I am designing a field charger for charging lipo batteries while I am out flying and on the go and I already have a huge 16000mah 4S lipo available that I could use to charge my 1300mah 4S and 2200mah 2S lipos while I am out and about. I want a cheap, but more importantly compact solution so although there are multiple battery chargers available, none seem to meet my needs.
Not wishing to re-invent the wheel, I have come across this charger that a friend uses that is actually very accurate despite its cheapness (around £9GBP delivered - https://hobbyking.com/en_us/hobbykingr-dc-4s-balance-charger-cell-checker-30w-2s-4s.html)
I am thinking of using 4 of the boards from these so I can charge 4 batteries added at different times while I am flying.
My issue is that the operating voltage is 9-16V, fully charged 4S lipo is 16.8V.
I understand there is usually some allowance made in this figure so I opened up my friend's unit to inspect. I cannot seem to find any components that are obvious to me that cannot handle 20V.
Would anyone be able to assist me in trying to assess before I either buy 4 of these, or otherwise try it out and blow up my friend's charger? (always a risk I know).
All of the caps are rated 25V, of the smd fets I can identify, those are rated at 20V. My question is if looking at these two photos I should pay close attention to any particular components.
I COULD use a buck converter to convert down to 12V first, but that will add bulk and reduce efficiency, so I wish to avoid that if possible. I will obviously check that all chargers are charging to accurate voltages before deployment. (I am experienced with lipos even down to a scientific level where I have designed some, so I am aware of the dangers, this is just for my hobby, nothing else.) I also wish to learn more about how to assess circuits that are not my own / well documented.
Many thanks in advance.
EDIT: The inductor for the buck/boost stage is pulsed using what appears to be an 'SI4606' - http://www.datasheetspdf.com/datasheet/search.php?sWord=Si4606
The control MCU is a Holtec HT46R066B - http://biakom.com/pdf/ht46r06xbv110.pdf But VSS is max 6V so like other ICs seems to be supplied by a reg... Other IC chips can be identified but they do not seem to see, from what I can tell, V_Input directly.