doubt regarding BQ24650EVM-639

Question

I've been working with BQ24650EVM-639 and have successfully charged a battery with a couple of solar cells. I've even done the test suggested on the User's Guide and everything works as expected. My only doubt is regarding VSYS, I don't understand what is it's use. I would like to understand because on a project I'm working, I need to charge a battery AND at the same time, have 12V available for another load, and I'm not sure if VSYS could be my solution. If it's not, I still think it's important to understand what is VSYS for. Thanks in advance!

jc2048 · Accepted Answer

VSYS is the output (system voltage). VOUT is the battery connection. It might have been better if they'd called VOUT 'VBAT'. As rsc says, VSYS is the diode-OR of the panel and the battery and is supplied by whichever has the higher voltage. In case you're tempted to try running a load directly from the battery (both supplied by the charger), I think you'll find you get to a stage where you can then overcharge the battery (as the current won't decline in the way the charger expects, the charger will keep going even though the battery has reached 100% capacity and is starting to work its way into the buffer region, and then you get to the point where the buffering runs out and you're just wrecking the chemistry and degrading the battery). You might get away with a light load that comes in a fair way below the end-point charge current, though.

rsc · Answer

From Figure 8. bq24650EVM Schematic pg17, VSYS = VIN - 1 diode drop (D1 - PDS1040)

http://www.ti.com/lit/ug/sluu444a/sluu444a.pdf

You should be able to use this for a general load < 8A if your power supply allows it.

Scott

jc2048 · Answer

The switchover occurs naturally, it doesn't need the charger chip to actively switch anything. When the panel is above the battery voltage it supplies the load and charges the battery. When the panel gets down to just above the battery voltage the charger shuts down and then the handover is done passively by the diodes. (The charger is a buck converter and only operates when the panel voltage is above the battery.) When you're designing your system, you'll need to look carefully at the panel characteristics and make sure it can support the load and the charging (or the battery may never charge or take an excessive time to do so). You'll also lose the adaption that the charger does to try and operate the panel at peak efficiency. You've got the test setup, so you can look at those things and satisfy yourself that it does what you want it to do.

doubt regarding BQ24650EVM-639

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