amperes ) that the battery is capable of putting out at the batteries rated voltage..........

Teddy

I have never lost a altimeter due to a e-match short or used a current limiting resistor. I might however be able to offer a bit of info on the amount of current a Li-Po can deliver.

As MickeyD stated the only way to determine the maximum current a battery can deliver is by knowing its internal resistance. Many of the better chargers measure I.R., and stand alone I.R. meters are available. I have two chargers that will measure I.R., and two stand alone meters. While I use them for keeping a watch on the health of my batteries they will also determine the maximum current a battery will deliver in a short circuit condition.

A nine volt battery has a I.R. of about 2 ohms, maybe a bit less, thus using ohms law, nine volts divided by 2 ohms gives us a maximum current of 4.5 amps. That is why many rocketers like them, even in a short circuit condition they cannot deliver more than 4.5-5 amps.

The I.R. of a Li-Po is measured in milliohms. In testing a "Zippy", 2200 mah, 20C, 3S battery, I measured a total of 29 milliohms between all three cells. Again using ohms law, 12.6 volts (the voltage of a fully charged 3s li-po) divided by .029 ohms, gives a current of 434 amps. That right, 434 amps. Now it will only deliver that for a very short time, the battery will heat up thus increasing the I.R., the leads will also heat up and melt, the solder used to connect the leads to the battery will likely melt and break the short, and the battery may burst a cell and catch fire.

In addition to rocketry I also do quadcopters, cars, and boats, thus at any time I have 30-40 Li-Po's on hand. In testing them, I have found that all of them will deliver between 100 and 200 times the rated mah current in a short circuit condition. The smaller 1s ,low mah batteries will deliver around 100 times the mah rating, while the 3-6s batteries all deliver about 200 times the rated mah of the batteries.

What many people forget is the "C" rating is the maximum amount of current that can be taken from the battery without damaging the battery, not the maximum amount that it can deliver in a short circuit condition. Thus that little 300 mah, 20c battery in a short circuit condition, could deliver 30 amps not 6.

catman001 said:

I have never lost a altimeter due to a e-match short or used a current limiting resistor.

Click to expand...

1. What altimeters/flight computers do you own?
2. What LiPos have you used with rocketry, what capacities, and with which altimeters?
3. What e-matches have you used in the past and with what combination of altimeters and LiPos?

Because I think some previous explanations may have been ambiguous (and misunderstood) on this point, I'll repeat that the "C" rating is a multiplier of the stated capacity, and indicate the maximum current that the battery can deliver without doing itself harm. So a hypothetical 300mAH 1C cell can deliver 300mA before damage. A 300mAH 10C cell can deliver 3A (3000mA), 300maH 20C can deliver 6A etc. And that doesn't mean they won't deliver more current - just that they may be toast if you draw more. For those who already got this, please forgive the repetition.

Mark

CORZERO, in answer to your questions.

1.With the exception of one "Raven", I have six Missile Works altimeters. Two of the RRC3's, two of the RRC2+, one RRC2 classsic, and one RRC2 mini. I started with a Missile Works altimeter and it worked well for me, so when I added to my collection, I just went with what I knew.

2. The batteries that I use are Nano-Tech, from HobbyKing They are 2s and either 180 or 260 mah, depending on what HK has in stock. While the RRC3 and the RRC2+ will work with a 1s battery, the older units will not. They require a higher voltage. The newer units require 3.7 to 10 volts. So a 2s battery which puts out 8.4 volts fully charged will work with all versions that I have. Going with one battery voltage helps keep it simple.

3. A few years back when the original Q2G2's were still available from Quest, every now and then they would have a sale on them. I bought a large number of the 24 count, thinking that I would be set for a few years and could sell them to fellow club members if they needed them. Little did I know that they would be discontinued. So while my inventory is getting low, I have only used Q2G2's for a number of years.

While I am aware that a fired e-match can create a plasma ball, which can conduct electricity and short things out, I have never seen a real study on it. How common is it? While metal charge holders are nice, I always worry a bit about using them. When the charges go off the insulation on the e-match that is inside the charge holder gets burned off most of the time. If the wires contact the metal holder for any amount of time, or each other, you have a short and there goes your altimeter. I often wonder if that is the cause of some burned out altimeters. One thing that I do is to add more insulation to the e-match wires. Small heat shrink works well, and seems to hold up to the heat. I get it as close to the head of the e-match as I can. While I use metal charge holders, I do like the PVC one's, as that is just one less thing to go wrong.YMMV, just what works for me.