# Buck converter charge battery control output voltage

I have a buck converter charging 8 series of Li-Ion batteries ( 8 x 4.2 voltage of charging) with a source of 36 V. I am using this schematic (from here):

I know that for a resistive load, the output is controlled as $$V_o=D V_{in}$$

But, here the load is not resistive. How to control the output voltage? can we control it as above without feedback?

Does duty-cycle D = 8 x 4.2 / 36 works here?

• Why don't you want to use feedback? That's both the standard and the best way to do it. And, without feedback you can't limit your charge current, so you're likely to blow up your batteries. Commented Jun 22, 2015 at 9:52
• @NickJohnson The batteries are protected. In the first step I just need something that works. First I should solve voltage control, then I will control the current. Do you see the duty-cycle formulation still valid for this problem too? Commented Jun 22, 2015 at 10:03
• Protected against what? Most LiPos have circuits to protect against overdischarge; that doesn't mean you can just hook them up to an arbitrary voltage source and expect them to charge fine. Commented Jun 22, 2015 at 10:29
• And you don't need to solve this yourself - there are plenty of charging solutions out there already. Commented Jun 22, 2015 at 10:29
• @NickJohnson They are protected against overcurrent too. So what is your suggestion? could you introduce me some of those solutions? Commented Jun 22, 2015 at 10:36

• Could you please mention the formula between Vin, Vout and the duty-cycle? Commented Jun 22, 2015 at 11:07
• Except for voltage drop at switch and diode, in my case is vout=vin x duty-cycle? Commented Jun 22, 2015 at 12:15