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i have a 4s4p 18650 lithium ion pack and i balance during charge (from 4 to 4.2V) down to ~5mv difference

on discharge i still see >100 mV difference at the end (2.5V). is it worth it to add a balancing circuit for discharge?

all cells are individually protected for under/over voltage.

where would i purchase such a board and how do i wire it with the main circuit?

ty for your time

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  • \$\begingroup\$ Balancing during discharge is not necessary. Only during charging. \$\endgroup\$
    – vini_i
    Jan 8 '17 at 13:59
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    \$\begingroup\$ You really shouldn't be running lithium-ion cells below 3V! \$\endgroup\$ Jan 8 '17 at 14:03
  • \$\begingroup\$ +1 Thanks for the question and especially the answer that received. \$\endgroup\$
    – User323693
    Jan 8 '17 at 14:19
  • \$\begingroup\$ You should not use "4P" configuration. Use "8S" and good DC-DC converter. \$\endgroup\$ Jan 8 '17 at 16:44
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    \$\begingroup\$ because unmatched cells connected in parallel tend to have strong self-discharge. Cells in series have no such problem, yet the capacity will be the same. \$\endgroup\$ Jan 9 '17 at 1:10
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You don't usually try to balance during discharge, just shut down when the lowest cell gets to the minimum voltage level.

Think about it. What exactly would you to do "balance" cells on discharge? On charging you turn on shunts across the cells that have higher voltage than the pack median. That slows down their charging, allowing the other cells to catch up.

Turning on a shunt during discharge makes no sense. That would only discharge the high cells to the same level as the low cells. The pack is still limited by the lowest cell, so you get no extra output energy.

You could, in theory, open the connection to any cell that hits the minimum voltage on discharge, then turn on a bypass around it. That would suddenly decrease the pack voltage as cells hit the minimum voltage, eventually going to 0 pack voltage when the last cell hits the limit. Such a thing would be complicated to do and probably of little use to the load. With cells reasonably matched in the first place, the extra energy available from the pack using this method compared to just shutting down the whole pack when the first cell hits the limit is minimal.

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  • \$\begingroup\$ Dear Olin, are there any active balancing boards commercial options for discharge balancing? Not based on shunts but on active transfer of capacity from cell to cell. \$\endgroup\$ Jan 10 '17 at 16:53
  • \$\begingroup\$ @guy: I don't know. I've never tried to look for such a thing. \$\endgroup\$ Jan 10 '17 at 17:31
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The amount of energy available between 3.25 and 3.0 V is only 2.6% of the total cell energy capacity. I suspect that between 3.0 and 2.5 V, there is only 1.0% energy available. There is no point running a cell below 3.25 V and below 3.0 V you are damaging the cell.

A 0.1 V mismatch between banks of 4P cells is not a big deal at 3.3 V (preferred cutoff). At 2.5 V, you are bound to get .1 V mismatch, but you should not be there in the first place.

The energy capacity between 3.4 V and 3.3 V is about 1.9% of the total cell energy capacity. And only 1.3% between 3.3 V and 3.2 V. You could conceivably cut off the lowest bank at 3.2 V instead of 3.3 V to capture this energy, which in a 4S configuration will be about 3/4 at 1.9% and 1/4 at 1.3% for an average of 1.75%. However, I do not think it is worth the effort to balance at discharge - you are only capturing 1.43% of the total system energy. If the 4P banks are so mismatched that at 3.4 V one of them is way below 3.3 V, then I would open the banks and rotate some cells, so at 3.3 V, no two banks are more than 0.1 V apart.

90% of the energy is between 4.05 V and 3.5 V. 80% of the energy is between 4.0 V and 3.55 V.

Note: All of the above voltages are running un-relaxed voltages at 0.5 A drain. Add 0.05 V to get the relaxed voltages.

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