more torque from 18v brushless trimmer

[banditman]

I have a 18v ryobi brush less trimmer that has an expand it capability so i bought the brush cutter to go along with it.  I installed a saw blade style cutter blade onto the brush cutter head but the trimmer really struggles to run this.  If i bought some 9 ah high output batteries will they increase the torque of the trimmer or just run time?  Would the 40v have more torque?

[paulengr]

Hard to say about any of it.

To begin with ALL electric trimmers are marginal at best. Landscapers use them because of noise rules but they are terrible. Everywhere else they use gas trimmers for a reason. And brush trimmer heads don’t work on “feather light” gas engines but what you have is evff Ed n lower torque than the lightest gas trimmer. It does what it does...makes a thin string go fast for LIGHT duty jobs. It’s not a chainsaw. Any of them, even Stihl.

I’m not a landscaper but I grew up on a farm and my current lawn is 3 acres so the light duty stuff for 1/2 acre lawns is useless. I learned long ago the brush cutter heads are marginally better. The biggest advantage is you don’t have to constantly replace string when brush cutting. The best thing to do is get better string. Either get the stuff with a triangular shape or get the stuff with the titanium steel reinforced core. Both do fantastically better than the round plastic string and the titanium string beats the brush cutter blades. Plus these are both compatible with the standard head.

As to your ideas...

The torque out of a brushless DC motor is related to the voltage at the motor leads. So if it’s the same motor you can get more out by raising the voltage. Or with the same voltage use more wire in the motor (more turns) but this requires more available current and thus bigger wire. Higher voltage keeps turns and wire size down but demands more insulation so has a similar effect on the motor. This is why for instance Dewalt created the Flexvolt batteries but Milwaukee stuck with the 18 V platform but just made larger (more current) batteries to achieve the same performance. You just can’t predict these things. So the 40 V version might help, or might not. It’s an expensive experiment. I guess you can always return it.

As to why the 40-60 V range the answer is Ego. They set out specifically to make battery powered landscaping tools. The competitors were awful at the time. Ego uses a 56 V battery but the thing is HUGE compared to even the biggest from anyone else with big contacts. I mean the batteries alone are bigger than most Ryobi tools. They were the first ones that did not suck. So suddenly everybody else had to step up their game. To date I think only Stihl matches them. So they upped both voltage and current (output). Unless Ryobi has a ten pound battery they just aren’t going to match that.

If the resistance in the battery itself limits output current, voltage will drop and torque is affected. This is where higher output batteries will help. But if the limitation is the controller or the motor itself all you get is a higher capacity...the internal resistance is less but the battery is not the limitation, Usually high output is useful where low speed torque is an issue like with a drill in steel or an impact gun. If there is a big difference on a fully charged battery compared to “1 bar”, high output will matter but otherwise it probably won’t. Again...you can’t predict these things. I have HO 6 Ah and regular 5 Ah Milwaukee batteries. I’m a contractor. I can’t tell the difference other than the HOs last longer.

Plus there is a big difference in terms of engines. Horsepower equals torque times speed. In a gas engine which is what the attachments are for horsepower is a curve. You don’t hit full horsepower and torque until around 2000 RPM and it goes up from there. Thus you HAVE to have a gearbox. Cars would be a lot simpler and lighter and faster without it. In a DC motor horsepower is essentially a constant. You get maximum torque at zero RPM (stall). If you increase speed, torque MUST decrease. So with a trimmer when it works best at top speed only the gas engine is best. Electric is always going to be a compromise at best. It’s great for things like drills where it can slow down and still work but not with trimmers.