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Powerboost 1000C shows low-power when connected to battery w
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Powerboost 1000C shows low-power when connected to battery w

by harleyfrazee on Tue Jul 28, 2020 9:35 pm

I have a system using a 1000C Powerboost. I started longevity testing connecting the Powerboost to a 18650 LIthium battery at 4.2V. When the battery was reduced to 3.7V, the low power light showed red and the power delivered to the circuitry became unstable. What should I be doing/testing? What would cause this problem?

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Re: Powerboost 1000C shows low-power when connected to batte

by adafruit_support_mike on Wed Jul 29, 2020 2:31 am

The low-voltage LED lights when the boost converter sees less than 2.5V at its input, so we can assume that's happening for at least short periods while the PowerBoost is operating.

I'd guess the PowerBoost wants more current from the battery than the battery's C value can support.

'C' is a theoretical current load that will completely discharge a fully-charged LiPo in 1 hour. For a 100mAh LiPo, C-100mA. For a 6600mAh LiPo pack, C=6.6A. The C value is determined by the way the battery is made. Low-C cells have a few relatively small anode and cathode plates with a relatively large space between them. High-C cells have lots of large anodes and cathodes with a relatively narrow space between them. It's basically a function of how much lithium polymer is in direct contact with one plate or another.

Boost converters like the TPS61090 put a heavy and choppy load on the battery. They basically short an inductor to GND to charge the main inductor's magnetic field, then open the switch so the magnetic field can increase the voltage of the current flowing through the inductor. As a rule of thumb, it's reasonable to assume the peak inrush current is about 4x higher than the average inductor current, and the average current is (strange as it seems) larger than the average input or output currents.

For a PowerBoost raising 3.7V to 5V, the ratio of output voltage to input voltage is about 1.5. That means the PowerBoost charges the inductor for 15us, then discharges it for 10uS.. the ratio of charge time to discharge time controls the input/output voltage ratio. Both the input and output current are PWM'd, so if the average input is 150mA and the average output is 100mA (typical values), the instantaneous current is 150mAx25uS/15uS=250mA, and the instantaneous output current is 100mAx25uS/10uS=250mA. The instantaneous currents are equal, but their PWM duty cycles make them average to different values.

So, using those numbers, a PowerBoost driving at 100mA @ 5V load could put a peak current load of 1A on the LiPo. It would only draw that much for a microsecond or so, but that peak would hit once every 25uS (the typical operating frequency is around 40kHz).

If the LiPo you're using can't provide the peak instantaneous current for the load you want to power, the input voltage will drop briefly when the boost converter shorts the inductor to GND. The TPS60190 will see that dip and light the low-voltage LED until the inrush current drops to a level the LiPo can support.

Basically it means you've hit a boundary condition where the circuit still operates, but at lower than normal efficiency. It isn't bad per se, but you probably don't want to increase the output load any further.

To eliminate it, you can lower the output load or switch to a LiPo with a higher C rating.

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Re: Powerboost 1000C shows low-power when connected to batte

by harleyfrazee on Fri Jul 31, 2020 12:51 pm

First, let me thank you for your wonderful reply. It was concise and extremely helpful.

To resolve the problem, I added an additional 18650 Lithium battery in parallel and it appeared to work. To mitigate problems of dual batteries, I use the same model of battery for both power sources. Further, all batteries are tested for aH and the closest two are used in this implementation. Please let me know your concerns if any,.

Further, my product contains both PowerBoost 1000C's and PowerBoost 500C's. To extend the time of use, I have tested using an external, solar battery pack plugged into the mini-USB port. My question is whether it is acceptable to use direct USB input power and/or recharge the attached Lithium battery while running.

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Re: Powerboost 1000C shows low-power when connected to batte

by pneumatic on Sat Aug 01, 2020 2:32 pm

Hmm..

My intuition says that an 18650 should be able to handle 1A for short bursts without too much of a dip, as they typically have a 2200 to 3000 mAh capacity, so that should be under 0.5C of load. Is it possible that you have a high-impedance connection somewhere? Even if it's only a fraction of an ohm, then that could cause a rather big dip under load.

Rather than add a second cell, you might want to add a BHC (big honkin' capacitor) across the power input. That will even out the current draw and help minimize the spikes. Maybe 1000 uF at 10V? That way you don't have to go to all the trouble of capacity matching cells.

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