Mixing protected and unprotected batteries

[OzzieDoc]

I have quite a few unprotected batteries in various sizes that have all been treated gently but I now have on order (from DX) some of the new protected ones in various sizes.

My question is whether protected and unprotected batteries of the same nominal size and capacity can be safely mixed in series such as in a multicell flashlight. If so does the protected battery effectively end up as a defacto protection circuit for the unprotected cells. Is there an optimum ratio, for example in a X4 RCR123 flashlight is one protected battery enough to provide a protective effect to the others (assuming this is the case at all) or would more be better. Thanks.

 

[flashlight]

I would say no.

 

[Norm]

I wouldn't mix batteries at all, I would only use batteries of the same brand and specification.
Norm

 

[DM51]

Do not try this in a built-up area, and make sure your flashlight has a large vent to let all the smoke and flames out.

 

[SilverFox]

Hello OzzieDoc,

Mixing cells is not recommended.

If you put one protected cell in series with 3 non protected cells, and if the protected cell had the lowest capacity and emptied first, you would run a risk of destroying the protection circuit when the other cells tried to reverse charge the weak cell.

On the other hand, if the protected cell was the highest capacity cell and emptied last, the other cells may be over discharged.

In an emergency, your can use your cells to power up a light, but you should limit your run time to half that of the weakest cell. This means that you need to know the capacities of your cells under the loads you are using, and that you use a timer to time the run time.

It also means that you should not let anyone but yourself use the light. If it is going to blow up, you might as well be holding on to it... This will keep you more attentive to the run time.

In general it is not recommended. In an emergency, it is possible with certain conditions being met.

Tom

 

[OzzieDoc]

Thanks Tom, but I'm still a bit confused. Apart from the risk of destroying the protection circuit in the scenario you mention, how are the risks any different from using multiple rechargeable Li batteries in any situation. The batteries would be from the same manufacturer and same nominal size and capacity.

 

[SilverFox]

Hello OzzieDoc,

When you purchase a set of cells to use in a multi cell application, I am assuming that you get similar cells (same brand and capacity) and put them into service at the same time. It is also suggested to do a capacity, mid point voltage, and rebound voltage test on them to make sure they are a good match.

Your question sounded like you wanted to put together several cells of different brands. I may have misunderstood you. Nominal capacity is not a reliable way to match cells, and different brands may have a slightly different chemical mix within the cells.

Let me try again...

If your cells are all the same age, same brand, same capacity, same type, and have very similar performance, there should be no problem at all.

Tom

 

[OzzieDoc]

Thanks again Tom. I already do all of that with my unprotected (near new) cells to balance them. These new protected cells will be different only by about 3 or 4 cycles "fresher" as well as the protection circuit. I would do the usual checks before and after use and charging. Does that seem OK.

 

[SilverFox]

Hello OzzieDoc,

I don't think protected and unprotected cells have similar capacities, but it is possible.

If you match everything up and don't run them down too low, it seems like it should work.

Tom

 

[OzzieDoc]

Ultrafire batteries list as follows;
18650 2400Mah #3287 protected # 1213 unprotected
16340 880Mah #3373 protected # 2086 unprotected.

It all seems too hard so I think I'll just use the protected cells from now on.

Thanks again.

 

[flashlight]

Always better to be safe than sorry. :tinfoil:

 

[DM51]

It all seems too hard so I think I'll just use the protected cells from now on.

If you are as careful as you have said in post #8, then you should be perfectly OK using matched unprotected cells. The thing I personally would never do is use a different type of cell with them, that is all. Unprotected cells in the hands of a careful user are no problem, so you should be fine.

A good safety precaution, once you have established they are well matched, is not to run them down too low. Recharge little and often, and keep checking the voltages before and after charging to make sure they are staying well balanced. If you aim to recharge them when they drop to 50% of charge (3.7v at rest) then you will know you are OK and you will still have spare capacity if you really need it. You say you have quite a number of these unprotected cells, so you could carry spares and just change them out more often. If any cell shows signs of losing capacity or "getting out of step" with the others, just discard it.

There are plenty of people using unprotected cells without problems. They are the careful ones. It is the slapdash guys who don't understand the potential dangers (or ignore them) who are a worry. Your post #8 gave a quite different impression from your post #1, so I am sure you will be perfectly OK.

 

[OzzieDoc]

Sorry if I didn't make myself perfectly clear with my original question. While I have lots of unprotected cells in various sizes I never mix sizes and capacities and am very careful with their handling. I have carefully studied the wealth of information available on this site and value the opinion of many who post here. I have just purchased a few of the new protected Ultrafire cells in a few different sizes and my question was whether it was safe to mix protected and unprotected cells provided they were of the same size and capacity (even the same manufacturer) and were balanced in terms of voltage etc. The only difference between the cells at least in theory was the presence of a protection circuit. I didn't think that this was any more dangerous than using unprotected cells in general but wanted to check with the experts here first in case I was missing something.

 

[DM51]

Even if your protected and unprotected cells are the same size and nominal capacity, are made by the same manufacturer and maybe even have the same color wrapper, the two types are likely to be quite different. For one thing, the protection circuit takes up ~2mm space at the end of the cell, so if the cells are the same size overall, the business part of a protected one is actually shorter, and they therefore have less physical capacity. Also, there are a number of different variations of the Li-Ion chemistry, with slightly different characteristics. You have no way of knowing what you have, unless you buy cells from the same batch. Then there is the actual age of the cells, the number of cycles they have been through… etc etc.

Of course if you test them thoroughly, and find that despite their differences they do seem to have identical characteristics, you maaaaaay be OK, if you are extremely careful. I may be being over-cautious in this, but it just isn't something I would do myself.