Advice on using NiMH Cells from the BRCA Electric Board
 

EXPLODING NiMH 23/43 CELLS


No doubt you will have read or heard the stories of batteries exploding and that the Norwegian Federation has temporarily suspended all electric racing. We have been monitoring these ‘incidents’ but obtaining hard facts is not easy, making it difficult to draw any meaningful conclusions.

Given that there are thousands of cells in circulation, such incidents have been exceedingly rare in the past. The only serious incident that has occurred to date in UK was when a receiver pack blew, that has permanently damaged a person’s eyesight. Although there will have been more injuries caused by scalpels or super glue in eyes than caused from exploding cells, you still need to take care!

The NiMh cells have a chemistry that allows the cell to 'self-discharge' over a relatively short period, (days as opposed to weeks). Lately, some of the 4200 cells seem to self-discharge more rapidly than other/previous versions. More importantly the amount each individual cell within a pack discharges varies (i.e. cell 1 might be .9v, cell 2 .8v, cell 3 .9v, cell 4 .7v, etc.)

If charged in this state, some chargers will not know when to stop the charge process, as some cells are demanding to continue with the charge, whilst others could be over charged. It is also important to control the delta peak (voltage drop-off after completed charge) to a low value when charging. Once the cell has achieved full charge, any further charging will produce gas. The cells have a pressure vent, but if that is blocked or not working, the gas pressure could cause an explosion.

High charge rates will also produce more gas and reduce the cell’s useful life. It is possible that internal gas pressure may cause distortion to some internal parts that could result in a short circuit within the cell. An internal short circuit combined with gas pressure is likely to be a 'big bang'.

Drivers need to recognise that the 'matchers' recommendations are intended to achieve maximum performance. It is the recommendation from the cell manufacturer that should be used.



Best Practice for use of NiMH cells.

1. Equalise cells before charging if they have been stored (with charge) for more than 2/3 days. If any of the cells in the pack show that they do not need to be equalised (i.e. the light does not show or goes out straight away) then remove the pack from the equaliser and charge for a short period (approx. 5 mins. should be adequate). Then carry out the equalising process.


2. Never exceed the manufacturers recommended fast charge rate. This should be 1C max., even if the manufacturer states higher. (C being rate of charge based on capacity of the cell, e.g. a 4200 mAh cell = 4.2 amps)


3. Disregard any charge rate recommendations by the 'matcher' if it is more than 1C.


4. Use a maximum Delta-Peak setting of 3mV per cell (6-cell pack = 3mV x 6 =0.018V).

5. Never repeak cells after main charge.


6. Use a temperature cut-out as an additional safety feature set at 42 deg. C max., located on the hottest cell (usually middle cell). If charging on a cold day then consider reducing this to around 35 deg. C.


7. Allow cells to fully cool to ambient temperature before further charges. Be aware the
centre of the cell cools slower than the outer casing. Do not put them in water to cool down; in an emergency cover the cells with a damp/wet cloth if you have to quickly cool them.


8. Store cells with some charge (30/50%).


BRCA Electric Board 16.11.07



UPDATE:


Updated news regarding NiMH 4200 cell faults.

The Intellect Battery Co. have now supplied the following cell Batch No's., which may contain cells that could explode when subjected to ‘race type’ conditions :-

ARAET, AZCWT, BNCWT, BNDST, BRAET, BZAWT.


We must stress that the Batch No's. detailed have been compiled only from returns by European customers to Intellect. There may be other batches that we have not yet been notified of. (So, do not assume that if a Batch No. is not detailed above, that it does not have any problem cells within it). Some EU drivers obtain cells directly from USA, etc. and it is possible that batches outside the EU could also be affected which have not been notified to us.

Intellect supply the same cell in 'Customer designed shrink'. Intellect has not confirmed if the same Batch Codes are used on these versions of cells. Most of the reports that have been received appear to be relating to cells in the Intellect shrink. Hopefully the above information will cover most of the problems.

BRCA Electric Board. 17.11.07

well, that seems like the best bit of advice yet... rather than a knee jerk reaction of banning cells, a far more level headed approach to the problem, and trying to minimise risk!

Defo this looks like excellent advice! Thanks to the BRCA (and obvious tech advisor) for compiling this and not banning BRCA meetings.

So if you have a charger that won't go below 3mv/cell or a very basic charger that you can't change it and don't know what it's set to, consider a better charger a wise investment.

It also shows how a decent equaliser is essential these days. Fortunatly there are a variety around now, though as with chargers you usually get what you pay for.

That may be the case

but

the BRCA info doesn't allow for those racers that use stick packs :(, think its nigh on impossible to balance them in that format

I'll look around the 12th national pits this weekend and I bet there'll be some charging @ over 1c

I guess you can assume that the people using stick packs are not working their cells so hard maybe, but when the BRCA speak, they take it as racers, and most use normal side by side cells.

well most serious racers run side by side formation.

And you can pick upa good tray from lrp/ nosram for just 15 pound and it does the job grand goes to 0.9v per cell cant ask for much more.

A

Updated the first post with info from the BRCA regarding Intellect cells.

Thanks for keeping us updated!!! Please keep up the good work mate.

The batch codes are really hard to read on the cells I have but I'm fairly sure mine aren't on that list ...... will feel a little better charging those tomorrow now!!!

BRCA 12th section mentioned the issue re charge rates before the meeting got under way @ the weekend.

For the next 12th National @ Tamworth the section requires that all batteries have to have some means of being unplugged from the esc.

Looked around the pit tables and charger rates from 4 to 6 amps in use both days with 6 amps being favoured for the 19t class on Saturday. I used 5 amps on Sat as thats all my charger goes to (left the 10 amp Pro-Traks @ home so I wasn't tempted) and 4.5 amps on Sunday for a bit more duration in the open motor class

I reset my charger to 3.5A and 5mV per pack delta and still didn't like to be in the same room as it on Sunday!!!!!

think i may get a lipo sack for charging cells, just to be safe.

found a video showing how effective they are:)

http://www.liposack.com/video.htm

Quick double take - 1 pack of mine are BZEWU not BZAWT.

Thanks for the info - I regularly charge at 6amp on race day and re-peak. Don't think I will be doing that anymore!

Just bought an equalizing tray too, (better looking than safety goggles!)

I dont think nimh's sorta catch fire like lipo i think they more build up pressure inside and as the pressure inside is greater than outsideit exsplodes not catching fire more just sending shards of nickel outwards and into people.

A

So a LiPo sack would still be useful, just no airflow around the cells to cool them.

but surely wouldnt the shards of metal just cut though a sack liek that.

A

IIRC its made of kevlar.....

where do you find the batch number and if its recall do we get em exchanged for non exploding ones

wat about the stick packs??
 

i use stick packs at the moment ( sanyo 3700 mah and nossram 3600mah and 2 crapy tamiya tf1400 ) do they explode cause u cant individualy discharge cells ??

w.b soon greg

Greg, the problems seem limited to IB4200. If you're running GP's then they're 99% right as rain.

Batch number is on the IB shrink in black under the '4' of 4200. However, it isn't always on the cells with another matcher/distributor's shrink, and I don't know how to find it! Make sure you peel all the labels off your cells (from the matcher, etc.) before you give up the search.

No problems have been reported with those packs on public forums, so I'd not be concerned. For peace of mind, charge at 1C (so 3.6 amps for the Nosram cells, 3.7amps for sanyo, etc.) and allow the time it will take to get a full charge. Use a good delta-peak charger, or, if you have it, a thermal cut-off set to 40degC, and placed in one end of the stick pack.

There is a known problem with IB cells, but previous issues with NiCad/NiMh going back 30 years have been due to users abusing their cells. I would not worry about your cells if I were you.