nickagian
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Series will give you more voltage, = 3.6 + 3.6 = 7V, parallel will give you 3.6V. If you need less voltage, then, the rest will be wasted
I suggest you use parallel. From 3.6V to 3.3V, you can use a 3.3V LDO, the effieciency is about 91%.
If series, the DC/DC converter from 7.2V to 3.3V has effieciency maybe less or about 91%, but the circuit is more complex, and would have some spurs.
The power consumption is 3.3V*2e-4A=6.6e-4W. Assume the baterry aging discount is 20%, and DC/DC efficiency is 90%, so the lifetime is (3.6V*2*1.2Ah)*90%*(1-20%)/6.6e-4W=9425hours. Consider the DC/DC voltage should be greater than 3.3V, so the real lifetime should be less than 9425hours. You can check the battery datasheet to find the degrade plot.
Parallel has a advantage that is more robust. In series mode, if one battery fail, the whole battery system fail. But parallel not.
I searched in the web, found Li-ion battery voltage has some component like regulator to make voltage stable, so pls confirm this point from the vendor of the battery.
). I didn't completely understand your last point. You mean that you found that some Li-ion batteries have an internal mechanism to stabilize battery voltage?
Yes, some web file saidl so.
And if the efficiencies (η_p, η_s) of the two different regulators is around the same, then I can choose either of the two connections without loosing lifetime.
I should recommened you that when battery voltage varies, the battery efficiency will decrease.
PS:
Because your current is very low, you can't neglect the GND current of the LDO. Such as ADP1711, the GND I=40uA @ load current =100uA.
If you use DC/DC converter, the efficiency is also very low at low load current and also can't neglect the GND current. Check the datasheet.
You can check TPS54620 efficiency vs. load current.
Let's check some facts,
First of all, explain a little concepts:
- Parallel batteries would give you more capacity if needed. If you need 2A output and have two batteries of 1.5Ah, certainly you cannot connect them in series to provide you the current. Naturally, they will give you more application lifetime and voltage stability. If your application requires let's say 150mA out of these batteries, you will have 20 hours of running time since each one will be providing 75mA and they're rated at 1.5A (in this example).
- Series is for voltage boost. Simple as that, 3.7+3.7 (at nominal voltage) = 7.4. Running time won't increase since each battery will be sourcing 150mA rather than 75mA, so, the running time will be reduced to 10 hours. (the same as one battery). What was your gain then ? Well, the increased voltage. But, in your case, you don't need it.
If you're DC/DC converter is a buck/boost configuration, there's no problem of using a single battery and letting it work all the battery voltage range, and, since you said something about 200uA comsumption.. it looks like for the user it won't make any difference putting them in parallel, it would be everlasting anyway. Moreover.. if you want more lifetime... why don't you buy a battery with double the C-rate ?
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