Battery current vs RRC3

I have a fairly new RRC3 and I was wondering this:

I have some li-po batteries that are rated 3.7 volts and 150mHa.
I was planing to connect them so that I would have a two cell battery pack with about 7.5 volts.
What I am unsure about is the current...the RRC3 is rated at 10 to 50ua, the batteries are using the mHa measurement and my voltmeter uses neither of these units.

What does all this translate into for the RRC3? I really don't want to fry it with too much current.
As you can see my electric theory is not very advanced.

Thanks
Julian

I have used a 180mah 7.4v lipo on many flights with a RRC3. Here is an applicable thread....

https://www.rocketryforum.com/showthread.php?57022-What-Lipo-batteries-are-people-using-for-the-RRC3

Titan II said:

I have used a 180mah 7.4v lipo on many flights with a RRC3. Here is an applicable thread....

https://www.rocketryforum.com/showthread.php?57022-What-Lipo-batteries-are-people-using-for-the-RRC3

Click to expand...

https://www.hobbyking.com/hobbyking...180mah_2S_25_40C_Lipo_Pack_AR_Warehouse_.html

Julian said:

I have a fairly new RRC3 and I was wondering this:

I have some li-po batteries that are rated 3.7 volts and 150mHa.
I was planing to connect them so that I would have a two cell battery pack with about 7.5 volts.
What I am unsure about is the current...the RRC3 is rated at 10 to 50ua, the batteries are using the mHa measurement and my voltmeter uses neither of these units.

What does all this translate into for the RRC3? I really don't want to fry it with too much current.
As you can see my electric theory is not very advanced.

Thanks
Julian

Click to expand...

The 10 to 50ua rating is probably the current draw of the RRC3 at the maximum and minimum voltages it can work with. If it is 3 to 15 volts, the 50ua would be the draw at 3 volts and the 10ua at 15 volts. Power or Watts is calculated as voltage times current. So in this case, the RRC3 draws .150 milliwatts. The current draw is a function of the RRC3, not the voltage source. You could connect a 800 cold crank amp car battery to run the RRC3 and if the voltage is 12 volts, it would only draw 13.5ua. You will only draw the amount of amps the RRC3 uses and would not damage it because the voltage source is capable of many more amps. That's the way light bulbs in your house work. You can install 25, 40, 60, or 100 watt bulbs and they all work. They draw the amount of current they need and no more. Just because you have a circuit capable of supplying 15 amps doesn't mean the bulb will draw that much.

Put whatever battery you want in the rocket to power the RRC3 as long that the voltage falls in the operating range.

Yah, but the max current a battery can out-put can go over the max operating current when the recovery charge is activated. That is the problem-current fine when operating without activating the ouputs.

One important thing with Lipo's is the "C" rating. This describes how fast they can discharge.

Those Hobby King Lipo's are rated at 40C max. You multiply the capacity by the c rating to get the max discharge current = 180mAh = 0.18Ah * 40C = 7.2A

The rating on the RRC3 is 10A if I recall correctly, which means those lipos are safe to use directly connected to it while setting off starters for dual deploy.

You'll need to do a similar calc for whatever you are using to check that it's max discharge current is lower than the rating for the RRC3.

Zebedee said:

One important thing with Lipo's is the "C" rating. This describes how fast they can discharge.

Those Hobby King Lipo's are rated at 40C max. You multiply the capacity by the c rating to get the max discharge current = 180mAh = 0.18Ah * 40C = 7.2A

The rating on the RRC3 is 10A if I recall correctly, which means those lipos are safe to use directly connected to it while setting off starters for dual deploy.

You'll need to do a similar calc for whatever you are using to check that it's max discharge current is lower than the rating for the RRC3.

Click to expand...

Correct except the ratings stated in the lipo thread for this Altimeter state 5amps So that battery is too much if you have a short or are using it to air start. Not to mention the C rating is what the battery is happy to do,, not what it will do in a short which is probably much higher.

73

Chris

There is a lot more to this problem than this. The C rating is a multiplier to the capacity for what the battery will put out without damage as mentioned above. It is also true that if your battery can put out more current than your altimeter will handle that during a short circuit you will damage the altimeter. However a non-shorted output will put out only as much current as you can push through the load at a given voltage determined by the load resistance. The problem with a short is that it you can put extremely high currents at even a tiny voltage. When this happens the current will shoot up beyond the C rated amperage. Beyond damaging the altimeter, this can permanently damage the battery, in the worst case it will cause a fire, lipo fires are scary. It will also cause the battery voltage to sag extremely low which can cause your altimeter to reset, called a brown-out.

A flier I know tried to avoid damaging their altimeter by using a battery that wasn't capable of delivering enough amperage to do damage. The drogue charge browned-out both altimeters resulting in a drogue only recovery and only one of 4 charges ever firing. Ideally you should size a battery to maintain a reasonable voltage under worst case load, in most applications (not ours) a short only happens after a major failure.

The RRC3s max current is 5A, there is a huge thread on RRC3s and Lipos here: https://www.rocketryforum.com/showthread.php?57022-What-Lipo-batteries-are-people-using-for-the-RRC3/page4&highlight=RRC3+lipo Jim Amos has posted a number of replies to it especially near the end when we started coming up with ideas on how to limit the Lipos max current to the max of the RRC3 using BECs from RC aircraft.

One thing that you can do to alleviate the concerns with the short circuit current of the battery being above the current rating of the altimeter is to wire a series resistor with the pyro outputs. I do this and it works fine. Currently I'm using a raven with a 2S Lipo. Wiring has the battery positive going to one side of a switch, and the other side of the switch going to the raven power supply input directly. Also coming off the output side of the switch is a 1 ohm power resistor that has a sufficient surge power rating to handle 8.4V being applied across it for more than one second. The one I use is a 7W Yageo part from Digikey: YAG1226CT-ND, rated for 70W surge applied for 5 seconds. The other end of the resistor goes to the "lighters" and the lighters return to the raven outputs. That limits the output current in the event of a short to something that won't destroy the altimeter but still allows plenty of current to make sure the lighters do their job. It also allows the battery voltage to remain much higher to the raven power input while the events happen - not that this would be a problem given the nice big hold up capacitor on the raven. For lower current rated outputs, a larger resistance could easily be substituted to keep short circuit current in check.

The thing I'm not sure most of you realize is that if you have a "short" in your "lighter" that allows enough current to damage the altimeter, the "lighter" probably won't fire anyway. The bridge wire is going to have about 1 ohm of resistance. If you have a "short" it's because the copper is shorted together and the bridge wire is out of the circuit. If that is the case, you have a lot more problems than frying your altimeter. You will probably lose it to a lawn dart anyway.

Handeman said:

The thing I'm not sure most of you realize is that if you have a "short" in your "lighter" that allows enough current to damage the altimeter, the "lighter" probably won't fire anyway. The bridge wire is going to have about 1 ohm of resistance. If you have a "short" it's because the copper is shorted together and the bridge wire is out of the circuit. If that is the case, you have a lot more problems than frying your altimeter. You will probably lose it to a lawn dart anyway.

Click to expand...

I guess I'm thinking DD. If you have a shorted lighter failure on both the apogee and main, then yep, it's lawn dart time. If just one fails, which I would consider the much more likely scenario, you either come in under drogue or rip up the rocket with a high speed main deployment. Either of these would most likely (but not necessarily) be less stressful to your electronics than a lawn dart. Protecting the altimeter electrically at least gives you a chance that you can use it again if it isn't physically destroyed in the landing. Just the way I think about it.

Handeman said:

The thing I'm not sure most of you realize is that if you have a "short" in your "lighter" that allows enough current to damage the altimeter, the "lighter" probably won't fire anyway. The bridge wire is going to have about 1 ohm of resistance. If you have a "short" it's because the copper is shorted together and the bridge wire is out of the circuit. If that is the case, you have a lot more problems than frying your altimeter. You will probably lose it to a lawn dart anyway.

Click to expand...

The stress on altimeters are those used for air-starts. The igniter in the combustion chamber experiences a dead short in that condition, so I have heard. I have not seen any actual data though.

Lurch said:

I guess I'm thinking DD. If you have a shorted lighter failure on both the apogee and main, then yep, it's lawn dart time. If just one fails, which I would consider the much more likely scenario, you either come in under drogue or rip up the rocket with a high speed main deployment. Either of these would most likely (but not necessarily) be less stressful to your electronics than a lawn dart. Protecting the altimeter electrically at least gives you a chance that you can use it again if it isn't physically destroyed in the landing. Just the way I think about it.

Click to expand...

I also thought of DD. If a shorted lighter on the drogue fries the altimeter, there wouldn't be a high speed deploy of the main. It it's just the main, then a drogue landing might not be too bad for the rocket, but the failed main deploy might still fry the altimeter. The point is, shorts are a very rare electronic fail condition compared to opens. Most electronic failures are caused when circuit open and fail to work correctly. Having a short cause a catastrophic failure is not a common occurrence. Although it is possible, there are a lot more common failure modes and I would spend my time and effort mitigating them first before I worried about a very low risk event like an electronic short.

jderimig said:

The stress on altimeters are those used for air-starts. The igniter in the combustion chamber experiences a dead short in that condition, so I have heard. I have not seen any actual data though.

Click to expand...

I have not seen any data either. It makes "sense" that as the flame burns off the insulation and pushes the igniter wire out the nozzle that it would experience a short. Then again, I've never heard of anyone complaining about their air start timer/altimeter being damaged from that condition. I'm not sure I would worry about it considering how many other more likely failure modes there are in an air start flight.

Handeman said:

I also thought of DD. If a shorted lighter on the drogue fries the altimeter, there wouldn't be a high speed deploy of the main.

Click to expand...

That's partly my point - with proper electrical protection in place a shorted drogue or main won't fry the altimeter and it lives on assuming it doesn't get crunched. And yes you are correct, opens would be a far more common failure mechanism in the deployment area and deserve more attention.

Thanks for the info...It seem that Lipos can be used, bit that they are not hassle free by any means.

It is really helpful to know that shorts are a very rare failure mode.