4xAA batteries, 4.5V, No components? - Page 1
Forum section: electronics / beginners

author: igendel
author_rank: Frequent Contributor
timestamp: June 15, 2023, 12:09:11 pm
content: [Edit: This question has been answered:https://www.eevblog.com/forum/beginners/4xaa-batteries-4-5v-no-components/msg4910851/#msg4910851, thanks! ]Hi,I was asked to design the cheapest possible device (think Dollar Shop gadgets), with the following restrictions:1. It has to have 4xAA alkaline batteries (for mechanical/balance reasons)2. It powers a standard LED chain that works on ~4.5VMy initial thought was to connect three of the batteries in series, leaving the fourth entirely unconnected, as a simple weight. After all, I was not asked to make the device energetically efficient!But I've been wondering, what would happen if I connect two of the batteries in parallel, and havethatin series with the remaining two ("AB-C-D"). The internet is full of warnings not to mix and match batteries in such configurations, but this is a unique case where all the cells are of the exact same chemistry and voltage - it's like 3 in series where one of them has higher capacity. I guess there will be some waste, but is there any actual danger? Chance of overheating? Also, what happens if a client decides to put in rechargeable AAs instead of Alkaline?Thanks,

author: mariush
author_rank: Super Contributor
timestamp: June 15, 2023, 12:33:10 pm
content: You can get step-down regulators for pennies .. for example M3406 costs 2 cents if you get 200+ :https://www.lcsc.com/product-detail/XI-AN-Aerosemi-Tech-M3406-ADJ_C83224.htmlInductor's another 2-3 cents and the surface mount resistors are nothing.May want to add a diode to drop some voltage and also act as reverse voltage protection (these regulators are good for max 6v in, with 4 alkaline that may be up to 1.65v fresh out the pack and no load you could hurt the regulator.Alternatively, spend 5-10 cents more and get a step-up (boost) dc-dc converter to boost 2 batteries to 4.5v - you save money on the battery holder and batteries if you have to include only 2 in the pack instead of 4example :https://www.lcsc.com/product-detail/XI-AN-Aerosemi-Tech-MT3608L_C2932326.html

author: Jeroen3
author_rank: Super Contributor
timestamp: June 15, 2023, 12:50:21 pm
content: With A|B+C+D bateries A and B would be discharged only half. Eventually batteries C and D would reverse polarity. (yes they can do that)To get from 6 to 4.5 just add one or two high Vf diodes?

author: Gyro
author_rank: Super Contributor
timestamp: June 15, 2023, 12:58:53 pm
content: Quote from: igendel on June 15, 2023, 12:09:11 pmI'm sorry, but what completely ridiculous ideas! Totally wasting one battery or creating uneven life of the batteries by series/parallel connection is pointless.Something that you clearly haven't considered is that Alkaline batteries only put out 1.5V for a relatively short part of their discharge curve. The end point voltage (to get full life) is 0.8 - 0.9V depending on load current. Even with all four batteries in series, your end point would only be 3.2-3.6V.As mariush points out you can get really cheap switching or LDO linear (yes, it does need to be LDO or you will waste battery voltage) regulator ICs for very little money. Obviously the switching regulator will require a few more external parts for the benefit of better efficiency.When designing something (sounds like a product), you need to put some thought into the process. Even Dollar shops expect some sort of intellgence to be put into the design.EDIT: If it really is just a simple LED chain, then just use all four batteries and a series resistor to limit the maximum current with new batteries. Then at least the LEDs will simply dim down and go out as the batteries discharge below their combined minimum forward voltage drops.

author: igendel
author_rank: Frequent Contributor
timestamp: June 15, 2023, 02:05:10 pm
content: Thanks for the answers, maybe a little more explaining is in order :-)First, if you saw what this gadget is supposed to be and do, you'd agree that I am still the wisest link in that specific chainSecond, I'm not as clueless as the above question may suggest - I've designed proper circuits before. But I don't know the theory of battery inner working so I posted this in "beginners".I've seen some horrible cheap gadgets out there; a good example is an LED "torch" where the very thin wire from the batteries was used as a current-limiting resistor. That is positively dangerous. I was just curious, since my client is so determined to have minimum components (let alone a PCB), what would actually happen to the batteries. Again, in terms of function, the waste of power is unimportant.

author: DavidAlfa
author_rank: Super Contributor
timestamp: June 15, 2023, 05:14:04 pm
content: "I want gas engine that runs on electric power", this is how this sounds.A led needs some sort of current limiting anyways, aka "electronics", at least a simple resistor.Try to get one of these led chains, it might work at 6V with no problem after all.

author: igendel
author_rank: Frequent Contributor
timestamp: June 15, 2023, 05:58:14 pm
content: Quote from: DavidAlfa on June 15, 2023, 05:14:04 pmOff-the-shelf LED chains and strips usually have resistors built-in. But given the quality of those cheap products, I'm afraid that 6V will fry an LED, and that would be worse than poor use of batteries.Of course, I could ask my client for a few samples and do a "stress test". Perhaps with a diode in series to drop a little voltage just in case

author: Nusa
author_rank: Super Contributor
timestamp: June 15, 2023, 06:07:03 pm
content: Quote from: igendel on June 15, 2023, 05:58:14 pmIf you're willing to use a diode, why not just add a an appropriate value series resistor instead? It does exactly what you're asking for, after all! You won't find a cheaper solution.

author: igendel
author_rank: Frequent Contributor
timestamp: June 15, 2023, 06:15:12 pm
content: Quote from: Nusa on June 15, 2023, 06:07:03 pmBecause my original question, that got a bit lost in this discussion, is simply what will happen to four AA batteries in a 2-1-1 configuration...

author: Gyro
author_rank: Super Contributor
timestamp: June 15, 2023, 06:16:51 pm
content: Quote from: igendel on June 15, 2023, 05:58:14 pmIf you don't know what LED chain you're going to be using then it's rather difficult to give advice... or design it. Providing as much information as possible (I note your previous reply) saves peoples' time all round.Using a series diode will only make the LEDs drop out earlier as the batteries age. Getting some idea of the LED chain construction (yes of course you should get some samples from your client) is essential to any design to drive it. You don't actually know whether it contains resistors or not. You don't know what the LED configuration is, you'll only get 2, maybe 3 if they're Red, LEDs in series on 4.5V so talking of a 'chain' implies parallel or series-parallel configuration. If you establish the current at 4.5V then you will know what value resistor to add to limit maximum current at 6V.Go and download some battery datasheets, you can find them on the Duracell and Energizer websites. They will allow you to predict useful (light producing) life.

author: Gyro
author_rank: Super Contributor
timestamp: June 15, 2023, 06:19:08 pm
content: Quote from: igendel on June 15, 2023, 06:15:12 pmYou will end up with the crappiest possible design, that will eat batteries at different rates, have poor battery life and possibly cause them to leak. That ought to be sufficient for you.

author: igendel
author_rank: Frequent Contributor
timestamp: June 15, 2023, 06:29:43 pm
content: Quote from: Gyro on June 15, 2023, 06:19:08 pmTo what degree of crappiness, that's the technical knowledge I seekQuote from: Gyro on June 15, 2023, 06:19:08 pmMy client's problem; I warned about this as soon as I heard his wish to have "zero extra components". And I won't be signing my name on this thing.Quote from: Gyro on June 15, 2023, 06:19:08 pmAh, ok, now that's an interesting point. Can you please give a hint about the mechanism behind this?

author: coppercone2
author_rank: Super Contributor
timestamp: June 15, 2023, 06:36:37 pm
content: lets see the device will draw substantial amounts of current because two batteries will drop but not the third in seriesI think you are imagining that the batteries ESR will magically rise when they are depleted and block the circuit but that wont happen so long there is potential for current flow from the pairthen two batteries will drastically drop in voltage and get to a voltage range where the cell is generating hydrogen (deep discharge) and hydrogen builds up and it leaks, but the device will maintain operation because there is still enough voltage from the doubled up battery pair, and there will be no warning to the user that 2/4 cells are depletedit sounds like you are dealing with a very cheap old person that thinks everything is a open dry cell from 1935

author: Gyro
author_rank: Super Contributor
timestamp: June 15, 2023, 06:41:42 pm
content: Quote from: igendel on June 15, 2023, 06:29:43 pmOh, stop trolling. There are real beginners who read this section in the hope of learning something.Quote from: Gyro on June 15, 2023, 06:19:08 pmQuoteThat shouldn't matter, it's no excuse to not do a respectable job.Quote from: Gyro on June 15, 2023, 06:19:08 pmQuoteYou already covered that yourself - accidental reverse insertion of a battery (potential fire risk), different battery states etc. Are you applying any thought to this.

author: coppercone2
author_rank: Super Contributor
timestamp: June 15, 2023, 06:43:54 pm
content: the solution is to solder a wire across one of the battery terminals because this sounds like its a enclosure already made issue. then you can use 3 batteries in exchange for a piece of wire and two solder joints. I have seen this done in ultra cheap blue tooth speakers from like 15 years ago, they just left empty slots with no contacts because I guess the power requirement changed after mechanical had their way.if you solder a wire in there they won't be able to put a battery in. If you can just remove the contacts and rewire it underside it would be better though. And put some hot glue over the wire that is inside of the battery slot, so they don't think its a mistake.

author: igendel
author_rank: Frequent Contributor
timestamp: June 15, 2023, 06:58:13 pm
content: Quote from: coppercone2 on June 15, 2023, 06:36:37 pmI see. Thank you, that's what I was missing - the technical explanation behind the warning "don't do it". Case closed(p.s. I can't just solder a wire instead of one battery because the battery's mass is needed for physical balance)

author: TimFox
author_rank: Super Contributor
timestamp: June 15, 2023, 07:01:26 pm
content: You can make a dummy battery from a plastic tube and wire, adding whatever weight is needed for balance.

author: igendel
author_rank: Frequent Contributor
timestamp: June 15, 2023, 07:04:32 pm
content: Quote from: TimFox on June 15, 2023, 07:01:26 pmHeh, tell that to the client who's wary of the cost of a single diode or resistor

author: Gyro
author_rank: Super Contributor
timestamp: June 15, 2023, 07:10:45 pm
content: If you have the battery contact wiring capability to be able to configure them in silly combinations, then you have the ability to replace one of the interconnections with a resistor. No added effort and effectively zero cost.

author: TimFox
author_rank: Super Contributor
timestamp: June 15, 2023, 07:12:59 pm
content: Quote from: igendel on June 15, 2023, 07:04:32 pmThen use the wrapper from a stick of Wrigley's gum.

author: Nusa
author_rank: Super Contributor
timestamp: June 15, 2023, 08:45:30 pm
content: How many is he planning to make? You can get a full reel of 5000 1/4 watt through-hole resistors (lcsc.com for example) for about $5 (plus shipping). Per unit, that's literally nothing compared to the cost of any other part in the unit, never mind the labor to assemble them.

author: Gyro
author_rank: Super Contributor
timestamp: June 15, 2023, 09:27:34 pm
content: Just by way of comparison, this is an example the sort of stuff that is coming out of China and going into Dollar stores. Ignoring the solar charging bit, it uses a single NiMh cell (included), a boost converter, and a microprocessor, driving multiple LEDs to produce a realistic flame effect.If your client is wary of putting a single resistor in his product then it, and your relationship, ain't going anywhere!

author: james_s
author_rank: Super Contributor
timestamp: June 15, 2023, 09:50:00 pm
content: Quote from: igendel on June 15, 2023, 06:29:43 pmA completely unusable degree. It will very likely destroy someone's expensive rechargeable batteries by reverse charging the weakest cell. If you have one cell that is two in parallel, that pair will always be the strongest as the other two cells drain, and the first cell to drain all the way to zero will be charged backwards by the stronger cells. It's hard enough to avoid this even with a sensible arrangement of single cells in series.

author: james_s
author_rank: Super Contributor
timestamp: June 15, 2023, 09:51:25 pm
content: Quote from: TimFox on June 15, 2023, 07:01:26 pmWhy bother even doing that? Just pop the contacts out of that location in the holder and attach the wire to the contact on the 3rd cell instead.

author: igendel
author_rank: Frequent Contributor
timestamp: June 15, 2023, 10:02:09 pm
content: Quote from: Nusa on June 15, 2023, 08:45:30 pmI guess he needs these extra $5 for a packet of Wrigley's gumI think he's thinking about several thousand units, not sure - see below.Quote from: Gyro on June 15, 2023, 09:27:34 pmTo be fair, the video title does say "Surprisingly good"I do like bigclives' videos, thanks.To make things clear, I did not sign any contract for this odd job, and so far did not invest anything in it beyond this thread. The general idea for the product was thrown at me, and it made me notice a gap in my understanding of batteries. Now with the help of your feedback I can explain to the client, more confidently, that there's no clever loophole to be exploited.

author: wraper
author_rank: Unknown
timestamp: June 15, 2023, 10:07:10 pm
content: Connect two parallel battery pairs in series to get 2-3V during their discharge cycle. And use a simple boost converter/LED driver IC to boost it to required voltage, the best would be to use current feedback and ignore actual output voltage.

author: Gyro
author_rank: Super Contributor
timestamp: June 16, 2023, 08:15:54 am
content: Whilst I agree on the boost converter and driving LED current mode, the paralleling of cells in a battery holder should be avoided. There is too much risk of customers reverse inserting a cell (most devices use alternate up and down). As previously mentioned, rechargeable cells, and possibly some alkalines, will source enough current to create a fire risk. There is the issue of possibly mismatched cells too.

author: wraper
author_rank: Unknown
timestamp: June 16, 2023, 10:11:11 am
content: Quote from: Gyro on June 16, 2023, 08:15:54 amIf cell compartment is not garbage, it won't not make an electric contact when a battery is inserted in opposite direction. Then let's assume it uses 2 cells only. If both batteries are connected in reverse, you will have a similar current flowing through the body diode of a MOSFET in boost converter.

author: magic
author_rank: Super Contributor
timestamp: June 16, 2023, 10:26:04 am
content: That's a big IF, considering what we know about OP's customer. I wouldn't even waste my time dealing with such a moron.What happens in the original scenario: the two series batteries discharge first, leaving the two parallel at 50% charge each. Output voltage is ~1.5V and the gadget stops working. If the LED still conducts anything at such low forward bias (but I think it really won't) then there is a risk of the parallel cells reverse charging the depleted series ones, with all the fun consequences.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 16, 2023, 10:38:47 am
content: Standard alkaline batteries have a voltage drop when they discharge, and they have useful energy in them until they are discharged to around 1V. This means that 3 batteries in series is not enough if you need 4.5V on the output, and even with all 4 batteries in series the output voltage will drop below 4.5V before the batteries are completely empty. I guess you get about 90% to 95% of the energy out of the batteries when you put them all in series.Download some datasheets of batteries to verify this yourself. Dave has also made some EEVblogs about batteries and their voltage drop over their life.But also, this is all under the assumption that your gadget needs the 4.5V you specified. But even when your gadget still "works" down to about 4V, then you still can not get all the energy out of your batteries if you only put 3 of them in series.

author: SmallCog
author_rank: Frequent Contributor
timestamp: June 16, 2023, 11:02:36 am
content: Green wash the product and recommend using rechargeable batteries4x1.2V=4.8V and the LED’s won’t care