Cells ??

[neiloliver]

I would suggest doing as mentioned above, equalise the cells on a board and then charge to maybe 1000mAh capacity before storing. If cells are being measured at 0.05V then they have an internal short of some sort which is depeleting the capacity faster than the other cells which are OK. This is not really a good thing as these cells will always lag behind the other in the pack, will not reach top of charge and will be over discharge in the next cycle which can mean those cells are driven negative and can vent... it becomes a downward spiral...

Never store your NiMH cells at voltages of less than 1V as this leads to permanent capacity loss due to internal cobalt migration (its a bad thing).

The crunch with these high capacity cells is life, you dont get such high capacity and voltage without trading something.. in this case (i) cycle life, (ii) reliability and (ii) self dischrage rate. Everyone wants the big number on the cell, and that is fine, but you wont get as good all round performance as you would with a Sanyo 2600mAh cell.. which would be good for 1000 cyles. (2600mAh you cry, how crap is that... maybe, put Sanyo have just launched a new version of it and I would want that in my cordless drill any day!)

I sell GP4300 cells and they are very popular, but you cant expect them to last forever.. its a case of the candle that burns twice as bright burns half as long (or something like that!).

As for the -dV question, I would personally use 5mV per cell. (30mV/pack) but this is a question of personal prefference - they longer the amount of overcharge the higher the higher the temperature and the shorter the life.

One last point. If you ever buy cells from a shop, before you do ANYTHING (in fact, do this in the shop!)... measure the voltage of each cell, if any are <1.0V give them back. If 5pcs are 1.20 V and one is 1.1V give them back. I am not saying that this has any indication on capacity because it does not, but the one cell that has the lowest voltage will die first. I would put money on it!

N

rant over..

[super__dan]

Are you guys using the Pulsar 2, I can only talk about the version 1 I have and used to use a lot. I do believe these mv settings can not always be comparred from charger to charger, same as total charge values cannot be comparred. Apparently just from changing to a Protrak I used to get nearly 600mah in my cells, I have no doubt this was not the case, just however the chargers measured it.

[LiamD]

I'm using the old-skool Pulsar 1. I've used it with these settings pretty much since I got it when they first came out.

[Richard Lowe]

I use the 10mv setting at 5 amps on my Pulsar 2 and they dont get overly toasty. I think the Pulsar 1 and 2 are identical apart from the 2's 3 profile memory and higher current capability on the 'motor run in' feature. AFAIK the actual charging circuitry is the same so the delta cutoff should be comparable between them.

Neil - I think the self discharging to almost nothing is normal with the IB's, in every one of the 4 packs I got at the start of the season if I store them for more than a week (even with charge in) one random cell dumps to nothing. The thing thats strange is it's a different cell each time and they've done this from new. Nick at Demon told me to store them with whatever was left in them after the run then equalise and charge straight after. He also confirmed quite a few of the IB's do what mine are doing.

[Dunc]

Originally Posted by super__dan

Hhhhmmm, that's interesting, Chris must have changed as I know he defo used to use 10mv. Doesn't the manual say 20mv for nimh on sheet supplied, maybe he was quoting the party line

Judge it on temperature though I say, all cells when they are charged start to get warm. With nimh as soon as they are warm they are charged, not like the old nicads which you had to charge past the peak voltage and get them warm.

As an aside, I consider some sort of equaliser more important now than ever before. Some of the nimh cells I've had have wandered all over the place whilst left both partially charged and discharged. On assembling my latest cells I equalised them before their first charge and they were all over the place, I guess due to when that cell was actually matched. Similairly I have discharged a pack to 5.4v, left them for over 2 months and had 2 cells read 0.05V purely through self discharge. I equalised the pack and seems to have been fine, but I wouldn't want to make a habbit of it. So now I deliberatly leave my cells with charge in them and equalise them before charging on race day!

Dan

Dan, you could well be right about Chris quoting the party line. He obviously needs to be responsible about the advice he dishes out to the general public.

Quite right about temperature too - it is thine enemy!

I've had some GP3700's since May, which are still going reasonably strong although I've only been using a Spintec discharger. I suspect an equaliser would help to restore a bit of 'zing', which is next on my (never ending ) shopping list.

Btw, I should make it clear I've never owned or used a Pulsar 1 or 2 and so my advice is not the best on the matter!

[tc2k]

Yes, I use the Pulsar 1. I think Im goingto go for a middleground of 20mv, I find that any higher should be used for my very old cells which are prone to false peaking.

[tyreman]

When I asked David Spashett what mv I should be charging at with my LRP Pulsar 1/2, he told me that at 25mv I was killing my cells, and he recommends no more that 10mv, for GP3700 or GP4300.

[neiloliver]

let me throw in a curved ball for reference only... -dV is not a great termination technique for NiMH as the cells are in overcharge when the termination occurs. Most professional charge systems use dt/DT as the primary charge termination technique for fast charging. -dV, Max T, Max mAh, Max t and Max V are redundant back-ups. The lower the -dV threhold the less damage you will do, so unless you have your cells bought for you by someone other than father Xmas, keep that -dV level low and protect your investment. The biggest issue really is that the cells diverege so much over time that they dont all reach top of charge at the same time, so some get overcharged more while others never reach top of charge. this leads to greater imbalance in the pack for the reasons I have described in my previous rant. One thing I did not mention is charge rate, I try to keep to 4A maximum. charging at rates higher than this may make you wheelie off the line and hit the guy in front of you, but it does nothing for the cell life... and at an hour a round you dont need to be silly with charge currents.

N

[MattW]

I don't claim to be a cell expert, but i've learnt a bit over the years!!!

I use a Pulsar and CE T35 GFX. On my pulsar i set the delta at 10-15mV, depending on cells and ambiant conditions. Generally newer cells are ok at lower delta's, where as older ones might need it a touch higher. On my GFX, i would usually use 30mV.

I like the Pulsar, i think it's a good charger, but to be totally honest, i don't really rate the peak detection software. I have had mine set at 15mV, but sat there watching the digital display showing pack voltage drop 30mV before it actually cuts off. For this reason, i think the delta settings on the Pulsar should be lower than for many other chargers.

There is no doubt that IB cells being below 1.0V is a serious problem. I would never knowingly take them below 0.9. I think you are ok discharging to 0.9 as they do not rest there - picking back to oer 1.0 after the load is removed. When i first had IB4200's i was equalising them on conventional resistor/diode trays to 0.7V. I had 2 packs die very quickly. I stopped this practice and the ones that survived have generally been ok.

I did lose one pack last week - but basically this was my fault as well. I had come close to dumping and then left them connected to the car for about 10 days. The entire pack voltage had therefor gone far too low. 5 cells survived (to a point) but one didn't and has gone internal short. There doesn't seem to be a way of reviving them when this happens. IB cells must be left with charge in them - i would say at least 500mAh, possibly more if they are to be left for a long period.

As a general rule of thumb, GP cells seem much more resiliant than IB. However, it is at a cost, as performance from IB's seems far superior.

[super__dan]

Good call Matt!

[tc2k]

Ah well, despite the worry over the cells, with a 19t in the buggy, they had three times the punch my old cells did, felt much nicer. Capacity isnt a problem in 2wd anyway.