Multiple 18650 light - questions about safety

[Bright Bird]

Yesterday I (pre-) ordered my first torch (Astrolux MF-01) that uses Li-Ion batteries in a serial configuration (2S2P). From what I understand, the batteries need to have the same capacity and voltage, but there is a chance that the light might drain them at different rates. Does that mean that I need to check the voltage every time before I switch it on? Or only if I use / store it for a longer period? How much variation in voltage is acceptable, if I don't want to risk that the light turns into a pipe bomb?

The other thing I was wondering about is what batteries are suitable. I ordered the purple efest that are rated at 20A. Are they safe to use? Or doesn't that leave enough overhead, considering that the light draws nearly 16A in turbo mode? Would I be dangerously pushing the cells to their limit? It seems that Samsung 30Q would be the best choice if they were available as button tops, but I've only been able to find them as flat tops.

 

[terjee]

I'm not familiar with the light, so I'm commenting generically on 2S2P in lights:

Use identical cells, not just cells with similar ratings. Keep the batteries as a fixed set, don't split one away from the set temporarily. You don't want different wear on them.

You don't need to check the voltage before using it every time, but you should recharge sooner rather than later. Undervoltage is one of the risks. If left alone for a while, recharge fully, or check voltage.

Consider that when you pair up a set of batteries in parallel, they will have the same voltage. Whatever needs to happen for that to be true, will happen. That might include moving a lot of energy from one to the other. Largely solved by keeping them at as close a voltage as you can when you insert them into the light.

16A is split over the two parallel sets, so that's 8A pr. set, and 8A pr. battery. That's not extreme, but it's enough that some protected batteries are ruled out. Normally protected would be recommended when using multiple batteries.

You CAN get the 30Q with button top, but it you're going with a high drain unprotected battery, might as well go with VTC5A or VTC6, both also available with button top, and support twice the current of the 30Q.

8A drain on a cell supporting 30A (VTC6) or 35A (VTC5A) isn't pushing anything to their limits.

(I normally avoid pointing at specific vendors, but NKON has all of those available with button tops).

For the Efest, some ratings are off. Look for reviews by HKJ (lygde-info.dk) or Mooch. Both are trusted and capable reviewers, with known testing methodologies. Both are well known and highly regarded in respectively the flashlight and vaping communities.

 

[Bright Bird]

Damn - just read HKJ's review, and they're not recommended for high current draws (although apparently they perform well at 5A). Looks like I've now got an excuse to find another light that they'll work in!

Yes, I know about matched sets. I always keep my batteries in labelled boxes, so I know which ones belong to which light. Although the only multiple 18650 light that I've got so far is the Olight SR52UT, but that uses the three cells in a parallel configuration. The first time that I used the light I let it run down pretty low, and when I tested the voltage each of them was 3.09V Since then I've always recharged the batteries much sooner.

Good to know that the drain is split if sets of multiple batteries are used parallel - should have really been able to work that out myself! :thinking: So I'll be looking at a drain per cell of just under 8A, presumably only for a few minutes before the temperature regulation kicks in and the light steps down. Will have another look for button top 30Q cells, as that seems like a safe bet, or maybe the 25A version.

 

[Bright Bird]

Another question - as you said, normally you'd use protected cells in a multi-cell light. On the other hand I remember reading that IMR batteries are apparently considered a 'safe' chemistry, but I've not been able to figure out what that means. How safe? And what makes them 'safe'?

 

[terjee]

They're not safe, they're less volatile. That's one of the reasons why you can push them harder, such as 30-40A for some. If they do go "rapid exothermal", the reaction should hopefully be less bad.

That said, there are also things protected cells can handle, that these won't. They don't have any undervoltage protection for example.

 

[Bright Bird]

They're not safe, they're less volatile. That's one of the reasons why you can push them harder, such as 30-40A for some. If they do go "rapid exothermal", the reaction should hopefully be less bad.

That said, there are also things protected cells can handle, that these won't. They don't have any undervoltage protection for example.

Thanks for the explanation. I'm not really worried about low voltage, as the light switches off at 2.7 or 2.8V IIRC, and I intend to recharge the batteries long before it comes to that. Just ordered a set of 4 30Q's from BG who had the best price - I guess they need to be sent by surface mail, but as the light probably won't be arriving anytime soon, I don't mind waiting.

 

[Bright Bird]

And four of the VTC6 that you recommended ordered as well.

 

[terjee]

Low voltage is extra hard with batteries in series. Unless the light has a sensor between the cells, it'll just see the total voltage of the series set. If it sees 6 volts, it won't know if that's 3V from each battery, or 4V from one, and 2V from the other. This is a major part of why protected are nice for batteries in series, since the protection of the lowest battery would have kicked in before it got down to 2V. Also illustrates why it's important to have identical cells. If you had a 3000mAh battery and a 2000mAh one, the weaker would get to low voltage much faster, and the light might not notice.

 

[Bright Bird]

Low voltage is extra hard with batteries in series. Unless the light has a sensor between the cells, it'll just see the total voltage of the series set. If it sees 6 volts, it won't know if that's 3V from each battery, or 4V from one, and 2V from the other. This is a major part of why protected are nice for batteries in series, since the protection of the lowest battery would have kicked in before it got down to 2V. Also illustrates why it's important to have identical cells. If you had a 3000mAh battery and a 2000mAh one, the weaker would get to low voltage much faster, and the light might not notice.

Are there any specific protected cells that are recommended for high drain applications? When I've searched for 'high drain protected button top 18650', Google gives me results such as the LG MH1, which is advertised as supporting 10A discharge, but then the description says '2.0ACC' (constant current?), so 10A only seem to be for pulse.

 

[terjee]

High drain and resistance are natural enemies, and protection adds resistance. Some Keeppower are said to go to 10A, but other than that I'm not aware of any, sorry.

 

[Bright Bird]

High drain and resistance are natural enemies, and protection adds resistance. Some Keeppower are said to go to 10A, but other than that I'm not aware of any, sorry.

OK I'm actually glad to hear that, after what I've already spent on batteries!

So I guess that IMR cells must be the standard choice for this kind of light, and if I take the necessary precautions (avoiding over-discharge, avoiding physical damage to the batteries, only using a matched set and checking the voltage regularly), i should hopefully be pretty safe.

 

[terjee]

So I guess that IMR cells must be the standard choice for this kind of light, and if I take the necessary precautions (avoiding over-discharge, avoiding physical damage to the batteries, only using a matched set and checking the voltage regularly), i should hopefully be pretty safe.

Yeah, that with keeping an eye on temp during charging, as well as inspecting them before charging, should cover you pretty well.

If you're still using this year in 6-12 months, you could also add a charger that'll measure battery capacity and internal resistance. Those are pretty good indicators that a battery is wearing out, and should be recycled or moved to lighter duties.

You're covering some of the same ground with checking temperature during charging, so I wouldn't suggest it's something you'd need to run out and get now.

About lighter duties btw. What I do is that I'll splits packs after a while (half a year to a few years, depending) and then use them in less demanding single 18650 lights, or simply as backup power. They'd still be useful to keep around if I'll ever see a longer power outage for lights, or to charge cellphones (Nitecore F1, about $8).

Currently have a bit of a flow like that, with batteries starting their life in high power flashlights or with vaping, moving to less powerful lights, and finally into the "backup/emergency"-pile.

 

[Bright Bird]

I got into the habit right from the start to never charge any batteries unattended (including NiMH AA cells - although I'm probably being overly cautious), and at least the first few times that I charge a battery (or set of batteries), I regularly check the temperature. But I guess that if over-discharge is avoided, accidents are probably very rare due to invisible damage to the batteries, or a faulty charger.

I've been thinking of getting a decent DMM (currently just got a very cheap one), but as you say it does make sense to get a good charger that can display all kinds of information about the batteries being charged, and the charging process. Considering what I've already spent on torches and batteries this year (I only became a flashaholic earlier this year, and have already bought a dozen lights!), it probably won't do any harm to invest in an MC3000 some time soon.

Thanks for the infos. I feel a lot more at ease now, and it is good to know (now and for future purchases) which type of batteries work in lights with a specific current draw. And I'm sure that I'll find something that is a good match for the efest cells.

 

[terjee]

Just a quick couple of things: I more often see excessive heating with NiMH than LiIon, so good choice.

Also, a cheap multimeter is often just fine. More expensive ones gives you more of one or more of features, more precision at the extremes, more accuracy in general, and more safety (important for mains), but for simple voltage measurements in the voltage range of batteries, cheap is often just fine.

 

[Bright Bird]

Just a quick couple of things: I more often see excessive heating with NiMH than LiIon, so good choice.

Good to know - that means I'll be continuing my current practice.

Also, a cheap multimeter is often just fine. More expensive ones gives you more of one or more of features, more precision at the extremes, more accuracy in general, and more safety (important for mains), but for simple voltage measurements in the voltage range of batteries, cheap is often just fine.

I found the following on HKJ's page about using a DMM:

Note: Many cheap meters will measure wrong, without giving any warning, when the battery is nearly empty

so now I'm not sure whether that might be a problem that is likely to occur with my DMM, which is a very cheap one that I picked up on eBay for £6. I noticed that quite a few of the ones that are available cost over ten times as much!

 

[HKJ]

so now I'm not sure whether that might be a problem that is likely to occur with my DMM, which is a very cheap one that I picked up on eBay for £6. I noticed that quite a few of the ones that are available cost over ten times as much!

I am looking at some DMM's at the current time, first review is here:
http://lygte-info.dk/review/DMMAnengAN8008 UK.html

More will follow, but I do not know how fast (I have a few more that is written), they will show up here:
http://www.lygte-info.dk/info/index UK.html

In these review I will check if the meter shows a wrong value with a low battery. For the ones I have tested I did only see one and it was a minor error.

The more expensive DMM's will be much safer and probably also more precise over time, but the precision is fairly good on new cheap DMM's. The safety is not an issue as long as the DMM is used for low voltage stuff.

 

[Bright Bird]

I am looking at some DMM's at the current time, first review is here:
http://lygte-info.dk/review/DMMAnengAN8008 UK.html

More will follow, but I do not know how fast (I have a few more that is written), they will show up here:
http://www.lygte-info.dk/info/index UK.html

In these review I will check if the meter shows a wrong value with a low battery. For the ones I have tested I did only see one and it was a minor error.

The more expensive DMM's will be much safer and probably also more precise over time, but the precision is fairly good on new cheap DMM's. The safety is not an issue as long as the DMM is used for low voltage stuff.

Nice, thanks!

 

[Bright Bird]

So from what i can gleam it seems that the main common issues with cheap DMM's are high amperage measurements, and AC (mains) current. At the moment measuring the voltage of batteries is what is most important to me, but as the batteries degrade at different rates I will obviously also want to be able to check their capacity precisely.

Which brings me to the next thing I was wondering about - at some point it will be a good idea to split the set, as terjee pointed out. What values are considered 'safe', i.e. at what difference in capacity between two or more cells should I reallocate the batteries to 2x18650 or 1x18650 lights?

 

[terjee]

Two cheap multimeters can be tested against each other as well, giving some degree of validations. Know voltages can also be tested, and you can compare voltages to freshly charged batteries.

Often 80% remaining capacity is used as a number for when to recycle a cell. For when to break up a pack, you could also look at differences between the batteries. If 3 of the cells give you 2700mAh +/- 20mAh, and the last shows 2500mAh, I'd consider splitting the pack. Another way to go about that would be to check cell voltage after usage. If three of them end at 3.3V, and one at 3.1V, might be good time to split them. This is is simplified just to give the general idea, with parallel cells it might be harder to spot a difference.

 

[Bright Bird]

Two cheap multimeters can be tested against each other as well, giving some degree of validations. Know voltages can also be tested, and you can compare voltages to freshly charged batteries.

Often 80% remaining capacity is used as a number for when to recycle a cell. For when to break up a pack, you could also look at differences between the batteries. If 3 of the cells give you 2700mAh +/- 20mAh, and the last shows 2500mAh, I'd consider splitting the pack. Another way to go about that would be to check cell voltage after usage. If three of them end at 3.3V, and one at 3.1V, might be good time to split them. This is is simplified just to give the general idea, with parallel cells it might be harder to spot a difference.

OK thanks. That's a larger difference than what I had expected. But if the individual capacities are within a few dozen mAh's of each other, and they're above around 2,400mAh (for cells that were originally 3,000mAh) I shouldn't have to worry. I'll also keep an eye on whether any are depleted faster than others each time before recharging them.

I guess my DMM should be pretty accurate for voltage measurements. Whenever I check the batteries from my Olight (3*18650 parallel), it always gives me exactly the same reading for each of the batteries, with 0.01V precision. So even if the actual result might be slightly inaccurate (which I would only be able to test with a second DMM), it still shows me the same reading for batteries that have the same capacity.

I really appreciate all the help with this ... that's some really valuable information! :thumbsup: