BQ24650 charging current and charging time help.

[Sunny55]

Hi, i am implementing a constant voltage MPPT charge controller using the BQ24650 IC. The problem i am facing currently is the charging current. For my solar panel, the maximum output current is around 5.3A @ 100W but i was recommended to set the IC's charging current to 4A.

Solar panel specs

Max voltage:17V( its a 12V panel)
Max output current:5.3A

Battery specs

12V, 100Ah SLA battery, DoD of 0.7


(1) Does this means my charging current is "locked" at 4A by the IC? Will there be any changes to voltage if the current is to be set constant?


(2) How long would it take to get my battery fully charged using 4A charge current with a DoD 0f 0.7 and 4-5 hours of sunshine/day? I am also using water cooling for the panel and i think it(charge current) will stay at the maximum charge current of 5.3A for some time.

Thanks.

 

[BradtheRad]

1.

I did not have a MPPT with my backup power system, so I don't know the specifics.
I understand a MPPT is designed to find the maximum charge current in any condition. It might take PV output of 18.9 V at 5.3A, and drop it to 16V at 6.25A (theoretically).

Your 100AH battery should easily be able to absorb 10A (going by a typical safe recommended charge rate of C/10).

The TI datasheet says the BQ24650 can handle up to 10A, and will taper down the current as battery becomes fullly charged.

https://www.ti.com/product/bq24650#modalWin

It's a question why you were told to set it for 4A charge output.
Did you report a problem with components overheating?
Did you report that the battery was overcharging, bubbling? Etc.?

2.

A 100 AH battery at depth of discharge = 0.7...
will need 70 AH to reach full charge.

Charging at 4A, it will take almost 18 hrs. (Going by the math only.)

 

[Sunny55]

Thanks for your reply. The reason for why i was recommended to set it at 4A is because the panel max current is 5.3A and the panel cant support charging current of 10A.

- - - Updated - - -

Now i am concerned with the charge current as constant voltage algorithm sets voltage at constant and current changes with sunlight . So for low su nlight i get low current and less watts. According to the datasheet, the next charging current 8A and does this means ithe 4A can support up to at least 5A which is near to panel's max current ?

 

[BradtheRad]

I've never heard of a PV panel which isn't designed to support whatever is the amount of current it can put out in bright sun. That is, with a 12V battery as the load.

I believe you should do some tests with the PV panel and battery alone. Find out how much charging current you get. This is the kind of thing I did with my setup. It will give you a better idea what's going on.

You can bet that PV panels are rated optimistically. I'm sure they are tested in an area which has the clearest skies and the brightest sun (such as southwest USA), so it will yield maximum current.

You may decide you need a second PV panel, in order to charge the battery in a shorter time.

 

[iukhan]

Thanks for your reply. The reason for why i was recommended to set it at 4A is because the panel max current is 5.3A and the panel cant support charging current of 10A.

I have the same question as asked by BradtheRad, who/how you were recomended to set 4A charging current. Your reply is still unclear.

1. You are claiming that the panel is 100W. But as per specification the max wattage could be around 80-85W. The peak power will be less than 17 x 5.3 = 90W.
2. Asuming that you are chargining a battery of 12V @ 4A. This means that you are only geting 48W max. In other words, it is un efficient design.
3. MPPT is used to extract the maximum power from the panel.
4. If the panel has a limitation of 5.3A, it doesnt mean that the same applies to MPPT controler. e.g. if a solar panel has a peak power point at 15.8V & 4.9A then MPPT controller will be able to provide 12V and current 6.13A (asuming 95% efficency).

 

[BradtheRad]

According to the datasheet, the next charging current 8A and does this means ithe 4A can support up to at least 5A which is near to panel's max current ?

As regarding the panel capacity...

I wonder if someone on the help line thought that 'panel' meant you were talking about a control panel? Rated for 5A?

That's what I thought for a moment (instead of PV panel). Because some people get an all-purpose solar power panel for their system... containing charge controller, meters, switches, fuses, etc. The chief spec is how many amperes it can carry.

So you can see it's easy to lose track as to what kind of panel someone is talking about.

I believe it will be okay if you adjust the MPPT amp setting up to 8A.

 

[Sunny55]

Reply to post #5 : it was recommended by the TI technical support engineer. What other information do you need ? I will try my best to provide you.

- - - Updated - - -

Hi all, the panel refers to solar panel only.

- - - Updated - - -

This is the additional information i found regarding to the CONSTANT VOLTAGE ALGORITHM implemented by the charge controller IC.

More information at: http://www.ti.com/lit/an/slva446/slva446.pdf

Look under section 3.1: Constant Voltage. It says " This means that for the various different irradiance variations, the method will collect about 80% of the
available maximum power. The actual performance will be determined by the average level of irradiance.
In the cases of low levels of irradiance the results can be better."

Since it says that under varying sunlight conditions, the algorithm can collect about 80W for battery charging and higher power is possible under low levels of sunlight. What is the function of the 4A charging current for?

 

[BradtheRad]

the method will collect about 80% of the available maximum power. The actual performance will be determined by the average level of irradiance. In the cases of low levels of irradiance the results can be better."

Since it says that under varying sunlight conditions, the algorithm can collect about 80W for battery charging and higher power is possible under low levels of sunlight.

In dim light you won't get 80W, but the benefit of a MPPT is to collect whatever energy does come from the PV panel, and step up/down its volt level so it will charge the battery with whatever amount of current it can manage. (I see it is described as a buck converter and I don't know what to say about that, because in low light I think sometimes you need a boost converter.)

So the 80% figure means the panel might provide 10 W in dim light, and the converter will take that amount and deliver 8 W to the battery.

What is the function of the 4A charging current for?

I'm pretty sure this is intended for a battery with a smaller capacity (say 20 or 30 or 40AH). Or for thin wires. Etc.

This is a case where you'll need to try a few adjustments, to see how it works out.
It will take a while to observe how your battery behaves in various conditions, at different discharge levels, sunny versus cloudy days, etc.

 

[iukhan]

it was recommended by the TI technical support engineer. ........ What is the function of the 4A charging current for?

I think you have missunderstood. May be the TI technical support engineer recommended in some other context. Generaly for 100AH you can use any charging current between 5 to 10A (C/20 to C/10)

According to the datasheet, the next charging current 8A and does this means ithe 4A can support up to at least 5A which is near to panel's max current ?

The datasheet only showed typical values of current (.5, 1, 2, 4, 8 and 10). But this doesnt mean that you cannt design circuit for 5A or 7A. To get these values you will just need to calculate Inductor,Capacitor and sense resistor using the given fromulas in the datasheets.

 

[Sunny55]

Reply to post #9 : Yes you are right i can just design for 5A as in the datasheet the values are not far from each other. But i m also using adjustable optimum tilt angle with water cooling for solar panel and does this helps get higher than the rated maximum current during a very sunny day?

 

[iukhan]

Reply to post #9 : But i m also using adjustable optimum tilt angle with water cooling for solar panel and does this helps get higher than the rated maximum current during a very sunny day?

The energy production efficiency of solar panels drops when the panel reaches hot temperatures. So the answer to your question is that you will not get higher than the rated maximum current but the efficiency loss due to temperature can be reduce by using water cooling for solar panel.