The Li-Ions march on...

[MaryinScotland]

I've just paid one of my occasional visits to the Heinzmann site ( Estelle – Electrobikes – Bicycles with Electric Motor – Auxiliary Drive – Power Assist Drive ) to see what they currently offer as complete bikes rather than kits. You can download 2 leaflets, one with the current models, one with the 2008 range.

The bikes are basically the same. However at the moment, you can get a Heinzmann bike (trade name Estelle) with either NiCad or NiMH batteries. From next year, all have Li-Ion batteries. The only decision is whether to have the basic 5.2 Ah, or the 9.6 Ah (an extra 200 Euros). All the bikes are quoted as being 3 kg lighter than previously. All still have a rear pannier for the battery, but it's a more conventional style of pannier than the previous wide shallow box, and looks like it would have some storage space.

For environmental reasons, I'm happy to see the demise of NiCad. But considering the evidence on this forum that the bugs are not yet out of Li-Ion technology for e-bikes, I can't help regretting the passing of NiMH.

Mary

 

[flecc]

Exactly my sentiments Mary.

Some if the best Li-ions have been performing well, but how long they last is yet to be seen. In heavy daily use the best ones will probably match NiMh in giving a couple of years (400 to 500 charges) usage.

It's the intermittent user who's likely to suffer though, since NiMh could last four years or more with care, while Li-ion deteriorate at the same rate whether used or not.

If current Li-ions don't perform and cost too much, the e-bike market here will just wither and die. I can't see people putting up with a £300 to £600 battery replacement every year or two, especially if usage isn't daily.

The iron-phosphate variations on Li-ion could yet save the day if they arrive quickly enough, perform ok and aren't too dear, since they have the potential for 2000 charges.
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[MaryinScotland]

Hi Flecc,

I wonder if you could explain something to me. My current bike has an NiMH battery. It's supposed to be good for 500 charges, and I don't charge until it's getting low, occasionally making the effort to run it down to empty. My normal round trip to/from work is just under half the range, so I charge it every second day, and I should get 1000 commutes from it.

However, I've read that Li-Ion batteries should be charged after every use, even a short run. Does even a small top-up charge count, when you're calculating how many charges the battery is good for? Say I had a Li-Ion battery that was supposed to last for 500 charges. If I charge it every day, does that mean I only get 500 commutes from it? Or would the daily top-up only count as "half a charge"?

Since it looks like we're all being steered to Li-Ion, like it or not, I guess I'd better know how to use one.

Mary

 

[flecc]

The top up is a part charge in theory Mary, the stated life being for 500 full charge spans. However, these indicators are very vague on all battery types, and there are big variations due to differing qualities and developments.

For example, NiMh have always been stated to be ok for 400 to 500 charges, but the original ones never reached that, often only getting around 100 charges before failure. After a complete change in the elements used in the construction, mainly rare earths used instead of rare metals, the life has been increasing with the years, and on the best examples appears to exceed 500 sometimes.
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[MaryinScotland]

Thanks, Flecc.
Mary

 

[Jeremy]

If a NiMH battery is operated over a small part of it's capacity range, between about 45% and 80%, then it will happily withstand many, many thousands of charge/discharge cycles and last for years. Toyota maintain the Prius NiMH battery between these limits and correspondingly provide an 8 year, 100,000 mile warranty. The battery will partially charge and discharge several times each mile in city driving, so this gives a good indication of the ability of the this battery technology to be long-lived.

Prius batterys are fairly mundane 6.5 Ah NiMH cells manufactured by Matsushita (Panasonic) so are comparable to the size of cells used in ebike battery packs.

My advice would be to avoid deep discharge and charge and try to keep your battery pack cycling in the "safe" range if you want to get the very best life from it. This only really applies to NiMH as far as I'm aware, certainly NiCds will get seriously upset by partial discharge/charge cycling.

Jeremy

 

[flecc]

I support what Jeremy has said about long life of NiMh with partial charge use, my usage being mostly like this and I've had no trouble going past 4.5 years, only the capacity loss with time being of any importance.

However, I'm inclined to dispute the contention of never doing a full discharge, since I have always intermittently done full discharges to keep those batteries in good condition, every battery expert in the industry recommends that, and the chargers Panasonic supply for charging their 6.5 Ah e-bike batteries have a discharge function called refresh which they recommend is used regularly.
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[Jeremy]

I'm guessing that there is a greater risk of cell imbalance if you only partially discharge and charge a multi cell pack, which perhaps an occasional deep discharge might correct reasonably well.

The Prius does something similar, but by doing a full charge and invoking some sort of battery management process. It seems to want to do this about once a month or so - occasionally you can detect it on the displays. I know that the battery pack has connections to every individual cell, so am guessing that it monitors and controls the state of charge pretty carefully to ensure longevity.

Jeremy

 

[flecc]

I think imbalance is an issue too, photovoltaic slow charging known to lead to this, though the official reason for the intermittent full discharge is the retention of some degree of memory effect in NiMh.

Ideally our e-bike batteries would have cell by cell charging and monitoring like the Prius, but the price would leap of course. Li-ions manage individual cell treatment for £50 extra, but only for 10 cells at most, not the 30 in our eZee NiMh batteries.

I wonder if those 6.5 Ah cells in the Prius are the same high quality 6.5 Ah D cells in the Panasonic battery used on the Giant Lafree models, those being the 4.5 year plus ones I referred to.
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[Jeremy]

I believe that the early Prius, with the lower voltage battery, used cells that were cylindrical and about D size, so it's possible that they were similar. The current model Prius uses prismatic cells, with what look like big screw terminals on top of each cell.

The Prius battery doesn't charge each cell individually, BTW, it charges the whole pack. When regenerating the charge currents can be very high, probably in the many tens of amps range, maybe over a hundred if you're braking fairly hard (the brakes are primarily regenerative, it only uses the disc brakes for very hard braking or below about 5mph).

I think that the battery management system works by periodically charging the whole pack at a modest charge rate, whilst shunting current around cells that have a high terminal voltage. The wires from each cell are very thin, whereas the main power leads are about the size of welding cable. The cells are all interconnected by big copper bus bars, quite impressive looking!

Jeremy

 

[flecc]

One thing's for certain Jeremy, we could never see anything like that on an e-bike.

Sounds very impressive, but far too heavy and costly in our two wheel world.
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[Tiberius]

Hi Jeremy,

Are you suggesting that the NiMH should not be charged to 100% as well as not discharged too far? How does the system (or the manual operator) detect the correct endpoints, or is it a case of keeping the Ahr in and out within certain limits and doing an occasional excursion to top and bottom charge?*

Nick

*Just realised that sounds like subatomic physics

 

[Jeremy]

Essentially yes, Nick.

Certainly all of the available information about the Prius NiMH battery makes it clear that the key to longevity is keeping state of charge between the 45% - 80% limits.

I'm guessing that it does it by measuring the cell voltage accurately, as I doubt that any other method would be accurate enough. Most probably Toyota have accurately mapped the specific terminal voltage versus state of charge characteristic for the cells they use to make the system adequately reliable.

From what I've seen on the display, the battery is never exercised downwards below the lower limit, just upwards. The "State of Charge" indicator on the Prius reads full at 80% and empty at 45% and I've never seen it get to empty in the two and a half years I've had the car, which implies that the battery is never discharged below 45%.

Jeremy

 

[Tiberius]

Jeremy,

As you will have seen from another thread, I've been looking into the charge/discharge process of NiMH packs. Certainly there are things going on at full charge (oxygen generation and recombination) and at full discharge (reversal of individual cells) that can't be good for them.

If the gauge reads full and empty, how do you know that is 80 and 45%?

Nick

 

[Jeremy]

Toyota have been very open about the technology inside the Prius and have published a lot of the detail, including the range of battery SOC and the fact that the charge state meter display only indicates between these limits.

I've seen the display read "full" a number of times, but have yet to see it drop to "empty", which leads me to believe that the car tries to keep SOC on the high side of the range.

Jeremy

 

[Tiberius]

There seems to be the standard wisdom that with NiMH you have a simple battery pack and a smart charger but with Li-Ion you have a simple charger and put a battery management system in the battery.

Like most conventional wisdom, its probably wrong. It looks like there are benefits to putting some intelligence with the battery pack as soon as you have multiple cells of any type, and Toyota have realised this.

Nick

 

[flecc]

It's not so much conventional wisdom with NiMh to use a smart charger and simple battery Nick, just a matter of the cost, weight and bulk of managing 20 or 30 cells individually.

It's long been accepted that it's best to monitor and charge NiMh cells individually, just not commercially feasible.

The safety imperative, only having to deal with 7 or 10 cells, and the ability to charge more for a newer technology made it possible in our Lithium batteries.
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