Solar pannel - Lithium Ion battery charging

Hello,
I am designing a solar street light controller. I have a solar panel which gives around 6V / 1.5 Amp in direct sun light. I have 2 lithium Ion battery pack each with 9 cells inside. and the load is 2 led panels with an array of SMD led's .

I tried searching for lithium ion charger IC's and found BQ24210 from Texas instruments. but the Max rated current is less than 1.5 Amps. some other ICs say they are for 1 or 2 lithium ion cells.

I also have to make a led driver and integrate a PIR sensor, so that light switches to bright mode when a person comes in range of sensor.

Can someone please guide me on making the charger. I have googled a lot, but i am not getting what i am looking for.

Awaiting your responses. if you need some clarifications please let me know.

Regards
Mihir

What is the configuration of the batteries in the pack (voltage and current)? Depending on that answer the BQ2410 may not work. For battery charging you need to have a higher voltage for the charger output than the battery to force current into the battery. That's the reason the BQ2410 is rated for 4.2V output; a Li-ion cell is 3.7V charged. The current rating isn't that crucial. The BQ2410 will regulate the input power to only allow for a draw of 800mA so it still would work fine with your solar panel.

So what you really need to do is the following:
1.) Identify the battery pack configurations to determine how much voltage you need to charge the pack
2.) Determine how fast you want the batteries to charge. This will determine what charger IC you select based on the output current capability

hello, thanks for your reply.

1. The voltage from battery pack is 3.7 volts when charged , so i believe all cells are in parallel.
2. As per the reading that i came across, Voltage on Li-ion battery may increase faster, but if constant current is not supplied the capacity is lowered.

I also came across bq24072 which can take supply current upto 1.5 Amps

Okay your pack should work fine then from a charging point of view. I am not really understanding what you are saying about the current. You have a voltage source that is higher than your battery that will cause current to flow into the Li-Ion battery. The amount of current flow capable determines how fast it will charge but it will charge it up to completion. After completion, most charges have a trickle mode that keep them topped off or close to max capacity. The other IC seems to be your golden ticket but I am just trying to understand what you mean on the 2nd part.

Max absolute current rating on Bq24210 is 1.25 amps, and i just figured out the datasheet says it is a 800 mAmp single cell charger. so the ic will get damaged if i give 1.5 amps at input.

If i want to avoid using any Charger IC , can i design a circuit along with my controller ( atmega 328) to charge the battery pack. I can read the analog ports to read the voltages and isolate with a FET to protect overcharge and under charge.

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What do you guys think of my above concept ? I dont know what will be the constant current circuit. also my controller will need an additional smaller battery which i want to keep isolated. Any suggestions , Or am i missing out on some important points ?? will wait for your responses

Thanks

Ah ic now. Well you can add some resistance to the input to sink away the extra amount of current but that will effect your performance. The constant current could be something like an LM317 in current mode. The system looks like it could work (pending some controller protection for the I/O). For Li-Ion I am not sure if you can just monitor the voltage because I believe it does odd things when under load and under charge. Just read up to make sure that it is a linear correlation of voltage both as an open load and under load.

You people should go to www.batteryuniversity.com and read about lithium rechargeable battery cells.
1) One cell has an average voltage about 3.7, it is half discharged.
2) The voltage of a completely discharged cell is 3.2V when the circuit should detect the low battery voltage and disconnect the load.
3) The maximum allowed voltage of one cell is 4.20V.
4) A cell is not fully charged until the voltage has reached 4.20V and the charger circuit senses that the charging current has dropped to about 3% of the normal charging current then turns off the charger. Over-charging is NOT allowed.
5) The input voltage to the charger circuit might be 5V or more.

That looks about right. Little bit rusty but I was just pointing out that those values are nominal and that batteries are not linear devices when under duress (charging/discharging). They kind be a little bit more tricky to accurately detect the voltage of them without a specialized feedback circuit that is taking into account the current and voltage rating. Thanks for the link.

Thanks for your comments.

http://www.ti.com/product/bq24100
http://uk.farnell.com/linear-technology/lt3650edd-4-1-pbf/batt-charger-2a-li-ion-12dfn/dp/2295366
http://www.linear.com/product/LTC4001-1
http://cds.linear.com/docs/en/datasheet/40011fa.pdf