Two Batteries in Parallel - Twice the Available Current?

[TruckinVaper]

The other thing you need to be very wary about taking this road is you have to be absolutely confident in both battery connections. If for any reason you should lose connection on one of your batteries the entire load will be taken up with the other one. I'm sure you know that already but thought it worth mentioning.

 

[concentriccircles]

The other thing you need to be very wary about taking this road is you have to be absolutely confident in both battery connections. If for any reason you should lose connection on one of your batteries the entire load will be taken up with the other one. I'm sure you know that already but thought it worth mentioning.

Yeah, might be sensible to put a 25A fuse on each battery connection. I'll open it up when it arrives and see what's what.


Sent from my iPad using Planet of the Vapes

 

[Bobw1951]

Might be an idea to look here: Battery drain | Steam Engine | free vaping calculators

and fill in the resistance, mode and Battery voltage (4.2 for parallel or 8.4 for serial) then look at the battery drain and Amp limit figures, even at 0.1ohm you would have a negative amp headroom - not a good idea.

 

[TruckinVaper]

Might be an idea to look here: Battery drain | Steam Engine | free vaping calculators

and fill in the resistance, mode and Battery voltage (4.2 for parallel or 8.4 for serial) then look at the battery drain and Amp limit figures, even at 0.1ohm you would have a negative amp headroom - not a good idea.

Up the battery capacity to 5000mah to match the parallel batteries and you pop back in to positive headroom, not by a comfortable margin but it's there.

 

[Bobw1951]

Up the battery capacity to 5000mah to match the parallel batteries and you pop back in to positive headroom, not by a comfortable margin but it's there.

Yup, if you think the batteries are okay at 25A then away you go - me I'd rather err on the side of caution and not tamper with what it tells me is 22A

 

[t_o_f]

In theory

Two cells in parallel doubles the single cell amp rating but keeps the single cell voltage

Two cells in series doubles the single cell voltage but keeps the single cell current limit

Both configurations have the same effective capacity ->
Consider two ideal batteries of 2000mAH at 4V delivering 80 Watts
In parallel you have a 4000mAH battery pack @4V. To drive 80 watts will require 20A depleting the battery in 4/20 of an hour
In series you have 2000mAH @8V. To drive 80Watts will require 10A depleting the battery in 2/10 of an hour
Ergo the effective capacity (in Watt Hours) is the same


In the real world

Parallel batteries should not be run to 2x the current for a single one. Batteries have their own internal resistance which will not be the same between batteries so the one with a lower resistance will do more work. You're probably safe at 80% rating but to be belt and braces safe you should not exceed the rating for a single cell

Series batteries should double the voltage but they don't. The same internal resistance that buggers up the parallel configuration also monkeys with the series setup too. It will cause more sag under load meaning you don't get quite the voltage doubling you might hope for
Series setups can be safely run right up to the single current cell limit


Providing you grasp and respect the doubling of voltage but maintaining of current limits a series configuration is inherently safer than a parallel one as Truckinvaper intimates above

 

[TruckinVaper]

Yup, if you think the batteries are okay at 25A then away you go - me I'd rather err on the side of caution and not tamper with what it tells me is 22A

Just hang on a minute,,,, I don't think I've ever said what it is and isn't safe to pull from the battery just discussing the properties, benefits or otherwise of a parallel battery set up as that was what the op was about.

Might be an idea to look here: Battery drain | Steam Engine | free vaping calculators

and fill in the resistance, mode and Battery voltage (4.2 for parallel or 8.4 for serial) then look at the battery drain and Amp limit figures, even at 0.1ohm you would have a negative amp headroom - not a good idea.

All I did here was look at your link and do as you said. I entered 0.1ohm in as you said and yes it gives you a negative 91% head room figure and I agree, this would not be a good idea with a single cell but that's not what we are discussing. What it doesn't do is give you the option for a parallel set up that I could find. What you can do though is double the mAh to 5000 as that is what is happening in the subject discussion, doing this gives a positive 5% head room figure simple as that.

No that's Serial - parallel means they last twice as long

Wrong and right statements from the start.

"No that's Serial"!!!!,,,, er no, no it's not. The op was talking about firing a 0.09ohm coil, serial?, seriously? 8.4V into a 0.09ohm coil not accounting for volt drop is 93.33A from an as you say 22A set up. The max amps of a serial set up are the same as for a single cell set up. You do get twice as many watts for those amps though. You could even say this set up lasts twice as long, build for the same amount of Watts and you get those Watts for half as many Amps.

"Parallel means they last twice as long". You got that one,,,, almost. Doing a safe build for a single cell and then putting that build on a parallel cell setup will indeed double your run time. However doing as the op is suggesting, right or wrong, wouldn't double the run time, it would be a maxed out set up which would last as long as a maxed out single cell set up.

I hate to rant but if you want to try and call me out on something make it something I have said or done and I'll admit it. You might want to know what your talking about too and not give out misinformation.

 

[TruckinVaper]

t_o_f

Nice post and some very good points.

In the following paragraph did you perhaps get blue words the wrong way round? I've corrected it here but If I've got it wrong I can edit it to avoid confusion

Both configurations have the same effective capacity ->
Consider two ideal batteries of 2000mAH at 4V delivering 80 Watts
In Parallel you have a 4000mAH battery pack @4V. To drive 80 watts will require 20A depleting the battery in 4/20 of an hour
In Series you have 2000mAH @8V. To drive 80Watts will require 10A depleting the battery in 2/10 of an hour
Ergo the effective capacity (in Watt Hours) is the same

 

[t_o_f]

Oh FFS,
I spent minutes proof reading that and completely missed the basic error. That's why tech. pubs people get paid the big money

Many thanks for pointing out the error

 

[concentriccircles]

Ok, so I think it's pretty clear who here knows what their talking about, and who (like me) isn't quite up to speed, so thanks to the helpful folks.
Be nice to come to a consensus though. Is a sensible figure to tune the current draw to something like 80 - 90% of the maximum rated discharge for a single cell? And is it ok to work on a doubling of that maximum rated discharge for a parallel setup, or is it the case that although that makes sense on paper it isn't sensible in the real world and you should take a belt and braces approach and stick to the single cell rating?

It isn't the end of the world either way, the setup I'm running these days on a single sell is more than adequate, and a reason to start researching a mod with a LIPO cell is pretty appealing. I'd rather have a safe setup and a new project than no new project and no face or fingers.