I need to build a 10 kWh lithium-ion pack, anywhere from 70 V to 200 V. This is to drive a motor. Power output will not exceed 15 kW. The cells will be Panasonic NCR 'B' at 3.4 Ah 18650. I would like to minimize space and weight and cost. :D Level of safety would be standard for a hobby application. Charging will be done in a safe area using two levels of overcharge and overcurrent protection. Charging is from house current, 110V, and multiple chargers may be employed to double or triple the charge rate.

  1. What is the best configuration for cells? I would prefer if the pack is broken down into units of 1 to 2 kWh each. Each unit may be charged separately.
  2. In addition to a balancing-protection board and a CCCV charger, what other protection or safety mechanisms are recommended, short of a smart BMS system?
  3. I have looked at a cheap Chinese balancing board that offers protection (thermal, overcharge, undercharge, overcurrent, short). A 20S board will offer up to 84 V and adequate current for the application, for a 1.5 kWh unit. At 1/3C, I would need a 83 V 6 A CCCV charger. How do I get one of these?
  4. Off-topic question: Can a balancing-protection board be used for a configuration with a lesser number of cells? Such as using a 10S board to control an 8S configuration. Or does this depend on the board?

closed as too broad by PeterJ, Voltage Spike, clabacchio Jul 17 '17 at 17:13

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    \$\begingroup\$ When you mention 12 kWh but you need to ask basically everything about how to build this and how to treat Li-Ion cells and also make an unrealistic proposal (see Glenn's answer) then you leave me no choice but sayin: NO you should not be doing this in the first place. Li-Ion cells are not like Lead-acid cells which can be mistreated relatively safely. Li-Ion cells are different beasts, even the engineers at Boeing had used Li-ion batteries and they had issues. What makes you think you doing this is a good idea ? \$\endgroup\$ – Bimpelrekkie Jul 16 '17 at 13:20
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    \$\begingroup\$ To explain: do this improperly and you'll have a fire hazard next to your house. Would you still sleep safely then ? I would not. You do not have the knowledge needed to build this, nor do I. Also, the knwoledge you need cannot be explained with answers to a few questions here on this site. So I would just buy an off-the-shelf solution and so should you. \$\endgroup\$ – Bimpelrekkie Jul 16 '17 at 13:23
  • \$\begingroup\$ If you look at my previous comments, you will see that I have built Li-ion power packs before. Several packs from 8 cells to 25 cells, with balancer and protection. I am well aware of their safety issues. If such a power pack is not to be discussed on this site, then can you or others recommend another site where they discuss large packs (and have a positive attitude)? \$\endgroup\$ – Tranceducer Jul 16 '17 at 18:23
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    \$\begingroup\$ Supply to motor load depends on start current or battery ESR/Motor DCR ratio where heat loss is I²(ESR+DCR) total. then consider thermal resistance of motor and Cells for T rise. I guess your question was too broad for the MODs. Defects either OC or SC must be considered with thermal distribution, protection. Try 4.1 instead of 4.2 for longevity or even lower. \$\endgroup\$ – Sunnyskyguy EE75 Jul 19 '17 at 1:00
  • \$\begingroup\$ Use an electric car? \$\endgroup\$ – KalleMP Oct 28 '18 at 14:45

I need to build a 12 kWh lithium-ion pack, anywhere from 70 V to 200 V. This is to drive a motor. Power output will not exceed 20 kW. The cells will be Panasonic NCR 'B' at 3.4 Ah 18650

The proposed application is unrealistic.

A 18650 cell has a nominal voltage of 3.7 volts and up to a 3.5 Ah rating. To compose a 70 volt battery, 19 cells would be required. This comprises a 245 Wh battery. To get to 12 kWh, 49 such batteries would need to be paralleled. This requires a total of 931 cells in the battery.

Assuming $5 USD per cell, the battery would cost $4,655. Add to that interconnects and charging circuits and the total cost will be north of $7,500.

  • \$\begingroup\$ $7,500 is not unrealistic. Depends on the person, and the benefits of the project. \$\endgroup\$ – Tranceducer Jul 17 '17 at 0:14
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    \$\begingroup\$ @Tranceducer a smart BMS should be a smart move - preferably with inductive push-pull balance bypass. rather than shunt bypass, The power Pd in the Bypass is limited by string CC current resulting in I²R drop during bypass, so switched L push pull bypass shunts the current reactively. Normally <1% imbalance occurs during CV mode demands less Pd, but as aging or quality issues arise, smart reactive balancer is needed. Also micro-fuses in cells... Consider load DCR of motor and ESR of array e.g. 5~10mΩ /cell for 19S49P array may best case reduce array ESR to 19/49*100% of cell.Compare to DCR \$\endgroup\$ – Sunnyskyguy EE75 Jul 19 '17 at 0:57
  • \$\begingroup\$ @TonyStewart.EEsince'75 - thanks - the motor has good cooling. Shall I configure for 5 x 19S10P to arrive at 400V? Why is the ESR/DCR important, and what is the ideal value? Can a 20S board do 19S? Any hint where to get the microfuses or just put in wire? \$\endgroup\$ – Tranceducer Jul 21 '17 at 7:41
  • \$\begingroup\$ The cell ought to contain microfuses as Tesla uses to isolate faults. ESR is important if you compare ratio of the heat loss in the supply to energy used by load, since current is shared, I^2R becomes an impedance ratio of ESR Bat/DCR motor. Then choose this ratio to be small as possible. Note that this ratio changes for constant power motors with different V. Some research is needed to compute this inefficiency ratio. There are other factors to limit V as well that has more to do with Insulation breakdown. \$\endgroup\$ – Sunnyskyguy EE75 Jul 21 '17 at 17:33
  • \$\begingroup\$ Another approach to reactive balancing is to sense cell voltage and switch from CC to CV with a reduction of 4.2 to 3.9V per cell instead of 4.2 then stop charging when I(CV) mode < 10% or Icc. This extends the charge cycle time but reduces acceleration of mismatched aging due to 1st one fully charged due to lower capacity from aging getting over charged and thus aging faster. (parabolic effect) \$\endgroup\$ – Sunnyskyguy EE75 Jul 21 '17 at 17:43

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