Power supply won't go into CC mode when charging li-ion cells with series diode. - Page 1
Forum section: electronics / testgear

author: tylerssims
author_rank: Contributor
timestamp: December 29, 2022, 07:33:21 pm
content: Hi all,I have a Kikusui PMC 18-5A power supply. I am trying to charge an individual li-ion cell with it. I've watched this video by Dave (https://youtu.be/jNmlxBXEqW0) where he sets the current limit to 1.7A and voltage limit to 4.2V and the supply works in CC mode until the cell reaches 4.2V and switches into CV mode. My cell has the same charging parameters.When I connect my cell to my supply I can get the supply to work fine in CC mode like described above. However, in the Kikusui manual, it recommends using a diode in series with the load to protect the power supply. They recommend for the diode to have a reverse voltage withstand capacity at least twicethe rated output voltage of the power supply, and the forward current capacity to be three to ten times the rated output current of the power supply, I am using this diode:https://www.mouser.com/ProductDetail/821-SRAF1640HC0.However, when I put the diode in series with the battery cell, the load will remain in CV mode and wont draw more than an amp.How can I use a diode to protect my power supply while still operating in CC mode? Can I use the remote sensing feature of my power supply to fix this? Or can I choose a diode that has less resistance?

author: Seekonk
author_rank: Super Contributor
timestamp: December 29, 2022, 07:55:17 pm
content: You will have constant current, but not constant voltage. The added forward voltage drop of the diode will not be constant and will vary with current and temperature. When charging just one cell that is significant.

author: bdunham7
author_rank: Super Contributor
timestamp: December 29, 2022, 08:18:03 pm
content: You can just turn the voltage up to 4.4V and you might get your CC operation. The voltage drop across the diode should be more than 0.2V down to 100mA or so. Remote sensing should also work, but be careful to make sure the wiring is solid otherwise you might fry your battery--losing the remote signal may cause the power supply to go to full output.

author: BeBuLamar
author_rank: Super Contributor
timestamp: December 29, 2022, 08:44:57 pm
content: The voltage drops across the diode is more than 0.5V so you will have to increase the voltage by that much.

author: Fungus
author_rank: Super Contributor
timestamp: December 29, 2022, 08:49:40 pm
content: What do you get if you measure the voltage on the battery?Quote from: tylerssims on December 29, 2022, 07:33:21 pmAnswer the question above and you'll find out...

author: The Doktor
author_rank: Regular Contributor
timestamp: December 29, 2022, 08:51:31 pm
content: The diode is not always needed, depends on the power supply. If your supply has an "OV Crowbar" that trips above a certain voltage or if it sinks current as well as sources, you need the diode. Otherwise just hook it up directly.Beware that even if crowbar is set well above battery voltage, it may still trip at very low voltage if it's turned off or loses power. I suspect I got my 1st PowerDesigns 6050 for just this reason. Wire to SCR melted, and looked like someone broke the wire to the power switch when looking for the source of the smoke.Ed

author: The Doktor
author_rank: Regular Contributor
timestamp: December 29, 2022, 09:04:39 pm
content: Remote sense at the battery should be OK. DO NOT add the diode voltage drop to the output setting. Diode voltage drop decreases with lower current, lithium cells get angry when overvolted. You definitely don't want an angry lithium cell in your house/workshop/car.

author: tylerssims
author_rank: Contributor
timestamp: December 29, 2022, 09:11:00 pm
content: Thanks for the advice! I have never tried remote sensing before but I will make sure my wiring is robust, I appreciate the heads up.

author: tylerssims
author_rank: Contributor
timestamp: December 29, 2022, 09:19:38 pm
content: Thanks for the heads up. I can see how the voltage drop decreasing with lower current would over volt the battery once current drops. I will try the remote sensing!

author: tylerssims
author_rank: Contributor
timestamp: December 29, 2022, 09:26:04 pm
content: This Kikusui power supply calls for a diode to be used in the manual. However, I do have another power supply and that may not be the case with that one- ill have to check. Thanks!

author: AVGresponding
author_rank: Super Contributor
timestamp: December 30, 2022, 12:18:15 pm
content: Bear in mind the drop across the diode will vary according to how much current it is passing. I almost always use a DMM to monitor the battery terminal voltage when I'm using a PSU as a charger. Obviously it's more convenient if you have a bench meter that you can leave on, but if you only have a handheld you'll be fine checking it every now and again.

author: Fungus
author_rank: Super Contributor
timestamp: December 30, 2022, 12:50:06 pm
content: Quote from: tylerssims on December 29, 2022, 09:11:00 pmThis is a perfect opportunity to learn!