BQ24450: BQ24450 24V Lead Acid Battery charger circuit Resistors Value

Actually the input voltage will also be 28-30V. I will charge the lead acid batteries from another gel battery or alternator

The power dissipation in the pass element will be high. Do you have a transistor and heat sink in mind.
Reducing the change current will reduce power dissipation, would 1A be enough charge current.

Hi Bill,

Thanks for fast response. Actually I have 2x12AH batteries and they will be used at least 1 hour a day with a 2.5-3A load. So I need to recharge the batteries as fast as I can to avoid fromfully discharge. I wish to use MJD127 PNP Darlington but it has a 120mA base current. I am not sure if BQ24450 can drive that BJT. I will use heat sink ob the PCB, I am thinking of providing a big cupper area for transistor without soldermask and I may place a heat sink onto it.

I need to calculate the required external resistor values and the most important thing I need to select a suitable BJT. Please warn me if I am wrong but I want 3A charging current so I need to pick up a BJT with at least 5A current support right?

Also I wish to place indicator LEDs.


With kind regards,

Ahmet

One more thing Mr. Bill, the batteries will be charged from another battery, so do I need to place a step-up before the charger?

Thank you very much.

King regards,

Ahmet

Dear Bill,

Do you have any suggestions in mind? I will use MJD127 or MJD2955 as pass transistor with heat sink. 

I will be glad for your answer.

Best regards

Hello
The gain for both transistors look good. I would recommend reducing the charge current to manage the thermal on device. Try 1A charge current, the power dissipation on the transistor will still be over 5W.
With boost of 15V for two cells you may need more than 30V input voltage. But this needs to be balanced against the loss in the pass transistor.

Dear Bill,

Thank you for your kind respond. I have reduced the current to 1A but still it does not work. I could not manage to charge the batteries. Can you help me please It is a little urgent?

I am attaching my circuit .

Any advice will be preciated.

Black boxes are DNP. 

With kind regards.

Can you describe the problem that you are seeing with the design?
Recommend compairing your circuit with the Evaluation board. Also for trouble shooting disconnect as much of the additional circuits as possible, U2, U3 Q4/R6 and Q4.

Dear Bill,

I removed the D7 PGOOD Led and it started charging. I was measuring VFB higher than it should be and discovered that LED causes that VFB to rise higher. Now I can charge the battery but I couldnt manage to get 3A charge current with lower sense resistors.


With kind regards

Ahmet

Dear Bill,

I am charging the batteries but it seems that BQ24450 is heating more than pass transistor. Is it normal? I am placing heat sink for both BQ24450 and pass transistor.

With kind regards

Ahmet

No the IC should not heat.
Couple of thing to check:
1.) Input current to the device
2.) Current and voltage drop on the internal driver transistor Q1, DRVC to DRVE