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I have two mobile batteries, one with 3.7v 1500mah and other is 3.7v 1300mah. I am using a DPDT switch to switch the two batteries in series and parallel. I use in parallel mode (3.7v) to charge the batteries and in series mode (7.4v) to power my amplifier. And is it good to connect two batteries with different mah?

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    \$\begingroup\$ This basically duplicates electronics.stackexchange.com/questions/574974/… . The difference being lead-acid cells reverse-charged rarely catch fire -- lithium cells are more likely to ignite when abused. \$\endgroup\$ Aug 31, 2021 at 3:26
  • \$\begingroup\$ You mean, the mobile batteries will likely catch up fire? \$\endgroup\$ Aug 31, 2021 at 4:28
  • \$\begingroup\$ Yes, if they're Li-ion cells. \$\endgroup\$ Aug 31, 2021 at 19:29

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While you obviously can and have done so, lithium batteries should not be simply switched between series and parallel like that, especially as the batteries are not identical to begin with.

The problem is that when draining different capacity batteries in series, the weaker cell will get drained first and there is a danger of draining it to much and lithium batteries don't handle that.

Then, when they have been drained to different terminal voltages, the problem then is connecting them in parallel. The cell with higher voltage will charge the cell with lower voltage via the short circuit switch, and the current can be too high for the components.

Lithium cells are not designed to be used the way you use them, for safety reasons you should not do that.

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  • \$\begingroup\$ Ok, my mistake. I am still a learning guy. Thank you for the answer. \$\endgroup\$ Aug 31, 2021 at 4:56
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You must use a suitable BMS for series packs as they are unlikely to be balanced <1% by coincidence and overcharge risks are high on the weakest pack. Also, undercharge risks accelerate aging on the same weakest pack.

Your charger is OK in parallel. But to use in series you must use a BMS to inform the user it is low voltage on the weakest pack and not on the string voltage or sound, that's pretty easy to add with a voltage comparator and LED for 3.1V for safety on aging or 3.2V for gentle use or less for desperate aggressive shorter lifespan.

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  • \$\begingroup\$ You're right but I dont have suitable charger for 7v. So I am using like this. \$\endgroup\$ Aug 31, 2021 at 4:57

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