current limiter?

[raphaelriv]

hello all, well here is my scenario, i have these battery packs that are recomended to be charged at 950mA rate for 3hrs, i have a charger that charges at 1.65mA max rate with a voltage that it seems to step up starting from 7.83V to a max of 8.36V at the time it switches to green. However this charger is a pulse charger and its current varies but I will like to limit its max to idealy .95Amps now I was looking at some voltage regulators that have a max current of 1amp but with a set regulated voltage, I was just thinking maybe a mosfet and some resistors to control the current going into the charger but its this a efficient solution, ... any suggestions will be helpfull i am willing to try different things ... as that's is what i am doing now

 

[dknguyen]

I would just get another charger- messing around with chargers isn't something that is particularily safe because mischarged batteries will explode or catch fire.

Are your units wrong? You say 1.65mA is the max but you want to limit it to .95A

What kind of charger is it and what type of battery are you charging? If it's any kind of lithium battery then get a propercommercial charger and absolutely do not mess with it. If it's a NiMH battery and the charger is NiCd, then the charger won't terminate properly even if current is limited because of the difference in the Voltage delta peak drop at the end of the charge cycle.

If it's computerized you can probably program it. I am not sure if it's a good idea to limit the current going into the charger because there are losses by the time it leaves the charger to enter the battery. It might damage or cause other problems to the charger when the charger tries to draw more current than is being supplied, but I do not know for sure.

 

[raphaelriv]

re:

yes you are right my units were wrong & i want to limit the current going to the battery from the charger, the charger is a pulse charger that gives of 1.65Amps max, is a NiMH and the charger does charge NiMH, the only problem is the new cells are recommended to be charged at .95Amps for 3hrs if i charge it at 1.65Amps it terminates charge before 3hrs and i don't get full capacity

now you said:

"charger won't terminate properly even if current is limited because of the difference in the Voltage delta peak drop at the end of the charge cycle"

just in general NiCD, NiMH and LiOns have all different voltage delta peaks after terminating charge ?

 

[StopGo]

Could you just strap a resistor in parallel ? I mean work out the peak voltage on a pulse and choose a resistor to sink the unwanted half an amp ? You might need to involve a didoe to prevent the battery flattening throught the resistor when the charger stops charging.

 

[dknguyen]

Yes, all batteries types has different delta-peaks. It makes sense really- different chemistries = different behaviour. The difference between NiCd and NIMH is the NiMH peak is much subtler and harder to detect but are otherwise pretty much the same. This means you can peak charge NiCds with an NiMh charger, but a NiCd charger will overcharge NiMh. Lithium batteries also have compltely different peaks, and LiPo will explode and produce lithium fires if overcharged by even 0.1V.

Connecting a resistor isn't such a good idea, because the resistor limits current by producing a voltage drop, which you don't want when charging batteries. You want a current source.

EDIT: I noticed you said in parallel, not series. But the battery impedence is not constant, so that would mean the current would change too. A resistor in parallel could also possibly cause the battery to discharge through the resistor in some cases. It's similar to hooking two sources and a resistor in parallel.