Aluminum electrolityc capacitors operated close to rated voltage

[brumbarchris]

Hello,
Whereas we do have a fair experience with ceramics, we find ourselves in the situation where we need to place some aluminum electrolytic capacitors on our PCBs, in order to ensure a low impedance of the battery pack we are using.

The battery pack consists of 6 x LR6 alkaline cells. Even if the cells are super-charged to begin with, their individual voltage would not exceed 1.65V, which means that the entire pack would be at 9.9V maximum.

Is it "safe" to use 10V rated electrolytic capacitors?

Sure, I know having margin is better, but here this is a "late" addition to the design and we are quite constrained in terms of volumetric space.
Having browsed some datasheets, I have not seen any temperature derating information, but as I said, our experience with this sort of caps is limited, hence the question.

Regards,
Cristian

[Siwastaja]

It should be fine. Aluminium electrolytic capacitors inherently have built-in voltage margin (i.e., the dielectric is formed at the factory at higher voltage rating), and leakage current starts to steeply increase only after some 20-30% overvoltage. Contrast this to tantalum capacitors, which have fallacious voltage ratings, and 40% derating is the norm as always explained in the usually separate application note.

For the same reason, alu elcaps used to come with separate working voltage ("WV") and surge/peak voltage ratings in the past. Haven't seen them since 1990's. Nowadays rated voltage is the working voltage.

I always try to derate aluminium elcaps by at least 10-15% as a general principle, but I would not hesitate not to derate it in situation like yours.

1.65V is a decent estimate for initial no-load voltage of an alkaline cell.

Voltage derating helps dissipate heat by increasing the package size, in large ripple current applications, and this way, increase the lifetime. This probably does not apply to your use case.

Go for it.

[dzseki]

alu elcaps used to come with separate working voltage ("WV") and surge/peak voltage ratings in the past. Haven't seen them since 1990's. Nowadays rated voltage is the working voltage.

I have made a new design recently where I selected the filter capacitor from CDE, I was delighted to see on the sleeve of the capacitor that the surge voltage rating was also indicated!

[Someone]

https://data.energizer.com/pdfs/l91.pdf
1.8V fresh and excitingly low impedance!

the next common step up at 16V seems a bit high, so a good 10V part that specifies transient or surge voltage rating would probably be the best bet (more common in higher voltage/mains parts but available in 10V).

[TimNJ]

Check this: https://www.eevblog.com/forum/projects/impact-to-lifespan-of-an-electrolytic-capacitor-run-near-rated-voltage/

It’s not a problem, in principle. The only thing you might want tp consider is whether or not you can actually trust the manufacturer’s ratings. Probably just don’t use some garbage bottom tier brand.

[Psi]

Probably just don’t use some garbage bottom tier brand.

yeah, i think that is the key

and if you're running them close to rated voltage don't also stress them in other ways at the same time. Like temp.

[Zero999]

It should be fine. The battery voltage will soon drop to 9V anyway and presumably it's not going to be used at high temperatures.

[Siwastaja]

https://data.energizer.com/pdfs/l91.pdf
1.8V fresh and excitingly low impedance!

Very good point about 1.5V lithium chemistry being used. 10.8V would be significantly over 10V rating. I'm still 99% sure it would work in practice without any problems but again this starts to sound like risk-taking.

A decent 16V part might be easier to find than one with surge ratings. Besides, "surge" rating means a short surge, but 10.8V could be applied for days or even weeks if the device sits unused with fresh batteries. So it would be still used outside of specs.

So I'd say it's all about the risk taking mentality of the OP. The risk is practically small, but no one can give a guarantee what will happen when some very unlucky individual inserts fresh lithium cells and applies 11V to the 10V capacitor. Very likely, nothing happens, but can you convince yourself or your managers to trust this educated guess?

The question is, how much capacitance does the OP actually need? What is the purpose of the added capacitor and how were the required C and ESR values calculated?

[TimFox]

In general, I hesitate to recommend operating anything above its rated voltage or power.
How much larger is a 16 V capacitor?  Are there no 12 V capacitors any more?

[TimNJ]

Ooof - I did not read closely enough - I did not know you wanted to use the capacitor above its rated voltage. Yes, that's a tricky one to recommend. 100% voltage will be OK. 150% voltage - that seems like a stretch, though yes the voltage will no sit there forever.

[TERRA Operative]

Is not the charging voltage higher than cell voltage? What is the maximum voltage that will be applied during the charging process?

[Siwastaja]

Ooof - I did not read closely enough - I did not know you wanted to use the capacitor above its rated voltage. Yes, that's a tricky one to recommend. 100% voltage will be OK. 150% voltage - that seems like a stretch, though yes the voltage will no sit there forever.

The original assumption was at 99% rated voltage. However, OP ignored possibility of other chemistries (than alkaline) in AA form factor, and with lithium cells, this could mean ~110% of rated voltage, and at lower impedance than alkaline cells (which drop quickly under any load).

[Siwastaja]

Is not the charging voltage higher than cell voltage? What is the maximum voltage that will be applied during the charging process?

I think it's fairly safe to assume no one attempts to charge AA cells when inserted in the product. There are special alkaline chargers (yes, these can be more-or-less charged a few times even if forbidden) that could output over 1.65V, but the cells would be removed from the product and inserted into the charger.

[TimNJ]

Ooof - I did not read closely enough - I did not know you wanted to use the capacitor above its rated voltage. Yes, that's a tricky one to recommend. 100% voltage will be OK. 150% voltage - that seems like a stretch, though yes the voltage will no sit there forever.

The original assumption was at 99% rated voltage. However, OP ignored possibility of other chemistries (than alkaline) in AA form factor, and with lithium cells, this could mean ~110% of rated voltage, and at lower impedance than alkaline cells (which drop quickly under any load).

Oh. I apparently am not awake. For some reason I saw 10V and interpreted it as 10x cells @ 1.65V each = 16.5V. Literally made that up in my head.

[mariush]

I wouldn't. Use 16v rated, shouldn't be much more thicker or taller.

You may also want to consider if it's feasible to have a cutout in the pcb and have that electrolytic lay flat partially inside that cutout. You could probably fit it around the battery compartment - with round batteries you could have some wasted space.

Any particular reason you need nearly 10v or 6 AA batteries ?
Maybe you could make your product more "green" by using a lithium battery and a charger chip (so that you'd charge your product from usb or something) and use a boost dc-dc converter circuit or voltage doubler to boost 3.7v..4.2v to your higher voltage, if something needs it.

With AA batteries, you'd gonna have people use rechargeable batteries which will go down to 1v each, so I assume your device already needs to work with as little as 7v.

[bdunham7]

Is it "safe" to use 10V rated electrolytic capacitors?

I agree with the comments that a 10V capacitor across 6 AA/LR6 cells is unlikely to fail even if primary lithium cells are used.  The way they are manufactured, it would be quite a challenge to make them so that the failure rate at 10V was effectively zero but the failure rate at 10.8V was significant.

However, I have two other thoughts.  The first is that even if the capacitors don't actually fail, might this lead someone else to assert that the design is faulty?  Is your device subject to regulatory compliance, approval, etc?  The second is that given your need to reduce the high frequency source impedance of the battery pack, are you first looking at the actual ESR of the capacitors?  I don't know what size or type of capacitor you are looking at, but for example if it were an SMT polymer cap, you might find that a 6.3 or 8.2µF 16V cap actually has the same or lower ESR than a 10µF 10V unit.  Or they might at least be comparable.

So, how much capacitance are we talking about here?  Are you needing a lower source impedance @ 100kHz or do you need extra current to start a motor? 

[jbb]

… aaand once again, engineering is all about trade offs :-)

At risk of sounding arrogant, maybe these questions will be helpful?
- what’s the consequence of a 10V part going poof? Is there enough room for a vent to open, or would it be jammed in so much that the thing builds up pressure until it bursts?
- is the nature of the application such that the occasional random failure is worse than losing battery life (Eg safety equipment)?
- what’s the consequence of using a 16V part with less capacitance / more ESR? Would it reduce the battery life a little? Or a maybe a lot?

[Zero999]

I haven't gone through data sheets, but E = 1⁄2CV², so a 16V capacitor is going to store 2.56 times as much energy as a 10V one. Of course this might not mean it will be 2.56 times the volume or mass.

I've seen 10V capacitors in plenty of devices operated from a 9V battery or 6 AA cells and not known it to be a problem.

[brumbarchris]

Hello again, and thank you for all your replies.
What we have here is an ''update'' of an existing product, which also in the past used 6x alkaline cells. Requirement is that it continues to do so, therefore upgrade to other battery types is excluded. We are aware of the risk that lithium based cells might be used in the same battery slot, but doing so would run against the user manual of the final product, therefore running the warranty void. Surely, user manual or not, there will be people doing that and maybe busting the caps; moreover, it would be difficult to prove they have done so, but this is a risk we are willing to take.

Regarding the value of the caps, we are looking at some 1000uF...2200uF... tentatively. Sure ESR is a concern and we will try out some different values, before comitting to an update.

Again, thank you all, you helped me decide to go ahead with using 10V rated capacitors.

Regards,
Cristian.

[Siwastaja]

What function does the 1000uF...2200uF have and how did you figure out you need this amount of capacitance?

Damping for hot plugging wire inductance transients? Much less is needed, say 100uF. Providing peak current for communications? You can probably calculate quite exact value for the capacitance and ESR needed, given maximum allowable voltage drop and magnitude + length of the current peak. It can be way below than way over 1000uF.

[bdunham7]

Regarding the value of the caps, we are looking at some 1000uF...2200uF... tentatively. Sure ESR is a concern and we will try out some different values, before comitting to an update.

 

That must be an interesting issue...

[Conrad Hoffman]

ESR tends to go down and reliability tends to go up, with increasing voltage rating. Maybe a 16V part will be effective even with a lower capacitance value. Don't know the product, but remember that cap performance falls off with falling temperature.

[mclute0]

For every 10 degree Centigrade decrease in operating temperature, the capacitor life is extended by a factor of two.

Capacitor life ratings generally are specified at their maximum rated temperature.

This life-temperature dependence actually impacts how you should derate the voltage on the capacitor.

In practice, aluminum electrolytic capacitors typically are used at about 80% of their rated voltage.

Always check with the manufacturer since all capacitors may vary.

http://www.interfacebus.com/How_to_Derate_Capacitor.html

[uer166]

This life-temperature dependence actually impacts how you should derate the voltage on the capacitor.

In practice, aluminum electrolytic capacitors typically are used at about 80% of their rated voltage.

I don't believe either of these statements are true..

[mclute0]

This life-temperature dependence actually impacts how you should derate the voltage on the capacitor.

In practice, aluminum electrolytic capacitors typically are used at about 80% of their rated voltage.

I don't believe either of these statements are true..

You are certainly entitled to an opinion. I would suggest you take it up with Cornell Dubilier and ask them to change their capacitor usage guide.

from https://www.cde.com/resources/technical-papers/AEappGuide.pdf

Estimating Lifetime for Capacitors without an Online
Calculator
We offer online calculators for many of our capacitor series such
as our screw-terminal, snapmount and flatpack capacitors. For
our capacitor series without a calculator, you will find on its
datasheet a “load life rating” with an ambient test temperature
and duration at the rated ripple current, which is tabulated for
each capacitor within that series. To estimate the minimum lifetime, you may select a capacitor whose tabulated ripple current
is at least equal to your application’s ripple current, and apply
the “doubles every 10 °C” rule between the load life test ambient temperature and your application’s ambient temperature. If
your DC voltage is derated, then you may multiply the lifetime
by the voltage multiplier Mv given in equation (6). This will be an
estimate of the minimum expected life

...

Operating Lifetime Model

Onset of wear-out is determined mainly by the capacitor’s average operating temperature. Operating voltage has some effect.
For capacitors operating at moderate temperatures the operating life doubles for each 10 °C that operating temperature is
reduced. Our online lifetime calculators available at
http://www.cde.com/technical-support/life-temperature-calculators
our best estimates of the useful lifetime for many of our more
popular capacitor series. The expected operating lifetime is approximately

Lop = Mv × Lb × 2((Tm – Tc)/10[°C]) (5)

The above equation is the simple, classic “doubles every 10 °C”
rule used for many decades, and Mv is a DC voltage derating
multiplier equal to

Mv = 4.3-3.3 VA / VR (6)

Thus for example when the DC voltage is derated by 10%, this
multiplier is equal to Mv = 1.33. As in the failure rate equation
discussed in the previous section, VA is the applied DC voltage,
VR is the rated DC voltage, Tc is the core temperature, Tm is the
maximum rated core temperature and Lb is the base lifetime.