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Discussion Starter #1 (Edited)
Lemme just start from the beginning. My little brother and I built a home made flamethrower using a electric fuel pump and an old compressor that we chopped up. It works great, but it has its flaws (mainly in the saftey department) so we are going to be designing and building a new one. The main mechanical issue we had was batteries. We have been using various iffy lead acid batteries to power the fuel pump, but the repeated deep discharges kill them (they already had a terminal in the grave) and not to mention they are bulky, heavy and just generally dangerous due to them having to be externally mounted and carried by a second person due to them not being able to be tipped on their sides.

Anyway, I have a few questions.

First off, does anyone have 'good enough' sticks? What I mean by this, is sticks not suitable to be used in a pack, but still have some capacity left to be used for what I need them for.

Secondly, what are the voltages per stick? I think it's 7.2 or something right?

Third Q, How would I go about charging them outside of a pack? Do I need a special charger, or could I rig something up?

4. How low of a voltage per stick before I start damaging the individual cells? I would only monitor the worst stick and cut it off when it goes below the 'danger' threshold.

My plan is to run 4 sticks, 2x2 parallel/series for a total of a 12Ah pack. We plan to encase them in a thin, openable metal box insulated by Styrofoam.

My little brother and I are not idiots, when testing our home made shotguns(another story) or the flamethrower we wear metal plates for armor as well as full welding gear, and he 3 fire extinguishers on standby. We are perhaps, maybe a little crazy.

Also, I COULD just go buy a car jumper pack for 60-100$, but where is the fun in that?

[Edit]: I forgot to mention, the fuel pump we are going to be using on the new build will pull 10-13A @100psi (what it will be pushing 100% of the time).
 

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Bad idea. Parallel NiMH can result in thermal runaway due to the -dV phenomenon. This becomes more likely as the stick deteriorates from optimal. You would need a special charger, and you would need to break them apart to safely charge them to full. For anything not suitable for use in a car, SD is going to be an issue and cell imbalance will require long/slow charges.

You would likely get better results from Lithium. Either a Lithium 12V replacement or rigging something up with R/C batteries. You could get a lot of life out of them by limiting their cycle range.

Or this:

https://www.ebay.com/itm/NEW-12V-20AH-SEALED-LEAD-ACID-BATTERY-CB19-12-ES1217-UB12200-LC-RD1217P/142332462108?hash=item2123ad041c:g:Db4AAOSwmgJY3t5K

Heavy-ish, but cheap. At 20Ah, you could keep the cycle depth to about 50% and get many hundreds of cycles out of it with 45-ish minutes of use per charge. That's a lot of flame!

EDIT: Personally, I think you should edit your post to cut out the meat of what you're doing... You just need to run a 10-13A fuel pump for about an hour in a portable configuration, right? :D
 

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Discussion Starter #3
Bad idea. Parallel NiMH can result in thermal runaway due to the -dV phenomenon. This becomes more likely as the stick deteriorates from optimal. You would need a special charger, and you would need to break them apart to safely charge them to full. For anything not suitable for use in a car, SD is going to be an issue and cell imbalance will require long/slow charges.

I had no idea, and I'm glad I asked, and it's also the reason I asked, because I had no idea.

You would likely get better results from Lithium. Either a Lithium 12V replacement or rigging something up with R/C batteries. You could get a lot of life out of them by limiting their cycle range.

I was thinking about getting 16 18650 batteries, and running them in series/parallel. That would only weigh about 4lbs (each 18650 is only about 45 grams). The problem with that is, I can't find anything that will hold 16 batteries, and I'm not sure if soldering direct connections onto the + and - will damage the battery.

Or this:

https://www.ebay.com/itm/NEW-12V-20AH-SEALED-LEAD-ACID-BATTERY-CB19-12-ES1217-UB12200-LC-RD1217P/142332462108?hash=item2123ad041c:g:Db4AAOSwmgJY3t5K

Heavy-ish, but cheap. At 20Ah, you could keep the cycle depth to about 50% and get many hundreds of cycles out of it with 45-ish minutes of use per charge. That's a lot of flame!

Meh, too heavy. That's a gallon and a half of fuel worth weight. I'll bookmark it for later. Just in case.

EDIT: Personally, I think you should edit your post to cut out the meat of what you're doing... You just need to run a 10-13A fuel pump for about an hour in a portable configuration, right? :D
But where is the fun in that? I like to have FUN!
Thanks for your response!
 

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I have 17 decent BB sticks that I've been wanting to make a project out of, but as seen here they don't make for easy conversion to fun projects. So for now they just sit on my work bench.
 

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My thinking is that you don't want to draw the attention of any "regulatory" agencies... :)

To state it more specifically, when you have NiMH cells in parallel, the phenomena of voltage drop when full means any parallel connected cells and any charge current will start dumping their current into the lower voltage (full) cell. Yes, I have done this many times. Yes, I did experience thermal runaway once... From that point forward I made sure to stay far away from 100% SoC. No, I would never recommend another potential catastrophic failure option in your particular application :)

"XS" indicates "X cells" or "X cells in series"
"XP" indicates "X cells in parallel"

LiPo 18650 might work. It depends on the operating voltage of the pump. 3S will get you 9-12.6V working range 4S will get you 12-16.8V. To get 12Ah, you'll need 6P, so either 18 or 24 total cells. 6P is about 12Ah capacity, so a 10-12A draw might be okay for your typical cells.

There are lots of custom 18650 cell holder options where you can snap them together in whatever arrangement you want. There are also LOTS of Youtube videos showing how to make your own packs, soldering and everything.

You could charge the pack with any suitable $20 LiPo charger, and you could easily add a balance cable for balance charging. If you limit peak charge to 4.1V/S, you'll only charge to about 80% SoC. I don't' recall the bottom voltage that corresponds to 20% SoC, but it's out there. Keep your SoC in that range, and the battery will last forever. If you need more operating capacity, you just add more cells in parallel to each serial string. 3-4S/8P will get you 16Ah.

This would be easy-mode if 4S works for you:

https://hobbyking.com/en_us/multistar-high-capacity-4s-20000mah-multi-rotor-lipo-pack.html

Brutally strong high discharge cells. At the lower currents you use, they should last forever if cycled in a very narrow range.

here's a 3S 8.4Ah:

https://hobbyking.com/en_us/turnigy-nano-tech-8400mah-3s-30c-lipoly-battery-w-traxxas-connector.html
 

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Discussion Starter #6 (Edited)
My thinking is that you don't want to draw the attention of any "regulatory" agencies... :)

To state it more specifically, when you have NiMH cells in parallel, the phenomena of voltage drop when full means any parallel connected cells and any charge current will start dumping their current into the lower voltage (full) cell. Yes, I have done this many times. Yes, I did experience thermal runaway once... From that point forward I made sure to stay far away from 100% SoC. No, I would never recommend another potential catastrophic failure option in your particular application :)

"XS" indicates "X cells" or "X cells in series"
"XP" indicates "X cells in parallel"

LiPo 18650 might work. It depends on the operating voltage of the pump. 3S will get you 9-12.6V working range 4S will get you 12-16.8V. To get 12Ah, you'll need 6P, so either 18 or 24 total cells. 6P is about 12Ah capacity, so a 10-12A draw might be okay for your typical cells.

There are lots of custom 18650 cell holder options where you can snap them together in whatever arrangement you want. There are also LOTS of Youtube videos showing how to make your own packs, soldering and everything.

You could charge the pack with any suitable $20 LiPo charger, and you could easily add a balance cable for balance charging. If you limit peak charge to 4.1V/S, you'll only charge to about 80% SoC. I don't' recall the bottom voltage that corresponds to 20% SoC, but it's out there. Keep your SoC in that range, and the battery will last forever. If you need more operating capacity, you just add more cells in parallel to each serial string. 3-4S/8P will get you 16Ah.

This would be easy-mode if 4S works for you:

https://hobbyking.com/en_us/multistar-high-capacity-4s-20000mah-multi-rotor-lipo-pack.html

Brutally strong high discharge cells. At the lower currents you use, they should last forever if cycled in a very narrow range.

here's a 3S 8.4Ah:

https://hobbyking.com/en_us/turnigy-nano-tech-8400mah-3s-30c-lipoly-battery-w-traxxas-connector.html
Of course! I would never make a product that violates any local laws. In california it can't be over 10 feet, and of course it doesn't shoot farther than that!

12-16v would be fine. 16v -might- be too high voltage (it would see 14.4 with an alternator running in a normal car) but idk.

Took a look at the links. I'm really liking that 14.8v 20,000ah battery. We only ever spray it for 15-20 mins at a time so... Yea. That's A LOT of fuel.
 

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Of course! I would never make a product that violates any local laws. In california it can't be over 10 feet, and of course it doesn't shoot farther than that!

12-16v would be fine. 16v -might- be too high voltage (it would see 14.4 with an alternator running in a normal car) but idk.

Took a look at the links. I'm really liking that 14.8v 20,000ah battery. We only ever spray it for 15-20 mins at a time so... Yea. That's A LOT of fuel.
The 14.8V is nominal. It's operating voltage range is 12-16.8. If the pump can take it, great. If you charge it to a peak of 16.4V and cut it off at 13.6V, you'll probably have about 45-60 minutes of squirting, and the battery will last for many thousands of cycles.

If the pump can't handle it, the 3S might be better. 10.2-12.3V would get you the same life, and about 15 minutes of use. 12V discharge termination is typically 10.5V, so I suspect the pump will work down to that level and a little below.

There are other batteries out there. I only checked Hobbyking.
 

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Discussion Starter #9
The 14.8V is nominal. It's operating voltage range is 12-16.8. If the pump can take it, great. If you charge it to a peak of 16.4V and cut it off at 13.6V, you'll probably have about 45-60 minutes of squirting, and the battery will last for many thousands of cycles.

If the pump can't handle it, the 3S might be better. 10.2-12.3V would get you the same life, and about 15 minutes of use. 12V discharge termination is typically 10.5V, so I suspect the pump will work down to that level and a little below.

There are other batteries out there. I only checked Hobbyking.
Yea the fuel pump will operate at 10v no problem, just won't flow as much. I'll probably end up getting that 14.8v 20000 pack just for ease of use. The longer the voltage can stay above 12v the farther it will spray.
 

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Yea the fuel pump will operate at 10v no problem, just won't flow as much. I'll probably end up getting that 14.8v 20000 pack just for ease of use. The longer the voltage can stay above 12v the farther it will spray.
Well, then the concern becomes will the pump blow up at 16.4V.

I would use it between 13.6 and 16.4V. That's about 20-80% SoC, and it should last a very long time.

We all expect videos... :)
 

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Oh... and to bring it back around... 5.5Ah of just 2 sticks in series is still about 15-20 minutes of squirting at 10-13A.

It's not that the NiMH is a bad idea, it's the parallel NiMH that's the bad idea.

A few pre-charged pairs of sticks could provide an hour of fun or more... :)
 

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Discussion Starter #12 (Edited)
Oh... and to bring it back around... 5.5Ah of just 2 sticks in series is still about 15-20 minutes of squirting at 10-13A.

It's not that the NiMH is a bad idea, it's the parallel NiMH that's the bad idea.

A few pre-charged pairs of sticks could provide an hour of fun or more...
Videos are a definite. I'm gonna be working on it off and on over the next couple of weeks. Still have to order the - AN like and stuff.

I still have the two sticks you gave me. 15-20 minutes is probably plenty enough. It uses about 2-3 gallons a minute so I doubt I would ever run out of battery before fuel. My only issue is charging. I have no way of charging them. You said I would have to build a special rig to charge?

Oh, I forgot, no the pump won't blow up at 16.4v. It will probably overspin a little bit, but because it's only 15-20 seconds run time at most between breaks, it should be ok.
 

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IIRC, those sticks were pretty decent. If they're still holding voltage, they're fantastical... :)

Two sticks in series would only require a NiMH charger capable of charging 12 NiMH cells. This would work:

https://hobbyking.com/en_us/accuell-s60-ac-charger-us-plug.html

With 60W charge power, it should be able to charge all 12 cells in less than 2 hours. It also has 10W discharging, which could discharge a stick at about 1.3A if you need to recondition them.

If you already have a 12V power supply, this would be a nice little unit:

https://hobbyking.com/en_us/turnigy-accucel-6-80w-10a-balancer-charger-lihv-capable.html
 
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