A question about parallel batteries..

I have a project i am working on for an 18650 box to run two batteries parallel with a juice feeder for some new atty's i got. Whats the real voltage with two parallel batteries at 3.7v's? I heard from elsewhere that it will be a true 3.7v with load, but a single battery gives about 3.3v's with load. will the other battery allow it to stay at 3.7ish range the whole time or will it drop like normal when the full charge drops down? I am doing it anyways for a bad ass 18650 mod but just wanna know what i am expecting.

 

Yeah I could see the voltage under load staying a bit higher. But I believe there are dangers to this approach ie one battery being drawn on more than the other. Probably better off putting them in series and copying someone elses VV for 3 to 6V. Would get 2 or 3 days out of 2 x 18650.

 

What would the dangers be Hopefull? I would be using 2 xsl ultrafire batteries, just curious before i make this mod. And if needed to to buy other components needed to prevent a failure mod.

 

Venting is the danger or the one battery going to a non reusable state. Far better going VV, with the technology it will give you much better battery life if dialled to 3.7V and the option to select other voltages.

 

Anthony, the true voltage will still be about 4.18 but the mAh will be doubled. An 18650 has about 4.18 volts fully charged so under load they should only drop to about 4volts since you are running 2 in parallel. The dangers that HopeFuLLL is talking about is if one of the batteries is not of equal strength which is why I always check voltage with a voltmeter before every charge. I want to make sure that the 2 batteries I am using run down at equal rates which why I usually pair batteries together after testing out a bunch.

I have 6 pairs of 18350's that I use with my Precious. When they were all brand new, I used them in random pairs but when it came time to recharge(Precious led turns orange), I checked the exact voltage for that pair and wrote it on each battery. After cycling through all 6 pairs, I took the ones with the most amount of voltage left over and paired them together accordingly, did the same with the one that had less voltages as well. Reason that I did this is because not all batteries are made equally even though they are the same brand and were all brand new.

The same can be said for car engines even when the cars are identical, no 2 engines will have the exact same torque and horsepower at the exact same rpm. Long story short, you want to pair batteries of equal strengths together so one doesn't haven't pull the others weight which would lead to one being over worked, heated up, venting and possible explosion. I know it is troublesome but I try to check the paired batteries before and after recharging just to make sure that one isn't dying faster or accepting more of a charge than the other. When that happens, I toss the batteries not only for safety but you already know the inevitable is near.

I am no electronic guru but I do have 4 years of high school and 2 years of college electronics knowledge. Granted it was 20 years ago, batteries have come a long way but the formulas and theories are still the same.

Aloha, Keith

 

View attachment 1286

Here's a simple circuit to copy. 3.6 - 6 volt.

 

Here is my advice,

If you are going to do this and run a box mod in parallel then take two brand new batteries and mate them by labeling them. This way the batteries see the same life cycles and should be very well matched.

Charge them together
use them together
never use one without the other
when wiring use the same gauge and lengths for both cells, this will help by allowing both cells to see the same resistance overall.

There is a danger in damaging batteries but if you follow those guide lines then you will eliminate the dangers. Just keep an periodic check on the V when you pull the batteries out of the charger and out of the mod, they should be charging and discharging at the same rate and be within about .2 V of each other when measured.

 

After looking at the box, I may have room on the top to input a VV regulator and components. In this case I would need to run them in series right? So would that matter if they are not paired as compared to running them in parallel.



That makes total sense!!

 

I've never paired mine or had issues... I'm lazy though.

 

They need to be the same type of batt. Aka both trustfire flames or xls's ect. and I would use new batts. That way you know they have been cycled the same amount of times.

 

no matter of how you do it, either parallel or series, I will stand by my recommendation of pairing them up for the life of the batteries never to be used separately unless you never intend on using them in the series or parallel again.

Also checking the used voltage and fully charged voltage will help you determine if they are being equally used. This will prevent one battery trying to "make up" for the bad battery.