Li-Poly Should packs be Series or Parallel
02-04-2004, 10:20 PM
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Li-Poly Should packs be Series or Parallel
Hi,
The conventional wisdom seems to be that packs are always built with cells in series to get the desired V, then paralleled to get the capacity/current, and the advise is NEVER charge cells in parallel. However when you look at the manufacturers data and limits it seems to me that there are a number of issues with series packs the main two are:
1: Cell voltage should never exceed 4.235 volts or cell damage will occur, however when charging multiple in series (ie a standard pack) the terminal voltage of an individual cell cannot be gauranteed and if one cell is down slightly it is easy to exceed this limit.(there are no 4.2 voltage zener diodes in any of the packs I have examined!)
2: The absolute minimum voltage of a cell should not fall below 2.5V, once again even with a voltage cut off of say 9.0 volts for a 3 cell pack it is easy to get two cells at 3.3 which means the other cell as it 2.4, and most cut off is below this.
A parallel pack would not suffer from these limitations, but it would be necessary to charge at a lower current to avoid exceeding the maximum charge rate of a single cell, however the batteries would never exceed the maximum voltage on charge, and would be much less likely to go below the minimum, so the upside is much better battery condition and life span.
Perhaps I'm missing something here, if so please let me know.
Phil
The conventional wisdom seems to be that packs are always built with cells in series to get the desired V, then paralleled to get the capacity/current, and the advise is NEVER charge cells in parallel. However when you look at the manufacturers data and limits it seems to me that there are a number of issues with series packs the main two are:
1: Cell voltage should never exceed 4.235 volts or cell damage will occur, however when charging multiple in series (ie a standard pack) the terminal voltage of an individual cell cannot be gauranteed and if one cell is down slightly it is easy to exceed this limit.(there are no 4.2 voltage zener diodes in any of the packs I have examined!)
2: The absolute minimum voltage of a cell should not fall below 2.5V, once again even with a voltage cut off of say 9.0 volts for a 3 cell pack it is easy to get two cells at 3.3 which means the other cell as it 2.4, and most cut off is below this.
A parallel pack would not suffer from these limitations, but it would be necessary to charge at a lower current to avoid exceeding the maximum charge rate of a single cell, however the batteries would never exceed the maximum voltage on charge, and would be much less likely to go below the minimum, so the upside is much better battery condition and life span.
Perhaps I'm missing something here, if so please let me know.
Phil
02-05-2004, 09:37 AM
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RE: Li-Poly Should packs be Series or Parallel
Where does it say to NEVER charge the cells/packs in parallel? That's simply incorrect, and completely opposite of current "conventional wisdom." Charging NiCd and NiMH packs in parallel is problematic due to the way the cells are charged (pure constant current charge).
Because of the way LiPoly cells are charged to a specific voltage, charging in parallel is not an issue, provided the individual packs are fairly close in voltage to begin with. Parallel charging ensures that the packs all remain in balance.
With LiPoly packs, cells charged in series have the potential to become unbalanced, and sometimes do, causing catastrophic failure of one or more cells.
Charging purely in parallel, that is, a whole bunch of single cells connected in parallel, is the safest way to charge, and the best way to ensure that all the cells remain balanced. However, with large-capacity packs this is not practical. To charge a 5S4P pack of 1200mAh cells in a "pure parallel" fashion, in a reasonable amount of time, you'd need a charger capable of charging a single LiPoly cell at 24 Amps. 8 or 9 Amps is tops with today's chargers. It'd take about 4 hours to charge this particular configuration with current technology. The only way to reduce charge time right now is to charge in a series AND parallel setup.
You can mitigate the risk by checking the individual cell voltages from time to time, and charging each cell individually when an imbalance is found. The jury is still out on this, but I believe there is minimal risk of imbalance when you charge packs in 2S or even 3S configurations. You could break apart your 5S4P into a 3S4P and 2S4P, and charge each pack separately, for example.
Because of the way LiPoly cells are charged to a specific voltage, charging in parallel is not an issue, provided the individual packs are fairly close in voltage to begin with. Parallel charging ensures that the packs all remain in balance.
With LiPoly packs, cells charged in series have the potential to become unbalanced, and sometimes do, causing catastrophic failure of one or more cells.
Charging purely in parallel, that is, a whole bunch of single cells connected in parallel, is the safest way to charge, and the best way to ensure that all the cells remain balanced. However, with large-capacity packs this is not practical. To charge a 5S4P pack of 1200mAh cells in a "pure parallel" fashion, in a reasonable amount of time, you'd need a charger capable of charging a single LiPoly cell at 24 Amps. 8 or 9 Amps is tops with today's chargers. It'd take about 4 hours to charge this particular configuration with current technology. The only way to reduce charge time right now is to charge in a series AND parallel setup.
You can mitigate the risk by checking the individual cell voltages from time to time, and charging each cell individually when an imbalance is found. The jury is still out on this, but I believe there is minimal risk of imbalance when you charge packs in 2S or even 3S configurations. You could break apart your 5S4P into a 3S4P and 2S4P, and charge each pack separately, for example.
02-06-2004, 08:11 PM
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RE: Li-Poly Should packs be Series or Parallel
Sounds like we are in agreement here then. The "NEVER" charge in parallel advise comes from Kokam, it's on there site and is emphasized in bold on in the instructions with each purchase, I even emailed them about this question I got the same response. In tests I found it was easy to over discharge or over charge a cell in a 3S pack. It's hard to see though why they don't stick a 4.2V zener diode across each cell in these commercial series packs. THis would at least prevent over charge and restore balance.
With regards parallel packs I agree the down side is charge time.
cheers PS
With regards parallel packs I agree the down side is charge time.
cheers PS
02-10-2004, 04:13 PM
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RE: Li-Poly Should packs be Series or Parallel
Guys--
I have been round and round about this.
Any two power sources in parallel is dangerous.
They should only be connected when in use, not in transition or charging. This is only my openion.
But, I see info in post's here that say it's safe so I dont know what to say. It sure goes against my electronic knowledge.
I have been round and round about this.
Any two power sources in parallel is dangerous.
They should only be connected when in use, not in transition or charging. This is only my openion.
But, I see info in post's here that say it's safe so I dont know what to say. It sure goes against my electronic knowledge.
