Modifying 4.5 V (alkaline) lamp to run brighter with 3.6 V (NiMH)

[seanspotatobusiness]

I have a lamp which takes three D-size batteries. It seems to have been designed to function best with alkaline batteries and is relatively dim (in fact the bright and dim settings both appear the same) using 1.2 V NiMH batteries. I was wondering what circuit I might use to increase the brightness of the lamp without draining the NiMH batteries below ~0.9 V to protect them from damage.

 

[kellys_eye]

Fit a 2.5V bulb....

 

[seanspotatobusiness]

It doesn't use a bulb but three SMD LEDs which are connected in parallel.

This is the circuit that feeds the LEDs (there are no other components on the board with the LEDs). I figured the simplest thing to do would be to add a circuit between the battery and the original circuit which would boost the voltage to 4.5 V but I need it to cut-out at ~2.7 V supply to protect the cells. Would such a circuit continue to use power whilst the lamp was off, though?

 

[(*steve*)]

Use 4 NiMH batteries.

 

[seanspotatobusiness]

The battery compartment is designed for three. A fourth cell would be an awkward thing to add to the outside of the case and I think would leave the cells vulnerable to reversing polarity during discharge, plus the voltage might be too high when freshly charged? It would also no longer be waterproof (I bought it for camping although it's probably seen more indoor use than outdoors).

I have a voltage boosting module that will maintain 4.5 V output while input drops down to ~2 V but that's still risking reversing polarity if my cells aren't well balanced. I regret buying this lamp!

 

[(*steve*)]

Use a number of lithium cells in parallel with a buck regulator. Find one that has a low voltage cutout.

 

[Audioguru]

The thing was designed for only 4.5V. Why do you want to destroy it with a higher voltage?
A higher voltage will cause a higher current which makes little electronic things smoke and burn.

Right now I am modifying a similar product. It is a solar garden light that has a real IC and I have its datasheet. It uses a voltage stepup circuit with an inductor determining its peak output current. It was dim with its original 330uH inductor but I am using an 82uH inductor to make it nice and bright. The datasheet shows that the IC can survive an inductor that produces 3 times higher current than my modification. I replaced the cheap weak Chinese Ni-Cad battery cell with a much higher capacity Western Ni-MH cell.

 

[seanspotatobusiness]

The thing was designed for only 4.5V. Why do you want to destroy it with a higher voltage?
A higher voltage will cause a higher current which makes little electronic things smoke and burn.

4.5 V is the voltage I want to run it at. I never said I want to run it higher than 4.5 V.

I tried a cheap variable voltage boost module set to 4.45 V and it seems to work pretty well. I was originally worried about accidentally overdischarging the cells but the lamp begins to flicker when the cells are nearing depletion so there's clear warning. The lamp is now very bright but the battery doesn't last long. They're fake D cells which actually just contain AA cells and air. Real NiMH D cells are pretty expensive though and relatively hard to find.

 

[Audioguru]

I mentioned a voltage higher than 4.5V because a Ni-MH cell is 1.4V or 1.5V when fully charged. Its voltage drops to 1.2V after discharging for a while. Then if you boost the fully charged 4.5V to 5.6V the circuit might smoke and burn.
You need a regulated 4.5V from the booster that still passes 4.5V through it.

Why do you buy cheap fake batteries? Instead of battery chemicals and air, some might just have potatoes inside.

 

[seanspotatobusiness]

Fresh alkaline batteries can have a voltage of 1.65. I'm not concerned. The voltage from the booster will not go above 4.5 V unless more than 4.5 V are supplied which is not going to happen.

 

[ChosunOne]

Use 4 NiMH batteries.

The battery compartment is designed for three. A fourth cell would be an awkward thing to add to the outside of the case and I think would leave the cells vulnerable to reversing polarity during discharge, plus the voltage might be too high when freshly charged? It would also no longer be waterproof (I bought it for camping although it's probably seen more indoor use than outdoors).

I have a voltage boosting module that will maintain 4.5 V output while input drops down to ~2 V but that's still risking reversing polarity if my cells aren't well balanced. I regret buying this lamp!

I did a simple modification to a hand lamp decades ago, when incandescent bulbs were all we had. The lamp was flat and originally used a 4.5V bulb with 3 cells of a size intermediate between C and AA.. I wanted a brighter light, so I switched up to a 4.5V bulb and replaced the battery with 4 AA cells in a 4-AA cell holder like this one,
https://www.ebay.com/itm/AA-Battery...US/192238085081?hash=item2cc24863d9:rk:1:pf:0
...that fit nicely into the space for the original battery; and I'm pretty sure would fit into the space occupied by 3 D cells, with room left over.

You might have to go with NiCd cells, which start out at 1.2V fully charged, IIRC. You would have to replace the battery more often, but there's going to be a tradeoff somewhere, no matter how you do this.

 

[BobK]

If you want more run time, go with a Lithium Ion battery + booster.

Bob