So for various tasks (more on that in a moment) I've been wanting to be able to run lithium-ion batteries in parallel for a time. However, it has been my understanding that ideally you need to impedance match cells when doing this. Certainly real companies (obviously excluding cheap Chinese generics) can do this as they have a large scale with a lot of stock and thus can just check them all and put together matches. A DIY home user like myself who finds it to be exceptionally costly to buy lithium ion batteries (I generally just buy two at a time) obviously can't do this. In fact, the older ones have to be treated as not matching even if the impedance comes up the same as they've been used and thus have more cycles on them. Now, one thing I would swear I've read in the past is that the primary reason for this matching is to keep one from simply outputting its own excess voltage into the other one. If I have since read correctly in various searches (basically running across things by accident here while looking for other things) it really sounds like once they're voltage matched, essentially it's less of an issue since they will charge and discharge together.
I presume this would mean it would be more wear on the weaker of the two cells though, correct? The question then is if this is a big issue here? The key thing that finally got me to break down and start looking into this after all of this time (I've been messing with lithium ion for quite some time now but always using one cell or two in series for simplicity) is I dragged out my netbook after a really long hiatus (I wouldn't even care to guess how long it has been since I last properly charged it) and discovered that the battery pack is essentially dying. I didn't feel like being bothered to rig up a proper test mechanism, so I plugged it into a cheap low end Kill-A-Watt model and removed the battery pack (so it's running a power supply unit which is obviously not 100% efficient but it's not charging a battery. Thus my wattage readings include a bit of loss due to inefficiencies, but I can at least assume they are within a reasonable percentage of the tested results.) The absolute maximum reading I ever got was 19 watts for a split second and I only saw that once. Generally speaking it hovered between 17 and 18 (this thing doesn't do decimals, so it rounds and thus may have been 17.5 or something -- I could do current, but that gets messier since it's testing on the AC end) so I treat it as being 18 at the most. Average during heavy real usage after everything settled down was around 15 or so, give or take (really once it fully settles it goes down to around 13, but I assume that if I've pulled it out I'll be at least doing a bit with harddrive access and such going on. It's probably 14 or less realistically speaking, but I caught it showing 15 a lot during a quick test and I'm going with that.) This means absolute worst case scenario for any sustained usage is roughly (18/6)/2 = 1.5a per cell (absolute worst case scenario being when the battery is critically low and that chip is just about to clamp down and shut it off forcibly, so 3.0V x 2. It may not even turn on at that low of a voltage though, I'm not sure.) More realistically I'd really be looking at a much more typical range averaging around (15/7.2)/2 = 1.04a per cell. In each case well below 1C for even some really cheapo batteries. However, I don't plan to go cheapo here.
I pulled the battery pack apart out of curiosity and I guess one of the cells (or one of the packs of cells in parallel) must have been damaged due to a too low voltage when sitting around for too long (in particular, probably when I forgot to pull the pack out for a long time. It seems the netbook drains a trace amount of power from the battery while turned off. It's very little, but it probably had months to do this and then even once the protection chip presumably shut off the circuit it had plenty of time to slowly self-drain that last tiny bit after the cutoff point before damage occurs.) I noticed of the two sets of cells, one set was of a considerably lower voltage even after charging for a while. Assuming this thing isn't horribly made (I don't know my chips enough to really tell just from looking, but I believe that there are enough chips for this) it should be balance charging (are there any laptop batteries made in the last like ten years or so which do not balance charge? Lol.) Basically I think what happened is it sat around so long that eventually even after the voltage cutoff point the cells self-drained very very slowly over time until eventually at least one of them was dangerously low and was damaged. Maybe two weeks ago (three at the most) I had actually completely charged the battery pack overnight in the system and then taken it out of the system and left it out this time, then I found that when I tried to use it last night it wouldn't even turn on until I plugged it in (at which point it indicated that the battery was extremely low,) so one way or another the battery pack does appear to be damaged.
This comes to my biggest reason for wanting to do this. I'm basically obsessed with Panasonic's NCR18650 cells. These are designed to drain down to 2.5V instead of 3.0V. Now, of course, few things can handle that. In most cases everything below 3V should be considered useless. Especially when using protection circuits designed to stop at 3V anyway. Looking at their discharge charts though, the NCR18650s are still quite impressive when stopping at 3V. (It's difficult to exactly visualize, but I'd guesstimate it is roughly in the area of 2800mAh. It's definitely more than 2600mAh as far as I can tell at the very least, so at a bare minimum this means they're at least equal to the competition. As I'm sure you people in particular will know, it's rare to squeeze more than 2600mAh out of a 4.2V cell and to get more you need to go up to 4.35V with one of the newer tech higher voltage cells that cellphones do love so very much.) The real point here though is that the NCR18650 has more tolerance for applications where one may not necessarily be able to watch them as carefully. For example, a netbook sitting around for a very long time without charging. Once the protection circuit shuts it off around 3V or so, a normal lithium ion only has to self-drain down to 2.8V or so before damage begins occurring (and at that voltage range they drop much more quickly.) These would have to drop to around 2.4V or lower (I forget where the actual point of damage was on these) and they have a considerable capacity from 3V down to 2.5V as well. Thus even if I screw up again and forget to charge it for a long time, the actual cells shouldn't be damaged unless I basically just never use the thing again (or have other issues like the harddrive dying from old age.) Anyway, my point here is that you just can't do this no matter what battery packs you find made for any specific device on Amazon or eBay or whatever.
Now, I want to be clear that I'm 100% aware that I can just buy another battery pack. In fact, I did buy an old used one as a backup in case I screw things up. However, my goal here is just to make one that is more durable in general. This should produce a higher capacity along the way as a nice little bonus though. (Like I said, they're probably closer to 2800mAh or so. Maybe slightly more even. That means my 5200mAh battery pack could theoretically become a 5600mAh one. Not a huge amount, but on this thing that really would make a difference. Just a bonus though, nothing more. I need that low voltage safety net more than anything else I think.)
I presume this would mean it would be more wear on the weaker of the two cells though, correct? The question then is if this is a big issue here? The key thing that finally got me to break down and start looking into this after all of this time (I've been messing with lithium ion for quite some time now but always using one cell or two in series for simplicity) is I dragged out my netbook after a really long hiatus (I wouldn't even care to guess how long it has been since I last properly charged it) and discovered that the battery pack is essentially dying. I didn't feel like being bothered to rig up a proper test mechanism, so I plugged it into a cheap low end Kill-A-Watt model and removed the battery pack (so it's running a power supply unit which is obviously not 100% efficient but it's not charging a battery. Thus my wattage readings include a bit of loss due to inefficiencies, but I can at least assume they are within a reasonable percentage of the tested results.) The absolute maximum reading I ever got was 19 watts for a split second and I only saw that once. Generally speaking it hovered between 17 and 18 (this thing doesn't do decimals, so it rounds and thus may have been 17.5 or something -- I could do current, but that gets messier since it's testing on the AC end) so I treat it as being 18 at the most. Average during heavy real usage after everything settled down was around 15 or so, give or take (really once it fully settles it goes down to around 13, but I assume that if I've pulled it out I'll be at least doing a bit with harddrive access and such going on. It's probably 14 or less realistically speaking, but I caught it showing 15 a lot during a quick test and I'm going with that.) This means absolute worst case scenario for any sustained usage is roughly (18/6)/2 = 1.5a per cell (absolute worst case scenario being when the battery is critically low and that chip is just about to clamp down and shut it off forcibly, so 3.0V x 2. It may not even turn on at that low of a voltage though, I'm not sure.) More realistically I'd really be looking at a much more typical range averaging around (15/7.2)/2 = 1.04a per cell. In each case well below 1C for even some really cheapo batteries. However, I don't plan to go cheapo here.
I pulled the battery pack apart out of curiosity and I guess one of the cells (or one of the packs of cells in parallel) must have been damaged due to a too low voltage when sitting around for too long (in particular, probably when I forgot to pull the pack out for a long time. It seems the netbook drains a trace amount of power from the battery while turned off. It's very little, but it probably had months to do this and then even once the protection chip presumably shut off the circuit it had plenty of time to slowly self-drain that last tiny bit after the cutoff point before damage occurs.) I noticed of the two sets of cells, one set was of a considerably lower voltage even after charging for a while. Assuming this thing isn't horribly made (I don't know my chips enough to really tell just from looking, but I believe that there are enough chips for this) it should be balance charging (are there any laptop batteries made in the last like ten years or so which do not balance charge? Lol.) Basically I think what happened is it sat around so long that eventually even after the voltage cutoff point the cells self-drained very very slowly over time until eventually at least one of them was dangerously low and was damaged. Maybe two weeks ago (three at the most) I had actually completely charged the battery pack overnight in the system and then taken it out of the system and left it out this time, then I found that when I tried to use it last night it wouldn't even turn on until I plugged it in (at which point it indicated that the battery was extremely low,) so one way or another the battery pack does appear to be damaged.
This comes to my biggest reason for wanting to do this. I'm basically obsessed with Panasonic's NCR18650 cells. These are designed to drain down to 2.5V instead of 3.0V. Now, of course, few things can handle that. In most cases everything below 3V should be considered useless. Especially when using protection circuits designed to stop at 3V anyway. Looking at their discharge charts though, the NCR18650s are still quite impressive when stopping at 3V. (It's difficult to exactly visualize, but I'd guesstimate it is roughly in the area of 2800mAh. It's definitely more than 2600mAh as far as I can tell at the very least, so at a bare minimum this means they're at least equal to the competition. As I'm sure you people in particular will know, it's rare to squeeze more than 2600mAh out of a 4.2V cell and to get more you need to go up to 4.35V with one of the newer tech higher voltage cells that cellphones do love so very much.) The real point here though is that the NCR18650 has more tolerance for applications where one may not necessarily be able to watch them as carefully. For example, a netbook sitting around for a very long time without charging. Once the protection circuit shuts it off around 3V or so, a normal lithium ion only has to self-drain down to 2.8V or so before damage begins occurring (and at that voltage range they drop much more quickly.) These would have to drop to around 2.4V or lower (I forget where the actual point of damage was on these) and they have a considerable capacity from 3V down to 2.5V as well. Thus even if I screw up again and forget to charge it for a long time, the actual cells shouldn't be damaged unless I basically just never use the thing again (or have other issues like the harddrive dying from old age.) Anyway, my point here is that you just can't do this no matter what battery packs you find made for any specific device on Amazon or eBay or whatever.
Now, I want to be clear that I'm 100% aware that I can just buy another battery pack. In fact, I did buy an old used one as a backup in case I screw things up. However, my goal here is just to make one that is more durable in general. This should produce a higher capacity along the way as a nice little bonus though. (Like I said, they're probably closer to 2800mAh or so. Maybe slightly more even. That means my 5200mAh battery pack could theoretically become a 5600mAh one. Not a huge amount, but on this thing that really would make a difference. Just a bonus though, nothing more. I need that low voltage safety net more than anything else I think.)