Buying a 2nd battery

[ebikeerwidnes]

Thanks - interesting
but I wonder if it is that easy for smaller systems
and also - how much heavier would it make it??

also - if it is 70-80% efficient - would that mean a 20-30% reduction in range??

any electrical experts out there know better than me - which ain't difficult

 

[neil_merseyside]

The Cruachan pump storage power station at Loch Awe does pretty well! They reckon it is at least 70-80% efficient and maybe better than that.

Erm no:-
From Wiki
"Cruachan is not a net generator of electricity: it uses more energy for pumping water and spinning its turbines than it generates "
I suppose you could nip onto Wiki and change that if you wanted... but it's just a pumped storage power station.
All it does is use cheap rate power (maybe wind/solar oversupply?) to fill the top reservoir to be released it when needed.
It's a water charged battery of sorts.

 

[ColinJ]

Erm no:-
From Wiki
"Cruachan is not a net generator of electricity: it uses more energy for pumping water and spinning its turbines than it generates "
I suppose you could nip onto Wiki and change that if you wanted... but it's just a pumped storage power station.
All it does is use cheap rate power (maybe wind/solar oversupply?) to fill the top reservoir to be released it when needed.
It's a water charged battery of sorts.

Erm yes!

I was pointing out (as a former electrical engineer!) that a pump (effectively a type of motor) can be a very efficient generator, which goes against what you posted earlier. I wasn't talking about a perpetual motion machine which magics energy up from nowhere.

if it is 70-80% efficient - would that mean a 20-30% reduction in range??

You would only use it for recharging the battery and the capacity of that wouldn't change. It means that you could in theory get back a useful amount of energy coming down a big hill, for instance. I did a quick calculation (which may be wrong!) and I think you could get back about 60 Wh of energy going down a steep 200 m descent using motor-generator as a brake. (That is assuming 120 kg of rider and bike and about 75% efficiency.)

You could cycle along on the flat charging the battery instead of using the battery to operate the motor. As Pale Rider pointed out though it would be hard work if you had it set up to generate a lot of power and you don't buy an e-bike to make cycling harder!

 

[confusedcyclist]

Battery technology will have improved a little, and I suspect modern laptops need less power to run.

My 2014 mac air only uses ~20 watts when idle which blows my mind, because my desktop varies from 250-600 depending on if it's idle or running an intense application, and that's before you factor in my display, which of course is integrated into the Mac.

 

[classic33]

The everlasting battery.

 

[confusedcyclist]

Bit off thread but why is there no dynamo charging system for ebike batteries topping it up as you ride?

You can do it, and in some circumstances it may make some sense, like retrofitting to a bike which can't be setup with integrated lighting for whatever reason. Purpose built ebikes supplied with lighting as standard will always utilise a direct feed from the battery because the drag and conversion losses caused by the dynamo light and hub combo would waste more energy than a direct feed from the battery to a lamp. As a rule of thumb, whenever you convert energy from one form to another you lose energy, usually through heat, light and sound, the rate of loss is known as efficiency. This is why light bulbs get hot and sometimes buzz, even though we usually only want them to produce light.

Lets assume for the sake of simplicity that the rate of loss is 20% at each stage of conversion, and we need 3 watts to power the lamp, you're going to need to supply more watts from the battery to ensure 3 watts makes it to the lamp after all conversion losses, with 5 steps of conversion (Battery - Elec - Kinetic - Elec - Light), you're going to need to supply 10 times as many watts from the battery. I'm not an electrical engineer and I'm not 100% sure on the actual loss %, it does vary at each stage. But that's in essence the reason you won't see dynamos on commercial ebike systems.

Of course, 30 watts is negligible on a 250w motor, you can expect the rider to put in another 100w easily, so you're looking at less than 1% of overall power, but still that is lost range on an ebike. I would not fit a dynamo hub to an ebike personally, it would make more sense for my kind of riding to fit removable battery powered lights and preserve the range and long term wear on the expensive bike battery. Charging the LED lights from my bike is still possible, as I can hook up 1 2AMP USB device to my Bosch Kiox Display, so I could charge in a bind.

 

[confusedcyclist]

...using motor-generator as a brake.

The problem with regenerative braking is a person on a bike weighs a lot less than a car. Therefore there is a lot less energy available to be captured (which is what regenerative braking does). When you cycle, about 90% of your effort and energy goes into pushing you through the air which resists you as you move. Perhaps less than 10% is lost because you brake – so even in the best case scenario when you capture all of your kinetic energy when you brake, you’ll only go 10% further as a result. Of course, we haven't even started to factor in conversion losses or weight and costs of such a system.

 

[ColinJ]

The problem with regenerative braking is a person on a bike weighs a lot less than a car. Therefore there is a lot less energy available to be captured (which is what regenerative braking does). When you cycle, about 90% of your effort and energy goes into pushing you through the air which resists you as you move. Perhaps less than 10% is lost because you brake – so even in the best case scenario when you capture all of your kinetic energy when you brake, you’ll only go 10% further as a result. Of course, we haven't even started to factor in conversion losses or weight and costs of such a system.

Yes, I'm sure if it were worth doing then somebody would already be doing it and making a big deal about it!

One day somebody very clever will come up with new battery technology which will make it possible to ride all day on one charge using a battery which doesn't weigh much and doesn't cost much either. Some interesting new technologies are mentioned in THIS ARTICLE.

 

[confusedcyclist]

Yes, I'm sure if it were worth doing then somebody would already be doing it and making a big deal about it!

One day somebody very clever will come up with new battery technology which will make it possible to ride all day on one charge using a battery which doesn't weigh much and doesn't cost much either. Some interesting new technologies are mentioned in THIS ARTICLE.

Its an exciting field, never say never as they say. We have been working on batteries for 200 years and we have only increased performance 5x in that period. I temper my own enthusiasm and don't expect quite so dramatic gains as you propose. We are close to limits of energy density that can be achieved with commercialised liquid battery chemistry. Much needed improvements at this stage would be to switch from rare earth minerals which are about to peak and become much more expensive, or improve recyclability, both of which are show stoppers as far as affordable batteries and green credentials go.

 

[CXRAndy]

We need the flux capacitor

 

[classic33]

Its an exciting field, never say never as they say. We have been working on batteries for 200 years and we have only increased performance 5x in that period. I temper my own enthusiasm and don't expect quite so dramatic gains as you propose. We are close to limits of energy density that can be achieved with commercialised liquid battery chemistry. Much needed improvements at this stage would be to switch from rare earth minerals which are about to peak and become much more expensive, or improve recyclability, both of which are show stoppers as far as affordable batteries and green credentials go.

"Baghdad Battery?

 

[jann71]

I'm looking at a few used ones. Quite a jump from a 400 to 500.
Different serial numbers, different year?

 

[Pale Rider]

The passage of time will eventually kill any battery, but pattern of use has a bigger impact.

Buying used it's very hard to get accurate information on either of those two things.

I would be inclined to ask the owner how the battery has been used or how many miles it has done.

But ultimately buying a second hand battery is a risk you will have to take.

 

[jann71]

They are saying it's brand new but without box. Offering return within 30 days.
So I'm thinking if buy it and take to dealer to plug in to check and if it's got any issues send it back.

 

[Pale Rider]

A full discharge test takes several hours/over night.

The dealer might charge anything up to £50, but you will get a print out giving details of use and condition, and a percentage efficiency figure.

That would be something to take to the seller if there was a problem.