Horsefly
Solar Wizard
So, I'm pretty new here (there, that's out of the way ).
I've been reading the board for a quite a while, from way before I ever posted. I'm amazed at the knowledge and sharing of information. It's awesome!
My background: I'm a retired Electrical Engineer, and I designed my own solar power system, but I used Lead-Acid AGM batteries in my system, so I'm mostly new to the idea of LiFePO4 batteries. I admit I don't really understand how charging and discharging of LiFePO4 (or battery in general) really work (there, that's out of the way too ?).
A few folks in this thread (and others) have said that when they have packs/batteries of mismatched Ah tied together in parallel, it all works great and they don't see any current / power moving from the bigger pack (say, 280Ah) to the smaller pack (say, 90Ah) during discharge. Further, they report that the current delivered from the two disparate packs is proportional to their overall contribution to the total. I am confused by this, but I have no doubt of the authenticity that it is true. I just don't understand how this works.
Let's walk through it...
A 280Ah pack and a 90Ah pack are wired in parallel. They are both nearly fully charged, and so are quite happy at say 3.5V. Then someone turns on the loads. Both packs seek to fill whatever they can of the current demand of the load. It appears to me that the cells in a 90Ah pack and those in a 280Ah pack are not significantly different in internal resistance (from what I have been able to find), or perhaps it is somewhat unpredictable. Nevertheless, it seems that the 90Ah pack will deliver pretty much (90/(280+90) of the drawn current. Why is that?
As the two packs are drawn down, the voltages obviously are held together. However, it would seem like the smaller pack would be depleted faster. Eventually the smaller pack would not be delivering any current towards the load, and to maintain the same voltage, it would have to be receiving what is effectively a charge from the larger pack, while the larger pack is also serving the demand of the loads. However, it has been reported that there is not current flowing INTO the smaller pack in this instance, so.... Why is that?
Eventually, the loads quit and a source (solar, generator, shore, etc.) start recharging the two packs. Again, the two packs are maintaining the same voltage. Since the larger pack will rise more slowly, this part maybe makes more sense to me: The small gets what it needs to be at 3.3V, but the larger pack needs more current to get it up past that. So, the larger pack takes more current. Do I have that right?
This is probably a little thing, and not worth much discussion. However it is driving me crazy! I just don't get it.
Caveat: I'm coming from the Lead-Acid / AGM world. In that world, there are lots of rules you hear when you are initiated. One is that you don't put batteries that are not of the same Ah and same age - and maybe not even if they are not the same brand and date - together. To do so is asking for unspecified catastrophes! I didn't really understand that much either, but I could some sense of it, but it was mostly just the magic of batteries. Now I'm more prone to question things, I guess...
Any thoughts? Again, I'm just trying to learn here, and you guys know what you are talking about.
I've been reading the board for a quite a while, from way before I ever posted. I'm amazed at the knowledge and sharing of information. It's awesome!
My background: I'm a retired Electrical Engineer, and I designed my own solar power system, but I used Lead-Acid AGM batteries in my system, so I'm mostly new to the idea of LiFePO4 batteries. I admit I don't really understand how charging and discharging of LiFePO4 (or battery in general) really work (there, that's out of the way too ?).
A few folks in this thread (and others) have said that when they have packs/batteries of mismatched Ah tied together in parallel, it all works great and they don't see any current / power moving from the bigger pack (say, 280Ah) to the smaller pack (say, 90Ah) during discharge. Further, they report that the current delivered from the two disparate packs is proportional to their overall contribution to the total. I am confused by this, but I have no doubt of the authenticity that it is true. I just don't understand how this works.
Let's walk through it...
A 280Ah pack and a 90Ah pack are wired in parallel. They are both nearly fully charged, and so are quite happy at say 3.5V. Then someone turns on the loads. Both packs seek to fill whatever they can of the current demand of the load. It appears to me that the cells in a 90Ah pack and those in a 280Ah pack are not significantly different in internal resistance (from what I have been able to find), or perhaps it is somewhat unpredictable. Nevertheless, it seems that the 90Ah pack will deliver pretty much (90/(280+90) of the drawn current. Why is that?
As the two packs are drawn down, the voltages obviously are held together. However, it would seem like the smaller pack would be depleted faster. Eventually the smaller pack would not be delivering any current towards the load, and to maintain the same voltage, it would have to be receiving what is effectively a charge from the larger pack, while the larger pack is also serving the demand of the loads. However, it has been reported that there is not current flowing INTO the smaller pack in this instance, so.... Why is that?
Eventually, the loads quit and a source (solar, generator, shore, etc.) start recharging the two packs. Again, the two packs are maintaining the same voltage. Since the larger pack will rise more slowly, this part maybe makes more sense to me: The small gets what it needs to be at 3.3V, but the larger pack needs more current to get it up past that. So, the larger pack takes more current. Do I have that right?
This is probably a little thing, and not worth much discussion. However it is driving me crazy! I just don't get it.
Caveat: I'm coming from the Lead-Acid / AGM world. In that world, there are lots of rules you hear when you are initiated. One is that you don't put batteries that are not of the same Ah and same age - and maybe not even if they are not the same brand and date - together. To do so is asking for unspecified catastrophes! I didn't really understand that much either, but I could some sense of it, but it was mostly just the magic of batteries. Now I'm more prone to question things, I guess...
Any thoughts? Again, I'm just trying to learn here, and you guys know what you are talking about.