Want to Add Current Limiting to Computer Power Supply - Page 1
Forum section: electronics / beginners

author: Mikek400
author_rank: Regular Contributor
timestamp: October 05, 2025, 07:35:58 pm
content: I'm looking build a Lithium Battery Charger using a 12V 60A Server Power Supply.Adjusting the voltage to the required 14.5V seems simple enough, but what is a good way to achieve constant current? Just a large resistor? Doesn't seem like the way to do it.How would you do this?Thanks in advance

author: BillyO
author_rank: Super Contributor
timestamp: October 05, 2025, 11:51:27 pm
content: A big resistor is one way. It won't keep the current constant, but it will work if teh value is chosen to ensure the make charge rate is not exceeded. The real problem would be finding one that will handle the power dissipation.The schematic show is probably a better way.

author: inse
author_rank: Super Contributor
timestamp: October 06, 2025, 05:12:46 am
content: The current regulation will need some overhead to work, so it would be good to implement a voltage regulation to control the terminal charge voltage.The transistor also requires a heatsink chosen for maximal power dissipation.(full current at full discharged battery)If it needs to be short circuit safe, additional measures would be necessary.

author: Psi
author_rank: Super Contributor
timestamp: October 06, 2025, 05:29:34 am
content: First thing to decide is if you want the current regulation to be linear or switching.Linear is simpler but you have to deal with the heat.How much current does your battery charger need, 60A is a lot of amps.You would also need thermal regulation too.ie, have a current limit threshold that is somewhat based on temperature of the heatsink/regulator. Or which folds back the threshold as the temp goes above a set point.That way it will keep lowering the current draw to keep the regulator temp under control.

author: Smokey
author_rank: Super Contributor
timestamp: October 06, 2025, 05:46:35 am
content: What batteries do you need to charge at 60A that don't already have their own charger to do that?Are you OK with your battery catching fire? If not, it might be a good idea to be exceedingly technically confident in whatever circuit you try to hack together here.

author: BillyO
author_rank: Super Contributor
timestamp: October 06, 2025, 01:55:55 pm
content: Quote from: inse on October 06, 2025, 05:12:46 amThe supply he intends to use is regulated, and we're talking battery chargers here, not some reference source. A few mA here or there are going to matter diddle-squat.Of course it will need a heat sink.The circuit I showed has all the measures it needs to be short circuit safe. It's a current limiter. Follow the circuit.

author: BillyO
author_rank: Super Contributor
timestamp: October 06, 2025, 02:03:45 pm
content: Quote from: Psi on October 06, 2025, 05:29:34 amWhy not just make the heatsinking (possibly including a fan) sufficient for the worst case?

author: inse
author_rank: Super Contributor
timestamp: October 06, 2025, 02:37:54 pm
content: I don’t want to be nitpicking but Lithium batteries are sensitive to overcharging.And after all we are in the Beginners section.P.S. a heat sink for short circuit (14.5V*60A=870W) might get a little out of hand

author: BillyO
author_rank: Super Contributor
timestamp: October 06, 2025, 02:40:55 pm
content: Yes, that's why you set the voltage of the supply to the maximum voltage of the battery pack, and the max current similarly.

author: inse
author_rank: Super Contributor
timestamp: October 06, 2025, 02:42:48 pm
content: Plus VCE drop (which we don’t know)

author: BillyO
author_rank: Super Contributor
timestamp: October 06, 2025, 02:49:20 pm
content: For the Darlington shown, it should be around 1.4V. However, my approach would be to start low, put it under a small load, then bring up Vo to the desired maximum.The same with the current. Start off with it trimmed all the way down, then bring it up to near the max charge current or, if he's not in a panic, bring it up to about 1/2 the max current for longer battery life.

author: Psi
author_rank: Super Contributor
timestamp: October 07, 2025, 08:57:10 am
content: Quote from: BillyO on October 06, 2025, 02:03:45 pmYou can... if you want to burn off 720Watts, but I wouldn't recommend it.A big power resistors get to 100C+ when burning off 100W

author: Psi
author_rank: Super Contributor
timestamp: October 07, 2025, 08:59:10 am
content: If it was a more typical psu you could interface to the opto/feedback to add current regulation, but computer psus have a lot of monitoring stuff so its not that easy to make them do non-standard things.

author: MathWizard
author_rank: Super Contributor
timestamp: October 09, 2025, 06:07:13 am
content: When charging Li batteries, aren't they supposed to have certain currents at different times, depending on the charge ? I don't see any OP response yet, about the battery size or other specs.OP are you familiar with how ATX PSU's really work? There would be far easier and safer PSU's to try and mod, especially if you don't need 100's of Watts, and don't want to burn down the house.

author: inse
author_rank: Super Contributor
timestamp: October 09, 2025, 06:40:41 am
content: Yes, he left us a little in the dark…The basis is a server power supply, so single voltage (12V) but lots of amps and a lot of fancy surveillance and protection circuits.If it was a AGM replacement LiFePo battery, it’s probably less demanding in terms of charge profile.But we lack details on almost everything, probably another abandoned thread.

author: Mikek400
author_rank: Regular Contributor
timestamp: «Reply #15 on:Todayat 01:42:29 pm »
content: Thanks everyone for chiming in and giving me suggestions!Sorry for being late responding. Had some things come up.I have a couple of KWH of LIFEP04 batteries I want to be able to top up fairly quickly when needed. For example recharging them from a generator during long power outages. Still not sure if I will build a charger or buy. But after recently repairing my old HP 6228B power supply and started thinking about how the current regulation works in it and how difficult it might be to implement my own current regulation. (The hp using linear regulation which I don't think feasible for this much power)I have a setup now utilizing an off the shelf Chinese 30A boost converter board with CC CV connected to a 12v 60A PSU and it works, ok... but the cheap converter gets pretty hot and seems like it won't last.The batteries will be monitored during charging and don't plan to just leave it charging. They all have a BMS as well.When I get some more free time I want to try building the circuit BillyO provided even if just for fun and education.

author: Mikek400
author_rank: Regular Contributor
timestamp: «Reply #16 on:Todayat 01:48:40 pm »
content: Quote from: inse on October 09, 2025, 06:40:41 amYes they are LIFEP04 drop in replacements for AGM batteries.