Weird issue with e-matches.

[cwbullet]

I buy procell Duracell 9v batteries like we use in medical equipment. I have never hand one fail.

 

[ozwald]

Awesome, today I learned a cheap lesson instead of an expensive one later.

I remember discussing this very same thing at the Hope site with you. I remember that your battery was a little disfigured after one of your flights when you were breaking down your AV bays. Very good info to know

 

[Titan II]

When I use use 9V batteries, I exclusively use Procells bought on Ebay. I just checked some I bought May, 2014. The three unused ones are 9.53, 9.58 and 9.58 volts. They never go below 9V after a flight.

 

[Handeman]

I have a threshold of 9.1 volts. But what altimeters and e-matches are you using? I don't think any of my batteries read even 9 volts after a single flight with dual deploy. Maybe they sit longer on the pad and I'm slower to recover? But I can't imagine using the same battery for more than one dual-deploy flight.

I use perfectflite altimeters. HiAlt45, SL100 and CF. They all seem to read about 0.1V less then my DMM reads, which is why I let them get below 9V as read by the altimeter before replacing. Most read 9.2V the first flight or two and 9.1 the rest of the season. They are usually at 9.1 or 9.0 the next year.

If it works for you, that's great, you've got more intestinal fortitude than I do.

An important point about AA's vs. 9 volts: AA's have about 2000mAh capacity vs. 450-500mAh for 9 volts. So a AA has far more capacity to set off e-matches that can be fired by low voltage. And don't forget that battery chemistry allows them to regain voltage levels that read high but drop quickly under load, so if not tested under load they can give a false sense of confidence.

Actually the amount of current going through the ematch has little or nothing to do with the battery capacity. It is a function of the match resistance, battery voltage and internal battery resistance. Not considering the battery resistance for now, a 9V battery would try to send 7.5 amps through a 1.2 ohm ematch. The 1.5 V AA would try to send 1.25 amp through the same 1.2 ohm ematch. Add internal battery resistance and those values get lower.

Finally, a check of prices of Duracell 9 volt (only ones I've ever used) shows just under $2 each at Costco, but a whopping $3.50 in a blister pack at Walmart. I won't by batteries over the internet - read some of the reviews in this example:

https://www.amazon.com/Duracell-Coppertop-9V-Size-Pack/dp/B0074M2J88/ref=sr_1_4_a_it?ie=UTF8&qid=1461280143&sr=8-4&keywords=duracell+9+volt

Batteries are like the motors, if it fails the results are rarely good.


Tony

I agree that battery failure is bad, but my personal experience has been that battery failure is much less likely then ematch failure. In my experience, I've had twice as many commercial ematches that tested good pre-flight come back with the pyrogen intact, but the bridge wire open then failures I could attribute to a bad battery. The post-flight resistance was measured from each side of the match head BTW.
Add in the chances of switch failure, bad recovery system setup, fouled chutes, etc. and I think the use of a previously flight proven battery with good voltage is less of a risk then almost everything else in the system. Just MHO YMMV

 

[rocket_troy]

Actually the amount of current going through the ematch has little or nothing to do with the battery capacity. It is a function of the match resistance, battery voltage and internal battery resistance. Not considering the battery resistance for now, a 9V battery would try to send 7.5 amps through a 1.2 ohm ematch. The 1.5 V AA would try to send 1.25 amp through the same 1.2 ohm ematch. Add internal battery resistance and those values get lower.

Careful here though. In these specific discussions (especially when 9V batteries are one of the comparative points) it's dangerous to say "Not considering the battery resistance for now" without addressing that specific issue at some point in the explanation as 9V batteries were (not so much now) notorious for their internal impedance. It's not likely to be an issue for those flyers happy to dunk the hand into their pockets to purchase a decent (not extravagant) 9V battery, but it's important to stay clear of some of the cheapies as their internal resistance can provide you with substantially lower current flow than a reasonably AA for even modest loads like Ematches.
https://www.learningaboutelectronics.com/Articles/Battery-internal-resistance

However, I'm in general agreement with all the points you make.

 

[Jeffmhopkins]

A battery's ability to source current can be offset by capacitor based pyro circuits though... I use them to avoid brown outs mainly, but I like them for that added safety. A completely secondary pyro ignition battery source would work as well though, particularly with how cheap some of these lipo packs are these days...

 

[rocket_troy]

A battery's ability to source current can be offset by capacitor based pyro circuits though... I use them to avoid brown outs mainly, but I like them for that added safety. A completely secondary pyro ignition battery source would work as well though, particularly with how cheap some of these lipo packs are these days...

Indeed, a good point. I remember 1st seeing that technique used back in the 90s with Adept's 1st rev of their Altimeter utilising a cap to provide a spike of current to complement the very high impedance 12V A23 (or whatever the code is for those little things you use in your garage door remote).

Troy

 

[manixFan]

...Actually the amount of current going through the ematch has little or nothing to do with the battery capacity. It is a function of the match resistance, battery voltage and internal battery resistance. Not considering the battery resistance for now, a 9V battery would try to send 7.5 amps through a 1.2 ohm ematch. The 1.5 V AA would try to send 1.25 amp through the same 1.2 ohm ematch. Add internal battery resistance and those values get lower....

I think you misunderstood my post as that's exactly my point - the AA doesn't send as much amperage through the e-match to fire it and has a far bigger supply. When I talk about capacity in this instance I mean the number of matches I can fire with a single battery. It was in reference to a post about being able to fire a lot of matches with just a 1.5 volt battery.

I used 6 altimeters at BALLS last September. I still have two of the batteries (used just once) and two unused on hand. I tested them with a digital volt meter and then with the same meter using the battery test circuit:

Used once:
              DVM       10mA Load
Battery #1   9.0 v        8.8 v
Battery #2   9.0 v        8.7 v

Unused, same batch
              DVM       10mA Load
Battery #3   9.6 v        9.4 v
Battery #4   9.6 v        9.4 v

I don't use the plain multimeter circuit to check voltage but always do it under a load using the battery test circuit. The meter I currently use is a fairly inexpensive Innova 3320 (about $30 on Amazon). It uses a 10mA load to test a 9 volt battery.

I've been flying for over 15 years and still have my very first altimeter, an RDAS Compact. I now have over a dozen altimeters including 3 Perfectflite CF's. Obviously the newer units have far less battery drain than the older units. Perhaps I'm just being way too conservative using a new battery each time but it's been working for 15 years so I'm reluctant to change my ways. As always, your milage may vary.


Tony

ps: I should have pointed out that the article rocket_troy linked to also does a good job explaining why a AA is good at firing e-matches and also why testing under a load is useful. Also why you can't ignore internal resistance, especially when firing off matches that typically have a resistance of just 1-3 ohms.

 

[UhClem]

I've been flying for over 15 years and still have my very first altimeter, an RDAS Compact. I now have over a dozen altimeters including 3 Perfectflite CF's. Obviously the newer units have far less battery drain than the older units.

The RDAS Compact was notorious for its high current requirements (70mA to 90mA) which were higher than pretty much every other altimeter by quite a bit. For comparison, the AltAcc needs less than 5mA.

 

[manixFan]

The RDAS Compact was notorious for its high current requirements (70mA to 90mA) which were higher than pretty much every other altimeter by quite a bit. For comparison, the AltAcc needs less than 5mA.

Dave,

Funny you mention the AltAcc. Below is a photo of one of mine. It's one of the reasons I developed my always test and never reuse a battery mentality. I was simply following Scott's instructions as printed on the unit itself. Obviously Scott knew the current draw of his units yet he still very clearly states to use a fresh battery each time. To me a fresh battery meant a new unused one, not one that had been flown before. It's possible I misread what he meant but it seems unlikely.


Tony

 

[thomas]

I use always e-matchs from different manufactures for the main and the redundant altimeter. And I prefer manufactures that also produce igniters for technical applications like airbags or blasting caps. The chinese