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My laptop battery died and I can't find a replacement (old laptop) so I'm going to need to replace myself the 18650 cells it has.

It is an 8-cell battery (a series of 4 banks of 2), making the whole package 14.8V and 4Ah.
So each individual 18650 cell is the typical 3.7V and 2Ah.

As a possible replacement I found these:
http://www.ebay.com/itm/2-x-3-7V-4000-mAH-18650-Protected-Lithium-Based-Rechargeable-Battery-Flashlight-/131347594474

3.7V 4Ah 18650 cells

Which are 4Ah (twice the capacity of my current ones) and also they are protected.
My current battery cells don't have any marking that indicates whether they are protected or not but I have the impression they are not since the battery charging circuit should provide such protection.

Also, the physical dimensions of the cells are exactly the same as my current ones.

My question is:
Would there be any problem that the new cells have twice as much capacity?
Also, does it affect in any way that the new cells are protected and the old ones are not?

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    \$\begingroup\$ A 4Ah 18650? No thanks, not interested in that bridge. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Feb 25, 2015 at 0:22
  • \$\begingroup\$ @IgnacioVazquez-Abrams, what do you mean? \$\endgroup\$
    – GetFree
    Feb 25, 2015 at 0:24
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    \$\begingroup\$ 2800-3200mAh is about the sane limit for 18650s; Li-ion does have a maximum energy density. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Feb 25, 2015 at 0:26
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    \$\begingroup\$ What it means is that if you purchase some of these cells and run tests on them, you will most likely find that the stated capacity has been exaggerated. \$\endgroup\$
    – Dwayne Reid
    Feb 25, 2015 at 0:26
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    \$\begingroup\$ I got some chinese batteries labeled as 5000mAh. I tested them as having 375mAh or about 7% of their actual rating. Don't even bother buying anything rated over 3200mAh. \$\endgroup\$
    – horta
    Feb 25, 2015 at 1:34

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Cells in devices that you do not wish to be a flaming ruin MUST have protection.
Having protection circuits does not guarantee it will not happen.

Cells without protection are intended for use by either manufacturers or experts or enthusiasts who add their own or for suckers.

Whole device protection and cell protection are complementary and serve overlapping but different roles.

4,000 mAh 18650 LiIon batteries are ~~~= 99.9% +0.1% - 0.0% sure to be rubbish.
ie not just < 4000 mAh but << 4000 mAh and low quality.
The people who bogus label cells

  • almost never feel an obligation to use the best cells they can and

  • almost always decide to add injury to insult by using junk as well as lying.

Real world experience shows that the value of 'almost' is very high in both cases.

Higher capacity cells can usually be fitted OK.
MUCH higher capacity will lead to long CC tails and overcharge but not an issue here as mAh_new is < to << 4000 mAh.

ADDED:

Notice that in this ad and all their other ads they ALWAYS show non-brand-label views of the battery.

However, you may find that the racing stripes and general colour scheme a good match for the well known "Ultrafire" brand batteries. This may in fact be a real brand and these may be real examples of it BUT you can buy empty shrink wrappers to apply to the battery of your choice with this (or other) branding on it , so caveat emptor. Better nullius emptor I'd hazard.

These ones are a stunning 6800 mAh - a steal at the price. Available here

enter image description here

You'll find others similarly arrayed here and
here - 3000 mAh and 4000 mAh and

6800 mAh !!!!!!!!!!!!!!!! {again}

and unspecified but with GENUINE CREE 2000 lumen {so 20+ Watt} flashlight for $9.27 and
only 4000 mAh and
that's better - 4200 mAh
and .............

Flee!

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  • \$\begingroup\$ Here they are selling a 6Ah cell. I don't know why ebay doesn't ban those things. \$\endgroup\$
    – GetFree
    Feb 25, 2015 at 4:42
  • \$\begingroup\$ The original poster is rebuilding a battery pack that already has appropriate protection circuits built onto the little PCB inside the battery case. The original cells don't have protection and I don't feel that the replacements need protection either. However, I do stand to be corrected if necessary. \$\endgroup\$
    – Dwayne Reid
    Feb 25, 2015 at 4:44
  • \$\begingroup\$ @GetFree: because they get money \$\endgroup\$
    – PlasmaHH
    Feb 25, 2015 at 9:29
  • \$\begingroup\$ @DwayneReid No "correction" intended. It's a little complex. My comment on people selling unprotected cells was general for the LiIon market and as you say, the new cells claim to be protected. I'd be a little careful [tm] about the protection claim given the 4Ah claim - once they have started down the road of less capacity with bigger labels it's uncertain what else they will or will not do. | Your comment re protection within the battery pack is probably correct - some manufacturers provide per cell protection inside the pack but not inside the cells per se. I'd guess that was due to .... \$\endgroup\$
    – Russell McMahon ♦
    Feb 25, 2015 at 11:52
  • \$\begingroup\$ .... the added flexibility it gave them. | A single protector per pack is not good enough to stave off worst case nasties that a protector per cell so you'd need to know what an in pack protection system did. | I note that he thinks his old batteries and pack did not have protection. The charger protection is not adequate as a replacement for these. \$\endgroup\$
    – Russell McMahon ♦
    Feb 25, 2015 at 11:55
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OK, I have the winning entry for exaggerated claims as to 18650 battery capacity, all the way from Phnom Penh. 7800 mA/h!

Yep that is what the label states. Brand is UitraFire (sic). Here in Cambodia we get all kinds of wonderful things from China. Someone told me that there are relabelling factories in the People's Republic who will sell you any colour, brand or capacity desired. Including of course, these bright side red 7.8 Ah beauties. Good news is they're only $1.50 each, but they're not new. Surprisingly they do work but capacity is around 1200-1500 mAh, from my relatively crude testing. Not bad considering they probably spent their first five years in laptop, or propelling a Prius.

Be wary. All is not as it seems.

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A few comments.

1) Usually no problem replacing old 18650 cells with new.

2) You really should take your new cells to a professional battery build company and have them do the welding for you. You can purchase cells with tabs already welded on but those may take up too much room in your existing battery case. Under no circumstances should you attempt to solder directly to the end terminals of a Li-Ion battery.

3) There are two types of "protected" cells. Those having only over-current protection and those having low-voltage cutoff boards built in. The cells having only over-current protection are a tiny bit longer than cells without; cells having low-voltage cutoff can be as much as 1.5mm longer than cells without. In general, you are better off purchasing cells that don't have any protection added. The battery management circuit within your battery pack will take care of that for you.

4) It's probably wise to actually test the cells you purchase before you spend the time and money to rebuild your battery pack. Charge them fully, then put a load on each cell and monitor the cell's performance as it discharges. This can be as simple as a resistor on each cell. Bonus points if you are able to actually match the capacity of all the cells going into your rebuilt pack.

You should immediately recharge your cells after discharge-testing them. You don't need to take them all the way to full - around 30% net charge is good.

Testing your cells first is a good way to weed out any duds.

5) Your battery pack may not work after you have replaced the cells. The cure is usually simple: apply a charge voltage to the end terminals on the pack connector. This voltage needs to be current-limited to a low value (50 - 100 mA) with the voltage high enough that to ensure that current flows into the battery. That usually resets the shutdown circuit on the protection board.

Note that when I say "end terminals" on the pack connector, what I mean is the terminals that correspond to the most positive and most negative pins in the connector. These are not necessarily the actual end pins on the connector.

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  • \$\begingroup\$ Is it that important that the metal tabs must be welded to each end of the cells? What if I just tie everything together very firmly with adhesive tape? \$\endgroup\$
    – GetFree
    Feb 25, 2015 at 0:56
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    \$\begingroup\$ @GetFree Do you want your laptop to catch fire? Because that's how you get your laptop to catch fire. \$\endgroup\$
    – user39382
    Feb 25, 2015 at 5:28
  • \$\begingroup\$ @duskwuff, why would that make it to catch fire? (it's an honest question) \$\endgroup\$
    – GetFree
    Feb 25, 2015 at 5:54
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    \$\begingroup\$ I am not an expert, but: At a guess, the current from a AA battery isn't enough to seriously heat up a high-resistance contact; additionally, AA batteries (NiMH or alkaline) are a bit more stable than LiPo's. The combination of higher current from a LiPo means a high-resistance contact will heat up more; and LiPo batteries have a reputation for bursting into flames if overheated. Not a good combo. \$\endgroup\$
    – Li-aung Yip
    Feb 25, 2015 at 11:55
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    \$\begingroup\$ Have done it a couple times now and the main problem is not the batteries itselfs however the board, IC on the board that remember the 'state'/age of the batteries. Replacing old cells with new cells is quite dangerous and probably damage the new cells very quickly because the board still thinks the old batteries are there. I think modern batteries are not smart and just programmed to last a period of time by 'counting' it's age. Replacing the battery doesn't make sense when you are not able to reset the board (by battery protocol if supported) or to replace the board + new batteries. \$\endgroup\$
    – Codebeat
    Sep 15, 2018 at 3:18
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Usually LiPo battery chargers implement a timeout feature. Since you are doubling the battery capacity, there is some remote chance that your charger will timeout before they completely recharge. Even if this does happen, it is probably not a big deal. You will still get a lot of life from the pack. Apart from that, I suspect it will be OK.

I recommend that you buy protected Panasonic cells. I bought mine from Orbtronic. Personally, it makes me nervous to be at the absolute cutting edge of high capacity LiPo battery technology, and also to use unknown vendors for the cells. So I would stay away from anything rated at 4 Ah, and I would stay away from any cell vendor I have never heard of.

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  • \$\begingroup\$ No LiIon charger should NEED a timeout cct. The opposite effectis more likely. If TRUE 2000 mAh cells were replaced with TRUE 4000 mAh cells then the CC (constant current tail which was designed to stop charging at say C/4 = 500 mA will now need to go to C/8 to get down to 500 mA and the cells will be charged significantly more heavily. This adds only a small amount to stored capacity BUT noticeably shortens cycle life. \$\endgroup\$
    – Russell McMahon ♦
    Feb 25, 2015 at 12:25
  • \$\begingroup\$ Not sure I understand your point. I have never seen a Li charge controller IC that did not have a timeout. If I did see one, I would not consider using it in a design. The timeout is, in my opinion, an essential safety feature to avoid indefinite CV charging if, for any reason, the charge does not terminate according to its primary termination criterion (current less than some threshold). \$\endgroup\$
    – user57037
    Feb 25, 2015 at 15:12
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    \$\begingroup\$ Oh, I think I see your point. If the termination criterion was C/4, it has now become C/8 with the doubled capacity. So the cells will be over-charged. Yeah, I agree. In my answer, I didn't mean cycle life, I just meant discharge time for the subsequent cycle. \$\endgroup\$
    – user57037
    Feb 25, 2015 at 15:21
  • \$\begingroup\$ Do all this mean that it's not a good idea to use higher capacity batteries? \$\endgroup\$
    – GetFree
    Feb 25, 2015 at 19:23
  • \$\begingroup\$ @GetFree, no, I think it is OK as long as they are not too much bigger. Please read Russel's answer below. \$\endgroup\$
    – user57037
    Feb 25, 2015 at 22:29
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The problem could be that most advanced chargers monitor the charge status by two criteria: the voltage and the temperature. A Li-ion cell's temperature rise sharply when the cell is fully charged, and the voltage drops very slightly. Both effects are being monitored for stopping the charge, but with charge current much lower that required for larger capacity cell the latter effect may be less pronounced and not trigger the protection.

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    \$\begingroup\$ This is NOT how Li-ion cells are charged. It is, however, how NiCd & NiMH cells are charged. Li-ion cells are charged using CC-CV (Constant Current/Constant Voltage). A safe charger should also monitor cell temperature, and minimum voltage. \$\endgroup\$
    – Guy Gordon
    Mar 4, 2018 at 3:51

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