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Main Discussion > NiMH battery gurus?
 
 
nivlek
rrProfessor
Location: Norfolk England

My Posts This: Topic  Forum

The reason that they need to be trickle charged for the the first couple of cycles is that most fast chargers use 'delta peak' detection to terminate the charge .
On the initial charge , there is no delta peak to detect so the charger may not stop charging .
Once the cells have been 'formed' by charging , they will exibit a delta peak during subsequent charges.

At the end of the day , it gets dark .
10-28-2009 07:17 PM
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Ace Dude
Elite Veteran
Location: USA

My Posts This: Topic  Forum
Quote 
But overcharging and heat are not always a bad thing.

I disagree. Overcharging and heat is what kills cells. There is a difference between warm and hot. YMMV.

Quote 
A new NiCd or NiMh pack should be slow charged at between 1/20C and 1/10C for at least 16 hours before the first use.

There's no reason to charge below C/10.

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If they are at different states of charge and peak at different times during a peak detecting fast charge, the charger may not be able to detect an overall pack voltage peak and either drive some cells too far into overcharge or worse, fail to terminate the charge at all. That will damage cells and make the pack VERY hot.

My Orbit charger will do a forming charge for NiXx in CC/CV mode with a configurable CV. Default is 1.45v/cell. CC will taper so voltage never goes above the set value.



10-28-2009 10:17 PM
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pgoelz
Veteran
Location: Rochester MI

My Posts This: Topic  Forum
Quote 
I disagree. Overcharging and heat is what kills cells. There is a difference between warm and hot. YMMV.
Depends on what you call "overcharging". In this context, "overcharging" means any time the charge percentage reaches 100%. Since there is no easy way to determine when the state of charge reaches 100%, the charger continues the charge into "overcharge" until the resulting temperature rise causes the voltage to begin to fall. This is detected as the "peak" and terminates the charge.

Yes, continuous or severe overcharging creates a very hot and damaged cell.

Quote 
There's no reason to charge below C/10.
Depends on how closely you monitor the charge. I believe most NiMh cells can tolerate a 1/10C overcharge for limited time. The last time I looked (and we're talking AA cells), the CONTINUOUS overcharge current for NiMh was 1/20C. Unless you are sure you will discontinue the charge after 16-24 hours, 1/20C for at least 24 hours is probably a better idea.

Paul

Paul Goelz
Rochester MI USA
http://www.pgoelz.com
10-29-2009 01:05 AM
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mckrackin
Key Veteran
Location: lucasville,ohio USA

My Posts This: Topic  Forum		I've been charging at 1C(3amps) and all is well.
My Turnigy and Triton chargers both kick off at 9.4 volts.

The battery stays cold on the Turnigy charger and gets a little warmer than the glass table it's on in my garage and that's cold right now.lol...

I'm not overly concerned anyway.A lipo upgrade is coming soon.
Right now the crawler runs about 30 or 35 minutes on a charge.

I'm satisfied for now
10-29-2009 01:06 AM
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pgoelz
Veteran
Location: Rochester MI

My Posts This: Topic  Forum
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My Orbit charger will do a forming charge for NiXx in CC/CV mode with a configurable CV. Default is 1.45v/cell. CC will taper so voltage never goes above the set value.
Just like charging lithiums without a balancer, if your charger is not monitoring each cell individually, it has no way of knowing if any given cell is being driven above 1.45V. If one or more cell(s) reaches full charge significantly before the others, it will be driven into serious overcharge at your CC rate until the aggregate pack voltage reaches your CV setpoint.

This can be a problem if the pack is new and the state of charge of each cell is unknown. In the real world, we get away with it most of the time because new packs are made from cells that (usually) are from the same batch and are the same age. They have all self discharged the same amount from wherever the manufacturing process left them.

Becasue NiCd and NiMh can tolerate some overcharge, CC charging until the whole pack is in overcharge causes the cells to remain balanced.

Paul

Paul Goelz
Rochester MI USA
http://www.pgoelz.com
10-29-2009 06:24 PM
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Ace Dude
Elite Veteran
Location: USA

My Posts This: Topic  Forum
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ust like charging lithiums without a balancer, if your charger is not monitoring each cell individually, it has no way of knowing if any given cell is being driven above 1.45V. If one or more cell(s) reaches full charge significantly before the others, it will be driven into serious overcharge at your CC rate until the aggregate pack voltage reaches your CV setpoint.

This can be a problem if the pack is new and the state of charge of each cell is unknown. In the real world, we get away with it most of the time because new packs are made from cells that (usually) are from the same batch and are the same age. They have all self discharged the same amount from wherever the manufacturing process left them.

Becasue NiCd and NiMh can tolerate some overcharge, CC charging until the whole pack is in overcharge causes the cells to remain balanced.

Please, if individual cell monitoring was that important for NiXx then it would have been incorporated many many years ago. Obviously, it has not and will not.

If you're that concerned about individual cell monitoring rip off the heat shrink and charge each cell individually and add balance taps.



10-29-2009 08:02 PM
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pgoelz
Veteran
Location: Rochester MI

My Posts This: Topic  Forum		I am talking about balance issues with NEW packs that are not necessarily in balance YET. For those packs, a fast charge can damage out of balance cells and the likelihood is greater the more cells there are in the pack. A slow charge is the recommended first charge protocol.

The only reason cell balance issues are generally an issue with NiCd or NiMh packs is that driving the cells into overcharge at the end of each peak detecting CC charge keeps them in balance without individual monitoring. A cell that peaks early dissipates the excess energy as heat as the rest of the cells continue to charge.

I'm not going to argue this any more. Seems to be the second most popular hobby on RunRyder. I'd rather fly. Do what you want. You'll most likely get away with it with decent cells.

Paul Goelz
Rochester MI USA
http://www.pgoelz.com
10-30-2009 01:21 AM
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Ace Dude
Elite Veteran
Location: USA

My Posts This: Topic  Forum
Quote 
I am talking about balance issues with NEW packs that are not necessarily in balance YET. For those packs, a fast charge can damage out of balance cells and the likelihood is greater the more cells there are in the pack. A slow charge is the recommended first charge protocol.

I said that a while back. C/10 for 14-16 hours. What part are you confused about? This is also called a formatting charge. I'd even recommend 2-3 formatting cycles on any brand new pack. C/10 for 14-16 hours followed by C/5 discharge to 0.9v/cell or slightly higher if you'd like.



10-30-2009 02:49 AM
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pgoelz
Veteran
Location: Rochester MI

My Posts This: Topic  Forum		I'm not confused at all, and you and I are on the same page when it comes to charging new packs. The reason most people who ignore the slow charge recommendation get away with it is because good quality cells are all pretty close to in balance to start with. The self balancing that occurs during overcharge following fast or slow charge takes care of the rest after a couple cycles.

I had a Walkera NiMh pack fail to "peak" and subsequently went into serious overcharge a while back. I am assuming it was because the pack was new and I did NOT slow charge it first (thought I could get away with it). The cells were low quality and probably too far out of balance and blurred the peak to the point the charger missed it.

Paul

Paul Goelz
Rochester MI USA
http://www.pgoelz.com
11-02-2009 08:14 PM
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