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I have a 3 AA battery powered LED camping light/lantern. It draws (measured with a multimeter in series) about 18 mA when ON and 0 mA when OFF (ON/OFF controlled by a switch button).

It’s ON all night for about 8 hours, so the batteries last only a few days.

If I use rechargable AA batteries the charge lasts less time, and with the charger I have they take a long time to charge.

My idea was to use a little SAI 7 Ah 12v AGM lead acid battery (a Salicru UBT 7 Ah), so I can power the lantern many more days with a single recharge, until the battery is 30%-50% discharged.

The battery will be recharged apart in a well vented locarion.

To convert the 12v to 4.5v I plan to use a DC/DC regulable step down converter.

I cannot use an AC power adapter.

My questions are:

  1. This will be used in a bedroom, and be ON all night. Is it safe to use an AGM battery? It will only be discharged in the bedroom, not charged.
  2. I plan to use a car 1 A fuse, so in case the lantern shorts, the thing doesn’t get fire. Is this a good size? Since the lantern only draws a few mA, would it be better a smaller fuse? Are there affordable smaller fuses for this kind of scenario?
  3. What do you think about my solution? I am sure you can propose far better ideas that I would want to know about :)

Edit: the lantern is used (discharged) indoors, and charged outdoors.

Edit: turns out I messured incorrectly the draw. It draws 18mA when the batteries are partially depleted (3v total the 3 AA in series). But this is interesting, because at 3v the light is enough, so I could tune the DC/DC converter to 3v in order to save battery life. Or simply use the pot on the converter to set brightness up to a max of 4.5v.

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  • \$\begingroup\$ You are fine. I would probably use as small a fuse as possible consistent with the need. But I wouldn't worry a lot if I were stuck with a 1 A fuse. That AGM battery is fine and will last a very long time. It is probably over-kill. Why might you be concerned?? \$\endgroup\$
    – jonk
    May 22, 2022 at 7:19
  • \$\begingroup\$ @jonk thanks for your comment. I suppose my concern comes from misunderstanding of how the fuse works. I am worried that with a 1 A fuse, if there is a short, the cables, the lantern and/or the DC/DC converter could get fire before the fuse opens the circuit. I imagine that if there is a short the battery can deliver A LOT of amps and the circuit is opened very fast, but I am not sure about it. \$\endgroup\$
    – ElectroNewbie
    May 22, 2022 at 7:35
  • \$\begingroup\$ An amp isn't all that much. Sure, it's possible to be a problem. For example, if you supplied 1 A to an Estes rocket igniter than that could be a problem. But broadly speaking the 1 A fuse is fine. If something shorts out, it is likely to far exceed that value. Just as you think. Anyway, I personally would not worry much. And if you are camping out, I'm also not worried about AGM gassing. Plenty of ventilation, I suspect. Just have fun! \$\endgroup\$
    – jonk
    May 22, 2022 at 7:37
  • \$\begingroup\$ @jonk thx for the fuse clarification. It’s a camping light but the use would be indoors, in an off the grid house. It’s for the kids, so they have a little light while sleeping :) So perhaps it’s a good idea to be paranoid about venting and put it outdoors in a vented case or directly use another type of battery as suggested in Passerby’s answer. \$\endgroup\$
    – ElectroNewbie
    May 22, 2022 at 7:45
  • \$\begingroup\$ Sure. No harm in being safer. Agreed. But I think you are pretty much fine with the approach. I think I personally would stick with the 1 A fuse because there, sometimes, is an inrush peak current when something starts up. If you select a fast-blow fuse, then I'd go 1 A. If you use a slow-blow, then sure.. something smaller would also be fine. I don't think I'd go crazy about the fuse though. Anything near or under 1 A is not likely to be a fire hazard. if you want, short things out and see how it goes. But I think you are very fine. \$\endgroup\$
    – jonk
    May 22, 2022 at 7:49

2 Answers 2

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AGM batteries do require venting. No one would recommend using one in a low/no ventilation area/container, even if you are not charging it in that area. May be better off putting it in a box outside of your tent.

If your usage is only 18 mA plus the converters current, then oversizing the fuse isn't ideal. Cheap auto fuses in small sizes like 0.5 A are available at most auto or hardware stores, but you can get other sizes online.

That said, a dead short will be significantly larger than 1 amp so it's unlikely to make a practical difference.

Practically speaking, instead of getting a AGM battery and converter get a 10 Ah usb power bank that has a trickle charge feature (because of the low current). And a usb cable to cut up plus a 1n4001 diode to drop the voltage down a bit. Wire it in series with the light. Simpler than the AGM.

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  • \$\begingroup\$ Thanks for your answer. The power bank suggestion is very interesting. In that case, perhaps it would be better to just use these little LEDs that attach directly to a USB port, and just attach 2-3 of them to the powerbank. \$\endgroup\$
    – ElectroNewbie
    May 22, 2022 at 7:47
  • \$\begingroup\$ I assume powerbanks do have some sort of short/obercurrent protection, don’t them? Even the cheap crappy ones? :) \$\endgroup\$
    – ElectroNewbie
    May 22, 2022 at 8:06
  • \$\begingroup\$ @ElectroNewbie, I can't speak for the really crappy ones, but all powerbanks should have some kind of BMS (battery Management System), which includes overcurrent protection. \$\endgroup\$
    – StarCat
    May 22, 2022 at 8:11
  • \$\begingroup\$ @ElectroNewbie most should. A DW01 or better ic tends to be used, as a battery protection ic including overcurrent detection. The multiple cell ones that are 10A or better tend to do much better, but obviously you get what you pay for. \$\endgroup\$
    – Passerby
    May 22, 2022 at 8:11
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    \$\begingroup\$ @Passerby just connected the lamp to a crappy 2000mAh powerbank I had lying around. Works like a charm. Will put the diode for dropping to 4.5v aprox. The charge should last for about 12 nights. No worries about deep discharging a lead acid battery. No worries about venting. No worries about fuses. Bigger powerbank, more nights. Thx to you all! :) \$\endgroup\$
    – ElectroNewbie
    May 22, 2022 at 8:57
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I have a lot of LED experience that I can share from the last 20 yrs.

Simple version

  1. Choose a Battery with adequate Ah capacity that is < 25% above the LED voltage if you would like to have a simple resistor & LED circuit. This tradeoff is to simplify the electrical and size. The moderate intensity variation that tells you when to conserve your power from gradual dimming.
  2. Choose the biggest White LED you can find as these also have the highest efficacy. You can run the high power 160 lumens per watt 330 ~350mA types on 20 mA and it will be closer to 200 lumens per watt and lower voltage to use more capacity in a matched battery.
  3. Good Alkaline batteries in bulk are still the best bang for the buck with > 2Ah. Many big box stores sell them by the hundred, but they don't have the Eveready 10 yr shelf life. Expect 2 yrs, hope to get more.
  4. Li Ion 18650 batteries are coming down in price with 3Ah capacity for a useful range from 3.7 to 2.5

My Recommendation

A small 1W finger-size flashlight with a single Li-Ion cell modified with a resistor to draw only 20 mA @ 2.5V (Est.) +/-0.05 and 350 mA @ 2.8V +/-0.2. The accuracy of voltage improves at lower currents since the efficiency goes up by eliminating bulk resistance loss that accounts for all LED tolerance variations in Vf.

Or the battery socket and LED and solder then hot glue the base.

Since the bulk of Li IOn battery life at low currents is 3.7 to 3.8, use that to calculate or measure series R to desired drop voltage to 2.5V . e.g. 1.2V/20mA= 60 Ohms. Choose a few options around there.

The torch lights with a Li Ion cell usually use a DCDC efficient regulator in constant current mode to power similar high efficacy LED's.

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  • \$\begingroup\$ hilarious facebook.com/agt/videos/232383298935972 \$\endgroup\$
    – Tony Stewart EE75
    May 22, 2022 at 11:41
  • \$\begingroup\$ Thank you very much for the time you spent to share your knowledge. I still don’t understand all your points but will try hard. As for the video… WTF!?!? :))))) lol \$\endgroup\$
    – ElectroNewbie
    May 22, 2022 at 15:35

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