Mobile Tool Power.

ns8814@comcast.net · March 2013

Ok I am not getting the grasp of the electricity thing that easily. Basically I am looking to get some power to my truck. I started thinking simple like going with 4 deep cycle batteries and a 5000w inverter. The main tools that I will need to use is a 15 saw and a 15 amp compressor. I was thinking of just having a standard battery charger that I can plug in when power is available. I have a hard time converting the amps x volt to watts and then back to amp hours. How many hours could I plan on using the saw and compressor before needing to recharge. I also have a generator available but hate listening to that thing all day. I was going to wire the truck so that I could plug it in when I get to the job site and use some of the tools I have setup in there as well as 2 30 watt florescent lamps. The problem with that is if I have a saw going and the air compressor turn on chances are it will trip the breaker. So does it make sense to have the 120 ac go the battery charger which charges the battery bank-> which goes to a 5000 watt inverter. Will a battery charger be able to charge a battery bank in parallel? I'm not looking to run an 8 hours on the battery bank I doubt I could afford that many batteries. I just need a kick in the right direction.

Any help would be greatly appreciated.

Cariboocoot · March 2013

Re: Mobile Tool Power.

Welcome to the forum.

Let's look at the "one standard outlet" numbers: 15 Amps @ 120 VAC = 1800 Watts.
Here's the thing; tools have motors and the motors do not actually have a fixed current draw. They will have a start surge, and then the current will vary according to how heavily loaded they are. Somewhere on here I tested a bunch of power tools to show this. I'll see if I can find that thread again.

The air compressor will be the worst because it is going to try to start against a load: whatever air pressure is left in the tank. Mine will stall the Honda 2000 gen when it is cold out; not enough power at 1600 Watts to provide the start surge with cold compressor.

So in reality you want something like a 2kW pure sine inverter with a good surge rating. Stay away from the very high Watt 12 Volt inverters: they will invariably be MSW (which motors don't like) and of questionable quality. When it comes to firing up even 2kW from 12 Volts you need really good battery behind it and really good wires to connect it. We are talking short 4/0 cable firmly attached at both ends. The current can be over 200 Amps. That is worse than jump-starting another vehicle by about 3X.

Batteries. That is one open-ended question. Not only do they have to provide the high current draw mentioned above, but also for some length of time not determinable. That makes it tricky.

Frankly it is usually better to buy a big gen (think Honda 3000 inverter-gen) to power the big tools, and then get a small inverter set-up to run things like lights or tool battery chargers, et cetera. Supplying the full power needed via batteries and inverter is $$$.

westbranch · March 2013

Re: Mobile Tool Power.

If you plan on using saw and compressor at the same time I would go for at least 5000W, Honda has a 6500w inverter type, they are very quiet and idle down if not loaded.

ns8814@comcast.net · March 2013

Re: Mobile Tool Power.

thanks, yeah I have a 7k generator. However I hate listening to the thing. Would a standard Car battery even be able to charge a series of batteries in parallel. Also if I am understanding it correctly. If I have a 15 amp saw at 120v. That equals 1800 watts. So a 12 volt 105 amp hour (AH) battery can supply 1260 watt hours (12 volts x 105 amp hours =1260 watt hours). So the battery should be able to run the saw for .7 hours. If that is correct then I might finally be starting to understanding this. Obviously I should not completely discharge my battery.

Cariboocoot · March 2013

Re: Mobile Tool Power.

Actually if you try to supply 1800 Watts from 12 Volts & 100 Amp hours what you will get is instant dead battery.

In reality that 1800 Watts with conversion loss and powering the inverter will be like a 200 Amp draw. Guess that that does to a 100 Amp hour battery? I'll give you a hint: it's about 40X the current that capacity is rated at.

Another problem is that you can't use 100% of the battery's capacity. The lower it goes the fewer cycles you'll get and the harder it will be to recharge it. Below 50% capacity recovery becomes a very iffy thing.

So you're 105 Amp hour battery can provide about (50 * 12) 600 Watt hours DC, and that providing the draw isn't too high; the greater the rate of discharge the lower the actual capacity is.

If you were to set up a battery bank of 440 Amp hours @ 12 Volts (or better; 220 @ 24 Volts) it would probably work.

westbranch · March 2013

Re: Mobile Tool Power.

ns8814@comcast.net wrote: »

thanks, yeah I have a 7k generator. However I hate listening to the thing.

thats why the Honda i Series is so nice.

ns8814@comcast.net wrote: »

So the battery should be able to run the saw for .7 hours.

Nope. cut that in half

ns8814@comcast.net wrote: »

Obviously I should not completely discharge my battery.

Right, you don't want to go below 50% discharge.

Mobile Tool Power.

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