Electronics > Beginners
Dropping voltage from a Li-ion battery
Cricri:
@Audioguru: I don't know how they manage to still procure NiCd, but I suppose that if you are willing to save every penny, then manufacturers will be happy to sell them to you.
I didn't want to switch to NiMH firstly because the charging circuit for the original would fry them (as Ian mentions), and secondly because I didn't want to have to unscrew the device sheel every time I need to access them.
@spec: good point, when I did it on the Dymo printer, it was right off the transformer, so I presume there were some caps somewhere anyway.
For this cheap device I'm thinking off, it's all surface mounted, so there probably isn't a big electrolytic cap already in there, so I might put one in. It's pretty much just a cheap DC motor though, so I won't bother with the ceramic cap. Does it matter if I put it between the battery - and the + before the diodes, or after the diodes?
6PTsocket:
In defence of NiCd's, I have them in a number of devices like an old 4.8 volt carpet sweeper, 7.2volt B&D dust buster, a 14.4 volt drill, an electric tooth brush and a beard trimmer. I may be stuck in the past but they all work and a few are on their second set of batteries. They are still used in many non critical, inexpensive applications.
According to Battery University, the educational site devoted to all things battery, the NiCd memory effect was a problem solved in the very early days of NiCd production and the real cause of the problem is something else entirely. NiCd's on trickle chargers are continuously over charged and loose capacity. Then again all rechargeable batteries loose capacity over time.
There is a new generation of low self discharge AA and AAA NiMh batteries with high capaity and high discharge rate ability that give lithiums a run for their money. I woukd use those for replacement of similar size NiCd's. They will work on the old NiCd/NiMh charger. One of the manufacturers is Tenergy, the other brand eludes me.
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spec:
--- Quote from: Cricri on December 25, 2018, 03:24:58 pm ---@spec: It's pretty much just a cheap DC motor though, so I won't bother with the ceramic cap. Does it matter if I put it between the battery - and the + before the diodes, or after the diodes?
--- End quote ---
No, capacitors almost certainly won't be needed to make the motor run OK. But they may be needed for noise suppression.
The capacitors must go across the load, that is after the voltage dropping elements (diodes).
spec:
NimH batteries have been mentioned in this thread, so here is a point about them. Out of all the common battery technologies: lead/acid, NiCad, NimH, LiIon, NmH are the hardest to charge properly. And the reason is the very slight drop in voltage (-dv/dt) near the fully charged state. This means that you have to charge NiMH at a higher current than you would otherwise like to get a measurable -dv/dt, and even then it is missed. I have a top of the line NimH charger and even that gets the charging wrong from time to time. As for cheap chargers- forget it. :--
Of course, the easiest batteries to charge are the LiIon types. They have revolutionized battery power technology, in spite of some of the messages of doom you see on the net. ;D
6PTsocket:
--- Quote from: spec on December 26, 2018, 12:07:35 pm ---NimH batteries have been mentioned in this thread, so here is a point about them. Out of all the common battery technologies: lead/acid, NiCad, NimH, LiIon, NmH are the hardest to charge properly. And the reason is the very slight drop in voltage (-dv/dt) near the fully charged state. This means that you have to charge NiMH at a higher current than you would otherwise like to get a measurable -dv/dt, and even then it is missed. I have a top of the line NimH charger and even that gets the charging wrong from time to time. As for cheap chargers- forget it. :--
Of course, the easiest batteries to charge are the LiIon types. They have revolutionized battery power technology, in spite of some of the messages of doom you see on the net. ;D
--- End quote ---
Funny, I emailed Tenergy on that very subject and they assured me that my Tenergy smart charger, that predates the new batteries would have no trouble detecting the small voltage change. The fact is, there are a lot of devices that use drop in AA and AAA cells that are not easily converted to lithium. These new NiMh cells are something completely different than the older ones.
I never thought a cell that catches fire if it is over charged or is permamantly damaged if allowed to discharge too far, is a particularly simple technology. My interest started out when I had a problem similar to the OP. I had a 2.4 volt soldering iron that needed cells. As I do not use it a lot and NiCds do not hold a charge well, I thought of an 18650 but it seemed wasteful to lose1.2 volts of the 3.6 volts in a couple of diodes that also had to dissipate a lot of heat. I had to waste 1/3 of the cell's capacity. There is no room for a buck converter even if one existed for those specs and all that electronics just to change battery chemistry made no sense.
There is another lithium chemistry that is 2.4 volts per cell but there is not a lot of info out there. I would need control circuitry that is not on ebay.
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