Diode for charging lithium batteries necessary? - Page 1
Forum section: electronics / beginners

author: Jenoin
author_rank: Newbie
timestamp: May 07, 2019, 12:06:36 am
content: I have read that for charging lithium batteries using a bench power supply, that it is important to use a diode or verify that the power supply in question has means to prevent high reverse current. I am converting a a golf cart to lithium ion batteries and I am planning to charge using a Sorensen DCS 80V 13A power supply that I picked up off of eBay. It appears to me that this particular power supply has diodes built into the DC output circuit and shouldn't need any additional protection. I have attached an image of the circuit. Feel free to let me know if you disagree or agree. According to the manual, you can hook multiple power supplies of this type together in parallel for high current applications as well, which also makes me think it shouldn't need an extra output diode to prevent reverse power.I have a BMS on my battery which is a 14s 58.8V leaf derived assembly.Thanks for any assistance

author: KC0PPH
author_rank: Unknown
timestamp: May 07, 2019, 02:04:08 am
content: You should not charge lithium batteries from a power supply. Either build a proper charger or buy one. I use ones from revoletrix for my Rc batteries. These batteries are no joke.

author: amyk
author_rank: Super Contributor
timestamp: May 07, 2019, 02:19:05 am
content: Quote from: KC0PPH on May 07, 2019, 02:04:08 amA lion chargerisa power supply. The only important things are voltage and current, as long as you keep those within the limits the battery will charge just fine.(Dave himself has done a video on this.)

author: KC0PPH
author_rank: Unknown
timestamp: May 07, 2019, 02:22:24 am
content: A power supply that monitors individual cell voltages, provides over voltage protection and can follow the correct charge curve. As I said build a correct one or buy one. Your an idiot if you think you can charge a multicell lithium battery with just a power supply.

author: KC0PPH
author_rank: Unknown
timestamp: May 07, 2019, 02:49:10 am
content: Is the BMS a Battery Management System? If so look up to see if it handles the charging of the batteries and you just have to supply it with a fixed voltage? It is common for chargers to require a DC supply. My Revo requires a 12-24V DC power supply and will charge up to 8s packs. (BuckBoost Converter)

author: MarkF
author_rank: Super Contributor
timestamp: May 07, 2019, 03:17:50 am
content: Photoshop please

author: james_s
author_rank: Super Contributor
timestamp: May 07, 2019, 05:19:29 am
content: Well you *can* charge one with just a power supply, however it's not a very good idea. You have to be very careful not to let any of the cell voltages get too high.

author: paulca
author_rank: Super Contributor
timestamp: May 07, 2019, 06:58:55 am
content: I have charged LiPos with my bench power supply. However I do have a cell monitor with a balance function to make sure things stay good.

author: Jenoin
author_rank: Newbie
timestamp: May 07, 2019, 01:36:40 pm
content: Sorry, I thought it would be clear. I have a cell balancing BMS that will automatically interrupt charge current if any cell exceeds my specified voltage (4.15V)I think safety of cells will be adequate, though I do appreciate your concern.So nobody has any comments on the schematic then, other than removing the border?To me it looks like there isn't a reverse current path available. I will probably just check if I can drive reverse current through it with a different power supply at a low voltage/current to be sure.Thanks

author: james_s
author_rank: Super Contributor
timestamp: May 07, 2019, 03:43:42 pm
content: Well it's not really a schematic, it's just a simplified diagram but generally decent bench power supplies are reasonably well protected.

author: helius
author_rank: Super Contributor
timestamp: May 07, 2019, 04:27:55 pm
content: The risk comes with supplies with a crowbar overvoltage protector. The idea is that when powering sensitive transistors, you want to prevent voltage excursions caused by inductive kickbacks etc. since the transistors could be damaged. These types of supplies are often identified by an analog pot on the front panel that needs to be turned with a screwdriver, but they may take other forms. The crowbar circuit uses a triac or diac to short the output terminals if the voltage across them exceeds the set point, thus protecting the DUT from overvoltage damage. This type of supply should not be used to charge a battery.From the circuit diagram I can't tell if that applies to your supply or not (does the "Voltage Control Circuit" include a triac?), but you can study the power supply for any method of setting the overvoltage level.

author: Jenoin
author_rank: Newbie
timestamp: May 07, 2019, 04:44:49 pm
content: Thanks Helius, that makes so much sense. In the manual it says that if you operate multiple power supplies in parallel that you must ensure that the OVP (over voltage protection) is set to the maximum on all the paralleled supplies. Based on what you said, this is to prevent a supply from being damaged by reverse current from the other supplies in the event that the OVP shorts it's output terminals.So if I set the OVP to 80V+ I should be good to charge up to 58.8 I think. I may incorporate a diode anyway.Thanks for explaining how the OVP functions.