The biggest mistake you can make if you plan to use them in series not to
equalize them before use. They come in different state of charge. Maybe
close if  from the same lot but not close enough and I do not count on the
equalizer. I  start with a discharge to 2.5 V and charge two at a time in
Parallel with a  holder that I modified. Set a power supply to 4.2 V precisely and
charge all  batteries for that particular application with that setting.
Bert Kehren

In a message dated 1/23/2017 4:12:08 A.M. Eastern Standard Time,
holrum@hotmail.com writes:

I built  a switched capacitor balancer.  It takes some time to balance a
wildly  out-of-balance pack but does a great job of maintaining balance during
charging and discharge.  I run the balancer during pack charging and
discharging.  I tried it charging an 8S LiFePO4 pack with 7 fully charged  cells
and one fully discharged cell and it worked fine... no cells were
over-volted during charging.

It does not waste energy/generate heat  like a resistive balancer.  An
inductive balancer can handle higher  balance currents for very high capacity
packs, but is also quite a bit more  complicated to do correctly.  Also built
a processor-per-cell BMS using  ATTINY-85's.  I've used both on  4S32P /
8S16P A123 packs and  ICR 18650 packs.

Those A123 cells can dump 200A (each!) into a short  circuit... you REALLY
don't want to short a 32P A123 pack...  They are  also rather tolerant of
abuse.  A friend made a video of charging one at  a 100C rate and it survived
without exploding/melting.  I once bought  some that had been sitting fully
charged for over 4 years and when tested  still retained over 80% of the
charge.  A couple of charge/discharge  cycles and they performed like new.  I
keep  4S1P packs in my cars  for jump starting.  There is a video of a guy
in Canada staring his  car over 10 times in sub-zero weather using one.  The
cells are 26650  size... are about the size of a D  cell.

energy

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Balancing series packs is indeed important. This is why laptop batteries have all those pins on their connectors, and why hobby type batteries have balance charge connectors. Although they are different chemistries (laptop and 16450/18650 batteries are typically Li-Ion while hobby type batteries are typically Li-Poly) they both have this requirement. And each chemistry has it’s own charge/discharge requirements. Note that within Li-Ion there are still different chemistries that again have their own requirements, which are different from LiFE which are different from A123. Be careful making blanket statements.

At the risk of violating my own comment above, if you are discharging to 2.5V under load, then that could be OK depending on the specific battery. If you are measuring 2.5V with no load, then that is over discharged for pretty much any chemistry. And there is really nothing to be gained by discharging to this level. 3V is considered to be a safe low level cutoff for most cases, and below about 3.2V there is little capacity remaining anyway.

Over discharge and over charge both wind up plating metallic Lithium on the electrodes. Doing both is asking for trouble. Trickle charging a Li battery is also bad news as it over charges and again starts to plate out Lithium.

Yes, you can use a power supply to charge Li batteries. But you must be using a current limited supply, and voltage regulated. 4.2V at say .5 to 1C should be fine as long as you are there to babysit. But if you forget and leave it go then you will be trickle charging. Do that for too long it may become a problem. You simply cannot treat Li batteries/cells as you did anything else.

Proper multi-chemistry Li chargers are not hard to find, nor are they horribly expensive. Please use one. (And not the cheapest one you can get from Hobby-King or eBay.)

References? Please see:
<http://powerelectronics.com/site-files/powerelectronics.com/files/archive/powerelectronics.com/portable_power_management/battery_charger_ics/804PET22li-ion-battery-life.pdf http://powerelectronics.com/site-files/powerelectronics.com/files/archive/powerelectronics.com/portable_power_management/battery_charger_ics/804PET22li-ion-battery-life.pdf>

NFPA: <http://www.prba.org/wp-content/uploads/Exponent_Report_for_NFPA_-_20111.pdf http://www.prba.org/wp-content/uploads/Exponent_Report_for_NFPA_-_20111.pdf>
Sony: <https://cdn.sparkfun.com/datasheets/Prototyping/Lithium%20Ion%20Battery%20MSDS.pdf https://cdn.sparkfun.com/datasheets/Prototyping/Lithium%20Ion%20Battery%20MSDS.pdf>
Panasonic: <http://actec.dk/media/wysiwyg/Actec/PDF/Li-Ion_handbook.pdf http://actec.dk/media/wysiwyg/Actec/PDF/Li-Ion_handbook.pdf>

Datasheets:
Different chemistry/application:
Li/SOCl2: <http://www.varta-microbattery.com/applications/mb_data/documents/sales_literature_varta/handbook_primary_lithium_cylindrical_series_er_en.pdf http://www.varta-microbattery.com/applications/mb_data/documents/sales_literature_varta/handbook_primary_lithium_cylindrical_series_er_en.pdf>

vs:

Li-MnO2: <http://www.varta-microbattery.com/fileadmin/user_upload/downloads/products/lithium-rundzellen/HANDBOOK_Primary_Lithium_Cells_en.pdf http://www.varta-microbattery.com/fileadmin/user_upload/downloads/products/lithium-rundzellen/HANDBOOK_Primary_Lithium_Cells_en.pdf>

vs:

<http://www.rosebatteries.com/pdfs/Panasonic%20CGR18650D.pdf http://www.rosebatteries.com/pdfs/Panasonic%20CGR18650D.pdf>
<http://www.rosebatteries.com/pdfs/Panasonic%20CGR18650C.pdf http://www.rosebatteries.com/pdfs/Panasonic%20CGR18650C.pdf>

Paul