Power tool batteries for power wall

Wattsup

Wattsup

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Feb 19, 2017
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Definitely a nerd in training here so bear with me please. I am new to electronics etc.


Ihave quite a few laptop andpower tool batteries which I am slowly charging/ discharging.

I have many questions but for now am wondering your opinions onsuitability of harvestedpower tool battery cells for power walls andbattery packs. I realise the two are very different.
It seems most are using laptop batteries, any reason for this..availability?


I plan tomake a 60 cell 12v test battery next,have already made a few smaller 12v packs.

Any thoughts or opinions appreciated.
 
Laptop are generally low current cells with higher capacity meanwhile power tools are made to be able to deliver higher current.

Just note that there is still many power tools with NiMh. Those should not be mixed with Liion that is most common type in laptops.
 
Another thing to note is, is that a lot of ppl (including my uncle) says that the "nature" of lithium batteries is to deliver full power till it's discharged, and it stops suddenly. I'm sure we're all aware of using a drill and just banging along and it just stops suddenly.

Some may think that this means the cells are being drained really low. However, I'm under the impression that the controller board in the battery pack monitors the voltage and when it drops below a set point, cuts power to preserve the pack. Then when the cells recover, you get a little more juice from them. So, I would almost say that powertool battery packs have a better life than laptop batteries.

Now, as daromer noted, powertool cells are designed for high amp discharge, while laptop cells are designed for high mAh capacity. We have to keep this in mind when building packs from different designs.
 
My powerwall 1.0 is 90% power tool batteries. I'm building up my supply for powerwall 2.0 and that will be almost all laptop cells. The advantage over powertool packs is higher capacity with a smaller footprint. Most powertool packs i've got have cells in them with capacities in the 1300-1500mah range. With my 420 cells in my current power wall i have about 2kwh worth of power. My new powerwall i'm estimating will have 4-6kwh for almost the same footprint.
 
AZ_Tekkie said:
My powerwall 1.0 is 90% power tool batteries.
Click to expand...

Are we to assume then, that these are NiMH or NiCd batteries?

Korishan said:
Some may think that this means the cells are being drained really low. However, I'm under the impression that the controller board in the battery pack monitors the voltage and when it drops below a set point, cuts power to preserve the pack.
Click to expand...

I think so, mostly. But I have noticed that cells being charged come up very quickly to 3.7V from whatever discharged state they were in. So, natively, without a BMS, I think they drop off quickly below 3.7V. They can go further down than that IF THE LOAD IS VERY LIGHT, but I wouldn't really want to go there.
 
alfu said:
AZ_Tekkie said:
My powerwall 1.0 is 90% power tool batteries.
Click to expand...

Are we to assume then, that these are NiMH or NiCd batteries?


Click to expand...

Nope all Li-ion. NiMh goes straight to the bin.
 
Li-ion packs I've seen for powertools are usually some sort of green. And that's not just for Ryobi. I've seen others as well being green. Or, they are grey. I took a ryobi li-ion pack apart and all were 3.8v. With a resistive load on them, they only dropped to 3.5v. But I haven't charged them yet since I don't have a charger yet.

A lot of newer powertool battery packs are now starting to be li-ion packs. The tech is moving away from Ni cells. Probably because of how cheap the 18650's are to make.
 
I have stripped many li ion power tool packs and have found it very rare for cells to be below 3 volts,
unlike laptop packs I have opened its been common to find them below 2 v.
I just picked up another 20 18v packs this morning. The first one (dewalt) I opened was sitting at 17.57 volts.

I recently acquired 2 x 58v AEG packs and all cells are fully charged
In my experience the circuitry rather than cells causes power tool packs to fail
 
One big diff is that a powertool doesnt work when the bats just loose slightly so much capacity and umpf... A laptop, people tend to run as long as they can untill the battery doesnt last 10 min even :D
 
Korishan said:
Now, as daromer noted, powertool cells are designed for high amp discharge, while laptop cells are designed for high mAh capacity.
Click to expand...
That explains why the 'good' cells from the Ryobi battery I took apart all accept under 800mAh (charged up from 3.7V), while quite a few laptop cells take up to 2Ah. The Ryobi battery is a beast to tear down: 12 beefy solder joints to liquify simultaneously if you don't want to wreck the BMS.
 
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