Battery Recharging Watts?

[Beemer]

Trying to figure out how much it's costing to recharge a tool's rechargeable batteries but not sure it can be done from the information known.
I have a 4Ahr battery and an ac charger that has 60 watt output for 18 volts.
what would be the math to figure out how many ac watts the charger is using for a full charge?

 

[Gozo]

4 amps @ 18volts is 72 watts. If it’s at full capacity for the 1 hour, that’s 0.072 kWh. If your charger is only 60 watts, it’ll take a bit more than the hour to charge. Figure in losses, the fact that AC is going to drop average vs peak voltage when converted to DC, and much less than 100% efficiency; it’ll be more of a drop. What was the question again?

 

[5ubtle]

It's costing you so little that figuring out 'how much' is a waste of time.

 

[Modern Garage]

Why do the math ( which is likely to be in error) when you can just plug it into a "Kill-a-Watt" meter and know for sure?

 

[Citation]

What's the battery capacity in either wh or ah? If you know it in watt hr just double that number to swag for loses and compare it to what you pay per kWh for electricity. If you have the amp hr number, multiply that by the battery voltage to get roughly the watt hr. Note that almost all lithium tools cheat on voltage and use peak vs average. While the old nicad pack had an average voltage of 12v or 18v (the average nicad cell was 1.2 V so those work out) the average lithium cell is 3.6 or 3.7V. Three cells at 3.6 each gives 10.8v but marketing doesn't want your new tool to have less voltage than the old one. Hence, peak voltage. Bit of a rant there and given all this is a swag calculation just use the started pack voltage.

 

[djbmw]

As Modern Garage said above, the only accurate way is to use a meter. The age of the battery will change its charging time, and curve, as well as the amount of amps it can take at various stages.

Having said that, the amount for a typical person is going to be low. And no, you dont just multiply by your electricity rate... you have to also include any servicing fees, taxes, distribution fees, etc. that your power company also charges. Still, you're likely paying less than $8/mo. to charge your batteries

 

[Citation]

As Modern Garage said above, the only accurate way is to use a meter. The age of the battery will change its charging time, and curve, as well as the amount of amps it can take at various stages.

Having said that, the amount for a typical person is going to be low. And no, you dont just multiply by your electricity rate... you have to also include any servicing fees, taxes, distribution fees, etc. that your power company also charges. Still, you're likely paying less than $8/mo. to charge your batteries

Using the rate does make a lot of sense though it would be take the amount used / rate (make sure the units are the same). Trying to allocate the fixed costs only makes sense if you are billing for this usage or otherwise wouldn't have power at all. I don't think it's likely they would otherwise not have electricity. Also, while my method isn't going to give you a meter correct answer, most people likely don't have a meter. In such a case you can make some guesses.

If you don't have a meter or perhaps are considering new tools I think it makes sense to look at battery capacity * a loss factor (2x in my swag) then say that is how much power it takes to charge the battery. Convert that to kwh then divide by your kwh rate. What most of us will likely find is the power consumption is pretty minor on our monthly bill.

 

[Citation]

Just to throw some numbers on here, I picked residential rates for Clinton, MA as a guess. They are ~$0.15/kWh. The battery at most is 4Ah*18V, 72wh of power. Note I've already said that the 18V is peak, not average voltage. But this is a ball park swag. So we have 0.072kWh* $0.15kWh = $0.0108. I would double that to make up for electrical losses during charging. So about $0.02 to charge the battery.

 

[Beemer]

It's costing you so little that figuring out 'how much' is a waste of time.

Not a waste of time.
The item can run off of either battery or ac power so the question is to compare the two methods.
There is a reason for interest in that, as mother says, "you can't fight city hall", and they really do take advantage on the electricity bills here.

 

[Beemer]

Just to throw some numbers on here, I picked residential rates for Clinton, MA as a guess. They are ~$0.15/kWh. The battery at most is 4Ah*18V, 72wh of power. Note I've already said that the 18V is peak, not average voltage. But this is a ball park swag. So we have 0.072kWh* $0.15kWh = $0.0108. I would double that to make up for electrical losses during charging. So about $0.02 to charge the battery.

All the added fees that have been allowed makes our's more like $.29 kWr.
It's not funny how tax cuts get hidden back in other places.

Using the does rate makes a lot of sense though it would be take the amount used / rate (make sure the units are the same). Trying to allocate the fixed costs only makes sense if you are billing for this usage or otherwise wouldn't have power at all. I don't think it's likely they would otherwise not have electricity. Also, while my method isn't going to give you a meter correct answer, most people likely don't have a meter. In such a case you can make some guesses.

If you don't have a meter or perhaps are considering new tools I think it makes sense to look at battery capacity * a loss factor (2x in my swag) then say that is how much power it takes to charge the battery. Convert that to kwh then divide by your kwh rate. What most of us will likely find is the power consumption is pretty minor on our monthly bill.

Yes, a meter is on the way, if for nothing more than education on how much electricity is used by things.

 

[kbeefy]

The Kill-a-watt meter is pretty handy. I think it even has a way to enter your $/kw and display the running cost of something.

 

[djbmw]

Not a waste of time.
The item can run off of either battery or ac power so the question is to compare the two methods.
There is a reason for interest in that, as mother says, "you can't fight city hall", and they really do take advantage on the electricity bills here.

Uhhh.. then its going to be cheaper using AC power directly! You dont need to 'calculate' anything for that conclusion.

Charging a battery has losses within the charger as well as the battery. The battery will also have natural loss over time in storage.

The only time this wouldnt be true is if you have "time of use billing" from your electricity company where its SIGNIFICANTLY cheaper to charge the batteries at a certain time (night time, for example), and then use the power during the day (when rates are generally more expensive).

But as far as electricity consumption, running a tool directly from AC power will consume less.

 

[theoldwizard1]

How fast you can SAFELY charge a battery depends a lot on the battery chemistry !

If it is a "dumb" charger (basically just a power supply) and it is outputing 18.0V @ 3.33A a 12v flooded, lead acid battery "can take it" but it won't be "happy" and you will shorten its life (if it doesn't explode from boiling all of the electrolyte out first !).

 

[toolenthusiast]

Trying to figure out how much it's costing to recharge a tool's rechargeable batteries

Cheaper than buying new batteries every time yours are dead

 

[Beemer]

How fast you can SAFELY charge a battery depends a lot on the battery chemistry !

If it is a "dumb" charger (basically just a power supply) and it is outputing 18.0V @ 3.33A a 12v flooded, lead acid battery "can take it" but it won't be "happy" and you will shorten its life (if it doesn't explode from boiling all of the electrolyte out first !).

It's the charger that came in the kit so we are at the mercy of the manufacturer and know little about it except that it shuts off when charged.

Just got a tester and the discharged battery is on a charge cycle test so a better idea will be known soon.

 

[Beemer]

Thanks all for the ideas.

The charger is on the new meter that includes a cost reading based on a rate input and then another test can be run on a direct ac feed into the fan (rather than the battery).

We know how long the fan continually runs to deplete the battery (overnight) and can compare the same time run on ac to compare the cost output on the meter.

 

[Rabid Badger]

If the fan can run on AC, running it on batteries is always going to use more electricity due to the losses in charging the batteries.

 

[djbmw]

Thanks all for the ideas.

The charger is on the new meter that includes a cost reading based on a rate input and then another test can be run on a direct ac feed into the fan (rather than the battery).

We know how long the fan continually runs to deplete the battery (overnight) and can compare the same time run on ac to compare the cost output on the meter.

If thats the test you have lined up then you will also need an RPM meter to detect when the battery starts providing less voltage and the fan blades slow (or run slower than the AC counterpart).
With AC you get the full hog all the way through.... but with batteries it will start off strong, then settle into its "comfort" zone and remain there for most of the run time, then start to slow down for the last ~15% of the test time.

But,... as has been mentioned, direct AC is more efficient at running a tool than charging a battery to use in the same tool.

 

[Beemer]

So I ran the test on charging the 4 Ahr battery.
The charger lists 60 watt output but the ac draw was 40-45 watts and it ran 1 hour and 52 minutes.
It used .077Kw so at 29 cents/KwHr electric rate that was all of 2.2 cents per charge.
Good information to find out but as another post noted the cost is in the replacement battery when needed and not the charging costs.

 

[Citation]

So I ran the test on charging the 4 Ahr battery.
The charger lists 60 watt output but the ac draw was 40-45 watts and it ran 1 hour and 52 minutes.
It used .077Kw so at 29 cents/KwHr electric rate that was all of 2.2 cents per charge.
Good information to find out but as another post noted the cost is in the replacement battery when needed and not the charging costs.

I feel pretty good about my swag guesses at this point.

 

[rust in the eye]

Not a waste of time.
The item can run off of either battery or ac power so the question is to compare the two methods.
There is a reason for interest in that, as mother says, "you can't fight city hall", and they really do take advantage on the electricity bills here.

Always losses when converting. Feel the warmth of that charger?
If "city hall" bothers you enough to worry about how or if you charge batteries then buy your own rooftop solar.

 

[Beemer]

Always losses when converting. Feel the warmth of that charger?
If "city hall" bothers you enough to worry about how or if you charge batteries then buy your own rooftop solar.

After a lifetime of building design there is no way those things would ever go on my house due to the negative effect on the Architecture.

And just imagine the organisms that grow in the shade of the panels that thrive on the menu of shingles.

 

[Beemer]

I feel pretty good about my swag guesses at this point.

Yes, I did take note of your estimate.
But the "double that for loss" part, was that a Volta theory?

 

[Citation]

Yes, I did take note of your estimate.
But the "double that for loss" part, was that a Volta theory?

No, all of it was very swaggy. For example, I used 4Ah*18V to get battery capacity in whr. That actually over estimates because it assumes the battery was actually fully discharged. So while I said it needed 72whr to change I suspect the actual amount was less. Another concern is charge efficiency. Li-ion is actually pretty good in this regard but consider that any heat you feel is power not going into the battery. That includes both the battery heating up and loses in the charger. I suspect the overage would be more like 30% and absolutely less than 2x. But I figured it was better to over vs under estimate.

The power rates were a true guess on my part. But the overly aggressive loss guess mixed with the wrong power rates ended up balancing in the end

 

[rust in the eye]

After a lifetime of building design there is no way those things would ever go on my house due to the negative effect on the Architecture.

And just imagine the organisms that grow in the shade of the panels that thrive on the menu of shingles.

I wouldn't put those scabs on my house either for the same reasons. Looks like we're stuck with city hall.

 

[Beemer]

I wouldn't put those scabs on my house either for the same reasons. Looks like we're stuck with city hall.

Until the next governor's election.
Our's lost us over taking advantage of us in the electric rates regardless of his party.
Despite residents' outcry we just get ignored. What goes around comes around.

 

[Beemer]

No, all of it was very swaggy. For example, I used 4Ah*18V to get battery capacity in whr. That actually over estimates because it assumes the battery was actually fully discharged. So while I said it needed 72whr to change I suspect the actual amount was less. Another concern is charge efficiency. Li-ion is actually pretty good in this regard but consider that any heat you feel is power not going into the battery. That includes both the battery heating up and loses in the charger. I suspect the overage would be more like 30% and absolutely less than 2x. But I figured it was better to over vs under estimate.

The power rates were a true guess on my part. But the overly aggressive loss guess mixed with the wrong power rates ended up balancing in the end

I was funning with you.
Mathematical derivations was always my thing so I can get skeptical on approximations.
Thanks for the input and you were close enough.