Parallel LiPO voltage gradient, another internet battery mystery.

xanadu

My amp meter has a crummy sample rate and no peak hold and I can't find a concrete answer on this online. I get the "err on the side of caution" but I want to know what is really going on.

Say you have two completely identical LiPO batteries with different charge levels. Pack A is 12.4v and pack B is 11.4v.

You connect the two packs in parallel. Voltage wise they equal out instantly, but what does this mean current wise between the two packs?

LoopyByteloose

You could put a load on the pair and read the ampmeter set to each cell.

Something is telling me that the higher voltage cell is discharging and the lower voltage cells is charging. So expect the need to reverse polarity on the ampmeter connections.

When the two cells hit equilbrium, they will approximate an equal current discharge. (That is the ideal, they may never resolve internal resistance variations in a perfect harmony).

I do see in my notebook computers that the six cell battery sets of Li Ion are actually three parallel pairs in series. I have begun to think that their actually may be an advantage to having paired cells discharge in parallel --- maybe the low voltage discharge limit is more predictable.

You seem to imply that you have two batteries with 3 cells in series and then connected in parallel. That may have no advantage other than more overall current capacity.

Dave Hein

This doesn't sound like a good thing to do. I came across this on one website

Use Equally Charged LiPo’s in Parallel Configurations - Some UAV battery
configurations require the parallel connection of two or more Lithium Polymer (LiPo) batteries. In
this case, LiPo batteries must have the same or very similar output voltage. Parallel connection
of two batteries of significantly different output voltage (one is more fully charged than the other)
will cause one battery to discharge into the other battery at a high current rate. This may cause
damage to either of the batteries and potentially cause fire or the battery to burst. It is strongly
recommended that in applications requiring parallel connection of LiPo batteries, only batteries
of equivalent technology, capacity, cell count, and output voltage be used. A good assumption is
that fully charged LiPo batteries of the same type are safe to connect in parallel.

If you are going to connect two LiPo's in parallel I would make sure that their voltages are as close as possible.

LoopyByteloose

Somewhere amongst my vast accumulation of dead-end brainstorms I came across a technical article about matching Lithium cells according to internal resistances.

In sum, it asserted that while a lot of documents strongly suggest matching lithium cells and most likely by starting with cells from the same batches of production runs, the research results are that the productions themselves don't not produce anything of close uniformity. IN other words, it is nearly impossible to match Lithium ion cells as there is no reliable method for doing so... it is all theory.

But I do suspect that having parallel pairs of individual may outperform single cells in terms of longevity and stability. I just can't find anything written up about this very common practice.

Tesla has avoided the whole theoretical mess by using Lithium Ion cells that have a button device on the end of each and every cell to monitor under-voltage, over-voltage, thermal hazard, and current discharge. Centralized monitoring and control just creates a rat's nest of wire and has never been proven to do much other than waste charging and discharging power in attempting to balance the process. Tesla's scheme merely safely shuts out a bad cell until a service visit monitors and replaces all failed cells.

Parallax sell the same 18650 Li Ion cells that Tesla prefers. There are other ones without the button that fail suddenly from excessive discharge.

xanadu

Loopy Byteloose wrote: »

You could put a load on the pair and read the ampmeter set to each cell.

Something is telling me that the higher voltage cell is discharging and the lower voltage cells is charging. So expect the need to reverse polarity on the ampmeter connections.

When the two cells hit equilbrium, they will approximate an equal current discharge. (That is the ideal, they may never resolve internal resistance variations in a perfect harmony).

I do see in my notebook computers that the six cell battery sets of Li Ion are actually three parallel pairs in series. I have begun to think that their actually may be an advantage to having paired cells discharge in parallel --- maybe the low voltage discharge limit is more predictable.

You seem to imply that you have two batteries with 3 cells in series and then connected in parallel. That may have no advantage other than more overall current capacity.

I should have been a little more clear. We're just talking about two packs, 3S packs - as a for example. There is no practical application, the entire experiment would consist of Pack A and Pack B and the wires to connect them. They wouldn't be setup like for charging or use, only resting on a bench. The way I see it everything else can be figured out from that baseline. Plus I don't have the mental capacity to add any extra variables.

I have tried using my multi-meter's 10A current but the spike is too fast to get the peak. It has a 10 amp fuse so I guess we aren't over 10 amps with a 1v difference.

Laptop batteries are great, I've taken apart tons. I don't know if I've seen any with that config is it an extended battery?

xanadu

Dave Hein wrote: »

This doesn't sound like a good thing to do. I came across this on one website

If you are going to connect two LiPo's in parallel I would make sure that their voltages are as close as possible.

Seems like common sense right? I thought so too. Even if it were fine to plug different voltage packs together it seems like a bad habit. I don't think people understand that you can parallel balance charge the cells, but the charger has no idea the different pack voltage. It's up to the person charging to understand there is a difference there.

All that aside though, I still need some numbers. It's time for some better test equipment

xanadu

Loopy Byteloose wrote: »

Somewhere amongst my vast accumulation of dead-end brainstorms I came across a technical article about matching Lithium cells according to internal resistances.

In sum, it asserted that while a lot of documents strongly suggest matching lithium cells and most likely by starting with cells from the same batches of production runs, the research results are that the productions themselves don't not produce anything of close uniformity. IN other words, it is nearly impossible to match Lithium ion cells as there is no reliable method for doing so... it is all theory.

But I do suspect that having parallel pairs of individual may outperform single cells in terms of longevity and stability. I just can't find anything written up about this very common practice.

Tesla has avoided the whole theoretical mess by using Lithium Ion cells that have a button device on the end of each and every cell to monitor under-voltage, over-voltage, thermal hazard, and current discharge. Centralized monitoring and control just creates a rat's nest of wire and has never been proven to do much other than waste charging and discharging power in attempting to balance the process. Tesla's scheme merely safely shuts out a bad cell until a service visit monitors and replaces all failed cells.

Parallax sell the same 18650 Li Ion cells that Tesla prefers. There are other ones without the button that fail suddenly from excessive discharge.

It's a good thing those fail-safes are there! A big difference in internal resistance is bad, jump start a motorcycle with a car and you'll find out haha.

Tor

Loopy Byteloose wrote: »

Parallax sell the same 18650 Li Ion cells that Tesla prefers. There are other ones without the button that fail suddenly from excessive discharge.

But they are not the same, are they? I thought the 18650 cells that Parallax sell are Chinese? Tesla uses Panasonic cells, they are much more expensive. It's unfortunate for everybody else because it's now nearly impossible to find the Panasonic batteries anywhere. Not a single cell to find even in Japan. The Panasonic ones are known to be maybe the best cell, or at least among the top two or three. But Tesla takes them all so nothing left for us mortals.

-Tor

LoopyByteloose

okay, not exactly the same -- but same or similar design or protection scheme.

The point is that Parallax chooses to sell the 18650 cells with the added button that prevents a variety of problems. The degree of sameness or similarity is up to the individual shopper to decide. I can't help it that Tesla's due diligence is buying up the safest lithium cells available.