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203 posts

Master Geek
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Topic # 102850 23-May-2012 21:25
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Ill try and keep it short.

Basically Im trying to run my Makita dropsaw off the van battery.
I used the power meter to test the wattage of the saw off the house mains, for about 2 seconds it runs at 1450watts then once its up to speed it drops down to a stead 450watts. I didnt have anything handy to cut but im sure power usage will go up when cutting through something.

Inverter is a 1000W continuous, 2000w peak
Now the instructions that came with the inverter were not specific at all, no installation or trouble shooting on it (its only 1 page) WITH the exception that it says if its was over a certain wattage it had to be direct wired to the battery. (generic instructions for smaller ones)
I have wired it up directly to the battery using the cables that came with it.

Ive procrastinated about 3weeks before writing this, and it works fine for charging batterys or anything else. But it wont power up the drop saw.

The batterys in the powermeter thing are flat, so it only turns on when plugged into the power.
When I try and run the saw, it powers up for about 1/2 a second then stops for about 2 seconds then powers up etc etc it loops.
The power meter gets too 700watts before the cutout, and then the powermeter turns off, so power is being completly shut off.
The saw does spin a little faster each loop but seems to peak, never reaching anywhere near full speed.

Im trying to trouble shoot why the inverter isnt 'peaking' to 2000watts like it says it will. The manual doesnt say how long a 'peak' should last, but I though 2 seconds should be within the time

Perhaps the 12volt battery cant take the required output? Altho its not flattening the battery as it still starts fine.
I have tried having the van running with idle control set to 3000rpm (its a diesel, so pretty high)
I tried to get a photo of it, but perhaps the connection of the red wire into the inverter itself, doesnt look like theres a lot of contact. Tho I show do it.

Has anyone else tried something like this?
In an ideal world id like to have a UPS to take the extra strain, but the price on one that will put out 1500watts is expensive, for something I only want 2 seconds of work out of

Edit: forgot the pictures.

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1569 posts

Uber Geek
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  Reply # 629671 23-May-2012 22:36
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You've got pretty much no show running a drop saw with a domestic inverter off a van battery. Your inverter is rated for 2kW peak load but a 255mm Makita is a 1650kW motor. Motor starting current is up to 6 times the rated current. The inverter is trying to provide enough current but it's tripping out.

I used to sell these cheap domestic inverters and we always told people not to use them for this type of task. Get a decent generator if you want to do these sort of site works.

Your power meter is next to useless for measuring motor start characteristics too. You need a decent clampon ammeter.

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Uber Geek
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  Reply # 629694 24-May-2012 00:24
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Not to mention i can see this causing the battery to drain quite fast?


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Uber Geek
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  Reply # 629696 24-May-2012 00:40
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I tried running two different drop saws off a 2kVA generator with Yamaha engine.  It will run pretty much everything else I've tried except a microwave oven, but the drop saws were basically no go.  They would start OK, but with any kind of load, the generator slowed down and couldn't hack the pace.

Got a new 6kVA generator and both drop saws run fine.  I can cut through wet wood with the wood saw, and thick aluminium with the metal saw.  Neither of these would work properly on the 2kVA generator.

Which makes it pretty obvious that your 1000W (2000W peak) inverter will not have any chance at all of running your saw.

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Ultimate Geek
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  Reply # 629763 24-May-2012 09:34
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First things first - GET A FUSE ON THAT RED LEAD!!!

With a cable that size, if it shorts out you will have a fire.

Secondly get a bigger (& better) inverter, trying to run something near its peak is always going to end in premature failure anyway.


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Uber Geek
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  Reply # 629802 24-May-2012 11:11
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A softstart module may help, but you are better off spending money on other things. They are not cheap new but you might find one secondhand.

Imho the 1000w inverter is too small to cope with the VA from the motor in operation even if you got it started.

But guess what - Dewalt, Makita, and Bosch all do 24V mitre saws. There are a fair few in the country.

This one might go for a good price:


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  Reply # 629805 24-May-2012 11:13
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[Edit: there are a lot of 18v Makita systems around as well]

203 posts

Master Geek
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  Reply # 629983 24-May-2012 16:46
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Yeah, I did think this would be the case.
The boss did suggest a generator, but as I already had the inverter by this stage I thought id ask on here first.
Good to know about the generator power requirements too. But it is also a lot bigger, which I was kinda trying to avoid.

And yes, there should be a fuse on the red wire, I looked for one but never ended up getting
As for flattening battery: in my job roll I dont use the saw a lot, to the point where ill do 2 or 3 cuts and thats it for the day depending on the job....Takes me long to run a lead and set everything up and pack up again then it does to actually do the work.

Thanks for the help guys, ill consider this issue solved = wont work.

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Uber Geek
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  Reply # 629991 24-May-2012 17:22
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I'm wondering if heavier gauge wire to the inverter would make any difference (assuming it's not the inverter topping out) I know that jump-starting a car using a set of cheap thin jump cables is near impossible, yet doubling the jump cables up or using a decent pair can make a HUGE difference.

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Uber Geek
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  Reply # 630012 24-May-2012 18:43
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I did this once with a 1000 Peak, 600 Constant Inverter and 2 500W halogens, they ran for 1/2 a second every 2 or 3 secs.


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  Reply # 630300 25-May-2012 12:51
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IME all inverters grossly overstate their capacity.

Had a 150w (300 peak) one that was failing to supply 5 25w lamps in a chandielear once, unless the car was running and being revved pretty hard.


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Master Geek
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  Reply # 630481 25-May-2012 17:25
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I took in a bit of paper I riped out of the Super Cheap thing this week regarding the 25% off generators to the boss, he said to just go in a measure them (size) and see if the one I need will fit in the van.

So I guess ill be going down that track. Id much rather the inverter idea, not sure I can justify the cost of the generator just for for saving a bit of time on setup and packing away. but hey, not me buying it.

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Uber Geek
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  Reply # 630527 25-May-2012 20:28
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The reason why high power inverters are "overrated" (which would be false advertising) is because of a poor battery connection. Despite how good a connection you think you have, for a 1000W inverter (is that input power or output power?) the current draw off the battery is almost 100A. Unless you have a large truck, your alternator is probably rated 80A or 100A so that tells you the size of cable needed for a short connection, a long connection needs thicker. An then you need a clamp to connect to the battery, not crocodile clips or (e.g.) M6 eyelets, because even a 0.1 Ohm contact resistance at 100A will give a 10V drop across the connection.

Recently my 60Ah battery would often run flat. Lots of diagnostic by different "pro's" could not find the fault and said it is either a leakage somewhere or intermittent alternator despite telling the guys it is a bad connection. Then I fixed it myself, one of the crimps on the alternator power cable had slight corrosion between the cable and the crimp and I just filled it up with solder. Issue found by measuring the voltage drop between the cable core and the eyelet about 2V.

The Dewalt battery power tools use LiFePO4 cells from A123. They are very safe (unlike Lithium Polymer which is not bad but can still be dangerous), extremely low series resistance, and can deliver over 100A for 10 seconds. Highly recommended. By comparison, NiCd and NiMH has about 100x the series resistance, and Lithium Polymer cannot be abused and has limited cycle life.

A generator is a petrol engine with a high voltage generator so the current is lower for the same power output so connections are not critical. Personally for your situation I would go for Dewalt battery power tools if I can, otherwise a generator.

You can never have enough Volvos!

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