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I am trying to design GPS tracker controled by chip from arduino with GPS/GSM module sim808.

I need to power it from lipo battery and i need 5V input for the chip which can give up to 2A peaks. What way is more easier, 1 cell lipo with step up DC/DC and single cell charger, or 2s lipo battery with 2s charger, balance circuit and linear step down DCDC?

Firstly i tried to find some DCDC step up, but the sim 808 can take up to 2A peak current and step up with at least 1A output current needs a lot of space (big capacitor and inductor) so i tried to find some solutions for charging 2s lipo battery. I even find IC charger (bq2057w) and IC with automatic balance (TI bq2920x) but they make the IC balance circuit only in VSON package which i´m not able to solder.

Can you please help me? Thanks a lot.

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A boost step up is very simple and efficient. There are many devices especially made to give you 5V from a 3.3V or 3.6V source. All you have to do is add an inductor and a couple of resistors. Efficiency is in the 80-90+ range depending on current.

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  • \$\begingroup\$ Can you direct me on some verified and simple DCDC step up schematic please? I found some schematic for example amarokcz.wz.cz/MC34063.htm (that with MOSFET) but i dont know if it is suitable powering sim808 + arduino chip or if i coud use some less complicated schematic. (Like i wrote below SIM808 needs 300 mA when comunicating with up to 2A peaks 500us long with 4 ms repeating.) \$\endgroup\$ – Jan Maloušek Mar 17 '17 at 19:37
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Using a linear regulator will reduce battery life. Let's compare:

  • one cell with capacity C
  • two cells of capacity C/2 in series

Both solutions will use about the same amount of space. However the linear regulator will only be 70% efficient. Really, forget about the linear regulator. The choice is between a 7.2V to 5V buck converter, and a 3.6V to 5V boost converter.

Balancing is annoying. Also, one cell can be charged from USB, which is ubiquitous, whereas a 2S battery will need another boost to charge from USB, or... YIKES! Yet Another Damn Wall Wart!

I look at the powerbar on the floor at my feet... It has 3 USB chargers in it already...

If it's a GPS tracker, maybe as a customer I'll take it on a trip to geotak my photos. However, I don't want to have 10 different wall warts in my luggage. If it can't be charged from USB, I will buy a competing product that can. Sorry.

Go with 1 cell, boost DCDC, and easy charging from 5V. Less problems.


EDIT: I checked SIM808...

It runs on Lipo voltage directly (which is... kinda expected from a mobile phone chipset...)

Why do you need 5V for, actually? I'm starting to think you don't...

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  • \$\begingroup\$ That's a good point. Nice catch: Supply voltage range 3.4 ~ 4.4V. Still 3.6 makes charging from 5v USB easy. Boost will still be needed because the end of discharge voltage is 3.0v or less. \$\endgroup\$ – Misunderstood Mar 17 '17 at 17:27
  • \$\begingroup\$ True. And since it doesn't even need to boost anything until battery falls below 3V5... it would be nice to select a synchronous boost which can keep its top switch always-on (or nearly always on). This way, when battery is above 3.5V all we have is the top MOS RdsON and inductor in series, no diode drop... \$\endgroup\$ – peufeu Mar 17 '17 at 18:04
  • \$\begingroup\$ We need 5V, beacouse we dont use bare SIM808 chip but whole board with this chip, which needs at least 5V, and i think arduino chip needs 5V too. But you are propably right, it would be better to use 1 cell lipo and step up. SIM808 needs 300 mA when comunicating with up to 2A peaks 500us long with 4 ms repeating. What output current the dcdc step up shoud have (assuming that the peaks can be covered by capacitor?). And would i need some filter to avoid interference from dcdc? \$\endgroup\$ – Jan Maloušek Mar 17 '17 at 19:13
  • \$\begingroup\$ "Arduino chip" (ie, Atmega328P) runs on lower voltages, too. If you can make your whole board run on a lower voltage you'll increase battery life. If you have opamps and such, this may not be possible. Could you post a block diagram of your board along with current draw? It would be kinda dumb to boost your LiPo to 5V then burn it in a 3.6V LDO for your GSM module! For your capacitor, remember i=C dV/dt so dV=i*dt/C. 2A*500µs/4700µF=0.2V drop. \$\endgroup\$ – peufeu Mar 17 '17 at 19:23
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Using 2 cells will be easier. That will only require a 3 terminal regulator and a few components to get your 5 Volts. A DC boost converter is more complicated and will require more parts and board space. Also, your 2 cell battery pack will last longer before recharge. For charging just use an external charger that has balance circuitry.

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  • \$\begingroup\$ It's not going to last longer, in terms of capacity, because the linear regulator is going to eat the extra voltage. With a 1-2Amp load, 2.2 volts (3.6+3.6-5) is a lot to waste. \$\endgroup\$ – Misunderstood Mar 17 '17 at 15:22

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