Batteries: Large vs Small capacity

[Pedalo]

I've read that in general you can hope/expect to get around 2 years life out of a battery. Can anyone tell me how the battery would deteriorate up to and after that time? i.e. for example is it a linear drop off from day 1 and by 2 years you're down to X% or does it stick about the same capacity and then one day just suddenly fail?

Reason I'm asking is that my daily commute is about 7.5 miles each way and I was wondering whether it would be more economical to go for a bike/battery with much higher range and expect it to last well beyond the 2 years, or go for something with a much cheaper battery which just covers the range and plan to replace it more often.

I'm in two minds between bikes such as Wisper (for which I've just read the battery is going to cost £599 from next year - Ouch!) and cytronex which has a much lower capacity battery but should easily cover the range. If a Wisper battery is unusable after 2 years it will be very expensive to maintain whereas if it actually still covers the required range after 4-5 years it wouldn't be so bad.

Any advice would be appreciated. Thanks.

 

[Tim]

Our experience with the Panasonic pedelec system batteries is that they last for years. We first sold them in the WiLL bike and it was four years before we started getting requests for replacement batteries.

So going on that evidence, and on the evidence so far from the thousands of Agattu, Pro Connect and Tasman owners have had so far, you should expect a battery life greater than the two years covered by the warranty, especially over a moderate daily distance of 15 miles.

A to B recently reported:

"After six months of fairly intensive all-weather use, our Kalkhoff Agattu battery hasn't deteriorated at all. We estimate the capacity to be 272Wh today, which is slightly ahead of Panasonic's claimed capacity [260Wh] and exactly the same as when we tested it last October" A to B magazine Battery Watch, May 2009

So if you're looking for reliability in the long term, a Kalkhoff electric bike is a good bet.

Tim Snaith
50cycles electric bikes

 

[HarryB]

This is quite a complex problems as you are talking about batteries using different technologies. The Cytronex uses NiMH and the Wisper uses Li-ion. The problem with Li-ion is that they start to lose capacity the moment they leave the factory - this loss worsens as the temperature the cells are kept at rises. Also Li-ion have an annoying habit when they age, of failing to deliver higher currents, so you end up with a battery that will cut out on hills. In the Wispers favour is its huge capacity if you could get a charger at work and charge every time you do a journey then (after the initial conditioning period) never discharge the cells completely then the battery will last a reasonable amount of time. How much longer than 2 years we don't know as this is evolving technology. When we first started with these cells they lasted 6 months - to give you an idea of how bad thing were.

If you are risk averse I can tell you that you can (almost) guarantee the Cytronex battery will last at least two years (again you will need to get a charger for work). So I think it will work out the cheaper option because at £599 the Wisper battery would have to last well over 6 years to match this (according to my maths anyway).

Hope this helps

 

[Fecn]

I've read that in general you can hope/expect to get around 2 years life out of a battery. Can anyone tell me how the battery would deteriorate up to and after that time? i.e. for example is it a linear drop off from day 1 and by 2 years you're down to X% or does it stick about the same capacity and then one day just suddenly fail?

I'm afraid the answer here can be both. I've been working on this analogy in my head for a while, so here goes....

It can help to think of a LiIon battery like a kettle full of water. When you charge-up/store energy, the water heats up. When you discharge/use the energy the water cools down. If you try to put too much energy in there when charging, the water will start to turn to steam and you loose the ability to store as much 'charge'. If you take too much energy out of the water, it turns to ice and your kettle breaks. Although water can exist as both steam and Ice, it's only useful to us in our kettle battery when it's liquid. However, even if you never switch the kettle on, the water will still eventually evaporate, and that rate of evaporation will be dictated by the temperature of the water.

LiIon batteries behave remarkably similarly. If you overcharge them, they explode/burn. If you discharge them too much, the chemicals inside change irreversibly into a different compound. If you leave them doing nothing, they'll still degrade. If you want the longest possible life from a LiIon pack, you want to keep it at around 5 degrees C and 40% charge. In those ideal conditions, you should only be loosing about 5% capacity per year, and potentially have a battery which lasts for decades.

Aside from the natural chemical wear and tear that's going on inside the cells of the battery, there's the possibility of manufacturing or mechanical defects occuring too. It only takes a single cell within a battery pack to fail, and the whole pack is effectively useless. To illustrate this point, I recently took apart a 9-year old 12-cell laptop battery pack. 5 of the cells inside were totally dead, the remaining 7 cells were still able to store 85% of their original rated capacity, so I rebuilt them into a new pack to use elsewhere.

I think LiIon batteries have improved massively and continue to do so. I know that the 2-year battery life figure is bounced around a lot, but I'd be very surprised if my Kalkhoff battery doesn't last a lot longer. It's had 54 charges over the past 8 months and shows no sign of any capacity reduction whatsoever yet.

 

[Tim]

The Panasonic batteries mitigate post-production decline through the hibernation function, which means deep-discharge caused by draw from the BMS circuity is prevented as the battery switches itself off after a few weeks of lying idle. It is revived by placing on the charger. That's another benefit of buying into mature, time-tested technology when you're considering which electric bicycle to invest in.

 

[flecc]

Lithium batteries normally lose capacity as a percentage within each year of use, so quantitatively the amount Ampere hours lost in each year reduces. The rate of loss depends on many factors, chief being the depth of discharge in typical use, the ambient temperatures when used and in standby, and the periods of use at very high discharge rates such as in hill climbing.

Earlier batteries used to lose capacity at a rate as high as 35% per annum in typical regular use and even 60% was possible in the worst circumstances, but newer designs have progressively improved. However, in the higher powered bikes using the latest generation batteries the loss rate can still easily be 20% due to the high discharge rates they are subjected to.

The Panasonic unit bikes mentioned above benefit in three ways, first by being moderately powered sufficient for the task, second by a very sophisticated battery management system both in charging and use, and third through the "sleep mode" which puts them into a very low level standby when not in use. They could well last for years beyond the nominal two, depending on the conditions of owner use.
.

 

[Alex728]

The Panasonic unit bikes mentioned above benefit in three ways, first by being moderately powered sufficient for the task, second by a very sophisticated battery management system both in charging and use, and third through the "sleep mode" which puts them into a very low level standby when not in use. They could well last for years beyond the nominal two, depending on the conditions of owner use.
.

do Panasonic do this on other portable kit as well? I recently brought back into use a Panasonic camcorder that had been idle for over a year since moving from Reading to Ipswich, and was pleased to find its Li-Ion batteries seemed to have as much capacity as when new..

 

[Fecn]

do Panasonic do this on other portable kit as well? I recently brought back into use a Panasonic camcorder that had been idle for over a year since moving from Reading to Ipswich, and was pleased to find its Li-Ion batteries seemed to have as much capacity as when new..

Most camcorders use a single cell 3.7V cell for the battery... Because it's just one cell, there's no need for BMS, and as such, there's nothing which will drawer current from the battery when it's switched off. Single LiIon batteries are very easy to charge. It's only when you connect multiple cells into packs that you need a fully featured BMS to manage them.

 

[Alex728]

these batteries are 7.2 V (just checked). The reason I am curious is when I first charged them after the long period out of use the charger lights flashed in a certain way before the main charge as if the batteries were being "woken up out of sleep mode" as has been mentioned with Panasonic e-bike batteries..

 

[Fecn]

In that case, I'd say there was a high likelyhood that they have a clever sleep-mode BMS inside them

 

[flecc]

Yes, a sleep mode is quite possible in those since Panasonic were one of the first in with lithium many years ago and have loads of experience with them. That said, small lithium battery cells behave very differently from the large ones we use and which I was speaking of above. I've got one in a portable minidisc unit that's nearly 11 years old now which still has over 80% of it's capacity.
.

 

[monster]

i would say buying a larger capacity battery than you need is the best way to prevent aging. if you buy just enough battery for your needs then you will be stressing it too much with your normal riding style and you will really notice the deterioration in performance. buy a large battery and treat it carefully and it wil last for years.

but no-one really knows how long some of these lithium cells are going to last because thay have not been around that long. manufactures guess the expected longevity based on accelerated aging at higher temperatures which may or may not be accurate?

 

[HarryB]

i would say buying a larger capacity battery than you need is the best way to prevent aging. if you buy just enough battery for your needs then you will be stressing it too much with your normal riding style and you will really notice the deterioration in performance. buy a large battery and treat it carefully and it wil last for years.

This is certainly true if you were comparing two batteries with the same chemistry and especially true of Li-ion batteries. However the original question was comparing the Cytronex and Wisper batteries. Provided that you are happy with the very limited range of the Cytronex then it will, in this comparison at least, be the cheaper option.

 

[jerrysimon]

Interesting thread. I am still looking at some 12V 4200 NiMh packs * 3 in series i.e. 36V. I am now looking at some RC model powerpacks with 35amp continuous draw not the cheaper ones from another thread I mentioned. I could get 4600 packs for a little more.

I reckon these would work on my DIY ebike 10 mile return journey to work.

So my question is what is my 250W motor hub drawing max 10 amps ?

There is a fuse in the 36v Li-Ion battery I am using but its not marked.

After using my current Li-Ion battery to work and back it takes about a 1.5 - 2 hour charge.

Regards

Jerry

 

[Fecn]

I reckon these would work on my DIY ebike 10 mile return journey to work.

So my questionis what is my 250W motor hub motor drawing max 10 amps ?

Yep.. about 10A sounds right. And yes... I reckon three of those 4600mAh packs in series should see you through the 10 miles to work and back.

 

[jerrysimon]

Fecn yer I think so

I just checked my current Li-Ion charger and it charges at a max 1.8amp. So at 2 hours thats 3.6amps. I live in Cambridge and its pretty flat. I am throttling a lot at the moment and reckon I could cut way back if I wanted.

I might even get away with 3*4200 pack though for a few pounds more probably worth getting the 4600s. At around 2kg for all three packs together, that would also knock 3kg off my current setup and bring my bike down to about 19kg with batteries.

I have checked I could even fit all three in a watterbottle

I have a good quality charger already from my RC model hobby. I would have to charge each 12v pack in turn about an hour each.

So do I risk it ? Its going to cost my about £90.


Regards

Jerry

 

[Fecn]

So do I risk it ? Its going to cost my about £90.

Speaking as a man who built his own 12Ah NiMH pack from cheap Aldi cells and is very happy with the results, I say "Yes".

 

[Wisper Bikes]

Batteries

Hi Guys

Just to say we have thousands of bikes on the road now and many are as much as five years old. We sell hardly any replacement batteries. Draw your own conclusions!

We now use massively superior batteries than we did on the early bikes,so based on experience I would predict that you will get at least 6 years good service from your wisper battery and even after that time when the capacity may be as much as 40 or even 50% down on the original it will still have a higher capacity than the vast majority of the batteries on the market.

Our two year warranty does not indicate the length of time a battery will last, my Volvo estate is 165,000 miles old now over 100,000 over it's warranty and is still going strong. Like wise the Wisper battery will give you years of service over its warranty period.

Best regards David

 

[Mike63]

"Like wise the Wisper battery will give you years of service over its warranty period."

....that's what we want to hear David.

....Mike

 

[HarryB]

Hi Guys

Just to say we have thousands of bikes on the road now and many are as much as five years old. We sell hardly any replacement batteries. Draw your own conclusions!

Anybody on the forum with a 2004 Wisper (Li-ion) battery still giving good service...would love to hear from you.