Makita 6851

[The Medway Handyman]

Just bough a Makita 6851 mains impact driver.

The reason being (not that I need much of a reason to buy a new power tool) is that driving in 6x90 coach screws works a treat with a 12v impact driver, but uses up the battery charge on the 1.3a/h batteries pretty quickly. Batteries are constantly on a charge/discharge cycle, so their life will be reduced.

Simple maths tells me that buying a new battery is going to cost me around £40, whereas the mains machine cost me £75 + I have the benefit of a machine always ready to go - sods law ensures that the battery will go at the most inconvenient moment.

What puzzles me though (& I haven't tried it yet) is that the mains jobby is

300w & rated at 100Nm, whereas the 12v jobby is rated at 135 Nm.

I would have assumed that any mains machine would automatically be more powerful that its battery equivalent?

[John Rumm]

Charging them while hot does not do them any favours, although if you have three batts that at least gives one a bit of cool down time.

Well if you assume that the mechanical power out is approx half the electrical power in - that gives you 150W ish for the mains one, whereas the cordless will probably develop in excess of 200W

Makita don't tend to publish output powers, but DeWalt do. Their equivalent tool would be something like:

mains is not automatically a guarantee of more useful power (not to mention it is harder to get smooth delivery of torque from an AC supplied motor).

Decent NiCd and NiMh cells can provide exceptionally high current delivery when required. (the top end electric modelling nuts will run match race packs at 50 - 60A discharge rates. I remember seeing one fast electric boat that was designed for a power draw of about 2kW! Needless to say the cordless tools will have more modest current requirements (and more than 30 secs of run time!)

[Rod]

Given what seems to happen between newly charged and discharged, perhaps a more accurate rating for the battery one would be 135 -> 40 (or some other low number)?

[dom]

I don't find it the torque that tails off that much as the battery discharges (maybe it looses 30%), but rather the rep rate of the impacts. It becomes unsable and much more prone to camming out (I always break bits when I'm thinking "time to change batteries").

[Rod]

Well 30% loss (and what I put was just a finger-in-the-air guess) would put it within spitting distance of the 100 that the mains one claims. So, perhaps they are fairly close functional equivalents over the full battery life?

We await Mr Medway's full comparison report. Hope he's got a few decks to do in the next few days.

[The Medway Handyman]

That makes sense, sounds like they are quoting the battery model max torque with a fresh battery.

Two very large jobs in the next fortnight!

[George]

You bought an air compressor didn't you?why not an air impact drill?

[The Medway Handyman]

I was looking at air operated tools as one option to speed up decking jobs, ended up with an autofeed screwdriver for the boards - much better than nailing. Been using a 12v impact driver for the frame bolts, bought the mains version to save battery wear & avoid waiting for re charges.

[SantaUK]

I asked the question months ago for doing my decking. I bought a Electrical Makita screwdriver for doing it. Wow - what a time saving. I think we have all went to the cordless a bit too much now - cordless lawn mowers etc. We've really forgotten how good electrical tools really are :-))

Regards

SantaUK

[John Rumm]

With decent cells there will be very little tail off in performance until the end - at which point you should stop using it anyway rather than run the cells completely flat. That has certainly been my experience with the 2.6Ah NiMh packs I have.