I know, such optimism...
Normally I should probably have all my ducks in a row and actually have something substantial with which to start this thread. But I don't really. I was just thinking about how pack management probably revolves a great deal around working voltages specific to the stock Panasonic NiMH cells; about how important those voltages are; yet also how voltage always got short shrift around IC and still remains such a haphazard affair around here...
I imagined how great it would be if we could have a single thread that was all about Insight NiMH voltages - pack voltages, cell voltages, stick voltages, tap voltages, full voltages, empty voltages, typical full self-discharge voltages, normal 'working' voltages, 75% voltages, 'bad' voltages, etc etc...
One working theory I have at the moment involves the importance of the 1.20 to 1.25V range: I think good cells will really stick to this tight range, even at pretty high discharges rates. That's one major difference I've been seeing with my latest good rebuilt pack versus my other couple or so crummy packs. We all know that the nominal voltage of the cells is 1.2V - though I think it's really supposed to be 1.25V. That means something. Cells should be working at that voltage (during discharge); it's what they're meant to do...
I think the Insight battery management probably relies a lot on that 'middle' voltage...
1.34V is also a key value; it's the idealized equilibrium potential for the main reaction on the positive electrode. In practice, one thing it means to 'us' is that resting, open circuit pack voltage is often hovering around 160-161V (120 cells X 1.34V)...
Full voltages? The Panasonic spec sheet I have for cells that have often been said to be 'our' cells, the HHR650D, show a charge curve at 1C rate peaking at 1.53V... Over the years of grid charging, I've honed-in on full charge values of about 174-176V, charging at typical room temp and typical grid charge rates 300-500mA... 174-176V=1.45V to 1.467V were it just a single cell... I made a chart a while back for temp adjustments; I'll add that later...
0.78V is supposedly the equilibrium potential for the unwanted reaction that causes 'voltage depression' or 'memory effect' - analogous to how 1.34V is the equilibrium potential for the wanted, good and normal reaction...
0.68V - That's the voltage I've seen many times on cells that have self-discharged for a long time; self-discharged cells will hover around that voltage for a long time...
I'll try to come back and add 'voltage' stuff when it comes to mind. If you have any "voltage stuff" please consider adding it to the thread - questions, comments, values to report (cell-level or otherwise), and stuff related to voltage, like battery management. I guess that's kind of my main concern, fleshing-out how the Insight BMS uses voltages to do its thing, and probably mainly that 1.25-1.20V range...
Basically, it's really looking to me like, if your pack is NOT holding tight to the 1.2-1.25V range (144V to 150V), it's a sign that something's amiss - cells are drifting out of balance (most likely and/or most frequent issue), maybe voltage depression is setting in, maybe cells are just getting old and internal resistance is getting high, a combination of these, etc. And the thing is, you can really spot when your pack is in top form (or not) if you just watch how voltage is holding up.
For instance, when the pack is cool it won't hold that voltage so well or it won't be able to hold it at relatively high currents. Likewise, if cells are out of balance, it won't hold that range so well, warm or not.
And really, it's not the whole range: in really top form your (stock) pack will be holding 1.25V (150V), not 1.2V (144V) at at least about a 3C (19.5A) rate. And it will do this even at relatively low charge states - say 50%, it doesn't have to be 'full' 75%...
This is all stuff I'm grappling with - trying to juggle charge state, temperature, balance state, overall cell condition, and voltage - to get a good handle on pack condition/state. I've generally been seeing something like 144V to 150V at up to about a 3C rate between about 45% and 75% charge state, pack warmed up to at least 70F. There's wiggle room in these numbers depending on the exact values, like the exact temp, SoC, and current rate. I'd like to tighten this up...
What voltages under various conditions should I be seeing if my pack is 'good'? If my pack is at a nominal 50% charge state, and I'm discharging at a 3C rate, pack temp 75F, what voltage should I see with a good pack? Or a bad pack? It's all pretty subtle - at this point I can pretty much tell when the cells have drifted about 20% out of balance. I'd like to lower that figure to some degree, plus be a bit more sure about it...
Right now, pack voltage sag at moderate charge states (maybe 50-60%) is my main realtime, tell-tale sign of imbalance. And I'm talking about only a few volts difference, like seeing about 139V instead of about 142V, warmed pack. Or maybe it's seeing that sag at slightly higher charge states; like instead of seeing it down around 45% SoC, it might be 60% SoC... I'd like to tighten-up these kinds of observations, like I'd like to be able to tell just what 'state' my pack is at with just a few charge/assist runs - like get the pack warmed up, do a couple assist runs, see the numbers, know the state... I'm almost there...
Normally I should probably have all my ducks in a row and actually have something substantial with which to start this thread. But I don't really. I was just thinking about how pack management probably revolves a great deal around working voltages specific to the stock Panasonic NiMH cells; about how important those voltages are; yet also how voltage always got short shrift around IC and still remains such a haphazard affair around here...
I imagined how great it would be if we could have a single thread that was all about Insight NiMH voltages - pack voltages, cell voltages, stick voltages, tap voltages, full voltages, empty voltages, typical full self-discharge voltages, normal 'working' voltages, 75% voltages, 'bad' voltages, etc etc...
One working theory I have at the moment involves the importance of the 1.20 to 1.25V range: I think good cells will really stick to this tight range, even at pretty high discharges rates. That's one major difference I've been seeing with my latest good rebuilt pack versus my other couple or so crummy packs. We all know that the nominal voltage of the cells is 1.2V - though I think it's really supposed to be 1.25V. That means something. Cells should be working at that voltage (during discharge); it's what they're meant to do...
I think the Insight battery management probably relies a lot on that 'middle' voltage...
1.34V is also a key value; it's the idealized equilibrium potential for the main reaction on the positive electrode. In practice, one thing it means to 'us' is that resting, open circuit pack voltage is often hovering around 160-161V (120 cells X 1.34V)...
Full voltages? The Panasonic spec sheet I have for cells that have often been said to be 'our' cells, the HHR650D, show a charge curve at 1C rate peaking at 1.53V... Over the years of grid charging, I've honed-in on full charge values of about 174-176V, charging at typical room temp and typical grid charge rates 300-500mA... 174-176V=1.45V to 1.467V were it just a single cell... I made a chart a while back for temp adjustments; I'll add that later...
0.78V is supposedly the equilibrium potential for the unwanted reaction that causes 'voltage depression' or 'memory effect' - analogous to how 1.34V is the equilibrium potential for the wanted, good and normal reaction...
0.68V - That's the voltage I've seen many times on cells that have self-discharged for a long time; self-discharged cells will hover around that voltage for a long time...
I'll try to come back and add 'voltage' stuff when it comes to mind. If you have any "voltage stuff" please consider adding it to the thread - questions, comments, values to report (cell-level or otherwise), and stuff related to voltage, like battery management. I guess that's kind of my main concern, fleshing-out how the Insight BMS uses voltages to do its thing, and probably mainly that 1.25-1.20V range...
Basically, it's really looking to me like, if your pack is NOT holding tight to the 1.2-1.25V range (144V to 150V), it's a sign that something's amiss - cells are drifting out of balance (most likely and/or most frequent issue), maybe voltage depression is setting in, maybe cells are just getting old and internal resistance is getting high, a combination of these, etc. And the thing is, you can really spot when your pack is in top form (or not) if you just watch how voltage is holding up.
For instance, when the pack is cool it won't hold that voltage so well or it won't be able to hold it at relatively high currents. Likewise, if cells are out of balance, it won't hold that range so well, warm or not.
And really, it's not the whole range: in really top form your (stock) pack will be holding 1.25V (150V), not 1.2V (144V) at at least about a 3C (19.5A) rate. And it will do this even at relatively low charge states - say 50%, it doesn't have to be 'full' 75%...
This is all stuff I'm grappling with - trying to juggle charge state, temperature, balance state, overall cell condition, and voltage - to get a good handle on pack condition/state. I've generally been seeing something like 144V to 150V at up to about a 3C rate between about 45% and 75% charge state, pack warmed up to at least 70F. There's wiggle room in these numbers depending on the exact values, like the exact temp, SoC, and current rate. I'd like to tighten this up...
What voltages under various conditions should I be seeing if my pack is 'good'? If my pack is at a nominal 50% charge state, and I'm discharging at a 3C rate, pack temp 75F, what voltage should I see with a good pack? Or a bad pack? It's all pretty subtle - at this point I can pretty much tell when the cells have drifted about 20% out of balance. I'd like to lower that figure to some degree, plus be a bit more sure about it...
Right now, pack voltage sag at moderate charge states (maybe 50-60%) is my main realtime, tell-tale sign of imbalance. And I'm talking about only a few volts difference, like seeing about 139V instead of about 142V, warmed pack. Or maybe it's seeing that sag at slightly higher charge states; like instead of seeing it down around 45% SoC, it might be 60% SoC... I'd like to tighten-up these kinds of observations, like I'd like to be able to tell just what 'state' my pack is at with just a few charge/assist runs - like get the pack warmed up, do a couple assist runs, see the numbers, know the state... I'm almost there...