BQ24650: Using the BQ24650 for Solar LiFePO4 charger - not utilizing full solar power

Tomer Bruchiel

Intellectual 620 points

Part Number: BQ24650

Hi,

I have designed a LiFePO4 battery charger based on the BQ24650.

The source power of the BQ24650 is a solar panel as this is its main purpose.

The battery spec is: 5 cells pack (approximately 17V nominal and 18V max) with a capacity of 12AH.

The solar panel is a 100W panel with an open-circuit voltage of 28 volts and 24V with load with about 4.16Amps.

To measure the actual power of the panel with load, I connected it to a power resistor of 7.3 Ohms. The voltage drop on it was 23.5V and the current through it was 3.22A so total power of about 75W.

I disconnected the resistor and connected the panel to the BQ24650.

I have two board designs that differ from one another only in their MPPT voltage - one is set to 19.9V and the other to 21.7V.

I measured the voltage and current on both designs and got these results:

First board: V=19.9, I=3.2A, P=63W --> Vbat = 17V, Ibat_charge = 3.46A, P=59W

Second board: V=21.7, I=2.9A, P=63W --> Vbat = 17V, Ibat_charge = 3.65A, P=62W

I have several questions:

1. The most important question from my side is why the used power was dropped to 63W when the solar panel can clearly produce 75W in these conditions.

2. (less important than the first question) Why there is a difference in the current produced to the battery between the two boards.

3. General question, if the solar panel introduces voltage lower than the MPPT voltage, the charge will stop, right?

See the attached schematic.

Any help is appreciated - this product is about to be manufactured and we need to resolve this quickly.

Thanks in advance,

Tomer

over 4 years ago

0 Tomer Bruchiel over 4 years ago

Intellectual 620 points

Hi,

Any ideas?

Thanks

0 Kedar Manishankar over 4 years ago in reply to Tomer Bruchiel

TI__Mastermind 24075 points

Hi Tomer,

When you are calculating power of solar panel, measure input voltage at VCC of IC, not directly at solar panel? The way MPPT works is that when solar panel voltage drops below the MPPT voltage set point, BQ24650 will regulate solar panel input voltage at MPPT set point by decreasing charge current, so that solar panel can operate at its maximum power point for maximum efficiency. This seems to be what you are observing with decrease in total power. When charger is in MPPT regulation, voltage on MPPSET will be 1.2V. It would be good to look at solar panel's IV curve as well to understand where to set MPPT voltage.

0 Tomer Bruchiel over 4 years ago in reply to Kedar Manishankar

Intellectual 620 points

Thank you Kedar,

What you described is how the BQ24650 works and that is fine.

This is why when I set the MPPT to 19.9V I got 3.2A and when I set it to 21.7V I got 2.9A, both cases produce 63W of power.

I have no issue with that,

I do have an issue with the fact that when I connect a 7.3Ohm resistor I measure 23.5V and the current through it was 3.22A so total power of about 75W.

The way I see it, the BQ24650 "wasted" some how 12W of power (75-63).

Can you please explain this and help me understand what am I doing wrong?

I would like to utilize the entire solar panel power.

Thanks,

Tomer

0 Tomer Bruchiel over 4 years ago in reply to Tomer Bruchiel

Intellectual 620 points

Hi Guys,

Any ideas?

0 Kedar Manishankar over 4 years ago in reply to Tomer Bruchiel

TI__Mastermind 24075 points

Hi Tomer,

Your charge current is set for 4A. As Ibat_charge is not reaching 4A, it is possible the charger is already operating in Constant Voltage mode. In this mode charger would bre regulating output voltage and letting current taper down.