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Lithium cells: Too many in series?

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DigiTan

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I need a circuit built into a very compact casing. It needs to operate for short periods on time on 12, 18, or 24 Volts, depending on how initial design testing goes.

Because of the small space, I need to use Lithium coin cells in place of the 12V "security/alarm" cells I typically use. Before I do anything, I need to know the max number of these button cells I can link in series before something dangerous happens. Anyone have an idea of what the upper limit is?


Specifically, these are CR1220...
Voltage: 3V
Continuous Drain: 0.1mA

As an alternative, I could take some other battery type.

[edit] A mod can delete this duplicate topic. The forum gave me some "Invalid form" jive when I tried to post the first one.
 
The voltage is going to be very unstable if the load varries a lot. Nothing harmful will come of it, but you'll have to try it to make sure it works they way you want.
 
how about using a single 'beefier' 1.5 or 3v battery and a boost converter? Maxim makes some very compact switching regulators, which for 100uA would require only tiny components. a single 4mm smt pot would let you dial any voltage you desire, toss in a few ceramic caps and a low profile inductor and you're set, probably consume less space than a big stack of little batteries.
 
justDIY said:
how about using a single 'beefier' 1.5 or 3v battery and a boost converter? Maxim makes some very compact switching regulators, which for 100uA would require only tiny components. a single 4mm smt pot would let you dial any voltage you desire, toss in a few ceramic caps and a low profile inductor and you're set, probably consume less space than a big stack of little batteries.

yes i think it's the best idea, because if you have many batteries in series.. if one of them is empty before the others, no current will flow at all...
 
The continuous current specification doesn't change by putting more cells in series and neither does the short circuit current because the impedance also increases.
 
justDIY said:
how about using a single 'beefier' 1.5 or 3v battery and a boost converter?
That's one of the options I'm weighting. It will really depend on the resistance of this device I'm trying to drive. (I might even be looking a buck conversion if the current demand is too great). I don't actually have the load device at the moment, so I'm just collecting ideas for now.

So with these boost converter ICs...do have I have to supply an AC signal in addition to the DC supply, or is the AC signal provided internally (preferred)?
 
as I understand the terminology:

boost converter = monolithic solution, contains everything you need, except usually the blocking diode is left out.

boost (or smps) controller = just regulation, feedback and switch driver circuits, you supply the switch(es) and blocking diode

there's no AC involved, unless you're looking at some sort of off-line configuration

check out the TPS61040 from Texas Instruments. It's a monolithic switcher that is very small (sot23-5)
 
DigiTan said:
That's one of the options I'm weighting. It will really depend on the resistance of this device I'm trying to drive. (I might even be looking a buck conversion if the current demand is too great). I don't actually have the load device at the moment, so I'm just collecting ideas for now.

So with these boost converter ICs...do have I have to supply an AC signal in addition to the DC supply, or is the AC signal provided internally (preferred)?

it's provided internally.
 
Considering how much extra weight and wasted space the casing of the coin cells make it'd probably be more space and energy efficient to use a single AA (Or maybe 1/3 or 1/4AA) and a boost converter. You could probably build it in the same maybe slightly larger size than the coin cells but dramatically increase battery life.
 
Ikalogic, if one cell runs out the entire pack will still source current the voltage will just drop, cells don't suddenly become non-conductive when they run out of chemical energy. That's what I meant when I said the voltage will be unstable. You'll also get significant loading effects. If the load has lots of peaks your voltage is going to be all over the place and likley effect the circuit it's powering. Basically if drawing 100ua from a single coin cell will drop the voltage by .2 volts then you're going to drop 1.2 volts from the same current on the 24 volt scenario. I don't know the voltage drop at those currents though so it could be much worse than .2 volts drop per cell.
 
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Just what are you trying to run here?
How much current do you need?

One suggestion, are you SURE you need 12V/18V/24V? You can't run on 3.3v or 5v?

Many of the common loads that come to mind that might require higher voltages may require too much current for a series of coin cells as well. For example, a relay, 12v bulbs, etc.

CR1220 are fairly expensive unless you buy in bulk. This could be expensive.

There's also a small "alarm battery"- ok I don't recall any numbers for it- that is less than half the length of a AA and a bit narrower. It's like 12V, or maybe I think I saw 22V. Not many amp-hrs or a low resistance.

As has been said, maybe just a boost converter.
 
For this project a AA, or even N an cell would be too large. But I'm up for using larger coin cells. I'll some tests on the load and go from there.
 
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