LPR: In Amperes We Trust: New Sealed Lead Acid Battery for Home-brew Relay-Based Cluster Ignition System

[brockrwood]

I was using a pair of 10 year old Energizer 6V lantern batteries (hooked up in series for 12 volts) as my ingnition power source for a home-brew, relay-style cluster engine launch controller.

One of the 6 volters finally went flat.

Got this Sealed Lead Acid battery on Amazon for $23. 12v, 9Ahr. Charges up to a healthy 13.7 volts on my Black and Decker trickle charger. The 9 amp-hours of current capacity should be enough current for black powder clusters of 2 to 4 engines, no? I think the rule of thumb is 2 amps of current per igniter for black powder engines. We shall see.

 

[UhClem]

You are confusing the Amp-Hour rating with internal resistance.

The battery is rated to supply 1A for 9 hours. Or 100mA for 90 hours. It would probably be a bit less than 9A for 1 hour because batteries usually supply less energy at higher currents.

If you supply 10A for 1 second per launch...

 

[neil_w]

Amp-hours is the total energy storage of the battery; that'll dictate how many launches you can get out of the battery. The 2A/igniter requirement is instantaneous current. A lead-acid battery can usually delivery *tons* more than you need, provided your wiring and relays are sufficiently robust.

(d'oh, ninja'd by @UhClem)

 

[brockrwood]

You are confusing the Amp-Hour rating with internal resistance.

The battery is rated to supply 1A for 9 hours. Or 100mA for 90 hours. It would probably be a bit less than 9A for 1 hour because batteries usually supply less energy at higher currents.

If you supply 10A for 1 second per launch...

So, if it can supply 1 amp for 9 hours, then for, say, a four engine cluster, it would need to supply 8 amps for just a few seconds. It should be able to do that, don’t you think? I guess I need to look at the current output over time chart. I need to find the datasheet for this battery.

 

[neil_w]

So, if it can supply 1 amp for 9 hours, then for, say, a four engine cluster, it would need to supply 8 amps for just a few seconds. It should be able to do that, don’t you think? I guess I need to look at the current output over time chart. I need to find the datasheet for this battery.

Lead acid batteries can supply tons of current, *way* more than 8 amps. Again, just ensure that your wiring and connections are sufficiently robust.

 

[brockrwood]

Datasheet for the Mightymax ML9-12 SLA battery. The discharge curve on page 2 shows that the battery can put out 9 amps for about 4 minutes while discharging from 13V to 12V.

All I want is for the battery put out 8 amps for 1 to 3 seconds (to launch a rocket). This battery should be able to do that easily, all day long, during a rocket launch.

Hmm. Maybe I can even use a 7 amp-hour battery? Nah. When in doubt, over-engineer the system rather than under-engineer it.

 

[DaHabes]

I've launched a Cluster Duck (7 BP motors) with a 12V SLA battery. Do it....

 

[brockrwood]

just ensure that your wiring and connections are sufficiently robust.

The 18 gauge lamp cord should handle the current, yes? The igniter clips were soldered on when I was in high school (1977 - 1981). I think they are on there pretty good!

(The igniter clip piece of lamp cord was recycled from a “doorbell button” launch controller I made in high school. One launch button. No on/off switch. No continuity indicator. Safety was not a big issue for me as a teen. I am lucky to be here today.)


The relay is a 12v automotive relay (12v to actuate it) that is pretty big. All I had handy when I made the launch controller. I figure if it can switch loads in a car, it can handle the current to launch rockets.

 

[neil_w]

The 18 gauge lamp cord should handle the current, yes? The igniter clips were soldered on when I was in high school (1977 - 1981). I think they are on there pretty good!

(The igniter clip piece of lamp cord was recycled from a “doorbell button” launch controller I made in high school. One launch button. No on/off switch. No continuity indicator. Safety was not a big issue for me as a teen. I am lucky to be here today.)

Short runs of 18 gauge should be fine. Verify that the connections are all very solid. Test the resistance of the entire loop.

 

[brockrwood]

View attachment 519366

Short runs of 18 gauge should be fine. Verify that the connections are all very solid. Test the resistance of the entire loop.

Tested the resistance. Zero ohms.

 

[mh9162013]

Tested the resistance. Zero ohms.

Double check the "decimal setting" on your device.

 

[brockrwood]

Double check the "decimal setting" on your device.

Auto-ranging multimeter.

 

[UhClem]

Tested the resistance. Zero ohms.

Unpossible. The best way to test is with a known load. I like a 1 Ohm 25W resistor. The RH25 series has very good short term overload capability.
Connect instead of an igniter and measure the voltage across it.

 

[FredA]

if it truly can dump 135A, I would stand back from that 25W resistor....

 

[neil_w]

Well it could only dump about 12A into that resistor since it’s a 12V battery…

 

[brockrwood]

Unpossible. The best way to test is with a known load. I like a 1 Ohm 25W resistor. The RH25 series has very good short term overload capability.
Connect instead of an igniter and measure the voltage across it.

Ok. Re-ran resistance check with the sensitive “LCR” meter on 20 ohms setting. The resistance in each half of the circuit, with whip clip attached, is .14 ohms. BUT, the resistance in the test leads themselves is .08 ohms. So the real resistance in each half of the circuit is .06 ohms. Total resistance in whole loop is .12 ohms. Not bad for stuff I soldered 40 years ago.

 

[FredA]

Well it could only dump about 12A into that resistor since it’s a 12V battery…

Duh.....sorry you are right....

 

[Steve Shannon]

Datasheet for the Mightymax ML9-12 SLA battery. The discharge curve on page 2 shows that the battery can put out 9 amps for about 4 minutes while discharging from 13V to 12V.

All I want is for the battery put out 8 amps for 1 to 3 seconds (to launch a rocket). This battery should be able to do that easily, all day long, during a rocket launch.

Hmm. Maybe I can even use a 7 amp-hour battery? Nah. When in doubt, over-engineer the system rather than under-engineer it.

Our club launch system has used 7 Ah SLA batteries for nearly 20 years. They last all day long and they last for years and yearsbefore needing replacement.

 

[brockrwood]

So, I sweated the math and did some circuit analysis of my cluster ignition system.

The objective was to determine how much electric current each rocket engine igniter will receive based on the number and types of igniters in my system and the characteristics of the system.

This required a basic, working knowledge of Ohm’s Law, how series and parallel resistances work in a simple DC circuit, and how to analyze a simple DC circuit that has both series and parallel resistances in it.

The results of my efforts are shown in the attached PDF.

Short Answer:
With my 13.7V sealed lead acid (SLA) battery, a resistance of 0.12 ohms in the wires and clips in the launch system, and 4 Estes igniters (for black powder rocket engines), each with a resistance of 0.8 ohms, the total current supplied to each igniter is 10.703125 amperes. That is far more than enough current to get each black powder engine started in a hurry. The rule of thumb is to supply at least 2 amps to each igniter.
This system can supply 5 times that. We should be good on launch day as long as the SLA battery is fully charged at the start of launching.

Long Answer:
Read the PDF.

(Note: As noted by another forum member, above, the datasheet for the SLA battery says that the battery can put out 135 amps for 5 seconds, so, it should be able to put out 42.8125 amps (4 x 10.703125 amps) for 1 to 3 seconds, to ignite each engine.)

 

[brockrwood]

Update: The homemade launch controller with the 12V, 9 Ah battery has now been used several times. Works great with the old type Estes igniters with the black pyrogen on three engine clusters. Have not tried it with the igniters that come with the “translucent” pyrogen.

 

[techrat]

FYI: in Ham Radio circles, the 12v 7amp/hr SLA is a known factor. I built a battery box of three tied together in parallel to get amps not volts. In other words, it was a 12v 21amp/hr battery. That can run a 100 watt ham radio in the field for more than a few hours before needing a charge. Your duty cycle of needing a few amps for a few seconds and then off for minutes at a time until the next launch is NOTHING to these batteries. Chances are after a full day out launching, check the voltage and you probably won't see more than a tenth of a volt dip.

 

[Grant_Edwards]

Well it could only dump about 12A into that resistor since it’s a 12V battery…

12A at 12V is 144W.

Running a 25W resistor at 144W won't take long until "something happens".

Most power resistors are wire-wound, and my guess is the wire will melt open (like a fuse).

 

[neil_w]

12A at 12V is 144W.

Running a 25W resistor at 144W won't take long until "something happens".

Most power resistors are wire-wound, and my guess is the wire will melt open (like a fuse).

Agreed.

 

[Grant_Edwards]

So, I sweated the math and did some circuit analysis of my cluster ignition system.
[...]

Short Answer:
With my 13.7V sealed lead acid (SLA) battery, a resistance of 0.12 ohms in the wires and clips in the launch system, and 4 Estes igniters (for black powder rocket engines), each with a resistance of 0.8 ohms, the total current supplied to each igniter is 10.703125 amperes.

Nit: you've ignored the battery's internal resistance. According to the datasheet that is "20mo", which I assume means 0.02 Ohms. That reduces your current by about 5% (still way more than enough).