Battery capacity and cell weight

[Russell]

Is cell weight an indicator of battery capacity?  I use Makita Li-ion 14.4v power tools and have had a spate of battery failures recently so have splashed out on some non OEM replacements.  They are labelled as 3000mAh which is the same as the Makita originals but weigh 355g as opposed to nearly 500g of the originals.  They use Sony cells, the reason for getting that type, so ought to be up to spec.

They only arrived this morning so I have no idea yet how they last in comparison but am puzzled in the difference.

Thanks

[grams]

Yes, cheap low capacity lithium cells are noticeably lighter than high capacity good ones.

I wouldn’t have any faith in the quality/capacity/etc of third party batteries without testing them or at least opening them up to see the markings on the actual cells (which may also be lies).

[Kim]

Yes, assuming the cells are using the same chemistry, and there aren't significant differences in construction.  For an 18650 type cell, there's the possibility of a built-in protection circuit, which takes up some volume.

Hang on, those numbers are huge.  An 18650 weighs about 40-50g.  You must be talking about the battery pack.  4 cells in series gives you your 14.8V.  That could be 4 3Ah cells, or it could be 8 1.5Ah cells.  Plus whatever the balancing circuitry and sturdy plastic case weighs.

My guess is that the original packs contained 2P4S configuration of 1.5Ah cells, and the new one has 4S of slightly heavier 3Ah cells (and correspondingly different internal construction).  That would account for the weight difference.


(My experience with non-OEM 18V Makita batteries was that they performed well enough at first, but the complete lack of cell balancing circuitry made for a limited lifespan.)

[Russell]

Thanks

Yes, the non-OEM ones I've had for a while are 2P4S, one of them has spacers to replace an additional pair presumably for the 18V version.

The OEMs and these new packs are assembled with those tri-lobe (tri-wing) screws (down a tunnel) so dissassembling them is difficult so cannot say whay the construction is but the external shape is very close to original.

The non-OEM packs that have failed do have a circuit board with wires diving into the pack of cells which I assumed were for the balancing circuitry.  They are charged on a Makita charger - wouldn't that throw a wobbly if it couldn't see the balancing circuit?

[Kim]

The non-OEM 18V pack had a board a bit like this:

https://www.aliexpress.com/item/4000095632360.html

It does over/under charge protection and thermal monitoring, but doesn't balance the cells.  You can tell because it only connects to the ends of the series string, with a tap off the first cell to power itself (guaranteeing imbalance if the pack spends enough time not being used).  The proper ones connect to each series cell for balancing.

(The OEM 18V cells use the same scheme to power the battery electronics.  There's a failure mode where the first cell gets deep-discharged by the quiescent current, the pack refuses to charge, and the controller bricks itself for 'safety' on the third attempt.  Therefore, if you find yourself in possession of a Makita 18V pack that refuses to charge, it's best to open it up and attempt to breathe life into the cells individually before re-trying with the official charger.  And to make sure you charge them every couple of months, obviously.)

[rafletcher]

  And to make sure you charge them every couple of months, obviously.)

Top tip 

[Russell]

I have checked the battery packs that have failed and you are of course correct Kimmykins, except for one tiny little insignificant detail.  The PCB is powered off 2 cells not just the first.   

[Feanor]

I'm just going to throw in a kudos to Makita here.
Last week, my battery angle grinder died.
I sent an email to their support address, and they asked me to send it to their base in Milton Keynes.
I got a call yesterday, saying the controller board in the tool had died, and they had replaced it FOC.
I had no proof of purchase, and it was out of warranty.

So a big thumbs-up from me.

[Valiant]

I had a 9 year old SP6000 Track Saw that they picked up, repaired and return F.O.C. I was so amazed by that service that I went and dropped serious Ps on a multi batt 5ah Drill/Driver set.