silly charging question

I bought a fancy charger to use with my bike lights and anything else I get that needs charging.
The charger can be seen here
http://www.alshobbies.com/shop/lookupstock.php?pc=6209&Desc=

I've modified a PC power supply to provide me with a 12v (or there abouts) power source for the charger (using a spare 6v MR16 to put a load on the 5v wire to get 12v from teh 12v rather then 10v)

I put 4mm plugs on the battery pack so can plug it directly into the charger. I just don't know if I'm meant to put the positive to negitive or vise versa.


Also my power pack is built from 24AA cells (better power cells claiming 2700mah) set up to give 14.4 nominal volts in two banks of 12. Should I charge each set of 12 cells on their own or leave all 24 wired up? I've heard it's bad to charge Nimh cells in parrallel.

If you can seperate the 2 parellel packs prior to charging , you will be better off.

i experienced this only once, as usually we would use a higher capacity battery , instead. like Cs instead of AAs.

One parellel SET of battereeis was fully charged, and had reached "V-Drop" this dropped the voltage on the ONE set, from the ~14.5 to ~13.5 and the power from the charger, was headed into the overCharged set at that point.
the set that needed to be charged LEAST, was the set that recieved the charge, because of the voltage drop of overcharge.

i solved the problem by using a dumb charge rate, that would be below overcharge for the ONE set, instead of both sets.
its is not "best" to "smart" charge or fast charge a series set anyways, because you need to balance the pack. ok to fast charge to 80% or so , then slow charge after that, to top off and balance all the cells.

in general its sure a lot easier to use one big fat single cell battery

I thought it might be better to change each set on their own. AA cells were used for a number of reasons (some of which aren't an issue anymore).

I have thought that if I can balance the 2 sets up so that each pack has the same capacity it might be safe to charge them togeather. I also thought I could set the charger to put 4.9ah (max) in from a discharged pack so that would cut of the charging

well parelleled they will basically have the same charge/discharge, but even something like a extra 5 inches of wire, could cause the seperated parellel sets to have different charge and dishcharge rates. a lesser connection somewhere, anywhere in the numerous connections that will then exist.

you can charge at the 5A max, as long as you have some Cutoff , or slow down at the end of charge to below the overcharge rate.
AND
that is why i mentioned the problem with parellel sets, and vdrop cutoff, or dumb slow charge, if ONE set is heading into overcharge it will vdrop, but the other set will NOT.
SO
on smart charging you have the possibility that the parellel SET will not display a good VDrop, so a smart charger might not terminate. and when it does, you still have not achieved balance (both sets full charged)

on dumb charging , when one (charged) set Vdrops, the parellel set will off balance, for the worser. sending more power to the overcharging set, and less power to the charged set. which will overcharge the overcharged worse, and still not bring the undercharged up to fully charged.

so if your going to charge really fast, just slow down near the end, like a Lame dumb charger would do, when the voltage differentials change.
AND
to fully balance the series set, and the parellel set, you kinda need a slow rate topping type charge, that should be below the overcharge rate for one of the sets. before a vdrop occurs, or after a long pause.

this is easier to display in person, than to attempt to explain.

VidPro said:

this is easier to display in person, than to attempt to explain.

Click to expand...

I was getting that impression when reading the post (sounds like you really know what your on about)

I hadn't thought about the differences in my solding (for example) being a problem when charging. I've been intending to charge the pack fairly slowly anyway (the charger recomends 1/10 the capacity of the pack).
If I'm understanding you correctly then I should be fine charging both sets of cells togeather for the main bit of charging and then seperate them and charge on the own for the last part. More fuss then I'd like but still much less fuss then taking all 24 cells out and charging in my vason 8bay charger.

PS the lights power a 20w IRC light used for a bit each day going from work and then for about an hour once or twice a week.I'm hoping to top up the power pack mid week and weekends and then doing a full discharge recharge every couple of months to help reduce memory effect

well there you go, if you keep the rate at the 1/10th thing, you have little to worry about.
and if you use it often, it will enjoy being cycled down.
and if you tear the whole thing apart, and singularly cycle it once in a blue moon, it will be rebalanced and work great.

i think you got it covered with what you just said.

so what was the 5 amps for? the discharge rate?

doh
I meant to say 5ah put into the pack so never fully charging it (or charging it to the capacity of the weakest cell). Thank you for your advice