Li-po charger/switching PS that can handle 2A current

Ok, pulling my hair out trying to find the best solution:

I need to build an Arduino that automatically switches from wall-power to battery power while at the same time allowing the GSM to draw max 2A @ 5V.

Here is what I have arrived on:

http://www.ti.com/tool/bq24032aevm#supportandcommunity

Am I correct however that the Vout pin for this is regulated to ~3.3V? If so, please direct me to a way to supply 5V (with 2A max) to the GSM module.

Also, please advise me as to whether I can configure this Eval board with an Arduino.

The main output of that chip is just what the attached single-cell lipo battery provides, which is less than 5V. If you need 5V, you'll have to get a DC/DC step-up converter as well. These modules can be found on ebay and some of them can supply 2A.

I'm a novice, but are you sure? It seems odd to drop the voltage when the whole point is "reliable" power, is it assuming you have a voltage regulator on the other end?

Either way, if I couple the TI eval board with something like this:

http://www.ebay.com/itm/DC-DC-Converter-Regulator-Step-up-Step-down-2-1-/160644749552?pt=LH_DefaultDomain_0&hash=item25672c58f0#ht_1990wt_905

I should be able to power the 2+ amps required when the GSM module is drawing max current? If I am reading correctly, the USB cable for the TI eval board is limited to 500mA, how does the board provide 2A of power if the power input is limited to 500mA? Do I need to use the AC wall charger to get that kind of power?

Here goes another 'beginner' question: how do I initialize all the pins on the TI chip? Do I just use a few Arduino digital pins which output the required "High" or "Low" signals to the chip?

THANKS for helping me out.

You've chosen one of the more expensive dc/dc converters. There are others for about 8$-ish that can do 5V 3A, but the general direction is correct ;-)

As far as I could extract from the datasheet (just a short glance), power switching between USB / battery / external power supply is done by the chip automatically once it is configured right.

The chip provides an additional 3.3V pin (30mA). The main output is just switched between the available power sources. The power supply must be strong enough to power the load + charging the battery. I guess 5V 3A would be sufficient.

I also gather from the eval board user guide that you don't need any external artificial 'intelligence' (arduino) to configure the board. You set jumpers and adjust potentiometers. It seems helpful (almost mandatory) to have a digital multimeter at hand.

It would be good if somebody else had a look at it as well. I may have overlooked crucial things.

Ok thanks I will probably post up at the TI forum to confirm some stuff.

Also, if I now have 5V coming out of the regulator, capable of providing 2A I can't run the GSM shield from the Arduino or I'll exceed the current limit right? So Should I create a 5V "rail" and power the GSM and Arduino separately from it?

Which GSM shield are you using? I ask because most GSM chips are not powered by 5 volts and I think you have too many regulators in your circuit. The Eval board you linked will regulate the output to 4.4v if you're running off DC or USB. If you remove both, the output is whatever the battery voltage is set too, which, on average will be around 3.7 volts for a large portion of the charge. So now you're looking to take this output and run it through a DC/DC converter to bring it up to 5 volts for your GSM shield. My guess is the GSM shield needs 5 volts because it's putting it through a regulator to drop the voltage down to 3.8 volts.

If the GSM shield allows you to bypass the regulator you can simply connect the Lipo battery directly to the GSM and power it that way. The GSM module should operate with no issues over the total charge of the battery.

There's plenty of inexpensive lipo charger IC's (I'm using a MCP73833 for something similar) that you can connect the battery and DC power cord to that will recharge the battery. You can connect the output of the battery to a boost converter to supply the 5v you need for the Arduino.

Would that prevent the system from working? I really like the feature set of the TI. I looked at the datasheet for the Sparkfun GSM module:

http://www.sparkfun.com/products/9607

and it does regulate the 5V input to the GSM module voltage to ~3-4V.

Alternatively, could I employ a setup like this:

Use an adafruit lipo charger which has a direct battery output connection (http://www.adafruit.com/products/280) and boost that to 5V to power the GSM.

I would then power the Arduino via the 'DC out' pin on the lipo charger.

HOWEVER, what happens when I have this unit plugged into the wall and the GSM draws 2A? Is it drawing from the battery or from the charger IC? Doesnt that exceed the IC current? If drawing from the battery, wouldn't that deplete my backup power supply?

With the TI module, all power comes from the wall source when plugged in, and battery when unplugged. It seems like a better design considering I will be powering multiple 5V modules in the end.

Use an adafruit lipo charger which has a direct battery output connection (http://www.adafruit.com/products/280) and boost that to 5V to power the GSM.

This is basically the same setup as what I said. That board doesn't have a direct battery output, both of those battery connectors are in parallel. Actually that board uses the MCP73861, which according to Microchip, is not recommended for new designs and it's replacement is the MCP73833.

I would then power the Arduino via the 'DC out' pin on the lipo charger.

The DC out pin is connected to the USB/DC input so if you disconnect the USB/DC then you will lose power to the Arduino.

HOWEVER, what happens when I have this unit plugged into the wall and the GSM draws 2A? Is it drawing from the battery or from the charger IC? Doesnt that exceed the IC current? If drawing from the battery, wouldn't that deplete my backup power supply?

Under your conditions the battery would be supplying the power. The charger IC works in constant current mode and is preset to a specific current. In this case let's say it's 500mA, so what happens is the charger IC supplies 500mA (no more no less) which is used to recharge your battery. Unless you total current draw is more than 500mA you won't deplete the battery. Remember the 2A current draw for the GSM is only bursts of current that typically last a few milliseconds. Outside of the bursts the shield probably only draws around 50-60mA so unless the Arduino and other electronics are pulling more that 450mA, you won't drain the battery.

Am I interpreting this incorrectly: 2 JST connections so you can keep the battery plugged in and powering your project

Can you direct me to a board that has a direct battery output?

So I will connect a DCDC regulator to the battery output of the Lipo charger and power everything (Arduino, GSM, etc) from this? The 2A bursts wont affect the other modules right? Considering they are in parallel I wouldn't think so, just want to make sure.

Sparkfun has a board http://www.sparkfun.com/products/10300 that has a charger and 5v boost converter on the same board. The only problem is the 5v boost converter will only supply 600mA. If there was a way you could connect the Lipo battery directly to the GSM, bypassing the regulator, this setup would work well for you because you could use the 5v to power the Arduino.