I understand that the title may sound ridiculous but for my project I need to connect two 18650 Li-Ion batteries in parallel (In parallel so I can easily and safely charge them using 2 charging ICs). Since I need to draw up to 10A from each battery I cannot use the protection board I was intending to use (it limits the draw to 4A). To protect the circuit from shorts I will put 4 6.5A fuses (connected in parallel) into the circuit.

However now I still have the problem of getting an undervoltage on the batteries. Is there a way to protect them without limiting the current?

the circuit

I have already done quite some research on these Li-Ion batteries and it seems that a lot of things can go wrong, so I am putting a lot of focus on safety.

  • \$\begingroup\$ Show how you measured undervoltage (UV) What is the rated ESR of battery ? 5? 20 50? mOhm \$\endgroup\$ – Tony Stewart EE75 Mar 6 '18 at 17:20
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    \$\begingroup\$ Charging with 2 chargers in parallel? Bad concept. Putting fuses in parallel? Bad concept. \$\endgroup\$ – Chupacabras Mar 6 '18 at 17:31
  • \$\begingroup\$ Well there is one charger connected to each battery. And could you please explain why parallel fuses are a bad idea? \$\endgroup\$ – 404response Mar 6 '18 at 17:35
  • \$\begingroup\$ @Chupacabras Fuses are copper which have a PTC so they share currents in steady state. But mismatch may result in early trip. The real issue is ESR vs Load DCR. \$\endgroup\$ – Tony Stewart EE75 Mar 6 '18 at 17:57
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    \$\begingroup\$ I recommend you add a schematic. \$\endgroup\$ – Selvek Mar 6 '18 at 18:09

If you want an easy solution, you can stop with your fuses and just put on a 1s BMS unit. Just get one that will protect against over and under voltage conditions, and short circuit conditions.

If you already knew that and you are asking because you want to build your own sort of BMS, then to answer your question: How to protect against under voltage, you would put a switch (a relay or equivalent) that can pass your discharge current in the discharge line. Monitoring the voltage of the cells and switching the switch when they go too low. Unless you are doing this just for a bit of fun though, then there is no need as these circuits are common already.

  • \$\begingroup\$ So i would in addition to the 2 chargin boards also connect a 1s BMS unit to each battery? \$\endgroup\$ – 404response Mar 6 '18 at 19:15
  • \$\begingroup\$ The problem is that I can only find BMS units that also limit the current to around 4A, I however need to draw up to 10A. \$\endgroup\$ – 404response Mar 6 '18 at 19:18
  • \$\begingroup\$ ebay.com/itm/… \$\endgroup\$ – Bruce Abbott Mar 6 '18 at 19:24
  • \$\begingroup\$ Stop thinking of 2 cells in parallel as 2 cells. They are now just one big cell when they are in parallel. There is no need for 2 bms units and 2 chargers. There should be lots of BMS units that will work for that. looks like bruce found one. \$\endgroup\$ – thejun Mar 10 '18 at 1:38

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