0
\$\begingroup\$

I built a 4S Li ion battery pack and I need 12V output at 16-20Amps but I have limited space (70mmx55mm). 12 V would be the best but I could also just reduce the max output from 16.8V (4x4.2V) to under 16V. I spent hours trying to find a good small DC converter without luck. is there another way to lower the voltage just a little without impacting the capacity to draw up to 16A or 20Amps ?

\$\endgroup\$
  • \$\begingroup\$ A 4 cells pack is only 16.8 V when fully charged, the voltage will quickly drop to about 3.6 V per cell so 14.4 V. That is only a 2.4 V difference to 12 V. Most switching regulators aren't efficient at such a small voltage difference. At 16 Amps I think it will be nearly impossible. Also the small space means you cannot dissipate much power so you need an efficient conversion which is not going to happen at such a small voltage difference. So I think there's a reason why you cannot find any suitable DCDC converter: you're on or over the edge of what's possible. \$\endgroup\$ – Bimpelrekkie Jun 12 '18 at 8:44
  • \$\begingroup\$ @Bimpelrekkie Why would high duty cycle be a problem or cause low efficiency? \$\endgroup\$ – winny Jun 12 '18 at 12:06
  • \$\begingroup\$ @winny Unfortuantely I don't have the theory behind this at hand but from practical experience as well as looking at datasheets of many DCDC buck converters, it can be seen that efficiency drops significantly as the input-to-output voltage difference drops. This is I think partly due to the fact that the voltage across the inductor becomes smaller as Vout-Vin decreases. Also as Vin goes down, the input current will increase. This means increased losses from the input switch and inductor series resistance. \$\endgroup\$ – Bimpelrekkie Jun 12 '18 at 12:23
  • \$\begingroup\$ ...Combine that with the high dutycycle you would need at a low Vin and the losses get even worse (compared to the high Vin and low DutyCycle situation). The losses happen at the inductor charging cycle and when the duty cycle is high, say 70% you're in that situation 70% of the whole cycle. \$\endgroup\$ – Bimpelrekkie Jun 12 '18 at 12:25
  • \$\begingroup\$ Thanks for the information, what would be the best option then? can I use a resistor to make sure the voltage drops below 16? another possibility could be to use Lifepo cells at 3.2V each but they are much more expensive \$\endgroup\$ – ozzy Jun 12 '18 at 17:19

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.