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I have this table lamp that uses 3 AAA batteries to function, but the batteries run out pretty fast, so I'd like to swap them for a rechargeable battery that I can plug into a USB charger without having to take it out, basically like charging a phone.

Manual Current batteries
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I looked it up and I think that I would need this 18650 batteries or a 14500 battery and a charging module. But I have 3 questions:

1 - Which battery would work better, the 14500 or the 18650? I'm just considering the 14500 because it's smaller and it would look better, but I don't know if it would work, and it's pretty hard to find a charging module for it.

2 - As you can see on the lamp manual, it says the input voltage is 4.5V, and I noticed the 18650 batteries have a maximum voltage of 4.2V, so if I use the 18650 ones, do I need two of this batteries or can I use just one? If I use just one, would the lamp light be less bright?

3 - Does the battery capacity matters?

I'm sorry if the questions are dumb, I know nothing about electronics.

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  • \$\begingroup\$ First you should buy better batteries. Zinc carbon "heavy duty" batteries are crap. Alkaline would last longer. And less likely to spew chemicals all over the place. That top battery looks pretty bad. \$\endgroup\$
    – Passerby
    Commented Dec 12, 2021 at 5:26
  • \$\begingroup\$ Specifically AAA alkaline have twice the capacity and a better discharge rate than zinc carbon. \$\endgroup\$
    – Passerby
    Commented Dec 12, 2021 at 5:46
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    \$\begingroup\$ If that is a night light for children, for safety reasons it should not be modified at all and perhaps using rechargeable batteries is also not a good idea. Many devices intended to use near children say that rechargeable batteries are not supposed to be used for safety reasons. If there ever is a malfunction in a device that shorts out the battery, alkaline batteries are much safer than any rechargeable battery. \$\endgroup\$
    – Justme
    Commented Dec 12, 2021 at 9:40
  • \$\begingroup\$ Can you note that "battery AAA" exist which can be charged with USB placed at "head". It is a li-ion battery 400 mAh with "in-battery container and Led and USB charger". Ouput is 1.5 V as AAA. amazon.nl/… \$\endgroup\$
    – Antonio51
    Commented Dec 12, 2021 at 19:28
  • \$\begingroup\$ Your questions aren't dumb, however they do not fit here well. This is a site dealing with electronic design questions, but your question relates to usage, modification/repair and buying recommendations. All of which are considered off-topic. However, if you were able to break down the question to more specific and design related elements, it might pass as on-topic. A schematic of that lamp might help. \$\endgroup\$
    – Ariser
    Commented Dec 21, 2021 at 12:14

2 Answers 2

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  1. The numbers 18650 and 14500 refer to the the size and style of the battery. 18 x 65 mm cylinder vs 14 x 50 mm cylinder. If they are the same chemistry/type, then the only difference is physical size and thus capacity. A 18650 battery is bigger and can hold more of a charge. The 14500 is sized the same as a AA battery and the capacity is smaller. You can also find other form factors like square or rectangle flat batteries that could work better for you.

  2. Yes the lower voltage will be an issue. 4.2V is the fully charged voltage and they will typically run at 3.8 to 3.2v before the shut off. That won't work well for you. You would need a boost circuit. There are battery charging modules that do output boosting for you too. Those typically use 5V output and are the same exact thing as your average usb power bank.

  3. The capacity matters. Low capacity means lower run time. Your 3x AAA carbon zinc batteries have 600mAh at 4.5V or 2.7Wh. A single 14500 900mAh 3.8V battery have 3.42 Wh. Not too much longer (and the numbers change as the battery runs out, this is just a rule of thumb calculation). A good 18650 is probably 3000mAh or 11.4Wh so will run a lot longer than either the other two.

If you really want to put it inside, a flag battery like a cell phone one would be better for you. And get a module or add a boost circuit to get a steady 4.3 to 4.5V out. The boost module will reduce your total run time but will allow the led lamp to work steady instead of dimming or shutting off sooner than expected.

Or just use an external USB power bank with a 1n4001 diode to the existing battery connections. This is known as a battery eliminator setup, simple and effective.

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  • \$\begingroup\$ This answers all my questions, thank you so much!! \$\endgroup\$
    – Ana Clara G
    Commented Dec 12, 2021 at 16:04
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    \$\begingroup\$ Just one more question, is there a problem if the out voltage of the booster is 5V? Could it damage the lamp since it needs just 4.5? \$\endgroup\$
    – Ana Clara G
    Commented Dec 12, 2021 at 16:07
  • \$\begingroup\$ @ana I can't say without know the specifics of electronics, but you can add a 1n4001 diode in series to reduce the voltage about 0.7V. \$\endgroup\$
    – Passerby
    Commented Dec 12, 2021 at 16:26
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You could wire a USB cable in place of the batteries, and use a USB-out power bank to run the lamp. The power bank will have all the battery management and charging issues sorted for you.

Feeling more ambitious? First, what are those issues:

  • Ensuring the battery doesn't get charged too quickly or too much (current and voltage limit)
  • Outputting your required 4.5 ~ 5V voltage from Li-ion 3 ~ 3.7V (step-up)
  • Support charge+run when plugged in

A charge control IC + step-up regulator can do these tasks. Besides the battery, this is the guts of a power bank.

IMPORTANT!: Overcharging a Li-ion battery can lead to an explosion or fire. Ensure that any Li-po or Li-ion battery you choose has built-in protection. This is the norm these days for batteries that you buy from most sources, but if you do get a battery that doesn't have such protection, get a protection board for it too.

That out of the way, here's some DIY hackery-dackery goodness...

A well-documented board you could use: https://www.sparkfun.com/products/14411

A project that integrates a Li-po cell inside a USB cable shroud: https://www.instructables.com/Power-Bank-Inside-a-USB-Cable/ This uses a very small power bank board that accepts USB-micro-B input

A teardown of a 'cheap' power bank (1x 18650 cell) that uses a single controller IC: https://sites.google.com/site/wayneholder/inside-an-inexpensive-usb-power-bank-charger

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  • \$\begingroup\$ I would add a 1n400x diode in line first. But yes. \$\endgroup\$
    – Passerby
    Commented Dec 12, 2021 at 5:27
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    \$\begingroup\$ Winning answer. Uses COTS parts, address safety, and most likely to be achievable with OP's skills. \$\endgroup\$
    – Harper - Reinstate Monica
    Commented Dec 12, 2021 at 8:05
  • \$\begingroup\$ I get that this is the easiest way, but it wouldn't solve my problem, cause it would still have to be connected to something all the time. \$\endgroup\$
    – Ana Clara G
    Commented Dec 12, 2021 at 15:54
  • \$\begingroup\$ It’s a bodge, but it will work, and you don’t risk blowing up the Li-ion battery. I’ve done this many times with light strips. If you’re up for more of a challenge, consider using a Li-po prismatic cell and a pre-made charger/booster module, bought on its own or harvested from a power bank. \$\endgroup\$
    – hacktastical
    Commented Dec 12, 2021 at 18:06

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