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

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.

 

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.

 

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.

 

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.

 

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.

 

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).

 

^ 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...

 

HT7330 3V LDO has a 100mV dropout @ 40mA and draws only uA's