Custom launch controller - battery question

I recently made my third custom launch controller. I didn't build it for car battery power sources (on purpose) as I just wanted a low power launch solution.

My question for those of you who have done this more is how long should two 9V batteries (wired in parallel) last? So far I've gotten 6 ignitions off it, but it's not instant. I'm thinking of either wiring in two more 9V battery clips in parallel, or getting another box to house several D-Cell batteries. But that would mean an external power source which I wanted to avoid for this low-power application.

Should I expect these 9V batts to die fairly quickly, or should they be good to go for 30+ launches?


Why custom? I was originally just going to do a quck and dirty solution involving touching a 9V battery to a wire running to the launch pad. But then I saw this switch at Radio Shack. $58 later I have all the parts for my custom launch controller complete with arming key (large mono headphone plug and jack), arming switch with safety cover, a buzzer that beeps every couple seconds when the safety key is in, a flashbulb safe continuity indicator, pushbutton launch, and plug-in launch cables (again, large mono headphone jack)

Edit: Any suggestions on where I can get pieces parts at more reasonable prices than those offered by Radio Shack? I imagine some people might be interested in a controller like this, but not at a price that would be mandated if parts cost $60...

Edit: I figured some of you may want to see what it looks like... or not... Anyhow, here's a picture.

Should I expect these 9V batts to die fairly quickly, or should they be good to go for 30+ launches?

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I use the Quest launch controller, which uses a 9V battery. I get over 50 launches from one battery when using Estes Solar igniters.

I also have Quest MicroMaxx launcher which uses a 9V battery. With the QMX igniters I get about 25 launches if I'm lucky.

I'm thinking of either wiring in two more 9V battery clips in parallel...

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I wouldn't recommend wiring batteries in parallel. You will get a bit more oomph to the igniter because the internal resistance of the batteries will be reduced, but the effect may not be very big. Wiring them in series will give you much more power to the igniter, resulting in faster ignition. Of course, you must make sure that your lights and beeper can handle the higher voltage.

One option is to make the voltage switchable for one or two batteries. For example, the Estes Command Controller has two 7.2V batteries. A switch allows it to use either 7.2V or 14.4V. The lower voltage works good for single Estes igniters. The higher voltage works great for firing clusters or Copperheads.


This was my 500th post.

Originally posted by BobCox

I wouldn't recommend wiring batteries in parallel. You will get a bit more oomph to the igniter because the internal resistance of the batteries will be reduced, but the effect may not be very big. Wiring them in series will give you much more power to the igniter, resulting in faster ignition. Of course, you must make sure that your lights and beeper can handle the higher voltage.

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I measured the current delivered to the igniter leads as about 1.5 amps for one battery, 2.5 for two in parallel. Hooking them in series would increase the voltage difference, but the current delivery may actually be worse - although I didn't test the current across two series 9V batteries.

I just measured my wires, and I have about 2.1 ohms of resistance. Figuring in perhaps another ohm for the controller, that gives me 3 ohms total resistance when the launch button is pressed. So theoretically, a 9V battery should give me 3A of current. But it doesn't because the tiny batteries inside the 9V case aren't capable of delivering that current. And so while doubling the voltage would theoretically double the current, that may not happen in practice. Anyhow, I'm pretty sure that that parallel batteries deliver more current (in a situation where a circuit provides low enough resistance that the current delivery capacity of a battery is exceeded)

Next time I build a controller, or if this one dies prematurely, I'll do some experaments with series/parallel battery hookups to see which configuration will give me the highest current delivery to the igniter.

But at least now I know I should expect the batteries to last a while.

Originally posted by Steward


Dan... If I may...

I built mine with the internal "D" cells. Wired in series, they have lasted the entire season, lighting low power motors with ease.

If you notice on the picture, there is a switch and inputs for using it as a twelve volt system.

It 's worked great for me going on about five years now...

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I didn't use D cells since I din't think I'd have the internal space without going with a much larger box. But next time I'll do that so I can fit one of those holders in there, since I'm sure D cells can provide much more current than those 9V jobs.

My other launchers do have external hookups and I've launched off a deep cycle 12V battery before. It's nice, but I didn't want to do that with this incarnation. I'll see how my current high power controller looks when I visit my parents next month.

But I would like to just do one "end all" controller really nice. I'd have internal and external hookups all with nice plug jacks and heavy 12 gague internal wiring. Also, I'd be sure to include internal connectors so that the top panel can be removed completely, along with a circuit card connector. In the past all my stuff has just had wires coming out of the box for external sources. But I guess that was before I was too terribly concerned about making it well built inside and out...

The controller looks nice. Where did you get the parts from?

Originally posted by DanLW
I recently made my third custom launch controller. I didn't build it for car battery power sources (on purpose) as I just wanted a low power launch solution.

My question for those of you who have done this more is how long should two 9V batteries (wired in parallel) last? So far I've gotten 6 ignitions off it, but it's not instant. I'm thinking of either wiring in two more 9V battery clips in parallel, or getting another box to house several D-Cell batteries. But that would mean an external power source which I wanted to avoid for this low-power application.

Should I expect these 9V batts to die fairly quickly, or should they be good to go for 30+ launches?


Why custom? I was originally just going to do a quck and dirty solution involving touching a 9V battery to a wire running to the launch pad. But then I saw this switch at Radio Shack. $58 later I have all the parts for my custom launch controller complete with arming key (large mono headphone plug and jack), arming switch with safety cover, a buzzer that beeps every couple seconds when the safety key is in, a flashbulb safe continuity indicator, pushbutton launch, and plug-in launch cables (again, large mono headphone jack)

Edit: Any suggestions on where I can get pieces parts at more reasonable prices than those offered by Radio Shack? I imagine some people might be interested in a controller like this, but not at a price that would be mandated if parts cost $60...

Edit: I figured some of you may want to see what it looks like... or not... Anyhow, here's a picture.

Click to expand...

I wouldn't use 9volt batteries at all. At least AAs would be better. 6 of them would be 9 volts or 8 would be 12 volts.

I highly recommend 9.6 volt NiCd radio-controlled car batteries. They light igniters instantly and last for hundreds of flights before they need charging(I think. I haven't flown that much).

I get my electronics form Digikey (https://www.digikey.com/). Their online store is not too easy to use so I got their 1680 page catalog (It's free!). You'll probably still end up getting a lot of stuff at Radioshack, but Digikey is great if you ever need something unusual, in bulk, or that costs more than $2 at Radioshack. For example white LEDs cost $5.29 at Radioshack, but only $1.16 from Digikey (depending on the brand you get). I just ordered LED holders from Digikey, 25 for $1.5, but Radioshack has 5 for $1.29.

It is really mostly semiconductors (ICs, diodes, transistors, LEDs, etc.) and other components (resistors, capacitors, inductors). They don't sell much wire (like speaker wire) and they don't have any incandescent light bulbs (flashlight bulbs).

The basic issue is that 9V batteries are not really intended to put out high current. A bit over an amp is usually about it. Anything from AA cells on up will source a lot more current. Current will light those small ignitors right up.

You might consider a small gel cell. Many folks use 7 amp/hr ones for composite ignitors with great sucess. For BP ignitors you could use a smaller 2 to 3 amp/hr battery.

A good source for misc parts and small gel cells is All Electronics. DigiKey as mentioned by others is good. Mouser Electronics is also great.

Just as an aside I bought 17 amp/hr gel cells for LDRS 23 in 2004 (about 1500 launches) plus this years launches and have not had to charge them yet.

Al
BRS VP
NAR L2

I had a gel cel for a while, I think. But like I said, the purpose of this particular launcher is for low power fun. I may take what I learned with this one and apply it to an all around high current bohemoth that uses a lantern battery or external hookups.

Hmmm. A 9.6V battery pack just may fit inside the case. I'll have to check it out... in a month or two. I just blew a lot of money today buying almost 15,000 christmas lights from Wal Mart at 50% off. (Doing a 32-channel computer controlled light show next year)

Originally posted by DanLW
I measured the current delivered to the igniter leads as about 1.5 amps for one battery, 2.5 for two in parallel. Hooking them in series would increase the voltage difference, but the current delivery may actually be worse - although I didn't test the current across two series 9V batteries.

I just measured my wires, and I have about 2.1 ohms of resistance. Figuring in perhaps another ohm for the controller, that gives me 3 ohms total resistance when the launch button is pressed. So theoretically, a 9V battery should give me 3A of current. But it doesn't because the tiny batteries inside the 9V case aren't capable of delivering that current. And so while doubling the voltage would theoretically double the current, that may not happen in practice. Anyhow, I'm pretty sure that that parallel batteries deliver more current (in a situation where a circuit provides low enough resistance that the current delivery capacity of a battery is exceeded)

Next time I build a controller, or if this one dies prematurely, I'll do some experaments with series/parallel battery hookups to see which configuration will give me the highest current delivery to the igniter.

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It looks like you are near that region where series batteries would deliver more current than parallel, but the difference is not dramatic. The formulas are:
(1 Battery) I = V / (Rb + Rl)
(2 Series) I = 2*V/ (2*Rb + Rl)
(2 Parallel) I = V / (Rb/2 + Rl)
where
I = current (amps),
V= battery voltage (volts),
Rb= battery internal resistance (ohms),
Rl= resistance of wiring + controller + igniter.

A parallel hookup has the same voltage as a single battery with half the internal resistance. Hooking the batteries up in series gives twice the voltage but twice the internal resistance compared to a single battery.

If you plot these formulas for various values of Rb and Rl, you will see that whenever Rl is larger than Rb the series connection will deliver more current. A typical Solar igniter is about 1 to 2 ohms, plus a wiring resistance of 0.5 to 3 ohms, for a total Rl of 1.5 to 5 ohms. Most batteries will have internal resistance far less than that so a series connection makes sense for them.

A Panasonic alkaline 9V is specified as having a nominal internal resistance of 2.67 ohms, which is pretty high compared to other battery types. Based on the measurements for your controller, you should be able to get a little more punch by hooking the batteries in series, but the difference may not be really dramatic. If you put heavier wiring on your launcher to lower the lead resistance or if you use multiple igniters in parallel, then a parallel battery would be better.

As others have pointed out, the 9V transistor battery was not designed for high current drain, so the internal resistance is pretty high. If you want to pump a lot of current, you should try a different type of battery.

Originally posted by BobCox
... One option is to make the voltage switchable for one or two batteries. For example, the Estes Command Controller has two 7.2V batteries. A switch allows it to use either 7.2V or 14.4V. The lower voltage works good for single Estes igniters. The higher voltage works great for firing clusters or Copperheads.

This was my 500th post.

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Bob,

I bought a Command Controller on clearance at Hobby Lobby a month or so ago. It didn't come with batteries. Or instructions, I don't think. While it looks easy enough to use, what 7.2 V batteries are meant to be used with it?

Congrats on the post number, btw.

When I started in model rocketry I decided to be clever and built a controller with a 9V battery. It only lasted a few firings. They're just not up to it.

Originally posted by JRThro
Bob,

I bought a Command Controller on clearance at Hobby Lobby a month or so ago. It didn't come with batteries. Or instructions, I don't think. While it looks easy enough to use, what 7.2 V batteries are meant to be used with it?

Congrats on the post number, btw.

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It is probably 7.2 volt RC car batteries. They are very common. You can probably buy them at Radioshack.

You can get a simple 7.2 volt battery and charger here:
https://www.radioshack.com/product/index.jsp?productId=2062121

I used a connector repair kit to connect one (I used 9.6v because I had some) to my controller.
https://www.radioshack.com/product/index.jsp?productId=2062132
If I ever get a 12 volt battery I can use the other half of the kit to connect it to my controller.

Originally posted by spacecadet
When I started in model rocketry I decided to be clever and built a controller with a 9V battery. It only lasted a few firings. They're just not up to it.

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I have two Quest controllers that use 9V batteries, and I love them. I replace the batteries once per year, which works out to about 50 launches from each one.

Originally posted by JRThro
... what 7.2 V batteries are meant to be used with it?

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As Matt already posted, the Command Controller uses one or two 7.2V RC car battery packs. I don't have mine with me at the moment, but I'm pretty sure it has Tamiya connectors. IIRC, I paid about $50 for two batteries and two chargers on eBay.

When you are comparing batteries, don't worry much about mAh ratings. In a perfect world, the higher the rating the more launches you could get out of each charge. In the real world, you will lose more to self-discharge of the battery while it is in storage than you will from launching rockets. It's a good idea to top up the batteries before a launch if they have not been charged in the last month or so.

Well, I modified my controller. I re-did the circuit card inside with a 555 timer instead of the TTL multivibrator I was using to turn the buzzer on and off. Now the function of the buzzer is more solid.

Also, I got a battery holder that holds 8 AA batteries, along with some more AA NiMH batteries and charger. Now my controller puts out a nice 4 amps of current. Even with a set of rechargables that needed a charge I got a little over 2A out of them. So now, I'm putting out about 1.5 amps more than with the two 9V batteries.

The nice thing is that the battery holder connects using a 9V clip, so if they go dead on me I can use a backup 9V battery inside my box in lieu of the battery pack.