Dropping voltage from a Li-ion battery - Page 1
Forum section: electronics / beginners

author: Cricri
author_rank: Contributor
timestamp: December 24, 2018, 12:25:12 pm
content: 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

author: spec
author_rank: Frequent Contributor
timestamp: December 24, 2018, 04:45:42 pm
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: December 24, 2018, 05:35:33 pm
content: 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

author: Cricri
author_rank: Contributor
timestamp: December 24, 2018, 06:00:10 pm
content: 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).

author: Ian.M
author_rank: Super Contributor
timestamp: December 24, 2018, 07:00:12 pm
content: 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.

author: Cricri
author_rank: Contributor
timestamp: December 24, 2018, 07:56:47 pm
content: Thanks Ian, I'm saure that even with 2 dioades, the battery will still last longer than the 2x NiCd!I've used the same charging board to recharge several Li-ion batteries, including a Kindle one that was well discharged and that I subsequently disposed of because even after recharging it was losing its charge quickly, so I kind of trust those boards.Besides, I never leave anything charging while I'm not in the house, better be safe than sorry!

author: spec
author_rank: Frequent Contributor
timestamp: December 24, 2018, 08:00:53 pm
content: Quote from: Cricri on December 24, 2018, 06:00:10 pmI would think that a 1N400x diode would be fine running from a battery. Rectifier diodes are pretty rugged and will handle high current surges, as shown on the 1N400x datasheet:https://www.vishay.com/docs/88503/1n4001.pdf

author: Audioguru
author_rank: Super Contributor
timestamp: December 24, 2018, 10:54:39 pm
content: Ni-Cads are ancient. Haven't you tried name brand new Ni-MH AA or AAA cells instead?

author: Ian.M
author_rank: Super Contributor
timestamp: December 25, 2018, 12:38:21 am
content: NiMH cells don't tolerate high trickle charging currents, and moderate over-charging as well as NiCd cells, so cruder NiCd chargers that lack effective end of charge detection are not NiMH compatible. There is no reason to suspect that a Wun-Hun-Lo motorised gadget would have a high quality smart charger.

author: spec
author_rank: Frequent Contributor
timestamp: December 25, 2018, 03:52:18 am
content: Just a little more information for anybody using the series diode/zener diode method to reduce voltage.Some loads do not like the asymmetrical source impedance that this arrangement gives. The answer is to place a large electrolytic capacitor (1mF minimum, for every amp of load current) across the load. This provides smoothing/decoupling for low to medium frequencies. If you have a load that operates at high frequencies, like logic or opamps, also put an X7R dielectric ceramic capacitor (large thru hole type, not surface mount) across the load, to provide high frequency decoupling. The minimum value of ceramic capacitor is 100nF, for loads of 0mA to 100mA, and add an additional 100nF to the value for every additional 100mA of load current. So, for example, a load current of 1A would require a 1uF ceramic capacitor.

author: Cricri
author_rank: Contributor
timestamp: December 25, 2018, 03:24:58 pm
content: @Audioguru: I don't know how they manage to still procure NiCd, but I suppose that if you are willing to save every penny, then manufacturers will be happy to sell them to you.I didn't want to switch to NiMH firstly because the charging circuit for the original would fry them (as Ian mentions), and secondly because I didn't want to have to unscrew the device sheel every time I need to access them.@spec: good point, when I did it on the Dymo printer, it was right off the transformer, so I presume there were some caps somewhere anyway.For this cheap device I'm thinking off, it's all surface mounted, so there probably isn't a big electrolytic cap already in there, so I might put one in. It's pretty much just a cheap DC motor though, so I won't bother with the ceramic cap. Does it matter if I put it between the battery - and the + before the diodes, or after the diodes?

author: 6PTsocket
author_rank: Regular Contributor
timestamp: December 26, 2018, 06:49:29 am
content: In defence of NiCd's, I have them in a number of devices like an old 4.8 volt carpet sweeper, 7.2volt B&D dust buster, a 14.4 volt drill, an electric tooth brush and a beard trimmer. I may be stuck in the past but they all work and a few are on their second set of batteries. They are still used in many non critical, inexpensive applications.According to Battery University, the educational site devoted to all things battery, the NiCd memory effect was a problem solved in the very early days of NiCd production and the real cause of the problem is something else entirely. NiCd's on trickle chargers are continuously over charged and loose capacity. Then again all rechargeable batteries loose capacity over time.There is a new generation of low self discharge AA and AAA NiMh batteries with high capaity and high discharge rate ability that give lithiums a run for their money. I woukd use those for replacement of similar size NiCd's. They will work on the old NiCd/NiMh charger. One of the manufacturers is Tenergy, the other brand eludes me.Sent from my SM-G900V using Tapatalk

author: spec
author_rank: Frequent Contributor
timestamp: December 26, 2018, 11:53:03 am
content: Quote from: Cricri on December 25, 2018, 03:24:58 pmNo, capacitors almost certainly won't be needed to make the motor run OK. But they may be needed for noise suppression.The capacitors must go across the load, that is after the voltage dropping elements (diodes).

author: spec
author_rank: Frequent Contributor
timestamp: December 26, 2018, 12:07:35 pm
content: NimH batteries have been mentioned in this thread, so here is a point about them. Out of all the common battery technologies: lead/acid, NiCad, NimH, LiIon, NmH are the hardest to charge properly. And the reason is the very slight drop in voltage (-dv/dt) near the fully charged state. This means that you have to charge NiMH at a higher current than you would otherwise like to get a measurable -dv/dt, and even then it is missed. I have a top of the line NimH charger and even that gets the charging wrong from time to time. As for cheap chargers- forget it.Of course, the easiest batteries to charge are the LiIon types. They have revolutionized battery power technology, in spite of some of the messages of doom you see on the net.

author: 6PTsocket
author_rank: Regular Contributor
timestamp: December 26, 2018, 02:07:52 pm
content: Quote from: spec on December 26, 2018, 12:07:35 pmFunny, I emailed Tenergy on that very subject and they assured me that my Tenergy smart charger, that predates the new batteries would have no trouble detecting the small voltage change. The fact is, there are a lot of devices that use drop in AA and AAA cells that are not easily converted to lithium. These new NiMh cells are something completely different than the older ones.I never thought a cell that catches fire if it is over charged or is permamantly damaged if allowed to discharge too far, is a particularly simple technology. My interest started out when I had a problem similar to the OP. I had a 2.4 volt soldering iron that needed cells. As I do not use it a lot and NiCds do not hold a charge well, I thought of an 18650 but it seemed wasteful to lose1.2 volts of the 3.6 volts in a couple of diodes that also had to dissipate a lot of heat. I had to waste 1/3 of the cell's capacity. There is no room for a buck converter even if one existed for those specs and all that electronics just to change battery chemistry made no sense.There is another lithium chemistry that is 2.4 volts per cell but there is not a lot of info out there. I would need control circuitry that is not on ebay.Sent from my SM-G900V using Tapatalk

author: spec
author_rank: Frequent Contributor
timestamp: December 26, 2018, 03:44:27 pm
content: Quote from: 6PTsocket on December 26, 2018, 02:07:52 pmDon't get me wrong, nearly all of our domestic low-power devices, from TV controllers, to clocks, to radios, to garden lights, are powered by NMH batteries.By new technology I assume you are referring to the Panasonic/Sanyo eneloop type which are excellent. But all the same, they do get into a state on occasions and I have to put them through a couple of discharge/charge cycles to get them 'standardized' again.You do know that you can get LiIon batteries in all sizes, not just 18650. For small applications I use rechargeable CR123 types which are really handy and dirt cheap.

author: Cricri
author_rank: Contributor
timestamp: December 26, 2018, 08:18:46 pm
content: Yes, when NiMH came out, I thought they were God sent. I bought a bunch of Fuji ones on eBay, and gave all my NiCd to a flat mate. And that was more than 10 years ago!Today, even my NiMH leave me unimpressed. My Fuji are long gone, and I swapped them for some Panasonic Infinium, which allegedly hold the charge longer when not in use (which is somewhat correct). But even then, I use them very little because they don't last that long, and they take several hours to charge properly. I used to have a Rezap charger (Australian I think?) and sold it off since I never recharged alkaline batteries. The few alkalines I tried to charge in there leaked soon after, wow, would have thought (besides everyone)? Now I've got a Tronic branded (=Lidl) slow charger, and that's pretty much the best charger I've ever had, but still I only use NiMH AAs in my son's toys because if it doesn't switch off the toy, I can blame it on him, delay changing the batteries ("they need charging") and save money on alkalines. Actually, I also have 4 in my scopemeter in C adapters because I don't use it much, so don't check it often, and I know NiMH won't leak!So when I found these 2 Chinese device THIS YEAR that are still shipped with NiCd, I was at the same time annoyed and impressed that they still manage to source them.Another funny thing about NiMH: people tell me "if the voltage drops very low, they're dead as a dodo". What I did on few occasions was "trickle"charge them in a cheap Sony charger that doesn't seem to do much checking before charging, leave them in there for like 10 minutes, then drop them in my Lidl charger which will then accept them. I know what you're thinking: they won't last. And actually I did have subsequent occasions of batteries in the same Infinium batch to go dead later. But, the weird thing is, I started marking them, and the ones which would later die were not the ones I resurrected! I mean, statistically, they could, but I still have some that I marked as dead years ago that still work like the rest of the crowd! Which suggests that NiMH going very low doesn't necessarily lead to long term damage, which I find puzzling and counter intuitive.

author: spec
author_rank: Frequent Contributor
timestamp: December 27, 2018, 02:21:03 am
content: + CricriInteresting. Before the arrival of LiIon batteries, rechargeable batteries always seemed to be source of problems, and no two batteries of the same type behaved the same. As you say though, NMH was a game-changer when it came out.The NMH batteries around the house and garden often get discharged to a low voltage, but they appear to be working OK, although I have never checked what their actual capacity is. That dodge of forcing some current into a battery, with a low terminal voltage, made me smile- I do the same thing. In the old days I used to revive NiCad batteries that had shorted, and been ditched at work, by momentarily connecting them to a 50V 40A supply. In most cases, a few tries and they came back to life.For new designs though, I now only use LiIon batteries of all sizes and silver oxide, non rechargeable, for micro power applications. I have two identical Casio fx-570W scientific calculators, bought in about 1981, and in nearly 2019 they are still going strong on the original silver oxide batteries: that's 38 years.The beauty of LiIon is its light-weight and relatively high terminal voltage range of 3V7 down to around 2V9, which means you can build circuits that run directly from a single cell. And if you need a higher voltage, or even split rails at a higher voltage, a simple switcher board does the job nicely. With the approximately 1V2 terminal voltage of NiCad, and NMH, it was difficult.The other advantages of LiIon are its consistency, very low self discharge, and simple charging: if you do not need to squeeze the last 10% of capacity out of LiIon batteries, just stick a 4V current limited source across them. This charging regime also extends the life of the batteries greatly.Incidentally, as you probably know, LiIon batteries, unlike other batteries, do not use a chemical reaction to generate a flow of electrons. Instead they rely on ion entrapment.

author: spec
author_rank: Frequent Contributor
timestamp: December 27, 2018, 03:55:31 am
content: + CricriDo you have problems with batteries due to the low temperatures in the Antarctic?

author: james_s
author_rank: Super Contributor
timestamp: December 27, 2018, 05:56:05 am
content: Quote from: 6PTsocket on December 26, 2018, 06:49:29 amThere is a Dust Buster at our family cabin that was installed back in the 80s when they were a new thing. It has sat on the charging cradle ever since and I was surprised to find recently that it still actually works. Doesn't run very long, but it has 30+ year old NiCd batteries in it.

author: Cricri
author_rank: Contributor
timestamp: December 27, 2018, 02:13:11 pm
content: I never said NiCd were rubbish. Ok, I did. But that's only because I think it.I don't think anyone will argue that Li-ion aren't way better, so given the opportunity to change a dead NiCD battery, I'd go for a Li-ion. They are normally expensive, but I work in a big company where people drop their "dead" batteries in bins. So I always check the bin, test the alkaline/coin cells (I've got dozen of batteries that were thrown away but still good), keep them if they are in date (I've got so many I don't take any chance taking ones that will soon leak), and I also take the Li-ion ones. I connect them to a cheap charging board at home, let them charge fully, and then check them again a month later. So far, I only got once a Kindle battery that dropped back to sub 3V after a month in storage, and of course, it went back into the bin. So since I got them for free, I don't mind using them in place of NiCd packs in El Cheapo made in China devices. The only bummer is getting one has big as possible that will fit. For the kid's RC car, I swapped the 700mAh NiCd with a 1700mAh li-ion, because that was the biggest I could fit and there was lots of space. For my shaver, I can only fit the smallest one I recycled, which is only 400+ mAh. That's because rechargeable AA are cylindrical, but li-ion packs are typically thin and flat, so obviously that's a big limitation.spec: on the contrary, Antartica is great for batteries, never heard of the trick of storing batteries in the freezer? When I need to use them, I use the same trick I use in the UK when I get out of the pub late at night and find out the car lock has frozen over. Not only do I recycle electro-magnetic energy sources, but i also recycle heat energy that would otherwise be wasted.

author: james_s
author_rank: Super Contributor
timestamp: December 27, 2018, 07:10:51 pm
content: NiCd are not great by modern standards, but I generally don't convert devices from one chemistry to another unless the original is really inadequate. When a NiCd powered device needs new batteries I normally replace them with NiCd or NiMH so I can continue using the original charger.

author: spec
author_rank: Frequent Contributor
timestamp: December 27, 2018, 09:18:43 pm
content: Quote from: Cricri on December 27, 2018, 02:13:11 pm

author: spec
author_rank: Frequent Contributor
timestamp: December 27, 2018, 09:51:00 pm
content: Quote from: james_s on December 27, 2018, 07:10:51 pmThat is the sensible approach.i was talking to a builder friend the other day and he is in the process of converting his old battery power tools to LiIon and mains- using a PSU. I didn't let on that I knew anything about batteries, but I am interested to know how he gets on.By the way, AFAIK the price/performance sweet spot at the moment in LiIon 18650s is the 3.2Ah, 10A Panasonic NCR18650BD from eu.nkon.nl at Euro3.35

author: Psi
author_rank: Super Contributor
timestamp: December 27, 2018, 10:01:56 pm
content: Do they say NiCa on them? im guessing they are actually NiMh.Best option is to just replace any crap NiCa/NiMh with good quality NiMh like eneloop.Any lithium + switchmode system you add will need to be disconnected to prevent the switchmode draining the battery all the time.And then you have to deal with charging the lithium batteryAnd if you don't do it properly there's a risk it will burn your house down in the middle of the night

author: Cricri
author_rank: Contributor
timestamp: December 27, 2018, 10:17:20 pm
content: FYI, I didn't do my conversion on the shaver. My wife is not an engineer and has this boring "if it ain't broken don't fix it" attitude. Granted, I "fixed" an Android box few months ago that was working alright, and by the time I finished improving it, it was dead. I'll just wait until the NiCd dies and swap them for the Li-ion + 2 x 1N4001 as planned.I was under the impression that NiMH need more gentle care than NiCd, so even if you were "upgrading" from crappy NiCd to a-wee-bit-less-crap NiMH, wouldn't you need to redesign/swap the charging circuit anyway?My first pair of NiMH I ever bought were charged in my NiCd charger, and when I got them out they were piping hot. The fact that they never worked properly was a hint that they didn't appreciate the treatment!@psi:>Any lithium + switchmode system you add will need to be disconnected to prevent the switchmode draining the battery all the time.I'm not sure if that is a problem with my charging boards, I did check the batteries months after recharging them, with the board still connected, and they were still charged.>And if you don't do it properly there's a risk it will burn your house down in the middle of the nightYes, I never leave the batteries charging when I'm not in the house. I'm even more careful when it's a battery salvaged from the bin!

author: Psi
author_rank: Super Contributor
timestamp: December 27, 2018, 10:22:21 pm
content: For low current devices you can swap between using NiCa and NiMh without any problems.NiCa can usually handle higher discharge current so if the device draws 5+Amps from a single cell then changing to NiMh may not be the best plan. That said, you can get special NiMh cells intended for high discharge.

author: 6PTsocket
author_rank: Regular Contributor
timestamp: December 27, 2018, 10:38:41 pm
content: Later, they offered NiMh for longer run time and eventually dropped the NiCd's. Now you can get generic 2000 mah. NiMh Versapak batteries. Since I owned a smart charger and several Versapak trickle chargers, I gutted one of the trickle chargers so it is just a charge stand for the smart charger. I can charge them in an hr. or so at 1.9 amps and it shuts off like a good charger should. With a set of generics a few spare filters and the brush and crevice tool there is no end in sight LOL!!Quote from: james_s on December 27, 2018, 05:56:05 amSent from my SM-G900V using Tapatalk

author: 6PTsocket
author_rank: Regular Contributor
timestamp: December 27, 2018, 10:44:32 pm
content: The eneloop pros can handle a pretty high discharge rateQuote from: Psi on December 27, 2018, 10:22:21 pmSent from my SM-G900V using Tapatalk

author: james_s
author_rank: Super Contributor
timestamp: December 28, 2018, 06:53:50 am
content: Quote from: 6PTsocket on December 27, 2018, 10:38:41 pm