delta V cut off advice on battery charger

[AndrewL]

my multi pupose battery charger doesn't seem to be cutting off the charge cycle and I'm not quite sure why. It's putting much much more juice into teh cells then they hold and seemed to be staying at around 1.56v (charger reading my volt meter gets less from the battery maybe 1.42v or so) for ages but still charging when I connected a single cell (and still saying it's putting 0.4 amps into the cell

I can alter the Delta V cut off and was wondering what sort of setting I should put in it

any advice

I'm normally charging 12AA cells connected in a battery pack, I use two of these packs in parrallel but thought it was best to charge the seperately and just get them to the same voltage when I connect them to the light

 

[wptski]

For Ni-MH it's 5mv/cell and 10mv/cell for Ni-CD.

 

[AndrewL]

thanks

I'm not sure what the defualt is but I will set it to that
I assume it's the same for one cell as it will be for a 12cell pack

 

[wptski]

thanks

I'm not sure what the defualt is but I will set it to that
I assume it's the same for one cell as it will be for a 12cell pack

Twelve cells in series would be 12x5mv.or 12x10mv. In parallel, it would just be 5mv or 10mv.

 

[Mike abcd]

Twelve cells in series would be 12x5mv.or 12x10mv. In parallel, it would just be 5mv or 10mv.

I'd suggest setting it considerably lower since it's unlikely that all the batteries will reach full charge at the same time.

I try to avoid charging in series whenever possible and avoid peak charging (-dV) when it's not. That said, I'd personally set the limit to something like 5-15 mV in this case so charging terminates pretty early.

Consider a pack with one or more cells with less capacity than the rest. I'd prefer charging terminates as soon as those cells reach full charge even if it leaves the rest less than fully charged. Continuing to fast charge past that point damages the lower capacity cells and turns into a vicious cycle. The weak cells lose capacity by being overcharged which results in them getting over charged more and more, losing capacity faster and faster...

Putting more capacity into the stronger cells in the pack also makes it more likely they'll have enough power left to reverse voltage weaker cells if you use the pack to near it's capacity.

Mike

 

[VidPro]

what mike said, you would never see a 5mv drop on every battery at once, and the brainey chargers will need it to occur in a TIME span that it can operate in. these microcontrollers are rarely capable or programmed to know what happened 10 minutes ago.

if your making the charger have it detect the very FIRST 5mv drop then top off the series pack with a very slow charge to finish, like 1/10thC or less.

the very first drop will be one battery in the series reaching overcharge state, IF you can detect that, then slowly topping off the rest of the pack WITHIN specs for overcharge would finish and balance out a Ni-??? pack, without ruining it.

OR
charge to some specific voltage, like 1.35-1.4 per cell, then go to a slow topping/balancing charge.

IF you wait untill all the batteries in series have reached V-drop, then if you were fast charging, you will certannnly overcharge many cells out of spec. because some will be overcharging, long before the last few are fully charged and started dropping.
also
the ones that do v-drop will v-drop further and further, while being excessivly overcharged, that wont nessisarily indicate the others are charged, just that the overcharged ones are being destroyed (so to speak).

 

[wptski]

I think the question is what the DeltaV value is!

 

[AndrewL]

true, but I might not have asked the right question.

My charger does say to charge at 1/10C if possible anyway which I don't quite do but I don't give it a fast charge. If I'm just topping the battery up then I'll put it on for a while in the everning and switch it off before it's fully charged since I'm charging two packs in parrallel (to make sure I don't overcharge anything). If I'm doing a full charge at the weekend then I charge each pack till the charger finishes and normally do this overnight and time it so that it will finish when I'll be awake. I then discharge the packs a tiny bit to get them to the same voltage under a bit of load before connecting them togeather in my bike lights.

Or should I do something else?

 

[VidPro]

if they were both fully charged or really close to the same voltage, i would not discharge them at all before slapping the 2 packs together, unless you had way small wire or something.
a breaker (or fuse) on each parellel pack would prevent any issues, if there was an error in judgement , and one pack was charged and the other was not.
but even if it was, the one would just charge the other, the speed of that, depending totally on the connecting wire (and springs) total resistances. but because the one pack would generally not incurr a "overcharge" state on the other pack, the charging of eachother should be uneventfull.

 

[AndrewL]

by discharge them I do only mean a tiny amount, one gets charged the day before the other so theer is normally a slight difference in the voltage

 

[wptski]

true, but I might not have asked the right question.

My charger does say to charge at 1/10C if possible anyway which I don't quite do but I don't give it a fast charge. If I'm just topping the battery up then I'll put it on for a while in the everning and switch it off before it's fully charged since I'm charging two packs in parrallel (to make sure I don't overcharge anything). If I'm doing a full charge at the weekend then I charge each pack till the charger finishes and normally do this overnight and time it so that it will finish when I'll be awake. I then discharge the packs a tiny bit to get them to the same voltage under a bit of load before connecting them togeather in my bike lights.

Or should I do something else?

Exactly what are your packs? Cells, voltage and capacity? What's your charger too?

 

[AndrewL]

packs are made from AA cells claiming 2700mah (better power ones like lighthound sells), 12cells in one pack so 14.4nominal and togeather a claimed 5.4ah (though I suspect it's really 5ah)

My charger is this one (powerd by a PC power supply)
http://www.alshobbies.com/shop/lookupstock.php?pc=6209&Desc=