Powerboost 500 charger

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BLOOD_FLUX
 
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Powerboost 500 charger

Post by BLOOD_FLUX »

Hello,

I was wondering if it would be possible to use 2 (or more) powerboost 500 chargers (each with its own lipo) with the outputs in parallel for a higher discharge rate? (up to 2A for 2 in parallel). I wish to power a 10"/7" hdmi screen, raspi and maybe a 2.5" hdd to make a tablet/laptop, i would like to have a 10AH lipo (in total) but my recharge time would be huge. With some Powerboost 500 in parallel i could increase the max discharge current and decrease charge time.

I understand that i would have issues charging multiple units from a USB (unless it could output more than the 500mA standard from a USB port) due to the high current draw the USB voltage would probably sag. But i could just make a cable that fork's off to multiple USB ports to charge my device.


Thanks for your help,
Aaron

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adafruit_support_mike
 
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Re: Powerboost 500 charger

Post by adafruit_support_mike »

BLOOD_FLUX wrote:I was wondering if it would be possible to use 2 (or more) powerboost 500 chargers (each with its own lipo) with the outputs in parallel for a higher discharge rate?
It's not safe.

No two power supplies have exactly the same output voltage, so if you put two of them in parallel, the one with higher voltage will try to push current backwards through the one with lower voltage.

At best, the combination will have less output power than you'd get from a single supply. At worst, something will short out and catch fire.

You can power multiple devices from separate supplies as long as you connect all the GND rails together, but as a rule you never want to have low-resistance connections between the positive *and* negative terminals of any two power sources.

BLOOD_FLUX
 
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Re: Powerboost 500 charger

Post by BLOOD_FLUX »

Thanks for your help.

Cerin
 
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Re: Powerboost 500 charger

Post by Cerin »

adafruit_support_mike wrote:
BLOOD_FLUX wrote:I was wondering if it would be possible to use 2 (or more) powerboost 500 chargers (each with its own lipo) with the outputs in parallel for a higher discharge rate?
It's not safe.

No two power supplies have exactly the same output voltage, so if you put two of them in parallel, the one with higher voltage will try to push current backwards through the one with lower voltage.

At best, the combination will have less output power than you'd get from a single supply. At worst, something will short out and catch fire.

You can power multiple devices from separate supplies as long as you connect all the GND rails together, but as a rule you never want to have low-resistance connections between the positive *and* negative terminals of any two power sources.
I'm interested in doing this as well, and searching the forums, I see you've cautioned against this several times. Why? You can prevent back current with a simple diode on each battery. Multi-cell Lipo batteries often have simple circuits built-in that do just this.

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adafruit_support_mike
 
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Re: Powerboost 500 charger

Post by adafruit_support_mike »

Diodes would prevent back-current, but you'd lose about 0.8v across a 1n4001 when the current is 500mA.

The bigger problem is that a diode's voltage-to-current curve is exponential. Changing the voltage by 18mV changes the current by a factor of 2, changing the voltage by 60mV changes the current by a factor of 10.

The tolerance for USB voltage is +/-150mV, and statistically you can expect the difference between objects with the same tolerance is the tolerance times the square root of the number you choose. 150mv*sqrt(2)=212mV, which translates to a current multiplier of about 3700 between two diodes.

You can reduce the exponential offset by putting resistors in series with the diodes, but the amount of cancellation you get depends on the ratio of resistor voltages. If you use 1-ohm resistors at 500mA, you could expect 500mA from the supply with higher voltage and about 300mA from a supply 200mV lower. That's better balance than you'd get from bare diodes, but still about 40% down on the lower-voltage supply. It would also cost you a total of about 1.3v from the higher supply voltage to the voltage reaching a load.

You can also trim the headroom by using Schottky diodes instead of silicon diodes (the drop will be about 300mV less) but those still have the exponential voltage-to-current behavior.

There's no easy way to prevent back-current and get decent current balancing without losing at least 1v of headroom.

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