4ah high power battery repair

[Mockbubbles]

Hello, I've come across 10 faulty festool 4ah high power batteries from late 2019 to early 2021. most of them just need a new cell put in but I have 3 with BMS failures (all cells are good but the BMS just refuses to output even once i've charged the cells manually). Does anyone know if removing the BMS from a working battery to put on these BMS failed batteries will kill the BMS? I've found talk online about the BMS killing itself if it sees 0v and would like to know if this is actually true

I must say the 4ah are extremely well built and cell failures seem to be in random positions so at least festool has done a good job with designing the boards.

Thanks for any help

 

[Cheese]

Recently I had a 4.0 HPC battery pack, dated 11/2020 go bad. It refuses to charge even though 2 of the green LED's still light up. I sent it back to Festool for warranty consideration but that was a no-go. I mentioned in a brief note to Festool Service that the battery probably had fewer than 15 charge cycles on it which I'm sure they were able to verify.

I know this doesn't answer your question but just thought I'd bring it up to see if my battery issue was similar to yours. This is the FIRST Festool battery I've ever had a problem with.

 

[Coen]

I have refurbished a bunch of rechargeable battery packs, including eBike batteries and various tool batteries. From the Lithium tool batteries I've done Bosch, Dewalt and Metabo. None of them were boobytrapped. The one battery I couldn't repair was a Shimano ebike battery; their BMS is 'designed to ""fail"" ' if you ask me.

I found this;

Expensive 18V Festool Powerpack Saved From The Dump | Electronics Repair And Technology News Expensive 18V Festool Powerpack Saved From The Dump

Fixing dead battery packs often is cumbersome and not without risk. Because explosion or fires are the result when cells are handled wrong. But when my nephew who is a professional kitchen installer recently rescued a defect and powerful Festool 18V battery powerpack from the bin that only was...

jestineyong.com

So not boobytrapped.

I have read about some laptop batteries being boobytrapped in the sense it will never report a working battery once it has seen less than xx volt once. But they are often re-settable.. just a matter of someone finding how to.

Before discarding a BMS you might want to check the temperature sensor that is attached to it. If that goes bad, the BMS will report it failed too.

Seems these Festool 4.0 HPC packs use the same Samsung 21700 40T cells as does Bosch in their Procore 4 Ah battery. I have had one of them leak too. And another pack I got that was dead had a completely dead cell too. The first one I replaced one cell, the other one five new cells. Both now work as new.

 

[mino]

Yeah, it seems they did not have much luck with the new batteries choice ... hopefully this will get sorted over time as is clearly a supplier-side issue.

S either has quality control issues (unlikely) or is over-speccing the packs (more likely). This should resolve over time hopefully. "Easiest" on the F side would be to use 4200 cells in the 4 Ah packs. The the good old way on how to "fix" overspecced but-otherwise-quality packs with modern BMSes and capacity cut-offs.

 

[Coen]

The BMS controlled cut-off isn't capacity based but voltage based. Nothing is measuring how much capacity was delivered. There is the INR21700-50S now that is 5 Ah and, like the INR21700-40T, also claims to be for 35A discharge. However, it's slightly longer and fatter...
In case of Bosch they have the cells press-fit naked into the plastic shell so there is maximum heatsinking. Going by the photos of the page I posted earlier, the Festool design is much more basic and would result in a much hotter battery (internally!) than the Bosch Procore design.

As for Samsung; I try to avoid everything they make. But in case of battery cells that isn't always possible.

 

[Cheese]

The BMS controlled cut-off isn't capacity based but voltage based. Nothing is measuring how much capacity was delivered. There is the INR21700-50S now that is 5 Ah and, like the INR21700-40T, also claims to be for 35A discharge. However, it's slightly longer and fatter...
In case of Bosch they have the cells press-fit naked into the plastic shell so there is maximum heatsinking. Going by the photos of the page I posted earlier, the Festool design is much more basic and would result in a much hotter battery (internally!) than the Bosch Procore design.

As for Samsung; I try to avoid everything they make. But in case of battery cells that isn't always possible.

I like that idea......they must be using a thermally conductive material like Makrolon for the battery holder. I wonder why Festool chose not to go down that road?

 

[Coen]

I like that idea......they must be using a thermally conductive material like Makrolon for the battery holder. I wonder why Festool chose not to go down that road?

No they are using HDPE.

Probably because it was easier to just keep on doing what they did for the past 15 years. If you look at the Festool pack you see the cells do have their original wrapper; that is the first layer of insulation. Then there is air around, also an insulator.

Anyway, the Festool design might be more resistant to drop-damage where plastic deformation on the outer shell absorbs more of the blow than the more rigid/massive Bosch design which probably transfers more shock to the cells.

But Festool's highest drain tool is probably a CTLC that is listed for 14 minutes for two 4 Ah batteries. Let's just assume flat out 100% efficiency and nominal 18V for sake of simplicity. That works out to a drain of 8,57A for the battery (each). That isn't anywhere near the high-drain devices Bosch has; they had a battery since way before Festool and it's listed at 4 minutes / Ah. That works out to 15A for the battery. They even have one at 2.5 min / Ah so 24A, but nobody is running that on 4Ah batteries. So there is no real need for Festool for that better heat transfer.

In reality there is no 100% efficiency and the higher the drain current, the more heat is generated in the battery.. and that doesn't scale linear.

A few years ago I bought a new EDC flashlight and some new 14500 lithium cells. There are some that are supposedly 1000 mAh, but at a 1A drain rate, they don't last any longer than 850 mAh cells that are build for the higher drain.