Backfeed protection module for lab power supply to charge batteries - Page 1
Forum section: electronics / projects

author: nightfire
author_rank: Frequent Contributor
timestamp: June 15, 2022, 02:35:27 am
content: As I am in the process to dive a bit deeper in batteries and their characteristics charging/discharging, I thought about being able for didactic purposes to charge batteries (and measure the charging current/voltage) via standard lab power supplies.As a lab PSU does not have backfeed/Polarity reversal protection and other gizmos that are needed in charging some batteries, I would like to build my own addon box, that interfaces via 4mm leads with a standard power supply, provides backfeed protection for the outputs of the PSU, and allows with integrated 4mm jacks for measurement of voltage/current via multimeter.Probably mostly NiMH/Lithium batteries would be charged with currents below 1A.My Idea would be to simply use some beefy rectifier diodes like the 1N5408 I have available, and maybe use some simple overvoltage protection like a Zener diode that shorts the rails above the critical voltage for the respective battery type.Question here: Are there any finished projects in this regard where I can draw some inspiration from?Aside from the voltage drop (therefore the measurement at the battery end of the circuit) are there some caveats I should/have to watch out?

author: jonpaul
author_rank: Super Contributor
timestamp: June 15, 2022, 06:21:36 am
content: Vintage lab supplies eg from Lambda and HP usually have a series diode and shunt diode to protect against reverse power faults.A Schoktty in series outside the FB loop and a shunt diode or transient absorbing Zener for overvoltage.Many had built in or accessory OVP crowbar overvoltage protection, an SCR and overvoltage detection.The constant current supplies are commonly used for battery charging.testing and industrial setups used HPIB/GPIB controlled PSU.Check the schematics of these old classic supplies.Bon ChanceJon

author: RoGeorge
author_rank: Super Contributor
timestamp: June 15, 2022, 12:04:45 pm
content: For big batteries, you must have a fusible near each battery pole to avoid fire, for just in case any short circuit happens by mishandling, by a component failure, or by other uncontrollable events (e.g. an earthquake).Other than that, I am using a diode when charging the car battery with a DP832 lab power supply. In fact, I'm using double diodes (scavenged from former PC power supplies) so I can parallel two channels of the DP832 power supply, and charge the battery faster, at 6A charging current instead of just 3A (3.2A would be with only one DP832 channel). The user manual of the DP832 explicitly tells to use a series diode while charging batteries, for just in case a power outage happens.I've put the double diodes on a former CPU radiator, because 2 diodes at 3A * 0.7V will dissipate more than 4W. Without any fan, the radiator only gets warm but not hot.Beware that a backfeed series diode won't protect against connecting the battery in reverse!When discharging small 1.2V battery cells, I'm using power LEDs as a discharging load, but with the discharging current controlled by the power supply. Note that this method can use a normal power supply to control the _discharging_ current (DP832 can not act as a load).The LEDs are scavenged from defective lightbulbs, because they are already placed on a radiator, and because they can also terminate the discharging, so the batteries under test won't discharge to zero, they'll discharge only to the desired minimum voltage, and that can be accurately dictated, too, by the power supply.When connected as in the next schematic, the power supply will control both the discharging current and the stop discharging voltage (using a CC/CV lab power supply that can not act as a load):https://www.eevblog.com/forum/projects/cc-cv-power-supply-as-a-contolled-load-for-batteries-(i-e-rigol-dp832)/msg4159222/#msg4159222

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 15, 2022, 12:06:00 pm
content: The guys above covered it better than I could, but just FYI, I once tried charging a battery from a big reference supply. I turned the supply voltage down slightly lower than the battery voltage and the supply instantly smoked. Expensive lesson- don't forget that reverse protection diode!

author: Terry Bites
author_rank: Super Contributor
timestamp: June 15, 2022, 01:42:31 pm
content: A relay?Disconnect when charger is not connected. No leakage.

author: nightfire
author_rank: Frequent Contributor
timestamp: June 16, 2022, 01:12:48 pm
content: Thanks a lot for all the answers, especially for the long elaboration about the ic based charging/protection circuits for batteries.I will definitely give them a closer look, to learn also about their capabilities.My original intention was to have some learning process in building things and doing some work on them, so that I really can see how a system behaves- and measure and plot the data associated with them.For discharging batteries I have a Korad KEL102 electronic load, so this is no problem to safely discharge them.And as I want to learn how charging systems behave, I would like to use standard components where I can put at several point some multimeters at neuralgic points (like measuring the voltage at the battery terminals or the current applied) and get to interactively work with them.As the whole setup would be under supervision whilst operating, I would like to put decent safety mechanisms in it, that prevents it from damage from decisions like lowering the voltage from the PSU below the voltage of the battery, so backfeed could destroy the PSU...From looking at the batteries I would want to work with, I see the following options:- single cell AA NiMH bases stuff, have tons of Eneloop around, some of them 10 years old- multiple AA NiMH, and closely watching which differences/balancing issues could arise when charging/discharging as a pack accurs- single unprotected Lithium cell, I salvaged some from broken camera batteries- protected Lithium batteries from DSLR cameras, that have protection circuits inside- which I want to re-use as low noise power sources (together with some buck converter or linear regulator) for some electronics projectsThose camera batteries usually have two Lithium cells, so about 8,4 Volts max.If everything goes well, I could think of upping the game to 12V lead batteries, salvaged from UPS systems- I have the idea of providing power to cameras for timelapse picture taking, where the embedded battery or even dual batteries from a battery grip are simply not enough...