Battery life extension calculation – need help
- by brady
- 2023-03-20 17:41:56
- Batteries & Leads
- 226 views
- 10 comments
I am confused and lost by how to calculate battery life extension and would welcome help.
As an example, my threshold voltage Vth@0.2ms=0.5V, pacing voltage V1@0.2ms =3V and battery life B1=6 year. (The house keeping current is in micro amp thus can be ignored for this purpose.)
I know that battery energy E=V*V*t/R
Where:
V is the pacing voltage,
t is the total time of pacing, which is, just equals to B and
R is the impedance at the tip of the electrode.
The purpose is to reduce the pacing voltage to V2=1V@0.2ms . The battery’s available energy is the same, thus we have
V1*V1*B1/R=V2*V2*B2/R
Or B2= B1*(V1/V2)**2=9*B1
That is by reducing the the pacing voltage from 3 to 1V, the battery life is longer by 9 times or 54 years. This cannot be correct. I must have done something wrong.
10 Comments
Wow
by docklock - 2023-03-20 19:00:06
Honestly have absolutely no idea what or why your concern about battery life to that extent. Using common sense, I guess it also depends on how many times battery is used. Also all batteries degrade during time. I got my PM installed on January 20 this year.
Medtronic tech said at my pace rate of 23% battery will last 14 years. I'm 78. So in 14 years I'll be 92 - that's only if I'm alive.
Sorry but just way above my simple brain. Got too many other things to worry/think about.
A lot of things wrong
by AgentX86 - 2023-03-20 23:08:57
You're way overthinking it all. First, if your capture threshold is 1.5V and you pace at 1V, you're battery will outlive you. Given a capture margin of 2x, we're right back to 3V. Do the same calculation with the timing and we get the three bears, again.
You assumed that pacing is all the device does. It has to know when and how much. There is a "constant" energy needed (to run the "computer" and the programs running on it). There is also the efficiency of the various parts.
In short, there are too many variables, few of them available to you, to back-o-the-envelope these things. I'm sure the manufacturer has good information for the design of the battery and unit. I can assure you, though, that even if you turn off pacing, the battery isn't going to last 50+ years. It wouldn't last that long laying out on the desk, turned off.
The whole thing is a pointless exercise. Pacing is set to what's needed, not to save power. If it did last 50 years but the pacing voltage wasn't enough to keep you alive, the 50 years isn't very important.
battery life new calculation --- it now makes sense
by brady - 2023-03-21 01:28:51
Hello AgentX86,
Thanks for your comment. What you said make sense.
So the power needed to run the chip is not negligible as I first thought it was. The revised formula is:
E=V*V*t1/R +PD*t2,
where:
E is the battery energy
V is the pacing voltage magnitude
PD is the power dissipation of the pacemaker
t2 is the battery life,
t1 is the time when V is on during t2.
The longest battery life will be when there is 0% pacing that is E=PD*t2, lets say it is 10 years.
For an order of magnitude estimation, we know that reducing (or/and narrowing) the pacing voltage spike or its frequency (in bpm) increases battery life.
The pacing current is ~2V/500 ohm (500 ohm is a typical impedance) or ~4mA @0.2ms.(0.2ms is the width of the spike)
Assume 100% pacing at 60 bpm, or one pacing pulse per second. The average of 4mA@0.2ms over one second is 20 micro amp per second, that is a DC current of 20 micro amp.
The house keeping current for the pacemaker is said to be of the order of micro amp, but is it 1 or 10 micro amp or more? We don't know the answer. Anyway this is just an order of magnitude estimation.
It seems that the current needed to run the chip is of the same order of magnitude as the averaged spike current. Thus, battery life definitely does not proportional to the square of the pacing voltage because there is constant power dissipation term in the equation. When the pacing pike height is further reduced, the power dissipation term becomes even more important.
It may be clearer to recast the equation using time average of the spike current, I. Then
E = [I*I*R +PD]*t2, where t2 is the battery life and R is the impedance.
When I is small that is having a small threshold voltage or pacing infrequently, it is PD (the power dissipation or consumption) which determines the battery life and when I is large, that is having a large threshold voltage and 100% pacing, it is the high pacing volatge (or current) that determines the battery life.
Make sense?
Brady
by piglet22 - 2023-03-21 12:05:53
I program the sort of devices used in pacemakers and use a lot of the supporting devices.
It's going to be next to impossible to calculate anything meaningful about battery/cell lifetime.
There are going to be devices like operational amplifiers to measure any residual waveforms, there may be memory chips, communication chips for Bluetooth or RFID/NFC communications.
The crucial component will be the central processor or microcontroller that does all the work and will hold some very sophisticated code (program).
Most certainly, the designers will be aiming to minimise the power consumption of all the components.
A typical amplifier might draw 1 to 2 microamps (uA). The battery/cell, probably lithium of some sort, might have a capcity of 500 milliamp hours. i.e.it will supply 500-mA for one hour or 1-mA for 500 hours.
Our amplifier will run for 28 years at 1-mA or 14 at 2-mA.
The processor will draw more power than that, but you can see that the components are going to be in the right ball park for lifetimes in the order of 10 years or so.
The only real way to find out is put a meter on it and measure the current and voltage, but that's not going to happen, unless of course the pacemaker does the measurements internally.
There are specialised chips for just that purpose under the name of fuel gauges, but I suspect that the processor chip would have that built in.
I don't think it would be possible to make a meaningful estimate of battery/cell life using what the pacemaker outputs because there are going to be a lot of other things going on.
I'm going to have a bash at putting a simple ECG machine togther so I might be able to throw a bit more light on it.
thank you all for your profession comments!
by brady - 2023-03-21 18:21:51
thank you all for your profession comments!
piglet22,
Thanks for you very technical response.
When you said,"Our amplifier will run for 28 years at 1-mA or 14 at 2-mA."
you mean micro amp not mA, yes?
I do have a question and hope you could answer. Is there any ferromagnetic material in the generator circuit? In the literature I read that there is ferromagnetic material in the voltage doubler circuit which I assume is a step-up transformer. Would you be kind enough to confirm that?
I had asked the Abbott engineer and he didn't know the answer.
When you said,"Our amplifier will run for 28 years at 1-mA or 14 at 2-mA."
you mean micro amp not mA, yes?
started another thread
by brady - 2023-03-21 20:56:19
Based on everyone's input, I have made a new calculation given in a new thread, as this thread with all the discussions becomes a bit hard to follow.
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