Converting electronics from alkaline to Li-Ion?

I need some new projects to keep me busy. I was thinking of converting some electronics designed to run off alkaline batteries to instead take rechargeable lithium ion batteries.

I have batteries lying around from old cell phones, digital cameras, etc that still hold a charge. I also have some very good 18650s from a laptop that went kaput a few months ago.

For 3x alkaline in series, the circuit is expecting 4.5v so 1s should work decently well. For 6x in series, the circuit is expecting 9v so 2s should work okay, yes?

I see some protection circuits like this one:
https://www.ebay.com/itm/6S-5S-4S-3...8650-Li-ion-Lithium-Battery-Cell/272724717408

Can I just hack off the power and ground leads and solder in one of those protection boards along with a battery holder (ex mouser part# 534-1098) and not burn my house down?

I'm aware the electronics are optimized for alkaline voltage, so they may complain about low battery quite early. I'm just mostly wondering if this is possible or if anyone has done something similar. I'd gladly do this if it means never buying an alkaline (better yet, any type of primary) battery again

If you want to do this to avoid buying alkalines anymore, just use NiMH cells like Eneloops. Piece of cake.

If you're doing it as a hobby project, you'd have to make sure the nominal voltage of the battery and the circuit match well enough, that the fully-charged voltage won't cause issues with the circuit, that the battery's mAh and current rating are adequate for powering whatever it is. More importantly, you'll need to make sure the circuit doesn't let the battery over-discharge which will damage LiIon cells (a protection circuit will prevent this). If you're using individual cells in series, such as those pulled from a laptop, you'll want to measure them and make sure they're close in capacity and internal resistance. Recharging would be an issue too, unless you plan on having the cells be removable and charging them separately in a dedicated charger.

What kinds of devices are you thinking of doing this with?

I have some led light strings which run on 3aa in series. Would run for about 4 hours before serious dimming. This Christmas I used a battery holder to put 4 18650s in parallel. Worked great and lasted all night. I'd just charge them with a hobby charger when it dimmed and it worked great

You'll get a lot more out of your hobby projects if you have a little basic test equipment. If you don't have a DMM, even the free or nearly free ones from Harbor Freight and similar outlets are quite reasonably accurate. The switches probably won't last long, but they'll provide decent readings. Another valuable piece of equipment is a variable voltage power supply. A basic one good for a few amps isn't too expensive, and even a one amp unit would almost certainly do for anything except high power flashlights and similar power hogs. With the power supply and meter you can see exactly what range of voltages and how much current your candidate projects require, and choose your batteries accordingly. Be aware that the voltage of any kind of battery drops as it discharges, so try to find one that retains a high enough voltage over nearly its whole discharge time to run your device. That way, you'll get from the battery the full amount of energy it contains. For example, a Li-ion cell is at 4.1 - 4.2 volts right off the charger, but (at low current) it drops to something like 3.8 or 3.9 volts within minutes -- If 4.1 volts is too much for your device, you can discharge the cell just a little before installing it. As the cell discharges, the voltage drops, and when it reaches about 3.0 volts very nearly all the energy is gone. So it would be ideal for use in a device that can function with a voltage over the range of about 3.0 to 3.8 or 3.9 volts.

A very few devices like el cheapo flashlights which run from three alkaline cells depend on the battery internal resistance to limit the current. If you want to fool with these, it's wise to put a resistor in series with the battery, and in series with the power supply while testing. You'll find dramatic changes in flashlight light output with very small changes in voltage when testing these devices, and I'd highly recommend either not messing with them or to put a current limiting resistor in series with any substitute battery.

c_c

vicv said:

I have some led light strings which run on 3aa in series. Would run for about 4 hours before serious dimming. This Christmas I used a battery holder to put 4 18650s in parallel. Worked great and lasted all night. I'd just charge them with a hobby charger when it dimmed and it worked great

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perfect, this is essentially what I'm going to do. thanks!

kpatz said:

More importantly, you'll need to make sure the circuit doesn't let the battery over-discharge which will damage LiIon cells (a protection circuit will prevent this).

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Oh I've got plenty of Eneloops

Will the circuit that I linked be sufficient or do I need something else? Got any examples / alternatives?