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# BQ25570EVM-206: BQ25570EVM-206

Part Number: BQ25570EVM-206
Other Parts Discussed in Thread: BQ25570

To charge a battery via bq25570 is to attach the battery to VBAT, My question is what is the nominal charging current comes out of the VBAT?

• Hi Kaleb,

The max possible charge current is 1/2 the peak inductor current, which is 230mA typical.  You likely will not get that much from a high output impedance source like a solar panel though because the charger sends a packet (burst) of energy (current), collapse its output capacitor below the MPPT point, pauses for the input capacitor to recharge and then sends a another packet.  This results in a lower average current.  You can estimate the maximum charge current from the datasheet efficiency curves if you know your power sources maximum output capability.

Regars,

Jeff

• Hi Jeff,

Massive thanks for replying to me. my input source is the solar panel with an open-circuit voltage of 3.4V and a short circuit current of 0.23mA under 500 lux.(80% mppt point) So, if, I would like to know how much exactly the power can be harvested into the battery (output of the solar panel via the bq25570 chip and into the battery), is that I can place a battery (80mAh) in series with a current meter and in parallel with a voltage meter to calculate the power on the input side of the battery?  Thanks again.

• HI Kaleb,

If you place a current meter in series, make sure you turn off auto-ranging or the charger will have problems charging due to the current meter impedance variations.  Because of the burst charging, the current meter needs to be set to the longest average measurement setting and still may not measure correctly.  The datasheet curves don't show efficiency for input voltage above 3.0V, which is 90%.  So if you uses 90%*(3.4V*0.8)*230mA/4.2V = 134mA, which is above the 1/2*230mA = 115mA max.  We used a Keithley sourcemeter that had very long averaging/measurement time when taking our efficiency measurements as explained in the app note at the link below.

https://www.ti.com/lit/pdf/slua691

Regards,

Jeff

• Massive Thanks, Jeff! It's extremely helpful!

• Hi Jeff,

After I went through the doc you linked above, I knew I could calculate the power output(Output of the Bat) by using the efficiency that the doc provided. But would you suggest any methods that I could really test the output power at the Bat on the two ends of the battery? (in that case, do I need to attach the battery when I try to measure the power that is charged into the battery) So what I would like to test is Under each Level of LUX, how much the real power I could get by using a solar cell plus bq25570. Massive thanks!

• Hi Kaleb,

In the lab we used Keithley Sourcemeters to simulate the solar panel and battery when measuring efficiency.  We didn't have DMMs that could average/filter long enough.  Worst case you could set up VIN_DC at each LUX and BAT with a mid-sized, low leakage capacitor, monitor on a scope and use I=Cdv/dt to compute the charge current.

Regards,

Jeff

• Hi Jeff,

What I try to do is the page 2 in this doc (https://www.ti.com/lit/ug/tidu382/tidu382.pdf?ts=1654703613944), Just would lik to confirm with you, is that the power you measue is at the end of Vbat or somewhere else? And when you measure the power of this, you calculate the power as: P= Cdv/dt* V?  By the way, when I measre the power at Vbat, is Vbat and Vstore pin interchangablly? Thanks

• Hi Kaleb,

I don't know how Gordon took those measurements.  I would assume that he took them at the VBAT output.  The undervoltage protection PFET between VSTOR and VBAT has Rdsonmax=1ohm.  If undervoltage protection is not needed, I suggest shorting VBAT=VSTOR.

Regards,

Jeff