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 NTEINC.com 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
5 days, 6 hours ago
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