Trying to convert external 24V PSU to 21V - Page 1
Forum section: electronics / projects

author: akatrinii
author_rank: Contributor
timestamp: July 26, 2025, 07:17:54 pm
content: Hi all,I was looking to find out if I could manage to convert a 24V external power adapter I had laying around into a 21V one. Need specific 21V because needs to charge a battery for a Drill (5x 18650 cells = 5x 4.2V = 21V). I've seen cheap ones on Aliexpress, but the build quality was horrible inside and also I'm looking not to spend money if possible. So I opened the casing and its pretty much standard switching inside. My idea was to find the datasheet for the main controller IC and see if there was any chance that by changing a resistor value I could change the output voltage. I can't seem to find a datasheet for it though.Any idea what this IC is? or if it is even possible what I'm trying to achieve. Any suggestion would be highly appreciated. Thanks in advance.

author: inse
author_rank: Super Contributor
timestamp: July 26, 2025, 07:22:38 pm
content: Does it have feedback via optocoupler from the primary side?Then it should be easy as the optocoupler is usually controlled by a TL431 or a fixed zener diode.If it receives its feedback via an auxiliary winding then it’s more difficult to trace out the circuit.But another problem is that such a simple approach will not make a decent charger.If your battery is discharged and you hook it up to a power supply, it will draw a lot of current, either making the power supply go into overload protection or destroying it.The charger also needs a current limitation which will be hard to implement into an existing SMPS.

author: akatrinii
author_rank: Contributor
timestamp: July 26, 2025, 07:28:07 pm
content: Quote from: inse on July 26, 2025, 07:22:38 pmHi, thank youYes it has optocoupler. Can't seem to find TL431, perhaps under the glue?

author: akatrinii
author_rank: Contributor
timestamp: July 26, 2025, 07:35:15 pm
content: Found the 431, its U5 on the secondary part on the bottom side of the PCB.

author: inse
author_rank: Super Contributor
timestamp: July 26, 2025, 07:35:23 pm
content: U5, I would guess.Btw, I made a charger for a hedge trimmer the same way.I added a 12V bulb as series resistor to limit the current in initial charge phase.It kinda works, the power supply is pulsing for some time until the bulb has heated up sufficiently.So, very botchy approach..

author: tunk
author_rank: Super Contributor
timestamp: July 26, 2025, 08:50:21 pm
content: Li-ion cells usually are charged in three stages:- constant current until end voltage is reached (usually 4.2V/cell)- constant voltage until- charge cut off, usually when the current declines to 10% of the value in the first stageI think most batteries do not do this, it's up to the charger.If this is the case, and as pointed out above, feeding it directly with 21Vis at best problematic.

author: mikerj
author_rank: Super Contributor
timestamp: July 27, 2025, 02:35:07 pm
content: Others have said this but you haven't acknowledged it: a basic power supply is not suitable for charging li-ion batteries by itself, and just setting the output voltage is not enough.This isn't just people being fussy, li-ion cells can quickly become extreme fire hazards if not charged correctly. The current must be limited to a value suitable for the cells for the constant current part of the charge cycle, as well as the final voltage which needs to be set accurately and not vary significantly with e.g. ambient temperature and line voltage variations. A proper charger will also implement a low current charge strategy if the voltage on the cells is below about 3v and will refuse to charge very low voltage cells since they can become compromised internally. It will also stop charging when the current has dropped below some threshold, since continuing to apply the maximum charge voltage is damaging to the cells.The easiest way to reach the bare minimum of these requirements (but no low voltage detect or charge termination) would be to leave the PSU unmodified with a 24v output and buy one of the very inexpensive buck regulator modules that are explicitly stated as being suitable for battery charging, often based on the XL4015. You would have to be careful to set the voltage and current limits appropriately and never leave batteries on charge for extended periods. Better still is just buy the correct charger.

author: tom66
author_rank: Super Contributor
timestamp: July 27, 2025, 04:10:12 pm
content: If you want to see what happens when people abuse lithium ion batteries, look for "DIY powerwall fires". Those are larger batteries, but a drill still usually has 5-10 x 18650 cells, easily enough to start a fire. If you do intend to charge the battery this way, please only do outside well away from other combustible things, so if the battery does catch fire, it does not burn your house down or kill someone.

author: akatrinii
author_rank: Contributor
timestamp: July 27, 2025, 10:04:59 pm
content: I appreciate all your suggestions. I have since abandoned the idea of using this modified charger.This is the cheap one from Aliexpress that people are currently using. Also there are others that cost about 2-3 times as much but I guess the insides are the same. Wish BigClivedotcom did a video on it on how dodgy it is(first photo is the product ad, the second photo is from a review)

author: xvr
author_rank: Frequent Contributor
timestamp: July 28, 2025, 11:26:45 am
content: QuoteI think they got wrong title. It should be not 'Lithium battery charger' but 'firestarter'

author: inse
author_rank: Super Contributor
timestamp: August 09, 2025, 04:10:57 am
content: How much do you expect for that price?