Small inverter to handle power tools..


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Wedde

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Aug 15, 2018
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Hi guys.

I,m on the planing stage to build a powerbank for power tools with 18650 batteries. So if i understand this right (pretty new in this game) i need a inverter that can handle around 1000w and preferable 1500w. Power drills for example is marked up to 1000w. I guess you want to go from 12v to 220v? or 48v to 220v?

The idea is to use a ammobox with limited space. It suppose to be simpel to carry.

I hope i can get some inverter ideas :idea:
Thanks! :)
 

Redpacket

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Feb 28, 2018
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If you're doing 1500W, you'd want to have a battery doing min 24V, the losses & currents are to high at 12V. Also a good "12V" is hard to get out of Li-Ion cells, 3S is too low, 4S too high.
7S is a good match for 24V.
Drills are doable via inverters but tools like circular saws, floor sanders, etc the startup current will be too much & most low cost inverters will likely overload & fail early.
Many drills seem to have soft start circuits which help too (my 1kW Metabo drill does).
 

Korishan

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Using an inverter with powertools, you have to remember you will be hammering the Surge section a lot. So, even though the inverter is rated 1000W, you will be pulling 2000W on a regular basis and this would shorten the life drastically of the inverter.

220V * 4A = 880W running. It's easily possible to expect close to 2000W on startup, especially with circular saws and when they get bound up in wood it spikes, too.
And what happens if you have two power tools running at the same time? Or, you have a shop light that would use the inverter (even some LED shop lights use AC to run and can't be rewired to DC).

I would recommend using a 2000W inverter that can handle 4000W surges. The inverter will thank you.
48V battery would be best as you can get away with smaller wiring, but more importantly, you can get away with 1/2 the amps on the cells. Meaning they can handle higher surges a lot easier, too.
 

Wedde

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Aug 15, 2018
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Thanks for the answers!

A 2000W that handles 4000W surges, is it even possible to find inverters that would fit a "carryable" box? The ones i looked at are pretty big for my project.
 

Korishan

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Not sure what your constraints are. But sometimes you have to sacrifice size for usability. You might need to have 2 carry luggages instead of just the 1.

Do you want to have inverter that keeps resetting every time you put a heavy load on it, or one that will run without a hiccup?
 

Wedde

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Aug 15, 2018
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11,8 x 5.9 x 7.1 inches (converted from cm)

Ok, thanks for the input. I will take my new knowledge and put it to use!
 

DarkRaven

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Sep 2, 2017
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It's important to look at the tools you will be running first. As you've already established it's the peaks that will be the biggest challenge. Many of the larger tools have a built in soft start to avoid the peaks when starting. I'm going to build something similar and aim for a 3kW inverter with 6kW peak. Such a device would be around 10kg, a 2kWh battery would be a further ~20kg. We're not really talking about a "powerbank" anymore, it's one heavy case or separated into two parts. 2kWh might not be needed but you need chunky cells to get the required currents.
 

Wedde

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Aug 15, 2018
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Well the weight is one thing, a large container is another. Its obvious i need to rethink the project, i thought i could be done smaller. But i like the idea to be able to use all of our powertools without long extension cords and be limited to the immediate surroundings.

Which tools could go as high as 8kW peak? its sounds extremely high...

I guess a powerbank like this would be perfect to power refrigerator and freezer in case of power failure, so it may have more uses than i first thought of..
 

DarkRaven

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The inverter I'm looking at can do 3kW continuous, 4.5kW (150%) for 10 seconds and 6kW (200%) for 2 seconds. It's not that I need so much power at any given time but I don't want to run the inverter too hard. There is no single tool I own and that runs from 230Vac that could fully load the inverter, but I will run combinations of tools like the table saw (rated 2100W) and the vacuum cleaner for dust extraction (1400W). The vacuum needs 1400W all the time while the table saw is rated for 2100W and actual power will depend on the load obviously. Running idle it needs about 800W and it will hardly exceed this value during startup because of the soft start. But in theory this combo is rated for 3500W (and not by coincidence as it is designed to run on a 230Vac circuit with a 16A breaker = 3680W). The saw will only go this high when, for example, cutting through thick hard wood so I'll very likely never exceed the inverters maximum continuous power output, even on peaks.

Weight is an issue for me as well. Obviously I want it to be as portable as possible with as little effort as possible. The problem is that while you probably won't need very much actual energy you do need the high currents. Taking my table saw and vacuum combo as an example, they only run for seconds or a minute or two to make some cuts but in that short time the power is needed. Therefore you need lots of cells, or a few big cells, and you automatically get the bigger capacity and therefore energy. For example, for 3000W AC load you need 3300W DC input to the inverter and at 24V that comes down to 137.5A which is a lot.

I thought about LiFePO cells (between 60Ah and 100Ah). With a 3C discharge I'd have between 180A and 300A which is my target zone for this application. Obviously as high as possible without making it too big and heavy. But definitely not under 50Ah so I'll have 24Vx50Ah = 1.2kWh at least which would be enough to run my setup for at least 20 minutes which is much more than I ever need but I have as much energy because I can't use smaller cells. In the end I will likely end up closer to 100Ah so that gives me 2.4kWh just because I don't want to stress the cells too much. I never had to think about runtime for this project, it's solved by itself. I even thought about RC LiPo batteries because they can handle the current and are very light.

But you might need less. That's why it's important to think about the tools first. Many tools will need less. Most power drills, saws, small angle grinders, sanders and so on will need (much) less than 1kW.

A fridge is a much smaller load. Except you have a huge and old one which is really inefficient. My fridge/freezer combination uses no more than 60W (just if it's very warm outside) and usually more like 30W. It's roughly 0.5kWh per day. A battery backup for it would be very small and light unless I need several days / weeks of runtime.
 
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