Li-ion batteries in series or parallel?

[nickagian]

Hi to all,

I am asking again this common question, because I want to make things clear in my mind. In the attached file you can see the two different configuration that I have in mind for my project. I want to deliver regulated 3.3V to the electronics from battery voltage. One solution is to use two batteries in parallel (actually one could do the same job, two are for increasing capacity) and use a step down/step up DC/DC regulator like TPS63001. The second solution is to put two batteries in series and then use a step-down DC/DC regulator. My question is which of the two configurations give bigger battery capacity and thus longer autonomy for my circuit.

I've tried to compute the available energy in the two configurations, assuming that both DC/DC regulator have an effiency of η and that the capacity of all batteries is 1.2Ah. For the parallel configuration, the available energy is

Jp = η * [(3.6V * 1.2Ah) * 2]

For the series configuration the available energy is

Js = η * [(3.6V * 2) * 1.2Ah]

According to the above calculations, it seems that the energy is the same for the two configurations, but my feeling is that I'm making some mistake. I have the impression the the parallel configuration gives more energy than the series one.

Can anybody help me with this?

Thanks,
Nikos

 

[LTX71CM]

For a longer runtime you'll want to connect the batteries in parallel.

The energy is the same, in both cases you have two batteries, the difference is how you make use of them.

 

[nickagian]

For a longer runtime you'll want to connect the batteries in parallel.

The energy is the same, in both cases you have two batteries, the difference is how you make use of them.

LTX71CM thanks for your reply. Can you please help me a little bit more, in order to justify why it gives me more lifetime if I use the batteries in parallel? For example, lets assume that the batteries have a capacity of 1000mAh each and that the electronics require 200uA per hour for operation. Can you give me an example that shows the difference between the two configurations? How much life expectancy do the two cases give for these numbers? Does the regulator after the batteries play any role or it's the same with and without it, concerning the lifetime?

Thanks a lot again.

Nikos