Batteries and power tools...

couchsachraga
couchsachraga Solar Expert Posts: 84 ✭✭
I've always had a fairly modest setup at my place in the woods (completely off-grid by several miles...). I'm upgrading to a larger system that fits my current needs and then some, but is still fairly modest (1kw of panels, 600Ah of batteries (300Ah useable to 50% discharge, not that I plan on going that deep frequently).). I have a use of a generator in there, typically for construction projects, but realized the other day that I could likely run my radial arm saw off the batteries if I was just making a few cuts (not a big project... for that I'd fire the generator up).

So, the question, assuming a peak load of 2000 watts (2.5hp motor... I can't find a lot of info on what it draws but found the 2kw number somewhere), is that big of an amp draw for a short period of time going to shorten the life of my batteries? As i have a 12v system for the most part I'd have to get a separate inverter (I'd put right with the batteries, and then run 110 to the saw). Ok idea? Bad idea? OK idea if it is a sunny day (1kw coming off panels remember)?

Thank you all for your help, past and future!

Comments

  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Batteries and power tools...

    Persoanlly, I wouldn't do it. I think drawing 2+ kw from 12 vdc is going to be tough. Not to mention that if you hve to go with a bigger inverter just to power the saw, it will run pretty inefficiently the rest of the time. For the few gallons of fuel you might use, I would stick with the genny.

    That said, there have been pretty good advances with cordless tools, including drills, sawzalls, and circular saws. I tend to us cordless tools for lots of stuff, and then fire the genny for the table saw, planer etc.

    One final note, you should probably check the strting current for the saw. Depending on how big it is, it might momentarily draw 10 times (or more) starting than it does running. So a 10 amp saw might draw 100 amps to start on 120 vc. On 12 vdc that would be 1000 amps! Pretty hard for a small batty to deliver that current even for a short time without dropping voltage drop enough to send the inverter to low voltage shut down.

    Tony
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: Batteries and power tools...
    I've always had a fairly modest setup at my place in the woods (completely off-grid by several miles...). I'm upgrading to a larger system that fits my current needs and then some,
    <snip>
    As i have a 12v system for the most part I'd have to get a separate inverter (I'd put right with the batteries, and then run 110 to the saw). Ok idea? Bad idea? OK idea if it is a sunny day (1kw coming off panels remember)?

    If you are upgrading why not do it right and go to 24 volts? .... you might be able to reconfigure your batteries. (it would help for you to describe your system in a signature). Most charge controllers capable of handling 1000 watts can also handle 24 volts and are happier doing so. For most folks the hang-up in upgrading from 12 to 24 volts is having to buy a new inverter... but you plan to buy one anyway, so why not buy a 24 volt inverter?

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • couchsachraga
    couchsachraga Solar Expert Posts: 84 ✭✭
    Re: Batteries and power tools...

    The entire structure, with the exception of a single outlet, is 12v. Every light, ceiling fans, and every appliance (not that there are many!). So if I went from 12v to 24v, I'd need to convert back to 12v anyway.

    As an occasional use structure I've gotten by using panels that when new were 180w... and they are 30+ years old, and 2 Trojan 1275's.

    I don't have the new system in my signature yet as it's not purchased yet...next month it will be in and paid for, and hopefully this fall set up. Right now it will consist of 4 SolarWorld 245 watt panels, a Midnite Classic 200 controller, and 4 Trojan 1275's. I'll keep my trimetric meter, which has been of great help. I'll also have 60 tubes of solar thermal (hot water) that will be used both for heating the structure and pre-heating hot water (I have a Rheem propane on-demand hot water heater, the sole reason for the current SureSine 300 inverter I have now).

    Thank you for the replies - sounds like I'm better off sticking to cordless and recharging them as I've been doing (and maybe get a cordless skill saw) and fire up the 5kw (8550 startup) generator when needed.
  • petertearai
    petertearai Solar Expert Posts: 471 ✭✭✭✭
    Re: Batteries and power tools...

    Hi I did a project lining with tg and v . Got a Makita slide compound mitre saw, from memory rated 1450 watts. Ran with no problems on my xantrex ps1800. My batts are 900ah 12volt.
    2225 wattts pv . Outback 2kw  fxr pure sine inverter . fm80 charge controller . Mate 3. victron battery monitor . 24 volts  in 2 volt Shoto lead carbon extreme batterys. off grid  holiday home 
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    Re: Batteries and power tools...

    I've had others here suggest that starting power saws is not typically a problem with an inverter that is rated only slightly above the saws rated load. This has to do with the saws maximum load being while sawing and it really has very little load when starting. It makes sense to me, but I haven't run any tests. With a 12 volt system, you need to be sure to have heavy wiring to prevent the inverter not seeing a huge voltage drop across the system and with just a 450 amp hour battery bank you will be drawing well above the 1/20th of the battery capacity(based on an hourly rate) and it will have a large drop by it's self, so it's doable but you might consider a larger battery bank, again hard to do on 12 volt system, you could go to 4 - L16 (@350 ah).

    When I hade a 12 volt system I happily used a dewalt battery system tools that even charge just fine on a MSW inverter(not all tools battery chargers enjoy MSW inverters)
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Batteries and power tools...

    Well I have run the tests - the results are on the forum here somewhere.

    Saws start "in free air" and as such have very little start-up demand, especially compared to a pump or compressor. But when you get into the wood the power demand goes up. Trying to predict how much it will be based on the manufacturer's ratings is about the same as guessing. For example I have an air compressor rated as 3.5 HP. Technically that is 2611 Watts, so no standard outlet could power it. In reality that is a "locked rotor" number which is meaningless in the real world. It will not start off the Honda 2000 when cold, and it can sometimes pop a 15 Amp breaker under the same circumstances.

    Don't guess; measure. Get the Kill-A-Watt out and run your saw through it from a household source or big gen and load it down with wood. See what sort of peak Amps it really comes up with.

    As far as powering anything off a 12 Volt 2kW inverter goes, the trick to getting that maximum power is to have good batteries and cables to supply it. At peak output it will be drawing about 167 Amps (providing the Voltage hasn't gone too low) and that is a significant amount of current. Keep the DC wires short, large (4/0), and well-connected and it should work for anything that a standard outlet can handle.

    BTW run time makes a big difference, as does the ability to replace that "used capacity". When I was doing the cabin roof last year I would run the gen first thing and bulk up the batteries. Then I could use the tools (including the compressor) all day from the solar and still have power enough to get through the night. The trick was to do that initial bulk-up from the gen. Saved running it all day just to have power available for the work.