Battery powered chainsaw

[4570Man]

I'd modify that, slightly. They should have a battery that lasts as long as the spare needs to reach a near-full charge, even after the batteries have been aged several years or hundreds of cycles. If charge time is 3 hours, then I need my other battery to last 3 hours of infrequent cutting, lest I have to carry a boatload of batteries everywhere with me.

Furthermore, they must offer a 12V charger, so you can charge it from a vehicle, when not near 115VAC power. I suspect a typical car or tractor battery has sufficient capacity to recharge a chainsaw battery, and still start the vehicle afterward, but haven't done the math on that.

There’s no way a regular sized tractor battery could recharge a battery the size that a larger chainsaw or other outdoor tool would have and still restart the tractor. You can’t discharge a starting battery very much and it still work. It’s also going to require an inverter to charge a higher voltage battery from 12 volts. That’s not a big deal in itself but it’s creating more inefficiency in the process.

 

[WinterDeere]

There’s no way a regular sized tractor battery could recharge a battery the size that a larger chainsaw or other outdoor tool would have and still restart the tractor. You can’t discharge a starting battery very much and it still work. It’s also going to require an inverter to charge a higher voltage battery from 12 volts. That’s not a big deal in itself but it’s creating more inefficiency in the process.

First, I said "charge it from a vehicle", not "tractor". I was thinking more along the lines of a pickup truck, although it's possible either could work. I'll also admit I'm no expert on battery tech, I work in a different EE specialty, but here's how I see it:

1. Charging off of a battery would require a DC-DC converter, not an inverter. Similar in operation, but not the same thing. Typical DC-DC converter efficiency is ~96%, way better than AC inverters.

2. Most pickup truck batteries are 60 - 100 Ah, and we're talking about charging a 6 Ah battery, less than 10% of truck battery capacity.

3. Lithium-ion batteries charge at nearly 100%, it's the fast-discharge where they take the efficiency hit, meaning you're only pulling about 6Ah out of the DC/DC converter to charge the battery.

4. All of the above = just over 6Ah of power required from the battery to charge a 6Ah Li-Ion battery.

5. A typical pickup truck battery has a reserve capacity of may be 2 hours at 25 amps, the measure of time at 25 amps required to drag charge below 10.5 volts, but a 6Ah chainsaw battery will only require ~3.1 amps to reach near rated charge in 2 hours.

6. Vehicle alternators are typically sized to get battery from zero to 80% in about 2 hours, at typical 95% (lead acid) charging efficiency. Ballpark estimate to replenish the 10% taken from vehicle battery to charge your little 6Ah chainsaw battery is only 15 minutes with engine running.

From where did you get your facts/numbers? Can you give any basis for your certainty that this could never work?

 

[4570Man]

First, I said "charge it from a vehicle", not "tractor". I was thinking more along the lines of a pickup truck, although it's possible either could work. I'll also admit I'm no expert on battery tech, I work in a different EE specialty, but here's how I see it:

1. Charging off of a battery would require a DC-DC converter, not an inverter. Similar in operation, but not the same thing. Typical DC-DC converter efficiency is ~96%, way better than AC inverters.

2. Most pickup truck batteries are 60 - 100 Ah, and we're talking about charging a 6 Ah battery, less than 10% of truck battery capacity.

3. Lithium-ion batteries charge at nearly 100%, it's the fast-discharge where they take the efficiency hit, meaning you're only pulling about 6Ah out of the DC/DC converter to charge the battery.

4. All of the above = just over 6Ah of power required from the battery to charge a 6Ah Li-Ion battery.

5. A typical pickup truck battery has a reserve capacity of may be 2 hours at 25 amps, the measure of time at 25 amps required to drag charge below 10.5 volts, but a 6Ah chainsaw battery will only require ~3.1 amps to reach near rated charge in 2 hours.

6. Vehicle alternators are typically sized to get battery from zero to 80% in about 2 hours, at typical 95% (lead acid) charging efficiency. Ballpark estimate to replenish the 10% taken from vehicle battery to charge your little 6Ah chainsaw battery is only 15 minutes with engine running.

From where did you get your facts/numbers? Can you give any basis for your certainty that this could never work?

Amp hours doesn’t mean a lot by itself. Watt hours is a more useful means of comparison. A 60-80 volt 6ah battery would be more like 1/3 the capacity of a 100ah 12v truck battery. And that’s before any inefficiency in the process is accounted for. You’d have to pull more from the power source than you’re putting back into the small battery. The alternator output is kinda irrelevant. You can’t charge the lithium batteries very fast. The green works 60v 5ah battery is over 100 minutes charge time. I don’t have any factual information on how much you could discharge the starting battery and still start but I stick to the belief of not much at all especially in cold weather.

 

[WinterDeere]

Amp hours doesn’t mean a lot by itself. Watt hours is a more useful means of comparison. A 60-80 volt 6ah battery would be more like 1/3 the capacity of a 100ah 12v truck battery. And that’s before any inefficiency in the process is accounted for. You’d have to pull more from the power source than you’re putting back into the small battery. The alternator output is kinda irrelevant. You can’t charge the lithium batteries very fast. The green works 60v 5ah battery is over 100 minutes charge time. I don’t have any factual information on how much you could discharge the starting battery and still start but I stick to the belief of not much at all especially in cold weather.

Good point. I had forgotten the voltage of the saw battery is 60 or 80 volts, that's a huge factor. The other points were already accounted for in my prior post, but become irrelevant when accounting for the voltage multiplier.

 

[Fuddyduddy1952]

43 year retired electronic technician here, and I have the Greenworks 80v saw that came with an 2AH battery and charger and I also bought an extra 2AH battery & charger.
This info is from Greenworks as well as trusted sites...from actual experience.
Greenworks says 2AH charge time is 30 minutes which (ironically) is also the run time (144 Wh) or 150 cuts of 4"×4" PT posts.
Charger has an idle current of 9W and charging current draw of 334-342 Watts.
I like original batteries for everything, don't like the cheap knock offs. Greenworks 80v 5AH=70 min., 6.4AH=90 min. run/charge time.
Two 2AH batteries fully charged, run one 1/2hr., swap then charging that one, etc. basically would mean continuous cutting. You have to pause any saw adding chain oil, adj. tension, sharpen chain, etc.
I wouldn't ever recommend DC-DC charging for multitude reasons, mainly the Greenworks is a "smart" charger, not overcharging. That ~340W charger draw means an 500W inverter should work just fine (they're inexpensive) and 75-95% efficient (95% pure sine wave Inverter draw is 41.66A full load or about 28.5A @ 340W.
That load even on a 75AH truck battery isn't trivial so as mentioned you'd have to start/idle occasionally.
Having said all that () I see it as another tool. For my use I'd have both charged batteries with me for a small less than an hour job. If I had lots of cutting to do a gas saw is a much better choice.

 

[ning]

A 40V 2AH battery charging at say 80% efficiency takes 100Wh of energy.
A fairly typical SUV battery (possibly a 1/2 ton truck battery) has ~65Ah, perhaps probably around 700Wh.
Shouldn't have too much trouble charging that.
Going up to a 40V 4AH or bigger battery you may start running into trouble with your starter unless the truck is on while charging (such as when travelling between jobs).

 

[ultrarunner]

First, I said "charge it from a vehicle", not "tractor". I was thinking more along the lines of a pickup truck, although it's possible either could work. I'll also admit I'm no expert on battery tech, I work in a different EE specialty, but here's how I see it:

1. Charging off of a battery would require a DC-DC converter, not an inverter. Similar in operation, but not the same thing. Typical DC-DC converter efficiency is ~96%, way better than AC inverters.

2. Most pickup truck batteries are 60 - 100 Ah, and we're talking about charging a 6 Ah battery, less than 10% of truck battery capacity.

3. Lithium-ion batteries charge at nearly 100%, it's the fast-discharge where they take the efficiency hit, meaning you're only pulling about 6Ah out of the DC/DC converter to charge the battery.

4. All of the above = just over 6Ah of power required from the battery to charge a 6Ah Li-Ion battery.

5. A typical pickup truck battery has a reserve capacity of may be 2 hours at 25 amps, the measure of time at 25 amps required to drag charge below 10.5 volts, but a 6Ah chainsaw battery will only require ~3.1 amps to reach near rated charge in 2 hours.

6. Vehicle alternators are typically sized to get battery from zero to 80% in about 2 hours, at typical 95% (lead acid) charging efficiency. Ballpark estimate to replenish the 10% taken from vehicle battery to charge your little 6Ah chainsaw battery is only 15 minutes with engine running.

From where did you get your facts/numbers? Can you give any basis for your certainty that this could never work?

Maybe a Ford Lightning Pickup is the ticket?

 

[two_bit_score]

“Battery saw makers” don’t invent battery technology. They buy batteries from suppliers. Can only use what is offered for sale to them.

What a way to shortchange the saw manufacturing industry. Truth is they absolutely can pressure their battery suppliers for better battery technology.

 

[WinterDeere]

Maybe a Ford Lightning Pickup is the ticket?

lol... an $80,000 solution to a $800 problem! Cheaper to just fill the glove box on your old F150 with spare pre-charged chainsaw batteries.

And Fuddy, when I mentioned DC-DC converters for charging, I was referring to a manufacturer-engineered smart charger based on DC-DC converter tech. Not just home-spinning your own.

 

[kenmbz]

While Manufacturers can put pressure on battery companies, the standard form factor for a battery is pretty stagnant right now due to weight/space limitations.

Laptop manufacturers went away from standard cell type batteries, and invested heavily due to competition being in weight, size and longevity (<3 lbs and 8 hour flight goals).
Also people like smaller lighter laptops.

Power tools weighing extra is not anywhere near as important, yet.

So far the tool companies seem quite happy not to invest billions to change to custom batteries.
That might change, but already we complain about batteries not being interchangeable between brands. Also using standard battery types allows easier conversion to newer tech.