First charge and subsequent care and feeding for 12-cell NiMH pack

[RickB]

Hi All,

I am putting together a 12-cell pack to run a 100 watt (Osram 64623) Mag mod. I have 16 Titanium 1800 AA cells and a fivemega 12AA holder. I'm looking for advice on how to do the initial charge, and then later on, what routine to use to get the best life and performance out of the pack. I have a Triton charger that I'm not too familiar with.

For the initial charge, I was thinking of charging the individual cells at 1C (1.8 amps), then assembling them into the holder. Should I also do a 0.1C slow charge on the assembled pack to balance the cells? And/or, should I use the Triton to pick out the 12 best cells out of the 16? If so, how do I do that?

And for long term care, how does routine 1C charges with occasional (every 10 charges or so) 0.1C charges sound for the assembled pack? Should I do discharge cycles occasionally? Would there ever be a need for disassembly and charging of the individual cells?

Thanks for any thoughts or pointers you might share!

-Rick

 

[SilverFox]

Hello Rick,

The first charge should be a 0.1C charge for 16 hours. You can do this with the whole pack, or individually. After that, 1C charging is great, and as you have indicated you need to balance the cells in the pack with a periodic 0.1C charge.

Cell matching is done by picking cells with similar mid point voltage under load, and similar capacity.

Tom

 

[RickB]

Tom,

Thanks for the info. Looks like I was pretty close, but off on the initial charge.

Would it be okay to split the 16-hour charge into two sessions? I'd have a tough time being around for 16 hours straight to keep my eye on things...

After reading the Triton manual a bit, it looks like I can get capacity for the cells by doing a charge > discharge cycle and then looking in the data menu for the discharge capacity. Will this give me the figure you were referring to?

Unfortunately, I'm not sure how to get a voltage reading under load since the cells will be in a pack inside a light... Or is there a way to check this using the Triton (of course, with individual cells)?

Finally, for those interested, of the 16 cells I got, 15 came with a rest voltage of 1.24-1.28. Most were at 1.25. However, one was at 1.03 and one of the 1.25 cells had a dented negative end. Should I try to charge up the 1.03 cell and see if it's okay, or is this a sign that it's DOA? Any danger in using the dented cell? Then dent is not really deep, but deep enough that I was concerned about it making a good contact in the battery holder...

Thanks again for the tips!

-Rick

 

[SilverFox]

Hello Rick,

I usually start the 16 hour charge with discharged cells at night. The most dangerous part of the charge is at termination, so that way I am up for that part.

Cell matching is done with individual cells prior to assembling them into a pack.

1.03 volts is still good, but I would mark the cell to keep track of it. It may have a slightly higher self discharge rate, but it is difficult to know for sure. Your should charge the individual cells, let them rest for around 30 minutes, then do a discharge. Keep track of the discharge capacity and use that for matching. The Triton should display the voltage of the cell during the discharge. When you start the discharge, set a timer for something like 30 minutes. When the timer goes off, write down the displayed voltage. This will give you a good comparison from cell to cell.

I don't use dented cells... Of course, your mileage may vary...

Tom

 

[LuxLuthor]

I don't use dented cells... Of course, your mileage may vary...

Tom

Funny you should mention that. I made a 15s pack with my welder, and had seen one cell with a small dent, and never considered that was a problem. Sure enough after several charge/discharges that cell did not perform as the others, and I replaced it.

 

[RickB]

Well, I set up my overnight charge, but goofed! I left the "NiMH Maximum Charge Input" setting on at 1000 mAh, so when it got there about 6 hours in, it stopped. Not sure if I should try it again and put another 10 hours on at 0.1C...

I have all my cells numbered and am recording data about them so hopefully I can notice any funny business, like if the 1.03 V cell is weaker than the others.

Thanks for the technique to determine midpoint voltage under load with the Triton. Once I'm sure I've got the cells fully charged and have some time, I'll give it a shot.

-Rick

 

[SilverFox]

Hello Rick,

I don't see any issues with continuing on for an additional 10 hours, but usually this forming charge is done in one shot.

Technically, you will be measuring the voltage after a 30 minute discharge. The only time this will be the mid point voltage is when the discharge is completed in 60 minutes. The true mid point voltage is usually picked off of a graph. If you use the same time and conditions for each cell, you should still get very good data, even if it is not technically the mid point data.

Tom

Tom

 

[wptski]

Any "C" or "D" cell MagLite will dent the cells from spring pressure! What ya do then?

 

[VidPro]

Any "C" or "D" cell MagLite will dent the cells from spring pressure! What ya do then?

cut the spring down a bit, :shrug: then grind it off.
my mags dont dent my cells, even when i drop them, as in normal drop. some "cut" mags are tighter, and would benifit from shorter springs.

some cells have a weak loose base plate and nipple that can bend a bit before it changes anything. if its bending that easily, it probably is not reaching the Stuff inside to damage it yet.

any side dents or corner dents or things that effect the electrolyte plate roll, will definatly effect the cells, and i would avoid using stuff that was destroyed in shipping.

here is a way to determine if your base dent is reaching product. tap on it with your fingernail, or a pen or something, if its hollow sounding, then it didnt sink into important places yet.

 

[LuxLuthor]

here is a way to determine if your base dent is reaching product. tap on it with your fingernail, or a pen or something, if its hollow sounding, then it didnt sink into important places yet.

BINGO on that bad cell I had. Dent area (which was not that deep) has a noticeably solid tap sound around the dent. Really great tip.

 

[wptski]

cut the spring down a bit, :shrug: then grind it off.
my mags dont dent my cells, even when i drop them, as in normal drop. some "cut" mags are tighter, and would benifit from shorter springs.

some cells have a weak loose base plate and nipple that can bend a bit before it changes anything. if its bending that easily, it probably is not reaching the Stuff inside to damage it yet.

any side dents or corner dents or things that effect the electrolyte plate roll, will definatly effect the cells, and i would avoid using stuff that was destroyed in shipping.

here is a way to determine if your base dent is reaching product. tap on it with your fingernail, or a pen or something, if its hollow sounding, then it didnt sink into important places yet.

Why would you need to modify something like a MagLite? Are they defective in design? More likely to be the design of a Ni-MH cell being more dent prone because it's fragile!