I've got a broken power supply for charging a lithium-ion battery with the following specifications: INPUT: 220-240VAC~50Hz MAX 11W OUTPUT: 4.2VDC 1200mA

As I'd like a new power supply and I don't find one on, e.g. amazon, my question is in what way do I have freedom in specifications of a new power supply?

What I found so far are specifications of switching power supplies like this:

INPUT: 100-240 Volt AC, 50/60 Hertz
OUTPUT: 3 / 4,5 / 5 / 6 / 7,5 / 9 / 12 VDC - 600 mA

INPUT: 110V-240V AC
OUTPUT: 3V / 4,5V / 5V / 6V / 7,5V / 9V / 12VDC
600mA at 3V-6V / 450mA at 7,5V / 400mA at 9V / 300mA at 12V

INPUT: 100-240 V AC
OUTPUT: 3-12 V DC (3,0/4,5/5,0/6,0/7,5/9,0/12,0 V) 1000mA

INPUT: 100 ~ 240V 50/60HZ, max. 500mA
OUTPUT: 5V/2,4A; 6V/2A; 7,5V/2A; 9V/2A; 10V/2A; 12V/1,5A; 15V/1,5A

Is one of these four power supplies eligible as a replacement?

  • \$\begingroup\$ 4.2V seems to be a voltage used for charging lithium-polymer batteries. Is that the application here? If so, it would need extra logic that properly controls the charge cycles. Also, the specifications posted are of the adapter, not the device being powered. What are the voltage ranges and currents required by the device being powered? \$\endgroup\$
    – Pigrew
    Commented Mar 12, 2016 at 20:49
  • \$\begingroup\$ It's for a Remington HC-350C hair trimmer. The device specifications are 4.2VDC. It's said that it's for use with adapter model HK41G-4.2-1200 (which is broken). Unfortunately there is nothing more mentioned on there. \$\endgroup\$
    – SemtexB
    Commented Mar 12, 2016 at 21:20
  • \$\begingroup\$ @Passerby - calling this a duplicate of that is likely to lead to dangerously wrong conclusions. Battery chargers, especially those for lithium chemistries are not ordinary power supplies. \$\endgroup\$ Commented Mar 12, 2016 at 22:27
  • \$\begingroup\$ @ChrisStratton the question lacks any details on the end use at the time. That was edited in 1.5 hours after I VTC as a dupe. \$\endgroup\$
    – Passerby
    Commented Mar 12, 2016 at 22:58

1 Answer 1


The power supply in question is for charging a lithium-ion battery. These sorts of batteries have special requirements for charging in which a constant current (1.2A) is applied until the battery reaches some voltage limit (4.2V) and will turn off once the current has decreased to some limit (unknown, but probably 10-100 mA).

It is unclear if this charging circuitry is inside the battery-operated-device or the adapter. For the following analysis, I'll assume that it's in the power adapter, which seems likely.

The adapters specified in the question are not battery chargers and also do not limit the output to precisely 4.2V, so would be dangerous to use in this application. An adapter sold as a lithium-ion charger would be appropriate.

The battery could be damaged by a battery charger that has a HIGHER current limit, but it would be safe to use a lithium-ion charger with a LOWER current limit. The lower current limit (such as 800 mA or 1000 mA) would charge the battery more slowly, but would be safe and actually allow the battery to have a longer life.

Additionally, you need to be aware of the physical shape of the charger port in order to ensure mechanical compatibility. It may be required to retrofit the new charger with the plug of the old charger (using crimp connectors/soldering/etc....). Be sure that the two wires are not reversed (as this would damage the battery, perhaps causing it to catch of fire). The old adapter may indicate which pin is (+) and which is (-). A voltmeter could be used on the new charger in order to determine its output polarity.

If the charging circuit is inside the battery-operated-device, there is a small chance that the external charger circuit could damage the device, but my guess is that it wouldn't be damaged.

  • \$\begingroup\$ So if I got a charger with lower current limit. How much attention should I pay on the voltage limit of the charger? As written in the other thread posted by Passerby it says, that it preferably needs to be exactly the same voltage. Is this also the case here for my device and charger? \$\endgroup\$
    – SemtexB
    Commented Mar 12, 2016 at 21:57
  • \$\begingroup\$ Furthermore, why is it in my case better if I get a charger with lower current? It also says in the thread mentioned by Passerby that I should a charger with at least the current of my broken charger? \$\endgroup\$
    – SemtexB
    Commented Mar 12, 2016 at 22:04
  • \$\begingroup\$ Most circuits will self-limit the amount of current that they draw (at some fixed input voltage). If an adapter with a higher current rating is used, that's fine since the circuit itself limits the current. Batteries are quite different; if 4.2V was directly applied to a lithium-ion battery, the battery would draw too much current, causing it to overheat and fail. The battery charger itself must limit the battery's current to some maximum amount (which is roughly proportional to the battery's capacity) in order to protect the battery. The adapter's current rating is that current limit. \$\endgroup\$
    – Pigrew
    Commented Mar 12, 2016 at 22:24
  • \$\begingroup\$ It's possible that this current limit is enforced within the shaver, so an adapter with a larger current limit would be fine, but without disassembling the razor or charger, I cannot be sure of that. \$\endgroup\$
    – Pigrew
    Commented Mar 12, 2016 at 22:28
  • \$\begingroup\$ So I just found one on eBay, which comes closest to the original charger (ebay.de/itm/…) . OUTPUT: 4,2VDC 770mA. As far as I understood your answer(s), I should be fine with it?! After retrofitting the plug of the charger with the original charger of course. \$\endgroup\$
    – SemtexB
    Commented Mar 12, 2016 at 22:39

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