Different voltages in rechargeable batteries

[NikNak]

I'm looking at getting some cordless garden tools and slightly confabulated by all the different voltages available.

I understand that the Amp Hour of the battery is the 'running time' of the tool... X no. Ah = number of working mins etc.? (depending on tool of course)

But all the different voltages....?

I get that 2 x 18v = 36v i.e. you'll need two 18v batteries to make said tool work. But then there are 40v, 60v and even 80v batteries.

Would i be correct in thinking that voltage = 'grunt' of the tool.? and a bigger amp hour battery making the 'grunt' deal with a bigger load or indeed 'grunt' a bit longer.?

Or am i grunting up the wrong tree


Nick

 

[D_W]

I'm guessing that you can use a low volage high AH battery to get a lot of total wattage (like milwaukee does or at least did with an 18V chainsaw), but that running high amp low volt vs. the other way around to the same wattage is a good way to make the transmission of said wattage more costly (requires a bigger conductor, and more complicated heat management).

Aside from that, I'm guessing it also complicates things to run multiple batteries (it would be nice for those of us with gobs of 4 or 6ah 18v batteries to be able to use them in pairs or triples on a larger tool), and isn't in the interest of the maker to make high amp tools that use groups of already sold batteries vs. making single higher voltage systems and even more expensive batteries.

One thing that strikes me as a bit annoying is that lithium ion battery capacity is a fraction of what it was cost-wise 12 years ago, but the cost of something like a 4ah battery is at least as much (in 18V) as it was 12 years ago. And many of the older battery part nos. are being replaced or supplemented by others. For example, ryobi has p10# 4ah batteries (not sure of the number, may be 108). When you go to shop for 4ah batteries, there are two different versions now, and the newer type for whatever reason has notably less capacity than the older ones after a very short period of time.

 

[Sideways]

Almost certainly anything labelled 20V puts out exactly the same as other people's 18V batteries. DeWalt for example are masters of this. The marketing oiks think putting a bigger number on it will make more people buy it. It's like hifi used to be. Pick a way of measuring something so you can advertise a bigger number than the next one on the shelf. Don't be conned. In fact the typical battery is 20V hot off the charger and settles to a steady 18V very quickly.
The only real exception are premium brand Hilti who do actually have more cells and more real volts in their battery packs than the standard but I don't think they make garden tools.

36V vs 40V is exactly the same.

If unfamilar with electricity. Think of voltage as like the water pressure coming out of a tap. Higher is generally better. Amps are like the flow. And Amp hours, how much flow for how long.
To compare, multiply the volts and amp hours.
18Volts x 5 Amp hours or 36Volts × 2.5 Amp hours will be essentially the same, with a nod to the 36V battery probably having the edge if they are both quality tools.

 

[PerryGunn]

Would i be correct in thinking that voltage = 'grunt' of the tool.? and a bigger amp hour battery making the 'grunt' deal with a bigger load or indeed 'grunt' a bit longer.?

Not quite, the 'grunt' of the tool is indicated by the Wattage (Power) that is produced to drive the tool

Power (Watts) is calculated by multiplying voltage (Volts) by current (Amps)

In order to prolong life, batteries tend to be current-limited i.e. they tend to be designed to provide a maximum sustained current draw, this limits the power available at any given Voltage.
e.g. 5A at 18V will produce 90W and an 18V/5Ah battery will last for around an hour

The use of higher voltage batteries means that the manufacturers can
- produce more power with the same runtime e.g. 5A at 36V will generate 180W and a 36V/5Ah battery will last for around an hour
- produce the same amount of power with longer runtime e.g. 2.5A at 36V will generate 90W and a 36V/5Ah battery will last for around two hours
- produce a bit more more power and have a bit more runtime e.g. 3.5A at 36V will generate 126W and a 36V/5Ah battery will last for around an hour and 25mins

The drawback is that, at the moment, within the same brand the higher voltage batteries tend to be heavier and more expensive

 

[artie]

In theory bigger is better(more powerful)

In practice, it doesn't always work out like that.

A few years ago I had to screw a wooden base down to a concrete yard.

I hired a 20v drill for the day but ended up using my own 12v dewalt.

 

[Jonm]

Perry Gunn is correct but the claimed voltage is a minefield, 10.8v and 12v is probably the same, as is 18v and 20v.

Li ion cells are generally considered to be 3.6 volts however the voltage varies with state of charge and temperature. I attach a graph below for a particular battery cell, probably rated at 2 amp hour.

Fully charged they are 4.1volts and just before the voltage drops rapidly they are 3.6 volts. So for a five cell battery the manufacturer can claim a voltage between 18 and 20.5 volts.

So the question to ask is “how many cells”. Fundamentally there is unlikely to be any difference between a 20 volt battery and an 18 volt battery of the same amp hour capacity ( because a six cell battery would give 21.6 volts based on 3.6 volts per cell).

When you get to to higher volts it gets more complicated, a 40 volt battery could be 10 cells or 11 cells.

A few years ago Dewalt brought out their 12 volt range ( actually 3 cell 10.8 volt). Makita responded by changing their 10.8 volt range to 12v max. They then tried to explain how the new and old ranges were compatible without saying the only difference is the label.

 

[Spectric]

All you need to know is that volts x amps = power in Watts. So a higher voltage can deliver the same power power for less amps but it is the amp hour rating of the battery that will determine how long it can supply a given current.

A comparison might be the battery on your car, it has a very low internal resistance so it can deliver a large current but only for a short duration and is termed as the CCA rating. It can supply hundreds of amps but for only 30sec's along with a large voltage drop. The battery in a camper van often called a leisure battery has a higher internal resistance so can supply a lower current but for a much longer duration but maintains the voltage.

Now a garden tool needs power, more than many cordless woodworking tools so if a petrol strimmer has a 25CC engine then your electric version could well have a comparable motor but would need a very large battery to keep it running for any length of time. So if you want cordless garden tools then the
Husqvarna range are good but make sure you are not in a rush or have spare batteries at hand because a gallon of petrol may last all day but four batteries will not. Also petrol or battery they are both cordless.

If I wanted to go battery cordless then rather than a battery in the tool I would put a larger battery in a rucksack and power it that way, but at the end of the day I have an Allen strimmer from the eighties that is still going strong, how many battery tools would I have got through in that same period.

 

[NikNak]

Cheers guys...

So if i'm interpreting this right a 80volt tools with a 5 or 7.5 Ah battery should give me the best of all worlds.?

 

[Jonm]

Cheers guys...

So if i'm interpreting this right a 80volt tools with a 5 or 7.5 Ah battery should give me the best of all worlds.?

If you think in terms of your household electricity, you buy a quantity of electricity measured in kilo watt hours. In other words the amount of electricity to run a 1000 watt fire for 1 hr. Using your above example the amount of electricity in the 80 volt, 7.5 Ah battery is 80x7.5/1000 which is 0.6 kWH. An 18 volt 5 AH Battery would be 18x5/1000 which is 0.09 kWH

The above is not exact, the voltage varies with state of discharge and temperature plus you do not get all the available power out of the battery, but it will give you an idea for comparison.

As to whether It is “the best of all worlds” only you can judge. There are other issues like cost, weight and if you want to use the batteries for anything else. I have a 36 volt lawnmower powered by two 18 volt batteries and an 18 volt strimmer, both Makita so the batteries can be used in other tools which are a lot more interesting to me. Mowing the lawn is a chore and I am in the process f edging the lawn with bricks to substantially reduce the strimming.

 

[D_W]

Illustration of what I think the issue is in a low volt high watt tool.



No need to watch the video too much - the guy talks about having the saw twice stop cutting after a cut, but not during. I think that's probably two safety features:
1) high wattage on low voltage equals battery with thermal issues
2) the saw doesn't automatically stop mid cut, though, as I guess a fellow falling a tree with a saw that suddenly stops cutting in the back cut could be dangerous


As to the above comments about capacity, as john did - just get voltamp/watts estimate, and then if there's huge disparity in price, look around online and see if anyone has tested the batteries to see if they are honest vs. their claims.

No comments on push mowers as I am the power for my push mower. My mower is very green.

Not in the way everyone thinks here (though I guess it is). It's literally painted green by the manufacturer.