STEVAL-ISV020V1 - min required voltage to be operated

[rsmqu]

Hi ST Community / @Peter BENSCH 

I use my solar panel connected to the STEVAL-ISV020V1 to charge my LiPo battery. Everything works fine and it charges the battery. However since I am trying to charge it indoors, during night-time, my solar panel doesn't reach the min required voltage of 2.6 to operate the STEVAL-ISV020V1. 

Now I am wondering if the STEVAL-ISV020V1 becomes a load instead of being a charge controller when this happens because it drains the battery with a very low voltage at a certain time. 

[Peter BENSCH]

Welcome @rsmqu, to the community!

Well, the data sheet for the SPV1050 explains this on the first page:

In buck-boost configuration (CONF pin connected to ground), the IC requires 2.6V and 5μA at Cold start; while after the first start-up input voltage can range between 150mV up to 18V.

Does this answer your question?

Regards
/Peter

[rsmqu]

Hi @Peter BENSCH,

Thanks for getting back to me.

As I mentioned previously, I am trying to charge my LiPo battery through my solar panel in an indoor setting (no direct sunlight here so we're expecting a low-voltage source). It works fine during the daytime (it charges the battery well) since it still meets the required minimum voltage (2.6V).

But during night-time, my solar panel doesn't reach the min required voltage of 2.6 and I observed that it drains the battery (around -19mV/hr)

So my main question is, what happens to the solar charger STEVAL-ISV020V1 when it does not meet the required 2.6V, does it become a load that drains the battery instead? If yes, are there any countermeasures to prevent this in terms of my project setting?

Looking forward to your assistance. Thanks so much!

Best regards.

[Peter BENSCH]

Is the battery connected to VBATT (CN2.1) or to STORE (CN4.1)?

What else is connected to the battery as a load?

[rsmqu]

Hi @Peter BENSCH ,

I followed this diagram below attached screenshot from its datasheet:

The Solar panel is connected to CN1 and the battery is connected to the outputs. No other connections were used to the battery.

Solar Panel -> STEVAL-ISV020V1 -> LiPo Battery

[Peter BENSCH]

The evalkit is the Buck-Boost variant, which only starts from 2.6V when a PV panel is first connected, but can then work down to 150mV and even charge the battery, even if only very low charging currents are flowing. The battery is disconnected from STORE as soon as its voltage exceeds EOC and connected when the voltage falls below EOC (see SPV1050 data sheet mentioned above).

However, I would also like to point out that the internal LDOs can certainly cause this discharge, as also mentioned in the data sheet, section 6.5:

...the LDOs are both supplied by the STORE rail: if the input source is unable to sustain the current required by the load, then the missing energy will be supplied by the battery connected to the BATT pin.

and

If VSTORE drops and UVP pin triggers the undevoltage threshold, then the pass transistor gets open and the load is no longer supplied until next end of charge condition is reached.

Even if switches SW1, SW2 are set to position 4 (GND) and the LDO are thus disabled, a standby current of typically 0.8µA can still flow, which can lead to significant discharge depending on the type and size of the LiPo. And please don't forget the self-discharge of your LiPo battery.

[rsmqu]

Hi @Peter BENSCH,

So it does discharge the battery when the PV panel does not meet the minimum voltage required. But can I clarify something regarding statement above from the datasheet, section 6.5:

"the LDOs are both supplied by the STORE rail: if the input source is unable to sustain the current required by the load, then the missing energy will be supplied by the battery connected to the BATT pin.

and

If VSTORE drops and UVP pin triggers the undevoltage threshold, then the pass transistor gets open and the load is no longer supplied until next end of charge condition is reached."

So for example, the PV panel can only supply let's say 1V, so it's lacking 1.6V to charge the battery. It will then get the missing energy from the LiPo battery (it will get the 1.6V lacking). It is now meeting the minimum required voltage of 2.6V (1V from the PV Panel, and 1.6V from the LiPo Battery). So then it can now start to charge the battery until the next EOC condition is reached?

Please correct me if I'm wrong, and I am more than happy to be enlightened. Thanks so much!

Best regards.

[Peter BENSCH]

No, your assumption is not correct. In boost mode, the SPV1050 only works from 2.6V when it is first switched on (or when the PV module is connected). However, once the SPV1050 is operating, the input voltage from the PV module can drop to 150mV before the SPV1050 stops to operate.

With regard to the text quoted in section 6.5, this refers to the current requirement of LDO1 and LDO2 (in the SPV1050), which is not negligible even when deactivated. As a rule, with PV modules, as the voltage decreases, the available current also decreases, so that the pass transistor may (still) be switched on and therefore the LDOs are supplied from the battery.

Regards
/Peter

[rsmqu]

Hi @Peter BENSCH ,

Thanks for your response. I just have some things to clarify if my understanding is correct and put them in bullet form below:

•  The 2.6V is only required at cold start, but during operation, it can continue to charge the battery at least the input PV is at a minimum of 150mV. So this means during normal operation (after the first-start up), if the input voltage suddenly drops below 150mV, it then triggers the LDOs to get the lacking energy from the battery and then will start to discharge the battery until the requirement of the LDOs is met. Is this correct?
  
  
• Concerning the first bullet, just to confirm, once the input PV supply does not meet the requirement of the LDOs (whether the cold start of 2.6V or the minimum 150mV while it is ongoing operating), it will get the lacking voltage from the battery. Is this correct?
  
  
• Is there something we can do to prevent the discharge of the battery from the LDOs or it is the limitation of the solar charger when it does not meet the input requirements?
  
  
• Lastly, let's say after the LDO got the lacking supply from the battery to meet its required current, what then will happen? Will it resume its normal operation to charge the battery?
  

Please correct me if I'm wrong, and I am more than happy to be enlightened. Thanks so much!

Best regards.

[rsmqu]

Hi @Peter BENSCH,

I hope all is well with you.

I am just sending a gentle follow-up and hoping for your response soon.

Your advice will be appreciated very much. Thank you!