|Aug 15, 2011, 10:48 AM|
Joined Aug 2011
IMAX B6 Settings for NIMH Cells
This is my first post. I have purchased ten IMAX B6 chargers to rebuild a 2006 Honda Civc Hybrid battery. I know this doesn't really fit your forum but this is the target audience for the IMAX and I could really use your expertise!
Most of you are charging LiPos. I am looking for a guru that knows it all about NiMH charging.
I own a Honda Civic Hybrid. With 120k miles on it, the battery is tired. The battery chemistry is good for 30 years but some cells can go bad, and since they are all in series, they take the whole pack down. There are a few people out there rebuilding these batteries and charging $1200 to do it. I've chosen to do this myself.
I have purchased a second used Civic Hybrid battery from a local junkyard. I plan to use cells from this one to replace bad cells from the one in my car.
The battery is made up of 18 "sticks" of six cells welded together in series (like a six "D" cell flashlight. The sticks of cells are shrink-wrapped together with the end electrodes exposed. I wish to use the IMAX to "excercise" the batteries, one "stick" at a time, to chart the charge and discharge watts over several automatic cycles. Once charted, each stick's output will be compared to the others and I will put together a balanced battery with good sticks of similar results.
That's my project. Because each stick can take a while to cycle multiple times, and between the two hybrid batteries, we're talking 36 sticks I've purchased ten chargers. The car needs a battery to be driveable.
I don't think that the accuracy of the chargers is a big deal. I'm looking for similar good results from the testing. I also hope to break down the hydride crystials and also the "memory" effect on the battery. Though not as prevalent as NiCad, it is still there for Nickel chemistries.
This is where I ask for a bit of hand-holding. I do not know the best charge discharge currents to plug in. I've been using 1 volt per cell (six volts for six cells) as the cut-off voltage. I'm unsure of that number. And so far the results have been very questionable.
I am using only one charger right now while I wait for the other nine to come in.
Any and all help from you is very much appreciated. There is very little about this on the web. I've searched for hours (and I consider myself good at searching this stuff). Your help would be greatly appreciated.
|Aug 15, 2011, 11:43 AM|
What's the individual capacity of the cells? They really are NiMH?
They must be WAY bigger than D-cells. At a max of 1A discharge with that charger it could take a good while.
1v/cell as a low is considered 'empty'. If you can isolate cells (not the whole pack) you can go lower. Not sure if that is useful or not as I've mostly worked with packs so never push past 0.8v/cell minimum.
I've had somewhat decent luck recovering (cycling) discharged/abused/old NiCads but the NiMh seem to not be nearly as robust-at least at the consumer grade level.
You'll probably have to play with the peak termination a bit to get proper results from you particular cells. Getting a temp probe might be useful.
There's some good reading at these sites:
Learn About Batteries – Contents
The Care and Feeding of NiCd Batteries (Not NiMh but good info)
If you haven't discovered [CandlePower Forums] check them out-good resource.
|Aug 15, 2011, 11:49 AM|
What is the rated capacity (mAh or Ah ) of each cell?
My first though is that that charger does not have enough output or discharge capability to do an y real good excersising these cells if they are any capacity much at all.
Not sure who came up with the statement
|Aug 15, 2011, 12:38 PM|
I wondered about the [30 year] claim myself-pretty preposterous. Prius uses NiCads, seems old school but they are a more robust chemistry, at least most of what I've read and my own limited experience seem to support that.
Maybe he got the info here? Seems they are D-cells, a bunch of them soldered together so I wonder if the individual cells actually can be isolated.
[IMA batteries are made up of 20 battery "sticks". Each stick is made up of six "D" cell NiMH (Nickle-Metal-Hydride) batteries that have been welded together. This means that the IMA battery is really just 120 "D" cell flashlight batteries.]
|Aug 15, 2011, 12:50 PM|
Joined Aug 2011
I don't know what the rated capacity is for the cells. They ARE the approximate size of "D" cells give or take. You can be sure they are very robust. The sticks are actually connected in pairs at one end with a welded plate. The other ends are tapped for a solid connection to the battery assembly.
I can't comment on the 30 year life span. Don't shoot me for sharing what I've read. I would imagine some thought has gone into these $3000 battery packs to give them more than a few years of life. I have more than one source that says the chemistry is time friendly.
Anyway, I'm searching for the capacity of the cells. I have an electrical shop manual at the house. It may be in there somewhere.
So, the first few battery sets I've tried to charge and discharge gave me numbers that made no sense. The first settings I used were...
Cut-off 9.6volts (for 12 cells). I know this was on the low side. I've since set it for 12 volts (1 volt per cell).
After three automatic cycles, It read aprox 1800mah charge and 750mah discharge. I got that on three different stick pairs (12 cells each). How could I put so much in and get so little out unless I don't have the correct settings, or have three equally bad groups of cells?
Input! I need input!!!
|Aug 15, 2011, 01:35 PM|
If they are D size then they are likely somewhere between 7 and 10 Ah. and the statement I made back
in post #3
You need a lot more charger than you have now,IMO.
If I were trying what you want to do I would be using one of my FMA POWERLAB8 Battery Work Station which has some serious charging and discharing capabilities as well as graphing capabilities.
I would also be installing leads / voltage test points between each cell so I could monitor each cells volate during charging and discharging. At very mininum you need to be using an IR Temp. gun and checking each cells tmeperature during the charge and discharge cycle.
I thought that electric cars were suppose to have an on-board battery management system that monitored each cell or atleast each group of cells and reported battery condition.
If they are indeed 6 cells per unit and are indeed 7 to 10A per cell then a 1/3 C to 1/2C charge and a 1/2C discharge may give you a token idea of their condition but I would not count on it to much.
My experience reviving dired Ni cells be they Nicad or NiMH takes high C rate charges to break up the salt crystal which form in them when they go south. Also I never declare a cell of anytype good until I test it at the load it will see in actual service.
|Aug 15, 2011, 01:59 PM|
Joined Aug 2011
Thank you Charles for your feedback.
All good information. I questioned the power requirements to recondition these cells by the IMAX B6 as well. Though there are others that have done it and have claimed good results. It may be that they are more likely finding bad cells with this process than really breaking up the salt crystals.
The battery does have charge management built in to the car but after so many were failing early, Honda chose to encourage owners to come in for a "computer update". What that would have seemed to do is bandaid the problem of failing batteries by charging and discharging the packs more frequently and in a smaller window of charge (so as to put less stress on the battery). This has really pissed off the Honda lovers out there that spend top dollar to buy this precious Hybrid for it's rated 40/51MPG only to get in the mid 30's after the computer update. I can feel my hackels rising as I type this.
(somebody find my YOGA stick!)
So... what I think the B6 does, is collect data on charge and discharge only and store that for retrieval automatically. Does that sound like a fair conclusion?
|Aug 15, 2011, 02:10 PM|
I have Thunder Power, Hyperion,FMA , iChargers and a few others but no IMAX so do not know how / if it stores data nor to what extent.
A good site for NiMH care is here
|Aug 16, 2011, 07:52 AM|
Joined Aug 2011
So I dug into the manual better and found a few settings that I needed to change. It has a timer shutoff and a max power shutoff (such as 5kmah).
I turned off the timer shutoff and bumped the max power shutoff to 15k. I also chose to work with only six cells instead of the 12 cell group (two co-joined sticks of cells). This will take longer, but my results are already looking much better. I don't think I was charging the cells nearly enough before now.
So, it's still running the automatic 3 cycle discharge/charge routine. I'll know more when it's finished.
Waiting for the other nine Imax chargers to arrive. That will speed things up big time. There are 22 sticks of six cells each in a single hybrid IMA pack. That's three days minimum with ten chargers. Whoa. I don't see how I could do it without them.
When I'm finished (in about two weeks), the chargers go up for sale.
|Aug 19, 2011, 02:01 PM|
Canada, BC, Victoria
Joined Mar 2004
Returning only 750mah after a charge of 1800mah is a sign of something really wrong. Either you've charged them with the wrong settings or the batteries are toast. Or both. I'd stop cycling the packs until you test the cell voltages under load to find bad cells.
You have a huge task in front of you; you are essentially going to make up a huge matched pack. I'm not sure how you should handle the case of cells of different capacities. ie: What will you do if some cells measure 5ah, some at 4ah, others at ....???
Wikipedia says your car is "158.4 V (132 x 1.2 V) Nickel-metal hydride batteries with 5.5 A·h. So Everyday flier was close with his estimate of 7-10ah.
If you proceed, I would:
- 1/10 c slow charge
- test cells under load
- replace bad cells
- cycle a few times
- re-test cells under load
- cycle and mark capacity.
So, charging.... lets set your charger for a nice slow charge to help your first 6 cell pack.
- Set for 6 cells
- Set capacity to 5500mah
- Set safety timer to 14 hrs
- Set charge rate to 1/10c, or 550 mah
- Set delta peak lowest possible (more on that later)
- Set trickle charge to 550mah, if possible.
This will essentially be a timed charge. From the charge length you can calculate how much has been put into the cells. (Less a bit for inefficiencies.)
Monitor this charge. In general, the cells should warm up when fully charged. However, at this slow charge rate, they may not warm up at all. If some cells warm up and others don't, you have suspect cells. This slow charge should help "balance" the 6 cell pack.
Pack charged up now? Good. Rather than discharge to measure the capacity, find a way to put a good load on the cells and check the voltage of each cell. Bad cells will drop voltage very quickly. Good cells won't. Your charger's 5w discharge rating may not be enough to show this. (5w/7.2v = .7 amps.) You can try it, but a 1c discharge rate would be more telling - so a 5.5 amp load, or close to 50 watts. Say a 50w-100w, 1.8ohm resistor? (Check my math.)
Since all these packs are suspect, I wonder if you should even worry about peak charging. Bad cells will make it behave strangely, and likely cause the charger to stop early. But in general, peak charging settings depend on the cells.
Too fast will make the cells heat up. (Bad.)
Too low of a delta peak will shut off the charge early. (Bad.)
Too high of a delta peak can overcharge the cells, or the charger could entirely miss the peak. (Then will rely on timer or capacity cut-off.) (Bad.)
You want to choose a charge rate where the cells only heat up near the end of the charge, and a delta peak where the charge turns off when the cells warm up. For my 2/3AA kan cells, that is .6C charge rate, and 3mv delta peak. Yours will be different.
Sorry for the long post, and good luck. It sounds like quite a project.
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