# replacing two D cells with pair of 18650 batteries but need voltage drop

Discussion in 'Power Electronics' started by jon5500, Feb 8, 2021.

1. ### jon5500

72
4
Oct 31, 2020
I have several geiger counters that use pairs of series D cells for 3 volts output. I would like to replace these with pairs of 18650 cells in parallel (3.7V, 3000 mA/hr), but there is the issue of dropping the voltage from 3.7V to 3V DC.

I wasn't sure at first if this was going to work, so I did a test set up overnight with the 18650's in series actually and then used a Drok DC-DC converter just for testing (I had to put the 18650's in series to get high enough input voltage for the converter to work). The converter output was set at 3V and the set up ran fine overnight. However, this is overkill since I am only wanting to swap out the original cells for these and I am inquiring as to a better, simpler way.

I understand silicon diodes placed in series with a battery will drop voltage by 0.7V. This might work, but I don't know if it's the best way for two reasons:

1) Testing with the only diode I have available (1N5400) only dropped a 3.7V battery to 3.3V. Not sure if I could run the diodes in series for greater drop. It doesn't matter if the geiger counters run a little less than three volts, in fact I prefer them a bit under ~2.5-2.7V.

2) I'm finding that the 18650's vary a bit in voltage output. One I had is at 3.7V, but another is at 3.9V. As I said, I would run them in parallel, but is there any danger if the voltages among the cells vary?

2. ### dave9

1,175
315
Mar 5, 2017
You should charge the cells before connecting them in parallel, or discharge to same voltage level, or very nearly so. However, 0.2V difference is not that much to be concerned about.

You should consider them not as 3.7V but rather 4.2V (peak charged voltage) for the purpose of determining if voltage is too high. It is possible your geiger counter would work fine at this voltage but I hate to risk your equipment finding out. You might contact the manufacturer and ask them the max voltage it can tolerate.

Otherwise, the forward drop of your diode depends on the current through it. Yes you can put two or more in series to drop more voltage, and if you're getting 0.4V drop each, 4.2V - 0.8V = 3.4V, is highly likely to be close enough to 3V to be safe, but again 4.2V might be too, just don't want to risk YOUR property.

The other issue is the low voltage side of it, how low a voltage it can tolerate and still run, versus % of Li-Ion battery capacity dropped by wasting 0.8V. It might be the best option for you but I would consider getting one of those tiny switching buck regulator boards off ebay/etc if there is room in the housing for it, as they are only \$1 or so. However their input voltage range is usually around 4V or higher so if using one, I'd put the batteries in series instead of parallel. Example, but there are more to choose from, this just happens to be a smaller design than many:

https://www.ebay.com/itm/Power-Supp...or-Racing-Drone-Step-Down-Module/154042281621

Keep in mind that you don't want to over-discharge the Li-Ion cells as you may damage them. Some say around 2.5V (per cell) is the bare minimum but others would state that 3.0V is a better limit, using a battery protection board, which can also be found cheap on ebay among other places. If you put the cells in series then you need two of them or a single board with two channels to handle both cells, or a less optimal solution is one that takes into account the voltage of two cells in series then depends on the cells having a fairly equal true capacity so they stay near the same voltage during their discharge.

Since you have the diodes, that is where I'd start with two in series for testing purposes, determine if your runtime is acceptable or if you instead want the higher efficiency of a switching buck regulator, but remember, you still shouldn't let the cells discharge below 3.0V each, and this includes if your load has a parasitic drain when supposedly turned off, so I would test for that factor too and not leave the batteries connected until you have determined that.

Last edited: Feb 8, 2021
3. ### jon5500

72
4
Oct 31, 2020
I wouldn't want to try it being Ludlum instruments. That's why I'd rather be a bit on the lower side than higher.

Ok, thanks, then the question becomes diode size. I was thinking of using 2 or 3 (whatever it takes) 1N5400's since they have a 3 amp rating. Of course, I don't have any but the one so would have to order them.

Not too concerned about the low voltage. If it drops too low, the counter won't work. It would only be used temporarily for surveys, checks, etc and not run continuously. I did look at those tiny buck regulators, but the Amazon reviews I read don't give them a good reputation having runaway/ instability issues and so forth. I figured the diodes would be the simplest, safest way. I do have individual chargers for the lithium cells. I'll have connectors set up in such a way as to allow this.

Unfortunately, only the one diode. Will order some more if this type is ok. I was initially looking at D sized 1.5V Li-ion cells which are now available, but they too seem to have an inconsistent reputation and they are expensive at like \$20+ for a pair of cells. Too bad as that would have been the way to go.

4. ### Harald KappModeratorModerator

12,458
2,986
Nov 17, 2011
I recommend you use a low drop 3 V regulator. Or an adjustable one set even lower to 2.8 V (2 alkaline batteries in series will quickly drop below 3 V when operated, so the circuit will work from 2.8 V almost certainly). What is the current drawn from the batteries?

5. ### jon5500

72
4
Oct 31, 2020
I ordered a few of these. 16V max input which means I can again run the batteries in series. The units draw up to 200 mA max from what I have gathered. Difficult to tell for certain as various geiger tubes, etc, will draw different currents depending on how radioactive of a source is encountered too.

6. ### dave9

1,175
315
Mar 5, 2017
I should have asked what the goal was. If it doesn't need to run long and is tolerant of fairly low voltage, then at a mere 200mA drain, I would just use some LSD (Eneloop, etc) rechargeable AA cells in D-adapter sleeves (since I already have some, though the sleeves, are only a dollar or two on ebay or could be made from wood or whatever).

This removes the issue of having a discharge cutoff protection circuit since NiMH are far more tolerant, and less dangerous from damage.

However if you already have a charger with big bays, and are in a location with reasonable supply access, Tenergy does make some LSD D cells... \$13 for a pair in the US, but when you consider cost per mAh, that doesn't work out much different than LSD rechargeable AA cells, and at the moment, there is an additional 10% off coupon on amazon for them, probably making them the cheapest they've ever been for quantity 2.

https://www.amazon.com/dp/B0041TEHMS

What you linked to, it is not better to run the cells in series with an LDO regulator, essentially you will be wasting more power as heat than using to power the equipment. Unlike with a switching buck regulator, an LDO sheds all voltage drop as heat.

If you are okay with that, then I am okay with that, but it's not how I'd do it, and you'll be a bit over 1W heat so use a modest heatsink. I'd run the cells in parallel if using an LDO.

Last edited: Feb 8, 2021
7. ### jon5500

72
4
Oct 31, 2020
I would certainly think that the DC DC buck regulator I used would have used more power overnight than the LDO will, but I still appreciate where you're coming from .

After bringing out the scintillation probe and a test source, I was able to saturate the probe and determine that there appears to be a maximum current draw of 90 mA @ 3 Vdc.

I've never been a big fan of NiMH batteries. I first got a taste of Li-ion with a brand called "Talentcell". I am an amateur astronomer and used a 12VDC, 12000 mA/h cell to power all of my telescope equipment during overnight sessions. I've never had a problem with charging/ discharging and the cells retain their charge for long periods.

I already use the 18650's in an electronic shocker wand I have. I collect various moths/ butterflies during hotter months in net traps, but extraction has to take place by hand. Many times, hornets, yellow jackets, and even cicada killers are attracted to the same bait used for the others, so I can shock them with the wand from a safe distance. The two 18650's were pulled from some dollar store USB cell phone charger units. I extracted the batteries and kept the charger units so that when I want to charge, the units are there. Since initial charge of the wand two summers back, and with frequent use during summer, I still haven't had to recharge the wand. And that's been with the wand stored in an outdoor shed with cold winter temps too.

I got tired of always trying to keep alkaline D's or AA/AAA's around the house. When I need them, they're either dead or already used for something else. None of the geiger counters will be modified so I will always be able to back to standard batteries if I have to. It's not wise to do so with the Ludlum, for example, as the battery compartment is sealed and set up that way to protect against leakage and extreme environments. Any holes would compromise that. I do plan on using some sort of D empty shells (probably from some round wooden posts I have here) and run the wires to connection points on the bottom.

8. ### dave9

1,175
315
Mar 5, 2017
I was assuming you would have a switch between the cells and any type of regulator after them, whether you have to add one or there is a reasonable way to hack an existing mechanical switch on the GC to put the regulation in series after it.

At only 90mA drain, it is no contest to me, I'd use low self discharge NiMH AA cells. I find them extremely handy for so many things so I buy them in bulk. To me it was pretty much a game changer when the LSD version became available, though I do still prefer 18650 for higher current draw equipment, but the majority of things I buy that need the consumer to supply batteries, are AA based besides a few smoke detectors (9V), or remote controls, computer keyboards and clock backups (AAA). I have LSD NiMH AAA for most of those.

A good charger is critical for a good NiMH user experience, but so is picking a high quality brand.

Last edited: Feb 8, 2021
9. ### jon5500

72
4
Oct 31, 2020
Interesting. I guess I didn't read this carefully enough the first time. Are you saying that this LDO will use more power than the dc-dc buck regulator I had tested with? And it's going to need a heatsink going from 3.7-4V down to 3V @ 100 mA draw? There's not going to be a whole lot of room unless I mount everything externally.

10. ### dave9

1,175
315
Mar 5, 2017
Yes, any LDO or other linear regulator, does not "convert" voltages, rather it just shunts excess voltage to ground with the associated waste of power = heat associated with that. Their efficiency depends on how much above the target voltage, the input voltage is, so ideally you want to have input voltage minus regulator forward drop voltage (at needed current which is on the datasheet) as close to output voltage as possible to minimize losses, while a switching regulator generally gets closer to 85% efficiency (more or less) regardless of the I/O voltage difference (within reason, very large changes can't help but suffer some additional loss).

I stated it would need a heatsink if you used two cells in series so you had an initial 8.4V dropping to 3V, but this also within the context of the initial 200mA stated, but now with your lower measurement, it depends some on which package your regulator has since you linked to a datasheet that shows several variants of that.

yes, there's usually not a whole lot of excess room in a product, except back in the old days when they just sourced some random rectangular box to throw everything into.

Even so, the switching regulator boards (or similar) that I linked previously are quite small, can be shoehorned into a lot of products but yours, I don't know since it isn't cracked open in front of me.

I still don't see why you are going down this far more difficult path when you STILL need an overdischarge preventative protection circuit too if you use Li-Ion cell(s), not just a buck regulation circuit. It's more work and possibly strapping something on external when all you really need is a couple AA, or C or D cell w/size adapters, LSD NiMH cells. They were developed to replace alkalines with minimum fuss, and seem to meet that goal in your case, since you have not expressed some situation where every last little bit of power density or runtime matters.

Last edited: Feb 9, 2021

72
4
Oct 31, 2020

72
4
Oct 31, 2020
13. ### Harald KappModeratorModerator

12,458
2,986
Nov 17, 2011
That will create a lot of wasted power in the regulators as they need to drop around 4 V (compared to about 1 V in parallel connection).

14. ### dave9

1,175
315
Mar 5, 2017
The EBL you linked in the post after this one quoted, have very high self discharge rate. I'd only use them for things you plan to need the most capacity out of possible, within a week of charging and even then, I've never had good luck with EBL batteries as far as # of recharge cycles before their capacity dropped like a rock. I can't explain how they get such good reviews unless they have a marketing team making those plus a lot of people who have never tried better batteries so are only constrasting with the cost of perpetually buying alkalines.

The Powerowl is a piece of junk, offensive to me that they build that many bays into something they don't even include a power supply with and want you to use USB. Yeah, USB to simultaneously charge EIGHT AA cells? They state it uses trickle charge as if some kind of virtue but all it really means is they built it the cheapest way possible and it will overcharge the batteries as it can't use Delta -V charge termination at the low rate it has to have with # of bays on USB 5V input.

I would instead recommend a major brand charger capable of not only NiMH but also your 18650 cells, but I had assumed that if you had an 18650 charger that it would be backwards compatible with NiMH as all the better ones are. Best value would be a 4 bay charger that comes with a power supply, and a digital display, that tells you IR, has a discharge function, etc. and at least a 1A charge rate for the larger cells you might want to use. Decent AA can also use a 1A rate but it's a bit much for AAA.

Now that other link, Tenergy AA. I've had good luck with that brand, and they were a low enough cost alternative to Eneloop for me to bulk a 24 pack of them, but only because they were very inexpensive per cell at the time, \$26 for a 24 pack about 5 years ago. They work acceptably, not quite as good as Eneloop but at a better cost per cell. However, they are slightly larger diameter so may not fit in devices with tight battery bays.

However it is easy to suggest paying more to get nearer max performance and lifespan out of batteries when your initial need was just to power a 2 x cell, 3V device that it turns out uses only 90mA, so it is hard to suggest what is the best value for you long term versus the short term of only needing a couple cells to get the job done. I'll at least state that I wouldn't buy a high bay count, low quality charger, that if you are going to use that many batteries it is worth getting a better charger. If you cut too many corners in product quality, you are far more likely to have a negative experience again with NiMH. There is an Eneloop multi-pack that includes AA, AAA, and a pretty low feature, but at least smart Delta -V type charger so that would be my conservative, cheapest way to a fair outcome suggestion unless you are patient and wait for product discounts. Time spent, could be worth more than the cost difference.

The best charger value at this moment in time, I couldn't tell you as I tend to wait and buy on amazon when I see a great deal with a discount, but the folks over at candlepowerforums should have some idea what is the current best value on a smart, dual chemistry (NiMH and Li-Ion) charger, and where to get it cheapest if you can wait on a slow delivery from a Chinese merchant.

Last edited: Feb 10, 2021
15. ### Audioguru

3,616
763
Sep 24, 2016
I figure that LSD Ni-MH cells are Low Self Discharge.
Sanyo/Panasonic Eneloop batteries are expensive.
I am glad to find that Tenergy Li-Ion batteries are not the Chinese Ternigy Li-PO batteries I had a bad experience with and Chinese low capacity Duracell Ni-MH batteries.
I recommend Energizer NI-MH batteries that are sold with good value all over North America and are made in Japan (maybe re-labelled Eneloops).

16. ### jon5500

72
4
Oct 31, 2020
I ended up ordering this:

https://www.amazon.com/Panasonic-K-...charger&qid=1612975936&sr=8-7&tag=googhydr-20

I certainly have enough other items around here also needing batteries. Up until now, I've always used lithium but even those seem to drain fast especially with digital cameras. At like \$9 for four AA's, cost ads up quickly. This kit also includes the AA to D adapters I'll need for the geiger counter. Dave had a good point about shelf life. These are touted to have ten years.

One question though, what about possible leakage? Almost every traditional battery I've owned, that I have forgotten about, has ended up leaking, many times into the contacts. It's not too bad of a baking soda cleaning job, but not everything fairs so well with leaking batteries. Just wondering how these will do. With a "10 year" shelf life, I assume leaking is not an issue.

17. ### dave9

1,175
315
Mar 5, 2017
^ I've never had a single NiMH cell leak in use, though one time I bought some old stock no-name NiMH for dirt cheap from an electronics surplus site that were already leaking in the blister pack... so it is possible, but pretty unlikely. I do mark my cells so I keep them in groups of same age and purpose, so they are more matched in remaining capacity than if I just grabbed two random loose cells out of a drawer.

I do have Eneloops more than 10 years old, with a noticeable degradation in remaining capacity, so I use them in very low drain devices like computer keyboards and mice and still get over a year between charges. I used to use them in TV remotes too, still do but now have an android TV with a bluetooth linked mic in the remote, that needs fresher cells for acceptable duration... and because [email protected]#\$ TV manufacturers are trending towards no longer putting more than one or two buttons on the set itself. I guess that started decades ago but my newest TV, ONE button, and it doesn't even turn off, just shuts the screen off. Progress...

Last edited: Feb 10, 2021
18. ### Sunnysky

495
130
Jul 15, 2016
HT7330 3V LDO has a 100mV dropout @ 40mA and draws only uA's

19. ### jon5500

72
4
Oct 31, 2020
Thanks for the info. Well, after Dave's response concerning the LDO, I tried cancelling the order with Mouser in order to redirect funding to the Eneloop kit, but wasn't able to. It's apparent Mouser doesn't check their email frequently or even has a phone system set up to do cancellations unlike other places I've dealt with. I work shifts, so calling after midnight is the only option. So, needless to say, the 3V LDO's along with associated capacitors, did arrive the other day. Rather than face restocking/ return fees, I decided to keep them and may try one out on an old civil defense CDV-700 unit I have. There's significantly more "play around" room in their interior than the Ludlum so it would be easy to put an LDO in there, have it switched on and off via the main power switch, and even add a small heatsink to the LDO if I have to. However, probably not. Since the CDV also takes 2 D cells, two of the D to AA adapters will do nicely. The only thing I might change on the CDV is the horrible existing D cell holder. Horrible contacts, but wouldn't be that difficult to switch out to a modern dual D holder or even dual AA since the unit draws very little current.

20. ### jon5500

72
4
Oct 31, 2020
Ok, so back, and maybe Dave can help with this. It has come to mind that I have several 9V battery devices that could benefit from the switchout to the Eneloops but I don't see Eneloops in 9V, at least not on Amazon unless I'm missing something. I did see some 9V in the brand that Dave recommended for the other batteries, Tenergy, but the reviews on those are horrible!

Needless to say, back for 9V good rated LSD's as well. Recommendations welcome.