Step-up vs Step-Down APV Battery Life

[1Cas]

I've been lurking here for a while now and have gotten a lot of useful information. I am an IT geek, tinkerer and vaper, so I want to know how things work. Now, I have a question of my own:

I have the IPV V2 and have been playing with maximizing the battery life. Even though the device is great, it does not last me a full day, so I have been eyeballing one of the newer 100-150W devices such as the Sigelei, Cloupor T8 or GOD180. I am not in it for the wattage, but am looking for longer battery life and so a dual (or triple) battery box seems to be the logical solution. My question is more scientific: How does this work in practice?

At this time, I am working under these assumptions:
- 2 batteries in parallel provide the SAME VOLTAGE as a single battery, but double the CAPACITY (mAh).
- 2 batteries in series provide the same CAPACITY (mAh) as a single battery, but DOUBLE THE VOLTAGE output.

So, on a straight mech mod, the choice would be simple: Put the batteries in parallel and double the battery life.

But I get lost with step-converters in VV/VW devices. Regulated VV/VW mods use a power converter to take the input voltage and perform a step-up OR step-down to the preferred output voltage. Understanding Ohm's law and Watt's law, increasing the voltage and the resistance will bring downs the current (AMP draw), which should, theoretically, increase battery life.

What I do not understand is what happens if I put (e.g.) a 1.7Ohm atty and run it at 25W on:
- an APV with parallel batteries and step-up, versus
- an APV with series batteries and step-down

Assuming that all other variables remain the same (same number of puffs, length, atty/resistance, etc) what is ultimately better for battery life (in my situation):

1) 2 batteries in series: An average input voltage of 7.4v which is stepped down to ~6.5v, putting less AMP drain on the 'regular' battery capacity, or
2) 2 batteries in parallel: An input voltage of 3.7v which is stepped up to ~6.5v, putting more AMP drain on the 'doubled' battery capacity

Does 2 batteries is series make sense on a VV/VW when power is not maxed out?

I appreciate any technical explanation or empirical evidence!

 

[p7willm]

Energy is always conserved. The batteries will store and release the same amount of energy no matter how you wire them up.

Batteries in series have more potential problems than batteries in parallel. In series the weaker cell will drop voltage sooner and as the lower voltage in the circuit it will be called on to do more work, dropping the voltage more. Also, when charging, the Sigelei avoids this by not charging, a weaker battery will charge first and then will heat up while the other one catches up. Try to keep the batteries matched, start with a new pair and keep them together, check that they are at the same voltage when you take them out and check that they charge to the same voltage before you put them back in. If they do not match get a new pair.

I think 2 batteries in parallel is the way to go for a regulated device but the IPV, Sigelei, and T8 are all in series. The regulation circuit only bucks, drops voltage, so they all start out at 8.4, with fresh batteries, and drop the voltage to what you want.

The DNA, and copies, start at 4.2 and only boost voltage.

The board I like is the SX350 which both bucks and boosts. I have a mod with a pair of 18650s in parallel that I love.

 

[edyle]

At this time, I am working under these assumptions:
- 2 batteries in parallel provide the SAME VOLTAGE as a single battery, but double the CAPACITY (mAh).
- 2 batteries in series provide the same CAPACITY (mAh) as a single battery, but DOUBLE THE VOLTAGE output.

2 batteries double the CAPACITY (mAh); whether in series or in parallel.
In series its higher voltage but lower current.
In parallel, its lower voltage but higher current.

Running parallel batteries is supposed to be less unsafe than running series batteries.

 

[edyle]

What I do not understand is what happens if I put (e.g.) a 1.7Ohm atty and run it at 25W on:
- an APV with parallel batteries and step-up, versus
- an APV with series batteries and step-down

Assuming that all other variables remain the same (same number of puffs, length, atty/resistance, etc) what is ultimately better for battery life (in my situation):

1) 2 batteries in series: An average input voltage of 7.4v which is stepped down to ~6.5v, putting less AMP drain on the 'regular' battery capacity, or
2) 2 batteries in parallel: An input voltage of 3.7v which is stepped up to ~6.5v, putting more AMP drain on the 'doubled' battery capacity

Does 2 batteries is series make sense on a VV/VW when power is not maxed out?

I appreciate any technical explanation or empirical evidence!

1.7 ohm 25watts.
watts = volts x volts / ohms
25 = v x v / 1.7
42.5 = v x v
v = 6.5 volts.

either way, each battery still supplies 12.5 watts.

 

[1Cas]

Running parallel batteries is supposed to be less unsafe than running series batteries.

I agree when it comes to unregulated devices because of inconsistent load or discharge between the two batteries, but I am not too worried about this with a protected, regulated device. Am I wrong about this?

 

[1Cas]

1.7 ohm 25watts.
watts = volts x volts / ohms
25 = v x v / 1.7
42.5 = v x v
v = 6.5 volts.

either way, each battery still supplies 12.5 watts.

Sure, I get that. What I am thinking about is the amp draw. Not the peak current, but the energy used (mAh) in either situation when the voltage is stepped up or stepped down.

I found this explanation on VapeScores that explains the difference by using a Supermax (step-down or bucking) and an IPV2 (step-up). However, both run on a single battery - The IPV2 on an 18650, the Supermax on a battery pack.

"From the above examples you can see stepping down, or bucking your voltage source has the benefit of decreasing the amount of amps drawn from it, thus increasing battery life and happiness."

So what happens if I use a dual battery device? Which setup will give me longer battery life? Or does it come out to about the same?

 

[1Cas]

p7willm - Thanks for the explanation.

The batteries will store and release the same amount of energy no matter how you wire them up.

Am I correct in stating that both battery setups will (approximately) last an equal amount time?

BTW: What is the mod you are using?

 

[rusirius]

I agree when it comes to unregulated devices because of inconsistent load or discharge between the two batteries, but I am not too worried about this with a protected, regulated device. Am I wrong about this?

Technically I'm not sold on the idea of parallel being "safer" than series... In some respects yes... but..... See here's the thing... when you have two batteries in series and one has a voltage lower than another, then it will drain much faster than the others. Likewise when you charge it (as a set) that cell will reach full charge faster... Now this DOES set up the potential for a problem... A) it drains faster, meaning in the case of lithium batteries it could get discharged BELOW the safe threshold, and you have no way of knowing or monitoring that.. B) it charges faster meaning the potential to overcharge and at the very least generate excess heat exists. Possibly damaging the cell further. The point is, it can lead to a worst case scenario of venting. On the other hand, with parallel batteries when you have one bad cell essentially your current capability is what suffers... Now sure, if you're pushing it right to the edge of what your batteries can handle, then yes this can be bad... But with a safe margin it's less of a concern, especially since short of a completely dead cell the reduction is probably going to be marginal especially due to the voltage drop that occurs from the bad cell...

So yes... technically in some small way parallel is kinda a little safer... But to be honest I think it often times gets blown out of proportion... But honestly protected or unprotected the same principles apply... And honestly, there are all sorts of devices that use lithium packs in all sorts of combinations of parallel and series... If you use a bit of common sense, keep them in a set, keep an eye on them and make sure they are performing decently, in general it isn't really going to make any difference rather they are series or parallel...

 

[rusirius]

p7willm - Thanks for the explanation.



Am I correct in stating that both battery setups will (approximately) last an equal amount time?

BTW: What is the mod you are using?

That is correct... barring any increases or decreases in efficiency... For example, if I were to take a 3.7 volt source, run it through an iron core transformer to bump it to 5v and then feed a circuit... and then take another 7.4 volts source, but run it through a simple 5v regulator (that essentially "vents" the excess voltage by releasing it as heat) then that's going to be a much less efficient process and will suffer poor battery life versus the transformer...

But with properly constructed devices, aside from small differences they are pretty much going to have the same amount of runtime, though their capabilities may differ.

 

[edyle]

I agree when it comes to unregulated devices because of inconsistent load or discharge between the two batteries, but I am not too worried about this with a protected, regulated device. Am I wrong about this?

If you put lithium batteries in series (this is called stacking batteries) you are supposed to follow a certain protocol.. like:
but the 2 as a pair together - 1 sealed package
keep them married for life: use together, charge together;
mark them A and B
alternate them A on top, then next time B on top.

In the other case, when rechargeable batteries are in parallel, I don't think you need to do all that;
if one is weakening faster, the other one charges it I guess, so they stay balanced.

 

[1Cas]

Thanks, everyone, for the feedback and explanation. I will keep monitoring this thread to see if any additional posts are made.

Thanks!

 

[DaHurt]

Ok here are my thoughts on the matter in a analogy. #1 Cars used to use 6v electrical systems but now use 12v and the reason is less robust wire and components are required. #2 when you get electrical service to your house it is sent at high voltages and then transformed to much lower voltage (yes this is AC) but the point is the same less energy wasted on heat in components and wire. So I like step down for battery life.