Hi there
I'm trying to figure out what the best solution for charging a 6V lead acid battery from a 9V peak power / 10.8V O.C circuit solar panel. On cloudy days, the 5W solar panel will put out anything from 20 - 60mA at 6V (tied to the battery). In the winter in some locations, these conditions will persist for weeks at a time, so making the most of the current all of the time rather than making the most when it is sunny is more important.
I started by looking through your complete charging range and idetifying two groups of charging products: UC and the BQ families. From what I can see, the BQ series are newer and supersede all of the UC series - there is nothing the UC series offers that the BQ doesn't. This left the parts:
BQ24650 - mentions specifically for solar applications
BQ24450
BQ2031
The BQ24650 can't be used because of the current required for it to operate in charging mode. It is listed as 'Adapter supply current' and says typically 25mA. This means on a cloudy day, lots of the energy will be eaten up by the chip. Am I misinterpreting this?
The BQ24450 looks promising. It is a basic shunt regulated design and I have a few questions:
1/ I assume it would require a minimum input voltage of:the battery charging voltage + the diode drop + the transistor / FET drop + the sense resistor(s) drop. I'm planning on charging at no more than 0.8A (with 2 x 5W panels). What would you suggest to get this down to a minimum for this application?
2/ apart from the typical 1.6mA current draw, are there any other hidden current requirements in the charging circuit? With the transistor driving current, how low could this be - what type should I use to minimise this?
3/ I presume that the maximum power dissipation will only need to be calculated during the charge - once the charge has finished, very little power will be dissipated.
4/ Can you confirm that when the solar panel stops giving enough energy to run the circuit, with the blocking diode in place, will any significant power will be drawn from the battery apart from through the resistor Ra? Additionally, are there likely to be any negative side effects of having this non-continuous input?
5/ Lastly, can you think of any other reason why the BQ24450 would not be suitable for this application?
The BQ2031 also looks like it may be a solution. I have read the application note for MPPT with the bq2031 - http://focus.ti.com/lit/an/slva378/slva378.pdf
1/ Apart form the 2mA current draw + transistor drive current, are there any other hidden power requirements from the charging circuit (without MPPT adjustment).
2/ What kind of efficiencies should I expect in my application?
Which of these would you suggest? What other alternatives are there?
I don't expect using a switching design vs a shunt design to give a large advantage - Assuming full sunlight, a shunt system this utilises approximately 68% of the energy (vs MPP) from the panel charging directly (running at 6V would mean 0.275V / cell vs MPP of 0.42V / cell, so reading off approximately from the chart in this datasheet http://focus.ti.com/lit/an/slva378/slva378.pdf and comparing 0.42 x 0.036 = 0.0152 vs 0.275 x 0.038 = 0.0104 per cell). Switching designs would have to exceed this efficiency figure across the range, especially at the low end.
I am open to other solutions to this problem and any help would be appreciated.
Many thanks
Oli
Edit: I came across this question http://e2e.ti.com/support/power_management/battery_management/w/design_notes/bq24450-as-a-charge-controller-with-mosfet-regarding.aspx I'm guessing using a P FET would help reduce the loss across Q. Can you confirm and provide a bit more detail?