Looking for the real answer about "Marriage" (batt's)

Married or matched batteries?

I understand that if your using a dual 18650 IMR light that the batteries should be married I.E. used and charged together, purchased at the same time ect . . .


Using any batteries we, by default assume at lease some risk. We mitigate that risk by understanding or trying to understand batteries,, Knowing the life span, using proper charger, checking for damage, and purchasing quality batt's in the first place.


My question is:

• How much risk are we adding using unmarried batteries of the same age and brand? Purchased together just rotated individually in our battery line up.
• Can that risk be mitigated using "Balanced Charging"?
• Are there actual real world examples of failures attributed specifically to a non-married set, or is this educated theory?

Interesting questions. I don't know if there is any firm facts behind the answers or just educated decision making.

I think that it is a best practice to use matched/married cells in series configurations. This is based on several pieces of logic:

• Batteries of different ages will have different levels of capacity loss, increased internal resistance, etc. Using them together will mean that one could drain faster than the other causing an unsafe setup.
• Batteries, even of the same age, used in different lights could also have different levels of wear and IR. If you use one battery in a high drain light and one in a low drain light they could have different wear profiles causing them to have different drain characteristics if used together.
• Batteries from different vendors, even if same rated capacity, will have different drain and IR characteristics.

Even batteries that are purchased, matched and married together in one light could age differently and cause a risk when they are a few years old.

How can you mitigate some of these risks:

• Use protected cells. This is the safest when using a series setup. The protection could fail and cause an unsafe condition - but that is a fringe case. At least, in this scenario, the cell protection will protect the weakest cell in the series setup.
• Don't fully drain series setups. Most of the concerns that are discussed here only come into play when the battery is becoming depleted and the differences between the cells plays a larger factor. If you only drain the cells halfway each time before recharging any cell differences will be less apparent.

I think for most low drain flashlight applications there won't be any problems. It's when you get into higher drain that problems can occur. For the few lights I have that I run 18650s in series I always get a fresh matched set with them. Not only will I know the cells are safe to use together, but I know that I am getting the proper cell with the right drain for a high current run.

When you go from things like flashlights to even higher drain applications like RC and vaping this becomes even more important. With RC and vaping you tend to run the cells down much further than with flashlights - so the risks get higher.

In my RC days with 6-cell series LiPo packs I've had some cells in the packs start to go bad. You don't see this while running the RC car, but during balance charging you start to see that one cell isn't keeping up with the other and is constantly being balanced with the rest of the pack. One of my RC setups would pull up to 2000 Watts during hard acceleration. That's asking 90 Amps from a 6-cell pack. Here a single bad cell could cause things to go wrong really fast.

There is another active thread right now about the kid who blew up his face vaping. I don't even want to have to read a thread about someone who blew up there hand from a flashlight cell going bad. It's only a few bucks to buy a new set of matched cells for a light and I think it's worth every penny to remain safe in our hobby.

Yeah I am in that thread also. Good to know your an RC guy. It has been my experience that the RC dude's accumulate battery knowledge. A large part of using this stuff safely is simply asking question's. I recently (two months ago) bought several (3 sets) brown lg's. I hate to buy more because regardless of dual or single use. I can only use so many in a day. I have not gotten a good answer and like you believe the concern is theory, albeit an educated logical theory.

I personally have not heard or seen a failure specifically attributed to using a non married set. I'm not saying it has never happened. Only that because of the site's I frequent, flashlight forum, vaping forum, and a little bit of RC (mainly drone) forums. And that I have been a regular at most of them for years. I would have thought if the risk were high I would have seen some examples.

I don't recall hearing of any cases where a failure was specifically put down to using unmatched Li-Ion cells, but there have been instances with unmatched CR123As.

Even taking your last unused CR123A from your previous packet, then putting it together with the first cell from your latest packet could cause problems. You could be using the same cell model with the same ratings from the same manufacturer via the same dealer, but you'd still be open to differences caused by age or manufacturing variations between batches.

The older cell might only have lost 5% of its charge whilst lying in your desk drawer, but that can still get you into trouble if your light runs its cells right down. Lights that can operate on either 18650s or pairs of CR123As are particularly prone to this.

I can easily imagine the same problem appearing with unmatched, unprotected Li-Ion cells being run in series.

As ShineOnYouCrazyDiamond says, you can mitigate the risk by using protected cells, subject to a small risk that a protection circuit will malfunction. However, using unmatched cells will still reduce the runtime and performance of your light, protected or not. If you regularly see the same cell's protection being triggered within a set, it's still a good idea to investigate and consider putting together a newly matched set.

Honestly I haven't heard of an issue occurring either but I'd prefer every use good practices and not set the precedent.

I think the worst that would generally happen is you get poor performance if you use one strong cell and one weak cell. But I wouldn't want to be the one to condone any dangerous practices that could end up hurting someone. Erring on the side of caution, especially with LiIon cells, is the safest practice IMO.

ShineOnYouCrazyDiamond said:

Honestly I haven't heard of an issue occurring either but I'd prefer every use good practices and not set the precedent

Click to expand...

I agree, I just hate spending money.

Hello Hangn 9,

In the past these failures have occurred frequently its just that the cells had little energy so the consequences were simply that the device stopped working. Today we have cells with higher energy densities and our expectations are higher.

When I was young I had a 2 D cell flashlight. It wasn't very bright but it did provide enough light to finish the task at hand. In those days the goal was to only replace the cells when they were totally drained. On a camping trip I knew that my cells were mostly used up so I brought along a replacement set. Sure enough part way through the trip my light output was reduced to the point where it was no longer useful. Problem was that I could only find 1 of my replacement cells.

My Dad came to the rescue and told me to conduct a test. Put the new cell in first, then pick one of the drained cells and put it in and turn the light on to see how bright it was. Change drained cells and try again. Pick the drained cell that gave the brightest light.

This actually worked, kind of, and in an effort to reduce the cost of running a flashlight it became a regular practice.

Alkaline cells vent and leak. The used cells would frequently leak inside the light and make a mess.

Li-Ion cells can rapidly vent sometimes with flame and that is a lot different from having an Alkaline cell vent and leak.

Failures have also happened when using 12 V lead acid batteries in series, as well as when using NiCd and/or NiMh cells in series. The failures are usually not "spectacular" so the failure results in the device stopping working.

The RC industry uses competition to drive the practices to newer levels. If my vehicle always beats yours and you invite me out for a drink to discuss racing, I may out of the goodness of my heart let you in on the idea of balancing and using matched cells...

The best is to used matched cells in a battery pack. This doesn't mean that you can't get by using unmatched cells. It simply means that when using unmatched cells you need to be aware that you are taking additional risks and you have to be aware of how your device is performing and have a plan in place should things end up with damage to your cells.

Tom

Thanks silver Fox. That sounds like a reasonable explanation. And what my guess would have been.

It has been surprisingly difficult to get a response from my normal battery guys.

I have noticed that even when I used batteries and charged them together from the very first time I got them there is always a slight difference between them. Even if I charge them up to 4,20V there is a slight difference every time I place them in the charger again.
And it can be half an hour difference of the fully charging process. I understand there will be at least a very slight difference because even high quality batteries of the same model are not 100% equal. But what is an acceptable difference? With two of the models I always used together the voltage was 3,15 and 3,18V after a runtime test. Is that normal difference?