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Author Topic: mainly NiMh batteries  (Read 271 times)

roycv

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mainly NiMh batteries
« on: January 12, 2022, 07:38:16 am »

Hi, originally this was a post on another thread that got a bit technical!  It is about battery packs that fail either through low capacity or low voltage.  I have the right meters and tools and electrical/electronic background etc. 

I have frequently stripped relatively newish battery packs down and isolated faulty cells.  With a view to putting them back together afterwards I carefully slit the covering film to expose the cells.  I can then use sticky tape to re-encase the battery.  The cells, if possible should be separately monitored for voltage under charging conditions, anything over 1.5 volts is bad news, 1.46 is about right.

Another cell fault is going short circuit internally i.e current flows but no voltage across the cell. 

A couple of years ago I was given a 12 volt 4600mAh pack which on stripping down had 2 cells failed, I bought 2 replacement cells and soldered them in place.  When taking out the faulty cell un-peal the joining strap rather than cutting through it.  You then have somewhere to solder a the new cell in place.  Best of course is welding but that is beyond me so I use a 75 Watt soldering iron on the basis of going in for a very quick solder joint and not heating up the cell. 

Sometimes the intelligent chargers will not charge up a battery pack as the battery has been left for a while.  I use any 'old type' or dumb charger to get the pack going.  For example maybe half an hour on a lead acid charger to wake up the battery and then use a modern charger.
It may have gone unnoticed but as the cells are marketted with very high amp hour capacities, try checking the weight of the cell as the higher the capacity the heavier the cell. If you still use AA cells try weighing the old 500mAh cells against the latest ones.  The new AA cells can also be 1 mm wider than the old ones and you may have found problems placing them in the back of an old transmitter.

A word of advice is do not buy battery packs of any sort from China.  They appear to be able to print anything they want on the battery as far as capacity goes. 

This includes the 18 / 650 (eighteen six fifty) cells as well I have tested and weighed and tested for capacity for myself and there are also tests on-line that bear me out on this point.  However they shoot themselves in the foot when they publish the battery specification as the weight of the battery reveals the true capacity.
The eighteen six fifty cells are LiFe and 3.6 volts. (Lithium iron phosphate).  The Component shop give maximum discharge rates and personally I would go with their figures.  Some 18650 batteries from China are marked as 10Ah which takes some believing!

Buy from the well known shops who specialise in batteries, they are dearer but you do get what what you pay for.  I have found in the past that a phone call is often welcomed with lots of advice.
regards
Roy



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Subculture

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Re: mainly NiMh batteries
« Reply #1 on: January 12, 2022, 09:54:31 am »

18650 cells come in a number of chemistries, including LiFE. LiFE cells are 3.2-3.3v nominal voltage. Far more common to find them in Li-ion chemistry- 3.6-3.7v nominal voltage, they give a higher power density but a shorter cycle life.

roycv

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Re: mainly NiMh batteries
« Reply #2 on: January 12, 2022, 11:04:24 am »

Hi subculture my interest was aroused when I realized how light in weight that they were and as I said my one was a 3.6 volt one.  I think these are the type that are used in car batteries, is that right?
Roy
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Re: mainly NiMh batteries
« Reply #3 on: January 12, 2022, 11:42:31 am »

If you mean electric cars, Tesla has used them in the past, but they tend to use slightly larger cells now, especially in their lower priced models like the Model 3. Larger/less cells reduces cost but also reduces the amount of power on tap.

I've not seen them used in starter batteries for internal combustion cars. Some people have used four Lithium Werks 26650 batteries to start up cars e.g. https://www.youtube.com/watch?v=X8HoW-qmxnU but those batteries have very high C ratings of over 50A per cell. Very good batteries but not really necessary for many scale modellers.  https://lithiumwerks.com/download/lithium-werks-26650-power-cell/


18650's are used a lot in laptop battery packs, cordless power tools, electric bicycle power packs, led torches etc.

They're not all equal though, the current sinking and capacity varies enormously even with bona fide manufacturers.

Cells with higher current sinking capability will tend to weigh more and have less capacity than cells with a high capacity. However even the weediest of lithium batteries will leave SLA or many NiMh's in the dust.

If you want more capacity, then look at 26650 size.

roycv

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Re: mainly NiMh batteries
« Reply #4 on: January 12, 2022, 12:08:51 pm »

Hi thanks for the info yes I was thinking about the drive batteries.  I can't get my head around the benefit of owning an electric car apart from the polution that I make here. 

I am afraid I also find that almost everything technical the government advises is usually either wrong (re we should all buy diesels) or comes with unintended consequences.  (Broadband for all, death of the High Street)

My son in law is a consultant to some of the motor industry on planning an electric future and when he goes electric then the case will be proved for me.
I am using the Nicads and NiMh batteries I still have.  I even have some working disc type DEAC cells I think they are 650mAh still take a charge and deliver.  Not found it necessary to go to Lithium cells but have charging facilities for them.
Regards
Roy
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Re: mainly NiMh batteries
« Reply #5 on: January 12, 2022, 06:17:35 pm »

Solid electrolyte batteries seem to be the tech being touted as the solution, if they can manufacture them in quantity.

Colin Bishop

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Re: mainly NiMh batteries
« Reply #6 on: January 12, 2022, 06:36:44 pm »

Hamsters on dynamo treadmills is the way forward to support a green environment. (very handy for model paddle steamers too!)

Like all these things there is no immediate magic bullet but technology does move on and it appears that there are some new battery designs which don't depend so much on the mining of toxic chemicals in dangerous conditions.

It still remains the case that there is far more energy bound up in petrol or diesel fuel than electric car batteries. Both generate pollution either in the manufacture or in the emissions. Nothing is black and white unfortunately, maybe until nuclear fusion becomes a reality.

Colin
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