Lithium Ion

[Wisper Bikes]

My concerns on Lithium batteries come from reading on this forum and in A to B magazine. They are:

Hi Frank as promised I have some answers for you from our battery guys.

1. Cut out on hills. Because the voltage of a lithium battery drops under loading, it can sometimes trigger a cutout. As peak loading occurs when you are going up a steep hill, it is pretty inconvenient. This seems to be more of a problem on higher powered bikes. People have reported it on Ezee bikes (and I think also on the Sparta Ion), but there have been no reports of this happening with Powacycles, where the much lower powered motor does not make such demands on the battery.

I think this probably comes as a result of companies not specifying the correct type of lithium ion battery. All battery technologies such as Nickel Cadmium, Nickel Metal Hydride, Lead Acid and Lithium ion have product variants designed for high temperature, long cycle life, high rate discharge etc etc. Many Lithium Ion cell suppliers in China do not have their technology optimised for applications such as E-Bikes and therefore although their product may appear attractive on paper with good capacity, light weight etc etc the performance under high rate discharge is not so good and therefore they can have a high voltage drop under high load conditions. A correctly designed lithium battery should however be at least as good and perhaps even better at high rate discharge than all other battery types. The hybrid cathode material we use in our E-Bike batteries allows peak discharge at up to 5 times to rated capacity of the battery with very little voltage drop. Based on our 36V 10 Ah this means the cells can easily deliver 1800 Watts of power if needed. The limitation is therefore usually caused by the battery protection module as follows :

1. If the cell is not well suited to high rate discharge then the voltage can drop under load. It may therefore momentarily drop below the minimum voltage level of the protection module and the battery will as a result shut down thinking that the battery has reached the end of its discharge.

2. If the current limit of the battery is set too low or the motor wattage too high then under high rate discharge conditions the current demand from the motor can exceed the maximum current setting and again the battery will shut down to protect that battery and load.

Our battery has a max current limit on the battery protection module of 20A so in principle the battery can deliver 36V x 20A = 720 Watts.

2. Rapid loss of capacity and hence range with normal usage. People talk about losing anything from 10%-100% of capacity per year of life. The problems seem to arise if you use the battery to anything near its capacity (ie run it down past half charge). However, that is what I would want to do as I have a 25-mile return trip to work. NiMH by contrast seems to be much more robust. People on this forum report no loss of range after 2 years with deep discharges being possible.

It is hard to make a comparison without testing products under controlled conditions but it is widely acknowledged by cell manufacturers that lithium ion cells display better cycle performance and lower capacity losses under almost any condition. Again this assumes that the cell type used is suited to continuous charge and discharge characteristics of the E-Bike. As detailed before all batteries will lose capacity as a result of being charged and discharged this cannot be avoided but based on our standard specification then you should expect around 500 charge and discharge cycles and still have 70% of the original capacity. Given a daily commute of 5 days a week this should work out to around 2 years use before a significant loss of capacity is noticed. I can only assume that if many lithium ion users are reporting rapid capacity loss then the quality of the cells they are using must be very poor.

Therefore my questions are how can I be sure that a lithium battery would give satisfactory operating performance - ie ability to climb long hills without cutting out and ability to get me to work and back without having a very short life?

You can only make a judgment based upon the specification supplied by the manufacturer and your belief in their figures.

I do hope this helps

Best regards David

 

[coops]

Hi David

Thanks for the information, that does help explain some aspects for me, but could you help fill in some of the "blanks" that still remain?

In particular, what depth of discharge do you specify for your batteries to get the results you have given? i.e.

it is widely acknowledged by cell manufacturers that lithium ion cells display better cycle performance and lower capacity losses under almost any condition.

Reading between the lines, what are the exceptional conditions where this doesn't apply? For instance, if the battery "ages" after 1-2yrs and/or with high depth of discharge in use? and are they likely to occur in ebike use?

As detailed before all batteries will lose capacity as a result of being charged and discharged this cannot be avoided...

I accept what you say here, but I missed where it was "detailed"?

...but based on our standard specification then you should expect around 500 charge and discharge cycles and still have 70% of the original capacity. Given a daily commute of 5 days a week this should work out to around 2 years use before a significant loss of capacity is noticed.

What is your "standard specification" i.e. to what depth do you discharge the battery to get these results, and do they allow for the effects of the lithium "ageing" (has this result been achieved in practice, or is it a predicted result)? These seem to be the two main causes of lithium batteries quickly losing capacity, but you have not specified how your batteries are affected by either?

Also, a 30% loss is already quite significant in my book! Even 20%!

You can only make a judgment based upon the specification supplied by the manufacturer and your belief in their figures.

I am willing to believe in figures (what else can one do?), where the information given is complete: number of charge cycles is usually quoted by manufacturers, but without a specification of projected realistic actual lifetime in years, not cycle numbers, and without a depth of discharge level quoted for given results, essential information is missing.

Can you please fill in the blanks?

Regards,

Stuart.

 

[Ian]

David, thanks for providing the information but as it comes from the manufacturer I suspect it's biased to show the batteries in their best possible light.

I've have owned a considerable number of NiMh and lithium batteries powering various devices and coming from various manufacturers and not one of those batteries has lasted as long as the suppliers claimed but the worst offenders by far have been the lithium variants. The latest case is my work computer, an 8 month old Dell laptop used on mains power 50% of the time, the battery has done at most 100 cycles and has already lost 1/3 of it's capacity. It's predecessor was an IBM, after 2 years and 300-400 cycles the battery was totally useless. Both of these examples fall a long way short of the makers claims. I have a 3 year Nokia phone with probably as many as 700 cycles on the battery, but it's standby time is down to 8 hours, about 10% of when it was new. An Apple laptop, 3 years of very light use, battery totally U/S. A Sony camcorder, Palm pilot, the list of Li-ion failures gets longer

In short, my cynical veiw of batteries is based on hard experience and the fact I've never ever seen one achieve it's claimed life (Except the NiCad in my Bosch drill) although of course all have lasted the warranty period After all how much capacity has a battery got to lose before it qualifies for a warranty claim.

So the real issue is when are manufacturers going to back up the claims with a realistic warranty, after all a bike battery that has lost 30% of it's capacity may not be a failure in the eyes of the manufacturer, but it certainly is to the commuter who can no longer make the full trip under power.

Ian

 

[frank9755]

David,

Thanks for your reply. I appreciate your willingness to engage with us on this subject.

I don't have any reason to doubt that your lithium batteries are at least as good as those used by other bike manufacturers. Given the higher capacity (13.8Ah) they may well be better.

However, as you can see, there is a considerable level of scepticism on this site around the ability of even the best lithium batteries to meet the needs of electric bikes. We can all believe they are great when new, but the evidence is mounting that after a relatively short period of time, given typical usage profiles, the capacity deteriorates. This evidence comes from first hand reports of lithium battery failures from people in this forum. It also comes from sites such as battery university, which are clearly of the view that any lithium ion battery will lose capacity if frequently subject to deep discharge.

In a previous post you ruled out exploring NiMH batteries. Why is that? Might you reconsider it as an option? These batteries are not perfect either but evidence from many users is that they have performed better in electric bikes.

If you are set against offering NiMH, would you sell empty battery cases, to enable customers to build their own Ni battery for the bike?

Frank

PS I hope the event this weekend went well. I was sorry I couldn't make it!

 

[Bikerbob]

These doubts about lithium batteries should be a concern to suppliers and it would be good if they offered the choice of NiMh. The NiMh battery on my Giant Suede is coming to the end of it's life and I had seriously thought of buying a new bike, with a Wisper being a strong possibility. However, in view of these concerns I've decided to get a new NiMh battery for the Suede and hope that matters might be clearer by the time that one expires. It's a difficult balance between manufacturers pushing the technology to bring us improved products, and the danger of customers losing a lot of money when things don't work out. Perhaps the customer should have the choice as to how much risk they want to take.

 

[Wisper Bikes]

Thanks for all your queries.

I will email all the points to Harvey from Lishen and he will answer them honestly and candidly. It is a good point that Hervey is Lishen's European Director and he is bound to believe in his product, otherwise I don't think he would work for them (he's that type of man). He did make some very good points at the New Forest Event, one of them being that next year the estimates are that Lithium Ion (including Lithium Manganese and Lithium Polymer) will contribute at least 80% of all high power batteries in the world.

As soon as I have his reply I will post it.

Best regards David

 

[fishingpaul]

when will harvey answer.

 

[Wisper Bikes]

Lishen

Sorry for the delay I will chase this up.

Regards David