Pairing Batteries on a VV

[supertrunker]

The issue with parallel wiring of batteries is that, as far as we are concerned you still have to stay withing the Amp limit of a single battery. Ramping up the Amps does not suddenly increase the ability of your batteries to cope with it!

T

 

[jkais3r]

I think you meant series. In parallel amp capacity goes up.

But yes, with 3.2v and 4v in series you have 7.2v total with a 1 ohm resistor, and voltage down to 5v the total ampacity of the circuit is 5 amps. With 4v and 4v in series you have 8v total with the same 1 ohm resistor and rheostat down to 5v you still have 5 amps. In a mech mod your ampacity would go down with voltage.

"Current draw is divided up amongst the batteries the same way that capacity is combined. In parallel, each battery only need supply a fraction of the total current drawn by the load; in series, each battery must supply the full current. Thus, a motor drawing 150 amps from three 12 volt batteries in parallel will draw 50 amps from each."

Notes on Batteries

So I believe that both batteries are at 5 amps regardless of voltage. In parallel where the load is shared a differing voltage could change your ampacity. I think that solves it.


From all the research I've just done paralleling batteries sounds much more dangerous than running them in series.

 

[MamaTried]

Here's what i think/read/personally observed...

matching batteries out of the chute is generally a good idea. the internal matched characteristics of the batts (max charge/charge rate/discharge rate/internal resistance, etc...) should match well. this is a no-brainer.

so, pairing a new battery A with a one year old battery B (which has many more charge cycles on it than A) -- even tho they both charge to 4.2 on your charger- might be a bad idea. why? the older battery's chemistry has changed - it's internal resistance is higher, and will likely discharge at a faster rate.

regarding rotating top and bottom batteries. like many, i have observed that the top battery discharges further/faster than the bottom one. others have seen no difference. my theory is that this effect gets magnified if your batteries get over-discharged. (not a theory i want to spend the $$$ to test, tho )

so, to me the bottom line is that not doing the pairing/rotating/happy crap will probably reduce the useful life of the batteries, but isn't the safety issue that some refer to...

although, i do believe reverse-charging is possible, i think the stacked-batt horror stories are pre-safe chemistry batteries utilized by fools in devices that were not designed for safe stacking.

 

[jkais3r]

Here's what i think/read/personally observed...

matching batteries out of the chute is generally a good idea. the internal matched characteristics of the batts (max charge/charge rate/discharge rate/internal resistance, etc...) should match well. this is a no-brainer.

so, pairing a new battery A with a one year old battery B (which has many more charge cycles on it than A) -- even tho they both charge to 4.2 on your charger- might be a bad idea. why? the older battery's chemistry has changed - it's internal resistance is higher, and will likely discharge at a faster rate.

regarding rotating top and bottom batteries. like many, i have observed that the top battery discharges further/faster than the bottom one. others have seen no difference. my theory is that this effect gets magnified if your batteries get over-discharged. (not a theory i want to spend the $$$ to test, tho )

so, to me the bottom line is that not doing the pairing/rotating/happy crap will probably reduce the useful life of the batteries, but isn't the safety issue that some refer to...

although, i do believe reverse-charging is possible, i think the stacked-batt horror stories are pre-safe chemistry batteries utilized by fools in devices that were not designed for safe stacking.

I think you've took everything out of this that I was trying to get across. But I think reverse charging is only possible in a parallel circuit. In parallel the voltages would try to regulate each other. Don't do the math but lets say a 24v battery and a 12v battery in parallel would only give 18v. So the drain would be greater on the higher battery and so would the ampacity that feeds the load. Power supplies in series with varying voltages are fine because of difference in potentials. This is also very complicated. But it has to do with difference in potentials, because a 12v power supply and 8v power supply in series the 12v will impose a 12v on the negative of the 8v so you have a difference in potential of 20v because the negative is at 12 and positive is at 8. Thats why the top wears faster as well I think.

I think pairing/changing orientation (top/bottom) will let your batteries decay at the same rate, like you said giving you the longest life possible out of the set.

Just when I think I've got it all figured out there are other things I need to learn lol. (reverse charging)

 

[MamaTried]

I think you've took everything out of this that I was trying to get across. But I think reverse charging is only possible in a parallel circuit. In parallel the voltages would try to regulate each other. Don't do the math but lets say a 24v battery and a 12v battery in parallel would only give 18v. So the drain would be greater on the higher battery and so would the ampacity that feeds the load. Power supplies in series with varying voltages are fine because of difference in potentials. This is also very complicated. But it has to do with difference in potentials, because a 12v power supply and 8v power supply in series the 12v will impose a 12v on the negative of the 8v so you have a difference in potential of 20v because the negative is at 12 and positive is at 8. Thats why the top wears faster as well I think.

I think pairing/changing orientation (top/bottom) will let your batteries decay at the same rate, like you said giving you the longest life possible out of the set.

Just when I think I've got it all figured out there are other things I need to learn lol. (reverse charging)

yah, it's been a few decades since i had a job designing power supplies (none of which used batteries), so i'm kinda spitballing...

i can say i once yanked a pair of severely discharged stacked batts out and one measured a pos voltage and the other was slightly negative. i couldn't believe it, but measured with 3 different meters. a while back someone posted an excellent explanation, but if i actually bookmarked it, i can't find it. but i spose it only matters out of intellectual curiosity, anyways...

 

[Alexander Mundy]

It is best to pair batteries so they drain at approximately the same rate since the more charge / discharge cycles they have the faster they drain down. What happens with those dead batteries is that if not exactly matched 1 battery will drain down at a faster rate than the other. The combined voltage may still be enough to power the converter, but the battery with the lower voltage can be below the point of return and will not charge again. (Typically 2.5V) I.E. One battery at 3.5V and one at 2.2V. The converter has no way of knowing this. It is also best to quit using them in series (stacked) before they get weak as they will eventually grow apart in capacity as they age. Taken to the extreme it is possible to reverse charge the weak battery, but that normally only happens with more than 2 cells in series.

Kirchhoff hasn't been proven wrong from 1845 to present. The very building blocks of circuit analysis are based on his and Ohms laws. Current in a series circuit must be the same at any point in that series circuit. That's all I'll say about that since some people can't seem to grasp that and perpetuate myths.

 

[Vapeaholic]

Batteries are similar to tires. You want them to wear out evenly. Keeping them matched and rotating them (top vs. bottom) after each subsequent recharge has proven to help them last considerably longer.

That's a really good analogy.

 

[MamaTried]

It is best to pair batteries so they drain at approximately the same rate since the more charge / discharge cycles they have the faster they drain down. What happens with those dead batteries is that if not exactly matched 1 battery will drain down at a faster rate than the other. The combined voltage may still be enough to power the converter, but the battery with the lower voltage can be below the point of return and will not charge again. (Typically 2.5V) I.E. One battery at 3.5V and one at 2.2V. The converter has no way of knowing this. It is also best to quit using them in series (stacked) before they get weak as they will eventually grow apart in capacity as they age. Taken to the extreme it is possible to reverse charge the weak battery, but that normally only happens with more than 2 cells in series.

Kirchhoff hasn't been proven wrong from 1845 to present. The very building blocks of circuit analysis are based on his and Ohms laws. Current in a series circuit must be the same at any point in that series circuit. That's all I'll say about that since some people can't seem to grasp that and perpetuate myths.

yeh, that is precisely what i meant to say