Quick nimh charging question please help asap
 

When I hook my new 3000mah nimh 7 cell 8.4v pack to my charger and start to Charge it the charing voltage is slot higher than 8.4v? It is reading almost 10.25 at some points then drops to 9.5 or so then back up. Why is this? Is something wrong? Help please. I have the accucel-6 turnigy charger

The voltage should be higher but it should not be fluctuating too much. Should just keep going up until it's about fully charged, then it will come back down a bit before peaking.

well I know my 7.4v lipos peak at 8.4 but what do the NiMh 8.4v packs peak at?
I run all lipo stuff with my RC's but my brother just bought a new Slash and came with the 7cell 8.4V NiMh so charging it for him on my charger instead of using the charger that came with it that takes 8 hours to charge a 3000mAh pack.
I may have exaggerated the Voltage drop some but I know the 10.25 is how high it was going

NiMh cells have internal resistance and to charge them, the charger has to overcome that internal resistance in the cells. Also the charger electrically has to be at a higher voltage than the batt just to do the charging. The
internal resistance of the cells adds to this voltage differential increasing the voltage the charger has to maintain to deliver the charging current.

Over time the cells IR will go up and you'll see this voltage differential increase over time as well.

When the voltage drops, its the charger's way of measuring the packs voltage so the charger knows where pack is at voltage wise.

My 6-cell NiMH's peak at around 9v (1.5V per cell) so I don't think 10.25 is abnormally high for a 7-cell pack.

thank you Duster that cleared it up for me

I agree the 10.25 peak voltage is not unrealistic for a 7 cell. But the voltage drop mentioned suggests the possibility of a less than ideal connection, or perhaps a cell that is getting flaky. I have experienced such behavior before, usually ended up with one of these issues.

I’m curious, what peak charger do you have that does this? Offhand I don’t recall noticing this behavior. If I remember correctly the ICE chargers I have do blip at 2 minutes on a discharge, (which I never understood) but I don’t recall anything on the charge side. Except for a reflex charge mode of course.

The feature that measures voltage drop at the end of the charge cycle is called Delta Peak, and is typically set to 4 or 5 mV unless your charger has the option to change it manually, like the ICE does

Oh I agree, I understand the peak detection thing fairly well. I remember doing this manually even.

I just didn't follow the other comment, thought maybe he was referring to the reported large 10.25 to 9.5 drop, suggesting that the charger paused to get a better voltage reading or something. Seemed to suggest that the charger dropped voltage to get a reading, as opposed to the pack dropping voltage when peaked. I was just curious.

Or maybe I just misunderstood, and he was just referring to delta peak, my bad perhaps.

i have a question i bought a turnigy accucel-6 charger the other day and im lost as how to use it i want to charge my rx pack which is 6v 1100mah nimh what current or amp should i charge it at and do i need to discharge it also i have a futaba tx and want to charge the batt in the tx usein the plug it is 9.6v 1200mah nimh same as before what do i charge this at or what do you recommend i do

i have been lookin on a few sites and all im seein is confusein i have charged the 6v 1100mah batt and i had it at 0.8a and the volts went upto 9.48v on the screen i think i overcharged it so i dont want to do it to any more can you tell me how to set it properly anything ya suggest would be great

also the peak is set at default i read above it should be at 5mv should i set it to this the manual is not helping me need someones better knowlage

No, you didn't overcharge it. The voltage you saw is what the charger needed to force current into your rx batt. NiMh has a relatively large internal resistance and that has to be overcome as the batt charges, so as the batt charges and its voltage rises, the charger needs a higher and higher voltage to force current into the battery. I usually charge my rx NiMhs at 1amp, but there is nothing wrong with 0.8a as a charge rate, it just takes a little longer to finish than 1amp does.

As far as discharging, NiMh works best if you can discharge the individual cells down to 0.9v before you recharge. Since you have no control over individual cells in a pac, all you can do is discharge the pac down to 5.4v and then re-charge. Since the cell voltages in the pac can vary by surprising amounts, you can potentially cause more harm discharging the pac than good. I don't discharge my rx pacs, I just use them until they have to be re-charged and then just fully charge them. 5mv for a sensitivity setting is fine. Sounds like you've been doing a good job, but just needed to understand why the voltage was higher than you thought it should be.

duster 360 thank you very much that has explained a lot to me :D