Electronics > Beginners

Dropping voltage from a Li-ion battery

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Cricri:
Hi all,
I bought a cheap RC car in China for the kid and found out it had 3xAA NiCd batteries. Needless to say, they didn't last long, took long to charge, and then there's the memory effect thing.
So, you might be outraged, but I'm not a EE (I am a IET member, but I'm actually a mechanical engineer), so I opened my bin of recycled electronics stuff, found a Li-Ion battery I salvaged from the battery bin at work, connected it to the car via a charging board (I think they were 5 for like £2 off eBay from China), and it works great.
Feeling like an experienced EE now, I thought "right, next I have this fabric shaver the wife also bought when we were in China", and when I opened it, low and behold, it uses AA NiCd batteries too. I'm not sure how they still manage to procure those, maybe they found an old stock in a warehouse from the 80's.
Anyway, to the point (finally):
- I saved more Li-Ion batteries from the battery bin at work.
- I've got more charging boards.
- BUT the shaver uses 2 AA batteries, so about 2.5V ish, and of course my Li-Ion battery that is small enough is 3.7V.

So I need to drop the voltage. What I thought was to use a diode (on the load side obviously, not on the charging side), and I checked google:
- I found a thread on Stack Exchange with pretty much the same question. In the guy/gal's case, it's to drive a ESP, so maybe he/she needs a more reliable way to drop the voltage, and indeed the consensus it to use a low drop regulator.
- In my case, it only lights up a power LED and run a small DC motor akin to a toothbrush, so I presume that I can go low tech.
- I'd also like (if possible) to have quite an efficient solution, so I'd rather use a diode than a resistor based voltage divider.
- I don't have LDOs.
- I do have a couple of buck converters from eBay too, but again, I'm thinking that might be overkill.

So would the diode do the trick in this low-tech situation, or do I need something better (I suppose a buck DC step down or a LDO, but I'd have to buy some of the latter)?

Thanks!

*EDIT* I've got 2 DC step down buck converters from eBay, but they are rated for 4.5-28V input, so I don't suppose I can use them either, bugger  :--

spec:
Hi Cricri,

If you use 1N400x, type diodes, each diode will drop 0.6V to 1V depending on the current draw. So I would use two 1N400x type diodes in series to replace two NiCad cells in series. The other common diodes, the small signal, 1N4148, 1N916, 1N914, etc , are not suitable for this application.

Ian.M:
Brushed DC motors aren't *that* voltage sensitive,  20% high or low voltage will just cause a corresponding change in speed.  Therefore  a single diode dropping about 0.6V to 0.7V on load should be suitable to reduce the 4.1V to 3.0V operating range of a LiPO cell to what the fabric shaver motor can handle.  However you need to have some idea of its running current, as the startup surge is typically an order of magnitude greater than the max running current, fully loaded, so may be large enough to blow a 1A diode like a 1N4001 ... 1N4007.   The LED may or may not be happy with the resulting voltage range as on the high end it may get 3.5V and a white or blue LED without a series resistor is likely to blow.

Also, you need to consider undervoltage protection - you need something to cut off power if the cell voltage drops below 3.0V or the cell's life *will* be shortened, and deep, repeated or prolonged over-discharge makes the cell a potential fire risk if you attempt to recharge it.

See: https://batteryuniversity.com/learn/article/safety_concerns_with_li_ion

Cricri:
Thanks guys! :-+
I've got a bunch of used 1N4001 I scavenged from boards dedicated to the skip, so I'll put a couple in series as per Spec's suggestion. In fact, I did that on a 10V power supply that I use on a Dymo labellers working of 6xAA=9V years ago, and it's still going strong.
If the upcycled 1N4001 are to give up the ghost as Ian pointed out the possibility, I'll swap them for some higher spec diodes. I think it'll be fine though, it's not a beefy motor, it's only to shave the sofa not my legs (in which case it'd need a three-phase AC motor anyway)  ;) I'll make sure to use all the heatshrink needed as well as I'm not a fan of electrical tape and hot glue.
No worries about not letting the battery drop too low, I'm the kind of guy who often checks his batteries and recharge them when they need to. Also, I don't have any undervoltage protection stuff lying around, and to be honest, I already need to fit the Li-ion battery, the charging board and a couple of 1N4001 in the handle (as this is where all that needs to fit, although I suppose once I removed the Ni-Cd AAs there should actually be plenty of space).

Ian.M:
Two series diodes will probably drop the voltage too low over much of the LiPO's discharge curve.  e.g, when its down to 3.5V on load, you'll only have about 2.1V at the motor.  OTOH that does make it less likely you'll want to run it till its over-discharged.

On the motor startup current side of things, it would be a good idea to measure the DC resistance of the motor across its terminals, (use a DMM on a low ohms range, and make sure the motor is off or disconnected).  Then you can work out the stall current which is the same as the peak startup current from the measured resistance and the max voltage, by Ohm's law.  Alternatively, use a cheap multimeter with an unfused 10A range (as NiCd cells can deliver quite large surge currents, so you risk popping an expensive fuse if you use a good meter) in series with the existing battery (which needs to be fully charged) to measure the running current, when cropping the pile of a scrap of towelling or flannel.  If its over 0.1A, then 1A diodes are likely to have problems with the startup surge.

You do need to confirm that your 40p charging boards wont do anything disastrous if you connect an over discharged battery so try one on 3x NICd in series, discharged to 0.8V/cell, with a 500mA fuse in series in case it fails badly.  It should just trickle charge it at a few mA, but if it tries to charge at 500mA, or worse, its pass transistor overheats and fails shorted then I'd stop  as if you use it on a LiPO without over-discharge protection, you are basically building an incendiary timebomb.

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