Two batteries

[cicatrix]

Imagine a situation - I have 2 rechargeable batteries of equal capacity - one full and one empty. I connect them and the empty one starts charging from the fully charged one. The question is - when will it stop? When both batteries have 50% of charge?

[Ten Key]

If nothing else, you're going to lose some of that energy to heat along the way.

[StrandedonEarth]

It'll stop when the dead battery has charged enough to put out almost enough voltage to equal the voltage from the good battery. The Voltage drop due to the internal resistance in the dead battery will continue to generate heat.

That's if it charges at all since you generally need more voltage than a battery produces to charge

[-Velocity-]

It'll stop when the dead battery has charged enough to put out almost enough voltage to equal the voltage from the good battery. The Voltage drop due to the internal resistance in the dead battery will continue to generate heat.

That's if it charges at all since you generally need more voltage than a battery produces to charge

Not quite, the internal resistance only dissipates power if there is current flowing.

The battery with less charge would stop charging when the open-circuit voltage of the two batteries is equal. In other words- equal voltage when disconnected (no current flowing) = no current flowing when the two batteries are connected together, because you need a voltage difference for there to be a current flow.

[InterCity]

Theoretically, the charge will stop at about 50%, depending on the losses caused by heat.

However, don't even think about doing that in the real world. Unless you plug in something that will consume it, the current will reach insane values once you connect the batteries together, since that's basically short circuit. Never connect power sources parallel to each other.

[-Velocity-]

Theoretically, the charge will stop at about 50%, depending on the losses caused by heat.

However, don't even think about doing that in the real world. Unless you plug in something that will consume it, the current will reach insane values once you connect the batteries together, since that's basically short circuit. Never connect power sources parallel to each other.

Not quite a short circuit, but you're right that it's generally a bad idea to connect a fully charged battery to a fully discharged battery.

However, there's nothing necessarily wrong with putting batteries of the same type and same charge level in parallel.

[Nuke]

ive read that if you leave nimh batteries together for too long, that one of the batteries will actually go negative and be permanently destroyed in the process. i have in fact found negative batteries in my junk boxes while going through and seeing whats still usable.

[cicatrix]

Wouldn't the empty battery provide enough 'load' so that connection is not a short circuit?

[-Velocity-]

Wouldn't the empty battery provide enough 'load' so that connection is not a short circuit?

Exactly, but it would be a fairly small resistance, so it might be possible to damage your good battery from having too high of a current.

[InterCity]

Wouldn't the empty battery provide enough 'load' so that connection is not a short circuit?

Okay, I've expressed myself wrong.

When I said that it's "Basically short circuit", I meant that it's not exactly a short circuit, but the current would be still insane enough (especially if you're charging a 0% battery with a 100% one and if both are Li-poly or Li-Ion) to damage one or both batteries and fry any protective or measurement circuitry accompanying the batteries. All and all, it's just (another brick in the wall ) a bad idea. Don't do that. Period.

I've heard rumors about people doing that back when I was building minisumo robots on the university, and it never ended well.

[K^2]

Oh, Li-Po will just straight out explode with that sort of abuse. You can't even charge them as a parallel or a series circuit when they are in a battery pack. Each cell needs to have its own current regulator. There are types of rechargeables that can handle recharging from each other, though.

[MircoMars]

so if I connect two 9V batteries directly, it goes click, smolder, boom?

[linkxsc]

so if I connect two 9V batteries directly, it goes click, smolder, boom?

Generally

[Xannari Ferrows]

Energy to move energy... the driving force of Entropy. [Were they intended to sound similar?]

You won't find yourself getting a total charge from one battery to the other. If your connection is 1 way, all available charge from the full battery will move into the empty one.

[K^2]

so if I connect two 9V batteries directly, it goes click, smolder, boom?

You might not get an actual boom, but you might, and there will definitely be heating, burning, and boiling electrolyte escaping the battery. Not a pleasant thing to be around.

[-Velocity-]

so if I connect two 9V batteries directly, it goes click, smolder, boom?

They will probably just get hot, as they're dropping all their voltage across their internal resistances, but some battery types can certainly catch fire or explode, as already mentioned by others. Several years back, I made the mistake of putting a mostly discharged 9 V battery in my pocket. There was some loose change in there too, and one of the coins shorted the two terminals together. Next thing I knew, something in my pocket was most uncomfortably warm (and getting warmer), and I had to empty my pocket pronto!

Edited March 8, 2015 by |Velocity|

[Nuke]

its interesting to note than 9v batteries are just six AAAA cells in series.

i personally like to use a single cell and a boost (or buck) regulator to get it up (or down) to the voltage i need. this is great if i need to get a lipoly cell up to 5v (sort of how i power my raspberry pi with an ubec). of course if the voltage i need is 3.3v, and the battery voltage is anywhere between 2.3 and 4.2 you have a problem. you could just buck regulate it down to 3.3v, and then have a cutoff divider to switch the smps ic off when the battery voltage drops below 3.5 or so (you usually need a little extra for the ic, how much depends on the chip). of course that really means you can only discharge about the top 40% of the cell's usable battery life (also the cuttoff divider will slowly discharge your battery below safe levels if you fail to charge it). turns out they have chips that can work in either direction, and so the problem now is a case of "i need parts". regulation is a lot simpler than battery arrays. the only time you really need an array is for high power applications.

Edited March 8, 2015 by Nuke

[Mazon Del]

In all of my robots that use Li-Po batteries I make sure to toss some diodes on them to prevent cross charging of the batteries. If you put two packs in parallel and one is significantly more/less charged than the other, the two batteries will attempt to charge each other as fast as they are capable of dumping/gaining energy, which is most emphatically NOT at levels the batteries were designed to handle. It is rather inaccurate to say, but a simplistic explanation is that its kind of like taking a battery, designed to discharge a lot of energy relatively slowly, and discharging it like a capacitor. Things don't like this.

This is actually the issue that the Dreamliner aircraft had with their Li-Pos. They assumed they could use QA controls to ensure that this would never happen, and in the end this IS possible. But it turned out their QA wasn't good enough to make sure of it. And you have this to thank for everybody panicking over lithium batteries in the mail and planes. But if the batteries are disconnected from anything (and prevented from shorting with each other or other things) they are perfectly safe. So you don't need to worry when you lie to the post office that you didn't include lithium batteries in your package that you might be risking the plane.

[cantab]

ive read that if you leave nimh batteries together for too long, that one of the batteries will actually go negative and be permanently destroyed in the process. i have in fact found negative batteries in my junk boxes while going through and seeing whats still usable.

If the batteries are connected in series this can certainly happen. I think it's more an issue over multiple charge/discharge cycles, eventually one of the cells is bound to go negative. That's one reason why when I wanted an extra battery pack for my ham radio I just used a multiple battery holder with spring terminals, and charged the cells individually each time. (The other reason being that soldering to ordinary batteries is a right pain.)

[Nuke]

i kinda want to get away from nimh. they have ok energy density but their inability to run slow draw devices and their high self dicharge makes them practically useless for most of the things i use them for. i might switch up to lithium iron, supposedly they have a better energy density and self discharge characteristics.

In all of my robots that use Li-Po batteries I make sure to toss some diodes on them to prevent cross charging of the batteries. If you put two packs in parallel and one is significantly more/less charged than the other, the two batteries will attempt to charge each other as fast as they are capable of dumping/gaining energy, which is most emphatically NOT at levels the batteries were designed to handle. It is rather inaccurate to say, but a simplistic explanation is that its kind of like taking a battery, designed to discharge a lot of energy relatively slowly, and discharging it like a capacitor. Things don't like this.

This is actually the issue that the Dreamliner aircraft had with their Li-Pos. They assumed they could use QA controls to ensure that this would never happen, and in the end this IS possible. But it turned out their QA wasn't good enough to make sure of it. And you have this to thank for everybody panicking over lithium batteries in the mail and planes. But if the batteries are disconnected from anything (and prevented from shorting with each other or other things) they are perfectly safe. So you don't need to worry when you lie to the post office that you didn't include lithium batteries in your package that you might be risking the plane.

this kind of thing is why ive resorted to making my own packs from ewaste. i used to be able to get lipos shipped at a sane rate, but now they want me to spend $50 for ground shipping (good luck with that i live on an island) for a $15 battery pack. china post really dont really care and will ship them, but no guarantee parts from china will be of any quality (ive received shoddy cells in the past). most battery packs fail with one of the cells, while the remaining cells are still quite healthy, so out of a discarded 6 cell back i get 5 good ones. these get turned into either 2s or 3s, packs, since i only have a 2s/3s balance charger. though they dont have any balancing or safety hardware of any kind. but for the most part the packs become incapable of powering the buck regulator before they reach their discharged voltage, and most projects dont really have much current draw either.

Edited March 9, 2015 by Nuke

[K^2]

A custom balanslcer is a very simple circuit to put together.

[Mazon Del]

this kind of thing is why ive resorted to making my own packs from ewaste. i used to be able to get lipos shipped at a sane rate, but now they want me to spend $50 for ground shipping (good luck with that i live on an island) for a $15 battery pack. china post really dont really care and will ship them, but no guarantee parts from china will be of any quality (ive received shoddy cells in the past). most battery packs fail with one of the cells, while the remaining cells are still quite healthy, so out of a discarded 6 cell back i get 5 good ones. these get turned into either 2s or 3s, packs, since i only have a 2s/3s balance charger. though they dont have any balancing or safety hardware of any kind. but for the most part the packs become incapable of powering the buck regulator before they reach their discharged voltage, and most projects dont really have much current draw either.

Last time I had snagged some packs, I went through HobbyKing International. I emptied Switzerland's supply of the 5700 mAh batteries. ^^ Shipped without any issue.

[MircoMars]

its interesting to note than 9v batteries are just six AAAA cells in series.

I meant these because of the poles being on the same side

and you can do a click and run

[Beowolf]

Sounds like this technique would reliably produce heat and eventually fire in certain types of batteries. How predictable would the time-to-fire be?

Was talking about this thread just now with a writer friend, and he immediately thought about using it in a story as a "time delay fuse" to start an arsonist's fire. A great example of our personality differences, as I'd thought of a Star Trek phaser on overload. My idea was for lithium batteries to come with a built-in teakettle whistle, so they'd make that whine that slowly rises in pitch as the battery got hotter, just like a phaser. And he thought of a slum-lord committing insurance fraud.

I don't generally watch "CSI" type shows. Anybody know if this has already been used in recent fiction? There have already been several news cycles about lithium-ion battery fires, so maybe someone's already used it.

Edited March 13, 2015 by Beowolf

[daniel l.]

Yay for random comments!!

Have you been acting like a heretic towards physics!