BQ24702 - Lead acid charger implementation

We recommend bq24650 to charge lead acid battery even if you are not using the solar panel. This device has lower IQ, smaller size, and implemented multiple safety feature that is essential for battery charging. 

Hello,

I like the suggestion but I have some concerns after reviewing the data sheet which I hope you can help me to clarify:

1) Our application does implement dual input voltage, AC adapter and external battery. It is desired that the internal battery can only be charged when AC is the power source, if a external battery is the power source (12V lead acid battery) the charger should be OFF. I read the IC have a detection of such condition using the VCC and SRN pins to enter in SLEEP mode (which I assume means the charge is suspended), my concern here is that the condition is if VCC < SRN, but this condition is based on the assumption the battery connected to the SRN pin is always charged and thus have a higher voltage that the battery connected to VCC. What happen when the battery connected to the charger is not fully charged and the input battery is, then SRN < VCC making the charger exit SLEEP mode and thus charge the battery when the input source is another battery...right?

2) it would be a better solution for my design to control the MPPSET pin and connect it to GND when AC is no longer present?

3) I do not quite understand how the charge regulate the charge current depending on the MPPSET. If the AC adapter is overloaded to the point its voltage reach the MPPSET voltage the charger reduce the current to maintain the AC adapter voltage at the trip point? Fro example, our AC Adapter have an output of 16V, if we set the MPPSET to regulate 1.2V when VCC reach 15V tihs means that when this condition occur due to a overload the charger start decreasing the charge until the MPPSET voltage reach again 1.2V, if the adapter keep being overloaded even more then the current decrease more until a point it could reach 0A...right?

4) Also I want to know in the case of a lead acid battery how the IC handles the float level. The data sheet explain about the float charge for lead acid batteries, it says that it should be around 100 to 200 mV below the constant voltage limit but it does not explain how to "program" such feature on the IC. Does this feature need to be implemented using and MCU by modifying R1 and R2 relationship?

Best regards

Can you please share your system block diagram? It is unclear to me what your system requirements are. 

Regarding 3, the MPPSET is used as feedback to the IC to indicate the input voltage. It is internally scaled to the input voltage. It does not mean the input will drop 1.2V for the input regulation to be active. If system + charge current exceeds what the input can supply, the IC then regulates the input voltage and pulls as much current from the input source as possible. This prevents the overloading condition to crash the input source. You can find detailed description on page 12 in the input regulation section.

Regarding 4, please find the application note in the link below for lead acid battery charging implementation.

http://www.ti.com/lit/an/slva437a/slva437a.pdf 

Hi,

Below a extremely simple diagram, basically this are the requirements:

1) the system load can be powered from 3 different power sources, AC, external battery and internal battery.

2) the power source has to be selected automatically, the diodes will do that since the highest voltage will power the system load.

3) the internal and external battery might be used together, again the diodes will do that.

4) The internal battery can ONLY be charged with AC, this is where my question about the battery input detection come from. I am not sure if this detection can work over all the possible voltages of the external and internal battery...so maybe is a safest way to set the MPPSET pin to 0V when the AC is not detected thus disabling the charger when the input source is a battery.

5) Sorry my bad, I did not mean that the MPPSET will regulate 1.2V or that the input must reach such voltage to activate the feature. Let me try to explain myself again, as I understood this function help to always drain from the input source the maximum current possible. This is achieved by sensing the voltage at the MPPSET pin, as long as this voltage remains in 1.2V or higher the charger will charge at full current. If the system load drain more power than the input source then the input source voltage drops, so it will the voltage at the MPPSET pin.If this voltage drop causes the voltage at the MPPSET pin to be lower than 1.2V, then the charger will decrease the charge current until the voltage on the MPPSET reach the 1.2V thershold. If the load keep draining even more, it could reach a point where the charger drops the charge current to 0A...I am right? If yes, then if the input is an AC adapter of 16V, setting R3 and R4 to produce a voltage on the MPPSET pin of 1.2V for an input voltage of 15.5V will work...right?

6) Reading the application note provided I have a question, in the case we also want to change the Iterm setting to a lower one...it can be done? For example, the IC implement a Iterm of 10% of the charge current, so if the charge current is 2A Iterm will be 0.2A. If we need a charge current of 2A but a Iterm of 0.05A...it can be done? I am asking this since according the the battery spec file we have the 100% percent capacity is reach when the charge current is almost 0A (http://www.vision-batt.com/site/product_files/CP1290.pdf), so it could be useful to lower the Iterm to a lower setting than 10%.

Regards