Piggy Backing a lithium battery (cell)

I have several questions.  But the basic idea is that I want to extend the runtime of a device (such as a cell phone, tablet, camcorder, whatever) by increasing the mah.  So here are the things I am wondering:

1)Many lipo batteries have 3 or 4 contacts, one is -, one is +, and the others are typically data or something.  can I simply connect additional lipo's (just the + and the -) in parallel with the corresponding contacts in the device?
2) Are there any ill effects of charging (presumably from the built in charger of the device, like a cell phone) and discharging (using the device) parallel lipo's?
3)  They will all have the same voltage rating, but do they all have to have the same mah rating?


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verence6 years ago
1) Put the parallel for discharge - should kind of work. As long as they have always the same charge level. If one is charged and the other is empty, a big current will flow between them. This will look like a short for the charged one and like a over current charging for the empty one. Even if they have the same charge level when paralleled, that might change during discharge.

2) Discharging might produce different charge levels, see above. Charging will most probably not work. If you connect the data lines (I guess that are the terminals of the charge controller inside the lipo pack) you can either only connect one of the pack, so the other is charging uncontrolled or connect them to both, what probably wont work at all.

3) Theoretically yes. I say theoretically, because I would not try this and do just strongly advice against paralleling lithium power packs.
guyfrom7up (author)  verence6 years ago
1) When initially put in parallel, yes, I believe they should be as near as possible to the same voltage as each other (initially a little bit of current will flow between eachother, but they should reach equilibrium, yes?)

During discharge, wouldn't all of the batteries be forced to stay at the same voltage level? For example, if you had a 1000mAh battery and a 100mAh in parallel, wouldn't the circuit just automatically draw more current from the larger battery and less from the smaller battery (since all the batteries are at the same potential).

2) I don't see how discharging would cause different charge levels. When discharging packs involving series cells, I can see how it would happen, but not with a parallel pack for the arguement above.

3) Already adressed

Oh, and I wouldn't be connecting data lines, I would only connect the battery terminals of the bare cell to the cell in the phone where that cell's circuitry will talk to the phone.

The only reason I'm asking is if I'm missing something intrinsic to lipo batteries, after all I wouldn't want to mess up a larger battery pack (for safety and financial reasons).

Also, the internet is not providing clear cut answers, there's people with the same reasoning as me, others who say it'll instantly blow up, others that say you need a boat load of saftey circuitry (which I believe is nice, but in all honesty I think can be skipped in low current draw situations like this, after all thin wires will be going from the cells, which'll melt before catastrophic failure), and others that say you need to have an even number of cells in parallel (which makes 0 sense). There's also products like this:
which is just 3 cells in parallel. They do say they were matched for internal impedence, but I'm wondering if that is even relevant for the arguement made in number 1

2) I don't see how discharging would cause different charge levels.

SInce the batteries are going to have different length wires, and therefore resistance, different voltages will become established over several charge/discharge cycles.

wouldn't all of the batteries be forced to stay at the same voltage level? For example, if you had a 1000mAh battery and a 100mAh in parallel, wouldn't the circuit just automatically draw more current from the larger battery and less from the smaller battery (since all the batteries are at the same potential).

Capacity is not equal to internal resistance.  Batteries of different capacities may or may not have identical internal resistance.

guyfrom7up (author)  steveastrouk6 years ago
I'll do some calculations later to check your statement, but doing homework now :D
guyfrom7up (author)  guyfrom7up6 years ago
My current conclusion after doing calculations:
I totally get what you mean with the internal resistance, however I do not believe it will provide unbalance in cells. If one cell has relatively high internal resistance, there will be a relatively large voltage drop across the internal resistance and have a lower output. However, this output must equal the output of all the other lipos which have are in a similar scenario. Therefore less current will flow from the battery with higher internal resistance.

Therefore the batteries with lower internal resistance will drain faster.

However, as they drain faster, their voltage output drops, creating a larger current draw from the battery with higher resistance.

hmmmm, now I'm starting to second guess myself.
I want to say it's fine as long as the internal resistances are fairly close, but how close is fairly close?

The internet doesn't provide a clear cut answer because there are to many variables. A battery pack might hold anything from only the cells to full-blown load-balancing-discharge-controller-cum-charger-circuits. The load schemes vary with the connected devices and the same for the chargers (unless built-in). So in some cases it might work, while in other cases something blows up.
(Another variable is the knowledge level of the internet commentators, of course.)

Go on, if you want to try - but don't rely on the on the thinness of wires as a fuse, put in some real fuses - you can get tiny ones that break down at a defined current.

What I don't get is 'why' you want to do it. Making a device run longer, that's clear, but if you connect two batteries, they will not fit into the case, making your cell phone, tablet whatever look quite ugly and probably no longer really portable.

If it is for use at home, just use a wall wart - cheap and no need to charge at all. If it has to be portable, buy an extra battery or two. Instant doubled (tripled) time of use. No safety concerns, no loss of warranty, no ugly add-ons. If the original batteries are to expensive, try to find some cheap copies, they might last not as long but normally work quite well.

Only an even number of cells parallel? Ridiculous. I'd say use only an odd number of cells parallel: One. Anyway, if you want to test it, go on, but play it safe.
caitlinsdad6 years ago
Hmm, I've got a few old cellphones that I could take the batteries out and hook them up together. But now that you have the discussion going, I don't want to be the first to duplicate the exploding laptop battery phenomenon. It would be interesting to boost up the power reserve of a Lipo cordless drill or something.
guyfrom7up (author)  caitlinsdad6 years ago
i hope we can come to a conclusion to this! I want to make an extended battery pack for a nook color, going from 4,000mAh to ~18,000 mAh!

Maybe that might help.
guyfrom7up (author)  caitlinsdad6 years ago
read that, didn't help, but then after reading that I googled something similar and got this:

It seems like it's fine, actually even more than find considering these are 100% in parallel rather than parallel series combos. It just says make sure they have similar charge when first connected so that there's not a dead short surge like verence said.
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