Run 18V Weed Eater off 36V Bike Battery?

[mancuz28]

Perhaps a newb question, but as I'm sure you know the 18V battery that comes with a B&D weed eater lasts about 10 minutes. I just happen to have a 36V 12 AH battery that I'm hoping I could put to use to help the situation...the eater's removable battery uses a fairly standard looking two pronged connection, so I'm wondering if I can rig my battery into the weed eater, and carry it in a backpack while I trim the yard.

1) Can this be done?
1a) How can this be done? (with a 36V to 18V step down converter? Our could I try running at 20V or 24V? Recommended hardware for the connection?)
2) Should this be done? I'm sure there is a risk of blowing up the weed eater motor which I'm not too concerned about...but I definitely don't want to risk my battery (or life).

My electrical experience is pretty much limited to installing and operating an ebike kit...but I'm very good at following instructions, so advice is much appreciated!

 

[Drunkskunk]

36v would probably melt it quickly. 24 volts might work. maybe. I have a 14v Black&Decker cordless drill I run on those same 18v packs. the motors they use seem to take some over volting ok, but try this with the full knowledge that you might destroy it.

Wiring is simple enough. use the battery and a meter to figure out which side is + and which is -. then wire your 24v pack up to it. I made a connector for my drill out of some spade terminals, and play-doh. I stuck the spade terminals in the drill, jammed some play-doh around them, and pulled it out when it was dry. coated it with epoxy, and I had a passable plug. Not the best, but it wasn't going to cause a short if the wires got yanked out.

 

[amberwolf]

Based on experiments with an old 12V ryobi SLA type, using a 6s RC LiPo pack, even without a load on it the motor will probably overheat and/or the bearings fail, going up at twice the voltage.

Under load it's almost a guarantee of failure if yours is built like mine (tiny brushed motor, no current limiting or controller).


If it has a controller, then it may just blow up under that high a voltage.


I'm sure you could get away with a bit more than the usual voltage...but without some active cooling you probably won't be able to push that much power thru what is likely a little motor.


But...even if the motor itself survives fine, the plastic mountings around it, and / or the plastic string holder on the motor shaft, may well melt from the motor heat. (ask me how I know )

 

[mancuz28]

Thanks....yes I figured 36V was going to be too much, but could I wire in something like this?
http://www.amazon.com/dp/B00C4QVTNU/

To reduce it to an appropriate voltage? Not sure if 8A is enough, but other than that?

 

[dogman dan]

Though it will cost you more initially, you could just set up to run the weed whacker on RC lipo, one 4s hardcase pack should do er. The cost will be more for a cheap charger than the pack. But then you are all set to run any other battery yard tools you may get later.

In my experience, if you over volt your weed whacker, even if you don't burn the motor, it will just eat string like mad. Slower works best on weed whackers, unless cutting very stubborn weeds. The guys who sell you the string won't tell you that though.

The other option is to just shop the sales. I just replaced my old 24v b&d with a Ryobi 18v lithium. Part of why I did this, is I had a Ryobi saw that needed a battery as well. It was pretty cheap, and the battery lasts a lot better than the old B&D nicads.