I have a couple of Li-Ion batteries I made up for a particular application. They each consist of four 18650 cells in series along with a 4S battery protection board. I have been using these two batteries in series to give me an 8S configuration without any problems. My question regards re-purposing these batteries. I'd like to use these batteries to make up a 2P4S battery but I don't want to strip the cells out of these batteries and reconfigure them. What would be the down-side(s) of putting these two batteries in parallel (effectively paralleling-up the two 4S protections boards)?


If by 'paralleling up' you mean just paralleling the battery + and - leads then it will be fine.

The protection circuit in each pack works independently of the other, so in a fault condition (over/under voltage, over current) one pack may disconnect first but the second one will follow shortly afterwards.

  • \$\begingroup\$ That's exactly what I was hoping for Bruce. Thanks for your input. \$\endgroup\$ – Andy Kay Mar 13 '17 at 19:15
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    \$\begingroup\$ I'd connect them via a resistor initially, because if you directly connect them the voltages are going to be minimally different, leading to a high current to equalize them. After they are at the same level, connect them directly and keep them connected -- repeat the bit with the resistor when they have been disconnected for some time. \$\endgroup\$ – Simon Richter Mar 13 '17 at 21:28
  • \$\begingroup\$ Thanks Simon. I built a board that permits me to bottom balance all eight cells in situ via the balance ports, and I plan to run all cells down to the same low voltage before connecting the batteries in parallel. Then I'll supply 16V across the entire pack to charge it up via a 6V lamp (current limiter and charging indicator) before checking the voltages on each individual cell. \$\endgroup\$ – Andy Kay Mar 13 '17 at 21:47
  • \$\begingroup\$ If the batteries had previously been used in series then they should have the same charge. I wouldn't bother discharging them, just measure the cell voltages and equalize if necessary (>0.05V variation). If one battery has markedly lower voltage than the other then charge it to the same voltage before connecting them together. \$\endgroup\$ – Bruce Abbott Mar 13 '17 at 23:29
  • \$\begingroup\$ I'd be more comfortable playing it safe with Lithium cells Bruce, which means connecting them in parallel when fully discharged. Since I intend to never top-balance, I think it would be good for the cells to begin their life in this new configuration in a bottom balanced state. Maybe overkill, but peace of mind too. \$\endgroup\$ – Andy Kay Mar 14 '17 at 1:49

You will definitely want to be sure the cells in parallel have a voltage close to each other before connecting them, otherwise you will get fairly large currents trying to balance the cells as they even out. Other than that there should be no issues. Side note: If you use cells of similar capacity in parallel it helps boost life and efficiency. We started capacity matching cells in one of our batteries that uses cells in parallel and it boosted the specs a fair bit.

  • \$\begingroup\$ Thanks Jared. See my reply to Simon above and I'd be interested to hear if you have any advice on that procedure. \$\endgroup\$ – Andy Kay Mar 13 '17 at 21:52
  • \$\begingroup\$ It appears to be a reasonable procedure, although you need to be sure that the voltages on specific cells don't go too high when charging the string. Even though you may be charging the string to only 10V, assumption based on 16V minus the 6V lamp, it is very easy for an individual cell to charge much faster than the faster than the rest which could lead to an unsafe condition. Also, what voltage are you planning on balancing down to? I would recommend staying above at least 2V to avoid plating lithium. \$\endgroup\$ – Redja Mar 15 '17 at 13:26
  • \$\begingroup\$ The 4S string charges from the 16V supply via the lamp. As the string voltage approaches the supply voltage of 16V, the voltage drop across the lamp will approach zero, so the lamp will eventually extinguish when the string is fully charged. Using identical cells, so I'm assuming they'll charge fairly evenly though I will keep a check on that. I'm using TP056 boards that cut out when the cell voltage drops to 2.5V. I would have preferred 3.0V cut-out but can't find anything to do that job. I'm not overly concerned about this since bottom-balancing is not frequently performed. \$\endgroup\$ – Andy Kay Mar 15 '17 at 23:13
  • \$\begingroup\$ I would not recommend charging them that way as it is almost certain they will not charge equally. All it takes is one cell to go to far and then they all "rapidly disassemble". Lithium batteries are not something to be taken lightly, you need to keep careful track of them and don't let them go too high. Some 18650s come with protection circuitry built in, if you have those you should be fine, otherwise I would be very cautious charging them that high with that method. \$\endgroup\$ – Redja Mar 17 '17 at 2:12
  • \$\begingroup\$ Thanks for your concern Jared. I used to give my lithium cells a top-balancing charge cycle, but I've been swayed by the people that promote an occasional bottom-balancing discharge cycle and whole-pack charging on the grounds that all cells reach the same low voltage together during a normal discharge. I use a cell logger to give me a visual indication of how much the cell voltages have diverged after a full-pack charge, and somewhat surprisingly I've never found a significant deviation (I suspect I'll see that when one of the cells is failing and needs replacing anyway). \$\endgroup\$ – Andy Kay Mar 17 '17 at 16:17

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