Discharge Rate
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What's a discharge rate? Not sure what it is and would like to know how it applies to mech mods please
There are actually two types of discharge rating on a battery, continuous discharge rate (safest) and pulse discharge rate (dangerous). We emphasize on CDR, as this is the rate the manufacturer rated the battery to discharge safely the entire charge of the battery without going into thermal runaway and destroying itself. Pulse Discharge (PDR), there is no set standard of how long this short, pulse should be from manufacturer to the next manufacturer, one manufacturer it might be 0.5seconds, the next it might be 2 seconds, most vapers do a draw of at least 3 to 5 seconds depending on their build. So yes, CDR and PDR are very, very applicable to a mech mod.
At fresh charge of a battery, 4.2volts, the most amps are pulled from a battery, the battery has the most energy stored at this level, get it unstable enough to overheat into thermal runaway in this state, literally, you have a roman candle going off, or almost the power of a M80 (1/4 stick of dynamite) going boom.
So, do your calculations of 4.2v/ohms of your coil=max amps, 4.2v/1ohm=4.2amps, 4.2v/0.5ohms=8.4amps, 4.2v/0.25ohms=16.8amps, 4.2v/0.1ohms=42amps, see the exponential increase the lower you go?? Good on you asking, be safe out there.
Oh ok thanks! So for me 3.7v/.25 ohms= 14.8 my battery is mxjo 18650 with cdr of 20 amps and pdr is 35 amps. Is this safe? Sorry still bit confused, does cdr and pdr work both at once or its one or the other?
This should help, very simple ohm calculator... just put in your ohm's and another number click calculate and poof whatever you need to know shows up...
Please Note: a fully charged battery will be 4.2 volts not 3.7...
Watts/Volts/Amps/Ohms conversion calculator
Yes your build is safe, as for your setup/equipment I couldn't say ^__^
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All 18650 batteries come off the charger at 4.2v, 3.7v is the nominal voltage, decrease from 4.2 to 3.7 pretty rapidly, then slow voltage discharge in the 3.9 to 3.5v range, so 3.7 is an average charge. So the figure of 4.2v/0.25=16.8amps is actually more true to apply. First amp discharge of a battery is at its PDR rating, then decreases to CDR after the PDR cutoff, so the mxjo is still pretty safe, giving you about 25% headroom of the CDR rating, but yes the 35amp rating is the PDR of the battery, believe it is a rewrapped LG HE2/HE4 under the wrapper, MXJO do not make their own batteries, just buy batteries from Sony, LG, Samsung, and Panasonic/Sanyo, re-wrap and respec the cells and sell them off for double. Not a bad cell, but a mech truly the 4 best batteries would be Sony VTC3, LG HB2, HB4, HB6 (all true 30amp batteries but low Mah rating, sacrifice run time for raw amp output), next would be the Sony VTC4 (20amp, 2100mah but can be wiggled up to 30amp from time to time) would handle a 0.25ohm build better, the MXJO or any 20amp battery, you'll want to max them out at 10amp CDR at the most. As a battery ages it loses Mah, over tax them they also lose C rating, decreasing either C or Mah rating decreases max CDR, 6months down the road, a 2500mah 20amp CDR battery could potentially only be a 1200mah 10amp battery, so 0.35 to 0.45 ohms would be safest in the long run on those MJXO's
I agree with IMFire's comments. 
Here are some articles which might help you:
Explain it to the Dumb Noob: Ohm's Law Calculations
Continuous Discharge Ratings vs Pulse (Burst) Discharge Ratings, Why You Need to Know the Difference
The "continuous discharge rating" in amps is the standard specification for amp limits within the battery industry. It is a determination made by the manufacturer and represents the amp limit a battery can be safely used before it will fail.
The "pulse or burst" discharge rating is not a specification standard within the battery industry. Every manufacturer or vendor seems to have their own definition of what the pulse rating is.
A pulse discharge rating is any use above the continuous discharge rating. It is never safe and not within the intended operating parameters of the battery. You should not operate your device above the continuous rating if you can help it. The pulse rating is a condition in which the battery is on basically a buildup to failure. It is exceeding the sustainable and intended discharge rate of the battery. It is inappropriate for a consumer device to operate in the pulse range of its battery.
Which would be why we shouldn't rely on any pulse rating. Any failure, mechanical or electronic, that fires the mod will operate in the 'continuous' mode. If your setup relies on a pulse rating, it's instantly over spec.
If your amp draw is safely in the continuous discharge range, your coil could act almost like a fuse and burn out before the battery is stressed. If you are already running the battery at the edge of it's limits (pulse), there is no margin of safety.
I am of the mindset that you should leave a margin of safety when deciding what resistance coil to use. We probably place too much faith into cheap ohm readers in being precise and accurate. Also, a RDA's post screw unknowingly coming loose can greatly change the coil's resistance.
Everyone is free to set their own parameters, and I can only say what mine are.
I try to never exceed 50% of the CDR (continuous discharge rating) of a fully charged battery (4.2v). So with a 20A batteries, that would be 10A. The above Ohm's Law Calculator tells me that a .4 ohm build is as low as I would want to use.
The reason that I place a 50% limit is because as a battery ages the mAh of the battery degrades, as the mAh degrades so does the batteries c rating (amp limit). So down the road, your 20A battery may only be a 10A battery.
Here are some articles which might help you:
Explain it to the Dumb Noob: Ohm's Law Calculations
- As simple as it is to use, some people have a tough time grasping the concept. Warning: Includes graphic photos of mod explosions.
- A popular and essential read to understand which batteries are safe to use in mechanical and regulated mods. Includes a frequently updated list of recommended safe-chemistry, high-drain batteries with their specifications.
Continuous Discharge Ratings vs Pulse (Burst) Discharge Ratings, Why You Need to Know the Difference
The "continuous discharge rating" in amps is the standard specification for amp limits within the battery industry. It is a determination made by the manufacturer and represents the amp limit a battery can be safely used before it will fail.
The "pulse or burst" discharge rating is not a specification standard within the battery industry. Every manufacturer or vendor seems to have their own definition of what the pulse rating is.
A pulse discharge rating is any use above the continuous discharge rating. It is never safe and not within the intended operating parameters of the battery. You should not operate your device above the continuous rating if you can help it. The pulse rating is a condition in which the battery is on basically a buildup to failure. It is exceeding the sustainable and intended discharge rate of the battery. It is inappropriate for a consumer device to operate in the pulse range of its battery.
Which would be why we shouldn't rely on any pulse rating. Any failure, mechanical or electronic, that fires the mod will operate in the 'continuous' mode. If your setup relies on a pulse rating, it's instantly over spec.
If your amp draw is safely in the continuous discharge range, your coil could act almost like a fuse and burn out before the battery is stressed. If you are already running the battery at the edge of it's limits (pulse), there is no margin of safety.
I am of the mindset that you should leave a margin of safety when deciding what resistance coil to use. We probably place too much faith into cheap ohm readers in being precise and accurate. Also, a RDA's post screw unknowingly coming loose can greatly change the coil's resistance.
Not long ago I was vaping on my rebuildable dripper at 0.6 ohms. Suddenly the vape was unusually harsh. I knew my wick was wet, so something else must be askew. I removed the atomizer, checked the resistance with my ohm reader, and was surprised to find it was 0.1 ohm. I checked the post screws, and one was loose. I retightened it, rechecked the resistance, and it was again 0.6 ohms. Had my original coil been built to 0.4 ohm, the loose screw would have dropped the resistance below 0.0 ohm, which is a hard short and would have sent my battery into thermal runaway.
Everyone is free to set their own parameters, and I can only say what mine are.
I try to never exceed 50% of the CDR (continuous discharge rating) of a fully charged battery (4.2v). So with a 20A batteries, that would be 10A. The above Ohm's Law Calculator tells me that a .4 ohm build is as low as I would want to use.
The reason that I place a 50% limit is because as a battery ages the mAh of the battery degrades, as the mAh degrades so does the batteries c rating (amp limit). So down the road, your 20A battery may only be a 10A battery.
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+1, CDR is what you're looking for. The PDR rate is about pulses, which are usually measured in microseconds, far shorter than a vape draw. Your MXJO battery is a re-wrap; that is, MXJO bought a battery from somewhere and put their own wrap on it. It is pretty much impossible to know what battery they used.
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