LiIon / LiPo Charger and Step Up

Hi lads,
this post might sound repetitive, but my searches didn’t produce any clear result, so here I am.

My goal is to have an Arduino project running off a rechargeable single cell Li-Ion or Li-Poly battery, in both cases 3.7V. Planned discharge rate is a lot less than 1C so I believe both battery types are applicable and I’ll go for cheapest.

My initial prototypes are going to be assembled out of cheap eBay StepUp and mini USB charger boards, but before wiring everything I’m seeking your advice about the correctness of my schema.

I was thinking about adding rectifier diodes but I’m concerned about voltage drops.

Is your board a 5V or 3.3V output? Either way, you need to feed it to the appropriate 5V or 3.3V pin on the Arduino, not the barrel plug (what you've labelled RAW, actually labeled Vin on the arduino). This will bypass the onboard linear regulator. Additionally, you should use a schottky (Low forward voltage) diode from your 5V or 3.3V to the VIN pin to avoid a possibly nasty inbalance.

The reason for the above is because the Vin (Barrel) input is designed to be 7-12V, and doesn't work well with less. And if you are stepping a 3.7V battery up that high, you are going to then burn the energy off anyways using the linear regulator. Since you are already using a regulator, you can skip using the linear regulator.

Also, my experience with the LiPo Chargers are that they tend to drain the battery when not charging it. Do you have a way to disconnect it? Also, Sparkfun sells a all-in-one Charger/Step-up board for 3.7V batteries that they sell.

mirith: Is your board a 5V or 3.3V output?

The Step Up is a 5V, the Arduino a 5V.

mirith: Either way, you need to feed it to the appropriate 5V or 3.3V pin on the Arduino, not the barrel plug (what you've labelled RAW, actually labeled Vin on the arduino). This will bypass the onboard linear regulator.

Good spot, thanks! Yes, I agree with you, no need for a further regulator.

mirith: Additionally, you should use a schottky (Low forward voltage) diode from your 5V or 3.3V to the VIN pin to avoid a possibly nasty inbalance.

What about an electrolytic capacitor? It doesn't provide the same protection of a diode, but should additionally provide some voltage buffer, something like a 100uF or greater. Is it a good idea?

mirith: The reason for the above is because the Vin (Barrel) input is designed to be 7-12V, and doesn't work well with less. And if you are stepping a 3.7V battery up that high, you are going to then burn the energy off anyways using the linear regulator. Since you are already using a regulator, you can skip using the linear regulator.

Yes, I agree, I'll use the 5V pin rather than Vin/RAW

mirith: Also, my experience with the LiPo Chargers are that they tend to drain the battery when not charging it. Do you have a way to disconnect it? Also, Sparkfun sells a all-in-one Charger/Step-up board for 3.7V batteries that they sell.

The plan is to have it as part of the design, now with cheap boards, on the final version directly on the board so that's not an option. I can try to measure the current drain from the battery during power off, but that might be tricky because of the probably very low currents involved... I might consider using a switch in case the current drained by the charger gets in the mA range, otherwise I believe it is an acceptable loss to maintain a low price.

About the Sparkfun one I started from there, but the cost is too high to get into the design as it is, so I looked at cheaper boards from China and I'll try to get somewhere from there. Looking at Sparkfun schematics it doesn't look they have any protection and the battery seems connected as it is in my design between the booster and the charger, may be the components they use are better and higher quality, but nothing fancy.

The diode prevents the voltage from the input to the output from differing too much (You shouldn't put 5V across the output while have a floating or 0V input on the linear regulator). A cap in series would allow for an AC connection but block DC and therefore not be useful, and it would just be a wire in parallel. The Cap is still a good idea, but the diode is just a simple bypass diode. They are cheap and easy to find.

mirith:
The diode prevents the voltage from the input to the output from differing too much (You shouldn’t put 5V across the output while have a floating or 0V input on the linear regulator).

I don’t get this part. Are you concerned about having 5V on the battery while the charger doesn’t have anything connected to the input side of the charging controller? I didn’t check the charging chip datasheet, but I don’t see a diode protecting its input even on Sparkfun circuits, so my assumption is it’s either embedded or the IC has an high impedence on it’s output line.

If your concern is about the regulator/step-up then the battery is connected to the its input and the Arduino to the output, I’ll probably put a switch to turn it on/off, but that will disconnect the battery from the step-up circuit

Oh, sorry, this is on the Arduino. You need a diode going from 5V to Vin on the Arduino.

That confuses me more :%, if possible :astonished:

I believe you are suggesting a diode reversed, so that it's not normally active and allows current to flow from GND to Vcc (when could that happen?) if necessary. That should prevent reverse voltage (and current) to reach the MCU. Am I correct?

The only situation when I can see this happening is with an inductive load applied on the same power lines of the Arduino and in that case I would use a diode close to each one of such loads, if present. I'm sure I'm missing something, but I just don't see it: apologies.

Thanks a million for your free advices ;)

rlogiacco:
That confuses me more :%, if possible :astonished:

I believe you are suggesting a diode reversed, so that it’s not normally active and allows current to flow from GND to Vcc (when could that happen?) if necessary. That should prevent reverse voltage (and current) to reach the MCU. Am I correct?

The only situation when I can see this happening is with an inductive load applied on the same power lines of the Arduino and in that case I would use a diode close to each one of such loads, if present. I’m sure I’m missing something, but I just don’t see it: apologies.

Thanks a million for your free advices :wink:

Not what I was suggesting. I was saying that to protect the Linear regulator on the arduino, you want a diode that goes from the 5V pin on the arduino (If you look, there is a pin that says “5v”) to the pin that says “VIN” This is basically going from the output of the linear regulator to the input. More clear?

My bad, I read GND where you wrote Vin. Apologies. :roll_eyes: