Another 18650 Question

Sorry to bother you folks with what must be some very basic questions but I'm curious. Comparing a 18650 rechargeable with lets say 2400MAH fresh off the charger with 4.1 volts to 2x RCR123A cells rated at 900MAH and 4.1 volts each, does the MAH of the RCR123A's double when combined as does their voltage? I've read elsewhere on the Forum that the difference between using the 18650 as compared to the 2x RCR123A's in a LED flashlight may be as little as 30 lumens, does this make sense since the voltage is almost half when using the 18650? Thank you.

It depends on the light in question if it boost the voltage, or only bucks it. It may mean loss of regulation, or no difference at all.


A 18650 has over 50% more energy than 2xRCR123's.

If you put 2 batteries in serial (end to end, +to-) the voltage doubles but the capacity stays the same thus 2 x 4.1 in serial = 8.2. If the batteries were in parallel (side by side, + to + and - to -) then the capacittance (mah) would double and the voltage would be equal to that of one battery. To double voltage AND capacitance you'd need 4 batteries in serial AND parallel.

As to your question regarding brightness this is impossible to answer with the information provided. Most LED's run at around 3.5V so most LED flashlights use a circuit to raise or lower (boost or bucb) the voltage that is delivered to the LED - some circuits can do both. Thus the question with regards to 1x18650 and 2xRCR123A and total output is flashlight and circuit specific. No lumen count can be assigned because of the factors just described and also because LED's are driven by current, not voltage.

Marduke said:

It depends on the light in question if it boost the voltage, or only bucks it. It may mean loss of regulation, or no difference at all.


A 18650 has over 50% more energy than 2xRCR123's.

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Thanks for the quick response. I like the idea of having greater energy (run time) but I'm not sure how many lumens I would lose in some of my LED flashlights. Currently I have a couple Olight T-20's and the run time with RCR123A's is not very good, so if I could fit a rechargeable 18650 and get a much greater run time without losing too many lumens I would consider the switch. I was also reading up on the new Olight M-20 with 250 lumens which can use either 123A's or the 18650, that is what got me thinking about the pro's and con's.

Ah Matt, you were just the guy I was thinking about when I wrote this question. That new Olight got me thinking batteries and possibilities. I may give in and order soon. I appreciate your input. Thanks.

Yea I don't think any of the current T20s will fit an 18650, I could be wrong on that...

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Most lights that support 2xCR123 or 2xRCR123 but happen to also fit an 18650 will have MUCH longer runtime on the 18650 for 2 reasons:

1. The 18650 actually has about double the watt-hours compared with 2 RCR123s, and slightly more watt-hours than 2xCR123s.
2. Lights that can tolerate this higher voltage input, are almost ALWAYS buck regulated only, meaning that when driven on a single 18650, they will have diminishing output through the run, this results in lower performance, but much extended runtimes, triple to quadruple the runtime that it would get on RCR123s, or close to double that of CR123s.

I'd have to double check but I don't believe the lights design would allow for a retrofit tube. I think a protected 17670 might fit the most current version though - will have to double check.

MattK said:

I'd have to double check but I don't believe the lights design would allow for a retrofit tube. I think a protected 17670 might fit the most current version though - will have to double check.

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Matt, thanks for looking into the possibilities. I was wondering about how such a larger diameter tube could fit into the head and tailcap. I think it could be done by machining a larger diameter tube which would hold the 18650 and simply reducing the end to fit the T-20's. The metal would have to be thicker so that the reduced ends would still be stable but I'm pretty sure this could be done. The end product would be much more versatile and attractive with not only the current slimmer fit of the 123A's but the bold and thicker look and feel of the 18650 along with the longer run times. If I'm not wrong this is what was so well thought of and done with the new Olight M-20 Warrior? I hope we get to see such a product for the current T-20's and if not I'm still happy with the two I have. Just picture a tube with a reducer on either end and an insert at both those ends which would complete the battery contact to the head and tail switch. The final length would be a little longer since the tail end would require a double thread, one to close in the 18650 and one for the tail cap switch. Thanks.

It seems highly unlikely to ever happen on a production scale...

Hi Matt. That's too bad, I thought it would make a nice addition to the Olight line. Thanks again.

Perhaps but they would undoubtedly lose a bunch of money doing it.

Matt, my thinking is sell the accessories to upgrade the products you have in use and improve the products being introduced into the market place. Customers such as myself who own the current T-20's will seriously welcome the versatility of a higher capacity battery because it improves our product, however such a purchase will not replace the features and improvements that come with the Warrior M-20. If I want those higher lumens, I have to pay for them,plain and simple. Many industries make more money in the aftermarket than with the original sale of a new item. Look at Terralux for example and how they used the popularity of the MagLite to their advantage. If I could influence the folks at Olight I'd say manufacture and offer any improvement to your current line that will be attractive to the consumer, be it drop ins, battery tubes, tail cap switches, or any other improvements. I admit I'm no marketing expert, but I am a consumer and the market is competitive. Thanks. Just wanted to add this as an after thought. Looking at this from your perspective it makes good business sense for a couple of reasons. First of all If I can purchase the battery tube upgrade I will need a charger and 18650 battery(batteries), so I will probably buy from Battery Junction. Now that I have the charger and 18650 batteries and I'm enjoying my older T-20's I'm thinking this is great but how about the Warrior M-20? I already have the battery and charger!

I have no doubt that you're right and there would be some interest, particularly among CPF buyers of the T-20. It's not a bad idea at all, the question Olight will ask is if it is a profitable idea.

I welcome, and I'm sure Olight welcomes, these sorts of ideas however in production-scale terms, in this instance, I believe Olight would be hard pressed to make a business case for the accessory being profitable after development, tooling and production costs.

I would agree with you if Olight did not offer the "Olight T-15 and T-10 battery tubes for the very purpose of allowing users the option of a different battery. The Warrior sounds like it has some great features and forward thinking built in such as the flexibility of both 123A and 18650 battery use, the higher lumens than previous Olights is a definite plus. Will such a light hurt sales of existing T-20's? I don't know Matt, but I'd sure like the option of that battery tube! I won't keep pushing this issue, you've been very patient and helpful with me. Take care and best of luck with BatteryJunction.com.

FWIWl those battery tubes didn't need to be explicitly created as an accessory - they're just extra tubes that are already produced as the standard item.

Matt, since battery tubes will be manufactured for the M-20 Warrior all that Olight would have to provide to make the fit would be a reducer and insert to extend the contact from the battery to the head. Consumers who own T-20's such as myself would buy the battery tube with reducer and tailcap switch. We would also need the 18650 battery and charger. Just think of all the sales for the above items and all they would really need to make would be the reducer from the larger M-20 tube to the smaller T-20 head. I would go for such an accessory to help mode my T-20 and it would justify buying the M-20 since I already had the battery and charger. Just a thought.

I'll pass along the idea to Olight and see if they feel it can be done.

Thank you Matt.