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I'm building a launch controller for a model rocket, and I've worked out that I need to draw 2 A for no more than 5 seconds. Given most commercial 9 V batteries can give me ~500 mAh, would a 9 V alkaline smoke alarm battery give me what I need?

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    \$\begingroup\$ A typical 9V battery has an internal resistance of 1~2 Ohms. So, drawing 2A current will cause a considerably amount of battery voltage drop. Parallelling a few 9V batteries may be a solution. \$\endgroup\$ – Rohat Kılıç Mar 26 at 5:16
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    \$\begingroup\$ Do you mean a Lithium 9V battery? They could probably supply more current than an alkaline 9V battery, but you should find the specification so you can verify. \$\endgroup\$ – Mattman944 Mar 26 at 7:36
  • \$\begingroup\$ Thanks for that - I've now specified that its alkaline. \$\endgroup\$ – Woodman Mar 26 at 7:41
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    \$\begingroup\$ Use an elco to give you that 5s buffer. Or actually, a small bank of them. Ever looked at hand-held magnetic guns? That's how they do it. \$\endgroup\$ – Mast 2 days ago
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    \$\begingroup\$ "Smoke alarm batteries" would be the exact opposite of what you need. They're supposed to provide a small current over a period of years. Practically speaking, their internal resistance does not matter as they only source a few microamp. \$\endgroup\$ – MSalters 2 days ago
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An Estes rocket engine squib looks about like this:

enter image description here

(The above picture comes from a document I wrote on the topic.)

Estes provides very clear all-fire specifications for their squibs:

  • \$\frac12\:\text{J}\$ within \$50\:\text{ms}\$ into a resistance that is spec'd at \$\frac23\:\Omega\$.

This implies an average of \$\sqrt{\frac{\left[\frac{\frac12\:\text{J}}{50\:\text{ms}}\right]}{\frac23\:\Omega}}\approx 3.9\:\text{A}\$.

As a \$9\:\text{V}\$ alkaline battery will have about 3 times that resistance, internal to the battery. This means that the supply voltage must be \$V \approx 10.4\:\text{V}\$. If you go review the charts at this site, Discharge tests of 9 Volt transistor radio style batteries, I think you'll find that even at just \$1\:\text{A}\$, a fresh \$9\:\text{V}\$ battery might deliver about \$6.5\:\text{V}\$ for a short time. (That chart on that web page is consistent with the figure of about \$2\:\Omega\$ for the internal battery resistance, fresh.) If you were to attempt to draw four times that much, you can be pretty sure that it won't meet the all-fire specification. Not even close.

If you do the calcs, you can perhaps consider adding a \$22\:\text{mF}\$ capacitor with a voltage rating of over \$10\:\text{V}\$, placed in parallel to the \$9\:\text{V}\$ battery. That may work. But they do cost some money and they aren't small.

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    \$\begingroup\$ I think the best answer would be to use a bunch of AAs, at minimum, instead of an alkaline 9V. A lithium or NiMH 9V might also work. \$\endgroup\$ – Hearth Mar 26 at 12:23
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    \$\begingroup\$ @Hearth I agree. What got me started on Estes was watching folks haul car batteries to a launch site. I wondered how I might use a CR2032! I contacted the primary engineer at Estes and we talked for a while. Afterwards, he sent me their specifications and a huge box full of thousands of igniters to test. I can now say that it is possible to use a \$500\:\mu\text{A}\$ draw from a CR2032 (well, well within its specs) and deliver the needed pulse to the igniter. There is a problem to solve. And the hard problem isn't the switcher. That's an easy part. I wonder who can finger the hard part. \$\endgroup\$ – jonk 2 days ago
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Back when I was into model rockets I had a commercially made launch controller. That used 4 D cells in series. These have a much higher capacity than the small 9 volt cells do. Push the button and the igniter in the rocket motor would burst into flames the instant I pushed the button. I have no idea how much current it actually drew. The igniters were destroyed in the process so holding down the button even for several seconds didn't matter as they stopped drawing current during the launch. If I remember correctly the ones I bought only needed 3 volts. I can't check my launch controller, tossed it decades ago.

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Its hard to say without knowing the details of the fuses you are trying to ignite.

Here are a few battery choices. In general rechargeable NiMh batteries will have much lower output impedance than alkaline. AA or AAA batteries will have lower output impedance than 9V batteries.

Taking the Duracell MN1604 as a typical example of an alkaline 9V battery. The internal resistance is about 2.3 ohms at full charge.

At 2A discharge the output voltage would be 9V - 2.3 ohms * 2A = 4.4V.

https://d2ei442zrkqy2u.cloudfront.net/wp-content/uploads/2016/03/MN1604_US_CT1.pdf


Taking the Energizer NH22NBP as a typical example of a NiMh 9V battery. The internal resistance is rated at 1 ohm. This is considerably less than the alkaline version.

At 2A discharge the output voltage would be 8.4V - 1 ohms * 2A = 6.4V.

https://data.energizer.com/pdfs/nh22-175.pdf


Even better yet use a AA batteries.

Taking the Duracell MN1500 as a typical example of an Alkaline AA battery. The cell voltage is 1.5V and the internal resistance is 0.071 ohms. A stack of six alkaline AA batteries in series would have an output voltage of 6 * 1.5V = 9V, and an output impedance of 6 * 0.071 ohms = 0.426 ohms.

With six batteries in series and a 2A discharge rate the output voltage would be 9V - 2A * 0.426 ohms = 8.148V.

https://d2ei442zrkqy2u.cloudfront.net/wp-content/uploads/2020/02/14085905/MN15US11191.pdf


Taking the Panasonic HHR150AA as a typical example of a rechargeable NiMh AA battery. The cell voltage is 1.2V and the internal resistance is 0.02 ohms. A stack of eight NiMh AA batteries in series would have an output voltage of 8 * 1.2V = 9.6V, and an output impedance of 8 * 0.02 ohms = 0.16 ohms.

With eight batteries in series and a 2A discharge rate the output voltage would be 9.6V - 2A * 0.16 ohms = 9.28V.

https://b2b-api.panasonic.eu/file_stream/pids/fileversion/3515

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There are several "9V" batteries. If you really want an alkaline primary battery, you could use a PP6 instead of the familiar PP3. It has about twice the capacity, but uses the same clips.

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    \$\begingroup\$ Afaict at least in the UK PP6 batteries are hard to find and expensive. The next realistic step up from a PP3 if you want 9V from alkaline batteries would be a battery holder with 6xAAA in it. \$\endgroup\$ – Peter Green 2 days ago
  • \$\begingroup\$ @PeterGreenthat's probably what I'd do myself, and some 6-cell holders have PP3-style battery clips. You could even get some big heatshrink and make a more robust battery that way. Last time I needed PP6s for something I found a sensibly-priced source, but that was several years ago \$\endgroup\$ – Chris H 2 days ago

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