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I'm putting some battery-powered lights into a boat, and I have the following question:

I bought a bunch of cheapo LED lights, as pictured:

lights

They run on 3 AAA batteries. They work fine using 1.2V NiMH rechargeable batteries, and I've wired them up just to share one set of batteries, rather than each containing their own (still works fine). Rather than recharging batteries all the time, I figured it would be more convenient to get one of those USB LiPo power-packs for phones, and charge that when needed.

My question: Is anything bad going to happen if I hook up 5V USB power-pack in place of the (3.6V total) batteries? If I need a resister in series, how do I tell what resistance I need?

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  • \$\begingroup\$ If I need a resister in series, how do I tell what resistance I need? If you cannot figure that out yourself then you should not even be thinking about hooking up that 5 V power pack. \$\endgroup\$ – Bimpelrekkie Nov 15 '16 at 12:52
  • \$\begingroup\$ When you tested with NiMH can you tell if they were any dimmer at 3.6V vs 4.5V? If not can you retest? \$\endgroup\$ – Sunnyskyguy EE75 Nov 15 '16 at 14:54
  • \$\begingroup\$ Not sure. Will respond if I do the test. \$\endgroup\$ – Peter Nov 15 '16 at 17:02
  • \$\begingroup\$ I take it that if the answer is "no, the brightness doesn't change" it means the LEDs are hooked up in series, and with a boost converter, so that switching the power sources is ok, and if the answer "yes, it's brighter with 4.5V" then they're in parallel and swapping to a 5V power source may burn out the LEDs? \$\endgroup\$ – Peter Nov 15 '16 at 17:19
  • \$\begingroup\$ not burn out going from a 2V drop from 4.5 to a 2.5V drop from 5V is like 20 to 25mA scaled, so I see no serious problem, 25% temp rise can be felt if too hot \$\endgroup\$ – Sunnyskyguy EE75 Nov 16 '16 at 3:54
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My question: Is anything bad going to happen if I hook up 5V USB power-pack in place of the (3.6V total) batteries?

The LEDs will likely fail

If I need a resister in series, how do I tell what resistance I need?

Measure the voltage when operating on new batteries and note it down. Swap to the 5V supply but put 1 kohm in series and gradually lower that 1 kohm until you see the same voltage as measured previously. Use that modified value of resistor for each LED lamp you have i.e. they will all require their own individual dropper resistor.

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If I'm not mistaken by the photo, it has 5 LEDs which are each around 3V and requires a 4.5V battery source but there is no indication of current or power or Lumens. So if 1.2V cells work, I conclude it uses an el cheapo boost regulator to drive the LEDs in series near 15V. So a battery pack from 3.3 to 5V will work as long as it has same Ah capacity or better than Alkaline for useage time.

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    \$\begingroup\$ It is more "cheapo" to just switch the LEDs in parallel without any boos converter. I have a similar product myself, it also uses 3 x 1.2 V NiMh cells and it does not have a boost converter. \$\endgroup\$ – Bimpelrekkie Nov 15 '16 at 14:47
  • \$\begingroup\$ Good point , let's ask. \$\endgroup\$ – Sunnyskyguy EE75 Nov 15 '16 at 14:52
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I tried it, and everything seems ok, LEDs have not burned out, and now I have a USB-power-pack-powered lighting system. Thanks Tony and Andy for your responses.

Notes:

  • If I open the light up, it does seem that the LEDs are wired in parallel. So I'm guessing Tony's el cheapo boost-regulator with LEDs in series hypothesis is not true.
  • I did not see a noticeable difference in LED brightness when comparing 3x 1.2V NiMH batteries to 3x 1.5V Alkaline batteries. Have not yet compared with 5V source (which leaves me a little confused, as I'd expect a change in brightness if there's no voltage regulator on this thing).
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