# Li-ion battery 2 in parallel or 2 in series - Capacity vs voltage, which one will give more battery life?

My circuit needs 3.3 volt, and I have 2 options with a Li-ion battery pack.

Option 1: I can put 2 cells in series and get approximately 4.4 to 6V (7.4 volt mean) and then using buck converter reduce it 3.3 V. In that case my battery capacity will be 2000 mAh.

Option 2: In another case, I can put the cells in parallel and get 4.2 to 3V (3.7 volt mean). In this case, I am going to get battery capacity of 2000+2000mAH i.e. 4000mAh.

My query here is, in Option 1, I will get more voltage so that I can use the battery pack from 7 volt to 3.7 volt. In Option 2, I have bigger capacity i.e. 4000mAh but cannot use the range from 3.3V to 3V without using a buck boost converter.

Which option will give me more battery life for my circuit?

• The problem you’ll have if you put them in parallel is that once they drop to the dropout voltage of your regulator, assuming you’re using either linear or buck, your 3.3 supply will start to drop in voltage. With a series configuration you’ll need some way (a BMS) to stop the cells getting out of balance, which would result in one of them getting over-discharged (this is bad). Personally I lean towards higher voltages because they can be more efficient in many scenarios- lower resistive losses and the odd diode drop being less significant.
– Frog
Feb 12, 2022 at 5:35
• When you say "Li-iron", is that a typo of li-ion, or are you referring to lithium iron phosphate (LiFePO₄) cells? Feb 12, 2022 at 5:52
• Neither is inherently better. If you need really high currents, parallel will get you there more cheaply and easily.
– user16324
Feb 12, 2022 at 14:09
• @Hearth 3.7V must be Li-ion, not LiFePO₄ which is 3.2V Feb 15, 2022 at 2:09
• Exactly which cells will you be using, and what is the load's peak and average current draw? Feb 15, 2022 at 2:11