How to reduce voltage from my DIY power supply. 24.9v to 24.5v

Hi, I'm fairly new to electronics. I am making a 24v charging power supply to charge a Tesla Model S battery module segment that is made up of 444 (18650) cells. I am using 2 12v HP DPS600 server power supplies in series to produce the 24v I need to charge my Tesla module. The measured voltage output go the combined supplies is 24.9 volts. I want to reduce that voltage to 24.5 volts at 1150 watts. Is there a simple way to either add a fixed reduction in voltage or add a + - 5% adjustment to my voltage?

Thanks in advance for your input!

by MrCap
October 29, 2019

The really simple, cheap and dirty way to do this is to add a diode in series with the battery. The forward drop of the diode is in the ballpark of what you need so just make sure the diode current rating is greater than the battery load. As I said this is cheap and dirty but it might be adequate

by Foxx
October 31, 2019

Thank you Foxx, I will look for components. Having it adjustable would be ideal but the diode sounds like a good approach to keep me in a safe charging range. I am not familiar with diodes but will read up and see if I can sort out which one I need. Thanks again!!!

by MrCap
October 31, 2019

After a closer look at this I see you are talking about 1150 watts and this works out to near 50 amps. This is not kid stuff on which you can use a "quick and dirty" solution. You need a proper current regulated charger designed for the battery, otherwise you risk damaging or destroying a probably expensive battery and starting a fire.

by Foxx
November 01, 2019

Hello: As others have said, this is some significant current you are dealing with so proceed with care.

An 'Industrial Rectifier' is going to be about your best bet for a low-cost way for voltage reduction. A stud mount 85 Amp rectifier has an average Vf@If of 1.15 to 1.05 volt drop. While more than you need, it's about the only way out. I just looked at the site and found the NTE5934 would be a good one. Just look for one that has a Prv of 50vdc or more and an If of 70 amps or more. (The more the better on all of those.) If the diode has a Vf of 1.15 volts and you are say drawing 50 amps, you will have a device dissipation of 75 watts. That is the same heat a 75 watt light bulb would put out so the diode should be mounted to an Alu. Heat-Sink. I hope this gives you some guidance in your quest. Short of spending a lot of $$$ on a Constant-Current / Constant Voltage power supply I don't see many options. p.s. I've been in the design and repair of electronics for 50 years and using a diode for a small voltage drop has always worked for me.

by rknudtson
November 09, 2019

1 Answer

Answer by lotron

I think, that most important thing is, that for charging Li-ion cells is not suitable fixed voltage source, but charger must be used. Part of process requires constant current.

+1 vote
by lotron
November 16, 2019

Your Answer

You must log in or create an account (free!) to answer a question.

Log in Create an account

Go Ad-Free. Activate your CircuitLab membership. No more ads. Save unlimited circuits. Run unlimited simulations.

Search Questions & Answers

Ask a Question

Anyone can ask a question.

Did you already search (see above) to see if a similar question has already been answered? If you can't find the answer, you may ask a question.

About This Site

CircuitLab's Q&A site is a FREE questions and answers forum for electronics and electrical engineering students, hobbyists, and professionals.

We encourage you to use our built-in schematic & simulation software to add more detail to your questions and answers.

Acceptable Questions:

  • Concept or theory questions
  • Practical engineering questions
  • “Homework” questions
  • Software/hardware intersection
  • Best practices
  • Design choices & component selection
  • Troubleshooting

Unacceptable Questions:

  • Non-English language content
  • Non-question discussion
  • Non-electronics questions
  • Vendor-specific topics
  • Pure software questions
  • CircuitLab software support

Please respect that there are both seasoned experts and total newbies here: please be nice, be constructive, and be specific!

About CircuitLab

CircuitLab is an in-browser schematic capture and circuit simulation software tool to help you rapidly design and analyze analog and digital electronics systems.