18v Power tool batteries ?

[2jzs4rx7]

Hi
New ev noob here.
So I've been looking into the options of building a light weight buggy or motorbike to start with to get my head around all the required parts and the go & nogo's of electric conversions.
I've done heaps of engine conversions on cars and buggies in the past but ev is something I've never really looked into.
I'm considering something like the motenergy 96v dc brushless 12kw constant 30kw peak.
And was looking at something like headway 15ah lipos
30cells to create approx 98v
But then I started thinking could off the shelf 18v Power tool batteries work in this case?
I understand the overall amp/Watt out put per cell etc could be a limiting factor to power output vs a larger number of 3.2v cells
But I am not sure if the 18v cells would work in a series arrangement to get the 98v ?
Wasnt sure if it would cause them to limp mode if they were linked to output a high voltage compared to the 18v they are designed to output as a unit.
I thought if the project doesn't work out or I get over it I can still use the Bosch batteries for my power tools
Cheers the noob

[Johny]

The power tool battery packs actually contain a number of lipo cells in series. These cells are not rated to put out high current and will sag (voltage drop) if asked to do so. They will also not last long. EV battery packs using these smaller cells usually place many of them in parallel - then then parallel cells sets are placed in series to get the required voltage. You can see it gets to be quite a significant part of the EV conversion.
Lithium cells also need careful monitoring and control to be safe and reliable. A Battery Management System is almost always incorporated into the complete battery pack for this purpose.
I'd go the Headways but you will still need/want a BMS.

[weber]

Hi Daniel,

It's not a completely crazy idea. I certainly wouldn't put more than 5 x 18 V modules in series, due to concerns over lethal voltages and inadequate insulation. But then you need to look at how many such strings-of-5 you would have to put in parallel so they could handle the peak current, which is about 300 amps for the motor you describe. Here's a back-of-the envelope calculation:

If we take a guess that they have a 5C rating, that means you would need 300/5 = 60 Ah. So you would need 10 x 6 Ah modules in parallel, for a total of 50 modules. But maybe the 4 Ah modules are less $/Ah because the sell more of them, in which case you'd need 75 of them. I see them for about $44 on eBay. So that would be $3300 for a 90 V × 60 Ah = 5.4 kWh battery. Not too bad, I think. Others please correct me.

But maybe they are only 1C, in which case you'd need 5 times as many modules.

The tricky part would be monitoring all those "third wires" from each module, that tell you when you have to stop discharging, or stop charging. If you don't do this, then when the first cell hits bottom and the entire rest of the pack keeps ramming current through it reversing its voltage, it will explode, setting off a chain reaction that makes the entire battery turn into a giant fireball.

Also, although I described it as paralleling strings-of-5 above, you would really need to have cross-ties that parallel them at the module level. You would need to pay a lot of attention to detail with this, so that the 300 amps gets shared as evenly as possible between the 15 modules in parallel. This depends heavily on ensuring that they are all at the same temperature.

[Richo]

Even the headway you'd need more like 66-100 to get 30kW peak.
The 10Ah headway would more like $2,650 for 30kW.
Not too sure on the cost for 15Ah cells.

The LTO 2.4V 2.9Ah that I mention in another thread would be about $900 for 30kW peak.
Downside is you'd only have like 5km range
But you'd have an absolute blast for all of 1min

[Richo]

I do remember some show TV where they used tool battery packs for a conversion.
Some of the packs were in parallel so they had common +/- rails mounted to some wood.

Well the wood had moisture in it and started smoldering.

[Simon]

Here is the show your thinking of.
https://youtu.be/evoMgyTuTEg
Not very practical to use that many packs but good for smaller projects.
Iam going to use 2 x 18V 4AH cordless tool batteries from Aldi to power my electric bike and can use them in my electric lawn mower as well.
I have tested them in series to power my 36V ebike but haven't used them in my 24V mower yet. The 4Ah packs are designed to be used in series for the Aldi electric lawn mower I believe.

[2jzs4rx7]

Ok thanks for the fast replys fellas pretty much as I thought the answer was going to be I thought there would be a limitation on each pack with its internals series structure trying to grid tie multiple packs together in series again. Got the idea from Tesla using the 18650 cells in there vehicle packs but 7000+ cells on their 90ah pack lol.
Another quick question with the motenergy 12kw/30kw
It shows a rating of .15nm/amp output
If I can jam 400amp peak into it ?
.15*400a = 60nm correct ?
Would this be at the motor output shaft and then multiplied by the gearing used in the conversion ?
I read the specialised ebike can develop 90nm peak out of its "250w atw Max rating"
Made me wonder if that's accurate and what sort of torque a 12kw motor could develop in a light motorbike or offroad gokart etc.
Thanks again dan.

[Richo]

Apart from reverse voltage fireballs mentioned above if parallel packs are not similar they can also unbalance which again ends up in fireballs.
This will become more noticeable the more it's used.

Another quick question with the motenergy 12kw/30kw
It shows a rating of .15nm/amp output
If I can jam 400amp peak into it ?
.15*400a = 60nm correct ?

Technically yes.

Would this be at the motor output shaft and then multiplied by the gearing used in the conversion ?

Yes 2 for 2.

...what sort of torque a 12kw motor could develop in a light motorbike or offroad gokart etc.

I see some peak Current of 450A thrown around on the net.
So 67.5Nm would be the claimed peak torque.
If you are using a 3:1 reduction ratio then it would be 200Nm at the wheel.

There are issues going higher than this such as thermal degradation of the wire insulation, magnet demagnetization etc...
Both kill the motor.

[Richo]

How does the price compare on a tool battery pack v's Headway,leaf,LTO's etc?