TobinGeorge(Offline)

Many many of the battery packs sold for NiMh lights are incorrectly designed, and are virtually assured of early failure. The flaw is the series parallel design. SLA batteries can be charged and discharged in parallel, Ni-Cads and Nimh cannot. This is no great secret, any applications guide from any cell manufacturer makes this point clear. This has not however kept Halcyon, (and others) from selling 4.5, 9.0 and 13.5 amphour Nimh packs. Notice a pattern? The 9.0 is two groups of ten cells in parallel and the 13.5 is three groups. Bad, bad, bad. When charged in parallel one series string can take all the current leaving the other(s) uncharged. The correct way to increase the capacity is to use larger cells so you have only one series string......Exactly what was done in the X scooter.

To condemn NiMh powered scooters based on the poor implementation in canister lights is unmerited.

For lead acid batteries either open circuit or loaded voltage is a meaningful measure of state of charge, but this cannot always be extrapolated to useable capacity. A SLA battery that has lost 20-30% of it's original capacity may well exhibit the same open circuit voltage and loaded voltage when fully charged as a battery with a higher percentage of original capacity still available.

Here again a "watt-hour" logger is the best way to know how much energy is stored in the pack.

Do you know if this was recent event, or during development?

Regards,


Tobin

TobinGeorge(Offline)

This is Ben's opinion, and perhaps inappropriate in a public forum, however your previous complaint was about posters mistaking Singapore for the PI..............

The X scooter uses a pure series pack, 20, 13.0 amphour x 1.2 volts to create a 24 volt 13 amphour pack. As far as I know the majority of the problems with NiMh packs are associated with the 9.0 and 13.5 amphour series parallel packs.


Correct, the very same thing that happens when one cell in a multicell SLA battery fails

You are talking about battery packs, and IMO, wrongly comparing poorly designed canister batteries with reasonably designed scooter batteries. I specifically drew a distinction between the two implementations, you continue to try to confuse the issue.

I too have spent the last twenty years making my living by designing, manufacturing and selling rechargeable battery packs. I've sold well over 100,000 high end rechargeable bicycle lights. The vast majority have used Ni-Cads, but I have also used SLA, Nimh, primary Lithiums and Lith-Ion....

That's one solution

No arguement on this point

Should I infer from this both of the scooters in question "blew up"?


Tobin

graham_hk(Offline)

I will take your word for it but for scientific interest why? I am having trouble visualising why 20 cells in one configuration (say series) would be worse than those in another (2 sets, of 10 in series, in parallel).

That is your opinion - personally I can't see how I am confusing the issue. I don't like NiMH - it is my opinion and others may have any opinion they wish. Since the X-scooter uses NiMH exclusively and therefore gives no other options I would not use it but I am talking about the battery.

I only know of one problem.

Since you appear to know something about batteries - what would be the problem of using Li cells (say C or D size) in series in dive lights. 4 x 3v cells [some of which have ridiculous capacity (say 10+Ah)] would give a tiny 12v battery for a dive light (low number of connections, cheap ~US100 and I understand charging is relatively uncomplicated). I have looked and the discharge curve appears to make 4 x 3.6v cells a better choice for a HID dive light (max voltage 14.4v - according to spec sheets and keeping voltage above 12v during the majority of the discharge)but I know virtually nothing about this. Obviously Li and water are not good companions (but you are making a scooter battery that goes in water like dive lights)... any other reasons why this wouldn't be a good idea

I am enjoying discussing this with you :D

Graham

fraser(Offline)

Excellent informative thread gents.

Slightly off topic, I have two NiMH lights, 9amphour and 13.5amphour batteries which have so far given me very few problems. However based on your comments what sort of life can I expect from these packs before they become troublesome?

Cheers,

Fraser.

__________________
Damn it feels good to be a gangsta

TobinGeorge(Offline)

Lets take a simple example, just two cells. First configuration is series, i.e. pos term of first cell is connected to the neg terminal of the second. When charged in this series config, all the current has to flow through both batteries, no place else to go. No consider a parallel configuration, both negative terminals connected together, both positive terminals together. In this configuration you can see at least the possibility that two paths exist for the charge current, i.e. all through one cell or all through the other.

Think about paralleled resistors, with only a minor difference in resistance, the current flows will be greatly unbalanced. For Nickle based chemistries the tendancy is for any cell impedance imbalance that exists at the strat of charging to be magnified by the charging. The net result is one cells (or string of cells) impedance drops and almost all the charge current flows through it and the other paralleled cells (or strings of cells) remain uncharged.

Other chemistries do not behave this way. Lead Acid and Lithium -Ion for example can be charged in parallel without a problem.

Lets now look at a 9.0 Ah x 12 volt pack made up of 20 cells. Configured as two series strings of 10 cells each, with the most negative, and most positive cells in seach string connected in parallel. This is a very common type of pack for can lights. (The 13.5 Ah is just one more series string)

When the cells are new and hopefully fairly close in original capacity (graded cells) there maynot be enough impedance imbalance to cause charging problems. However, in time, well before the pack has lost any significant useable capacity, enough imbalance is likely to develop (remember this is an imperfect electrochemical system) to make parallel charging probelmatic.

Lets look at the typical charger used for Nimh. The chargers sense the change in voltage relative to time (Delta V / Delta T) For Nimh they are attempting to detect when the voltage stops changing (zero delta v) which is pretty hard to do, and requires a fairly sensitive circuit. When charging a parallel series pack the first string (the one that's actually accepting current) reachs this zero delta v condition, and the charge terminates.

In most chargers that use thermistor feedback the thermistor is an over temp guage, i.e. a failsafe, used to stop charging if the pack temp exceeds a certain temperature. They are not the primary "full charge" sensor.

Now lets look at some of the typical trouble reports from the perspective of bad pack design.

"My burn time was only half what it should be" Really? no kidding?

"I had to restart my charge x times! " The chargers typically turn off the zero delta v sensing for the first 30 minutes or so because batteries at a low state of charge can give false reading. Repeated restarting is one (bad) way to fool the charger.

"I had to charge in the refigerator!" One series string taking all the charge intended for 2 or 3 strings overheaded tripping the thermistor.

Cooling by the way can limit the cell impedance variation.

Do these problems seem predictable now?

Why did this happen in the industry? My guess is that the designers were incorrectly applying their previous experience with Lead Acid. Again let me repeat that the prohibitions against parallel recharging of Ni-Cads and Nimh cells is not buried in some NASA research paper, it's as plain as the ass on a goat to anybody reading application guidelines from any of the cell makers.

What is the correct way to use Nickle batteries? For greater capacity use pure parallel strings. If you need greater Ah use bigger cells. If you absolutely need to use parallel series pack provide a means to charge them string by string seperately. Parallel discharge is no problem.

Consider for a moment if your 9.0 Ah pack was really to 4.5Ah packs that had a harness that would allow parallel conection for discharge, and could be unplugged from each other for series only charging. All for the cost of a few connectors.

Let me say in the Bicycle business one could never get away with a 90 day warranty on batteries. A decent NiMh pack should last atleast a few years, and the duty cycle on bike lights is typically far higher than the typical dive light.

It appears you "dislike" of Nimh batteries is based on your experience with poorly designed packs. I don't care for neoprene Drysuits, should I also avoid neoprene hoods and gloves?



Thanks for clearing this up.

Forgive me I may be a bit brief here, I pressed for time. First Lith-Ion and salt water are not especially troublesome. A flood will damage the pack, but should not result in a dramatic event. The salt water simply provides a path for slow discharge. Lith-Ion batteries are currently used U/W for DOD DPV's and some video lighting.

The real problem with using Li-Ion for can lights is voltage matching. Most ballasts are nominally 12 vdc (others are available) at 3.6 volts per cell there is no good match. Additonally the current ballasts have a fairly narrow range of input voltages and exceeding the "do not exceed" voltage causes predictable failure.

Trust me I've studied this problem a bit. My solution was to build a "boost regulator" i.e. a DC-DC converter with PWM output regulation. This allows a 3 cell Li-Ion battery (3.6 x 3 = 10.8 vdc) battery to power a 13.2 volt input WA ballast with out fault. It is not however cheap or easy.


Me too

Tobin

Joe Hesketh(Offline)

This is gold. Thanks for all the info guys.

This fits with my observations of charging my 9Ah pack. My old charger was a Maha 777-something-or-other which had an LCD readout in mAh for the amount of charge. Even with my pack fairly well discharged, it was nye on impossible to get any more than about 5,000mAh into the pack before the charger would beep loudly at me and proudly announce that it was "full". 3,000mAh was more typical and I usually always had to charge it twice.

What are the pros and cons of quicker vs slower charging? My new charger is a 'Universal' charger (one of these which now ship with the newer Salvo lights) which at the setting Barry recommends (1.9A) is faster than my old charger I think. The charger has no read out and I've yet to do a burn test to see how much charge I'm getting.

TobinGeorge(Offline)

Basically impossible to predict. Much depends on how well graded the cells were when new. and to a certain extent, how deeply you have cycled them. Deep cycling will execerbate cell capacity variations.

Side note: the surest way to shorten the life of almost all rechargeable batteries is to deep cycle them. This may run counter to what you have been told, but it's the truth. The series / parallel packs in can lights actually increas the likelyhood for over discharge. Due to poor design, you olny managed to charge 1/2 the pack. What the chance that you will run this "half a pack" pack 'till the it goes out?

Now do you see why 90 days is the warranty period?

Not all can Hid's use series / parallel packs. Can you count how many cells each pack has? If more than ten, you have series / parallel pack.


Tobin

TobinGeorge(Offline)

Slow vs fast charging:

It's can be counter intuitive, but for NiMh fast can be better. Why? Because is makes the "full charge detection" at zero delta V more reliable.

One is looking a the change in voltage vs time. If one slow charges then the change in voltage vs time can be extremely subtle and very very hard to detect, it's really a signal to noise ratio problem.

If one fast charges (~4C) then the transition from a positive slope to a zero slope becomes more distinct. In the real world this is the preferred technique.

Having said that, one can actually return a bit more to a given pack by slow charging at a lower temperature, but charge termination becomes much more involved. Current integration is used, which a means of actually measuring the Amphours returned, and terminating charge at a given capacity. This also demands that you know with some certainty the state of charge of the battery at the begnning of charge, and the actual capacity of the pack. Rarely used.


Bottom line, Barry's recommendation of a faster charger is sound.



Tobin

graham_hk(Offline)

Tobin,

Thanks for the answers and makes sense regarding use/charging of NiMH. So in your experience a reliable NiMH pack can be as reliable as a SLA? Would you bet your life on it (ie an extensive cave penetration where scooter exits were essential)? I know not many people do these dives but...

Also, the Li Ion packs for HID lights. It was my understanding (from WA spec sheets - http://www.walamp.com/lpd/files/data...l018cP1-P2.pdf) that 18W 12 V ballasts could accept 9-16v therefore one could choose either 3 or 4 3.6v cells or (3) 4-5 3v cells. Obviously there is something that I am missing.

Cheers, Graham

TobinGeorge(Offline)

The long proven use of SLA certainly argues in their favor, and the same data pool simply does not yet exist for Nimh. Having said that, if I knew the source and quality of the Nimh cells, that they were well graded, and I had confidence in the charger used, I would have no more concerns about Nimh than SLA.

Most of my first hand experience with the WA line centered on their 10 watt lamps and ballasts. These ballasts have a narrow input range, and no internal regulation. If the wide input voltage range you cite is correct for the 18 watt ballasts and has no effect on output, or lamp life, then a Li-Ion pack can be fairly simply produced.


Regards,



Tobin