[chbot] High voltage batteries and electrolytic capacitors

[Helmut Walle]

A big echo for that comment! I would not only say it is "also 
important", but it actually is essential. And one reason for saying this 
is that, while explosions are often spectacular and hazardous, and 
sometimes even fun (type 3 fun usually), most electrolytic capacitors do 
not actually fail in an explosive way in real life. The more common case 
is that the designer heeds at least the maximum voltage rating with a 
more or less conservative rating, and possibly also the ripple current 
rating. But even so, electrolytic capacitors are one of the things in 
life that show a very continuous range of characteristics in a number of 
ways, and one of them is how they age. With a liquid electrolyte inside, 
and the can plugged with a rubber bung, there is that possibility that 
some solvent molecules escape as the inside pressure increases. There 
are also some score marks on the top of the aluminium can to allow for 
spontaneous decompression when the pressure accidentally increases so 
sharply that it cannot be released past the rubber bung. Many normally 
operating designs will eventually lead to capacitor failure as the 
electrolyte dries out due to the loss of solvent over the years.

Soooo... the loss of solvent is mainly driven by temperature, and 
particularly high temperatures. The electrolyte temperature is the sum 
of the ambient temperature and the differential achieved through 
internal heating. A constant and perfectly smooth DC voltage applied to 
an intact capacitor (one without DC shorts) contributes nothing to 
internal heating (but an excessive DC voltage can still lead to 
catastrophic failure by breaking through the oxide layers on, and the 
spacer between, the electrodes). But the ripple current obviously 
generates heat inside the capacitor. And while it isn't hard to 
deliberately feed through a ripple current high enough to explode a 
capacitor, even a moderate to high current well within spec will 
contribute to the ageing of the component.

So where does this lead? Well, it would be easy to over-spec 
conservatively for both voltage and ripple current. But this has 
numerous disadvantages: it costs, it adds size and weight, and it may 
come at the expense of increased parasitic properties that are 
counterproductive for other reasons. But electrolytic capacitors also 
come in different temperature ratings, and that helps where things may 
get warm... Everything else being equal, a 105C rated component will 
live longer than an 85C rated one. It may cost more, but depending on 
design life that may be well worth it.

Capacitor de-rating calculations should be easy to find somewhere 
between component data sheets and application notes...

Kind regards,

Helmut.

On 1/10/2020 15:22, Richard Jones wrote:
> In relation to the explosion part of the question...
> It is also important to check that the ripple current rating of 
> capacitors is not exceeded.
>
> Richard
>
> On Thu, Oct 1, 2020 at 11:19 AM Stephen Irons <stephen at irons.nz 
> <mailto:stephen at irons.nz>> wrote:
>
>     At a previous company, when I did hardware develoment, the policy
>     was 200% maximum design voltage. For example
>
>       * Nominal 12 V, with 5% regulator -> maximum design voltage is
>         12 V x 1.05 = 12.6 V. Minimum capacitor voltage rating: 25.2
>         V, so you would have to use a 35 V capacitor, not the 25 V one.
>
>     There was some flexibility. If space or cost were tight, we could
>     probably negotiate the smaller one in this example, because it is
>     all under our control. But for external inputs, where things are
>     less controlled, we would go the other way.
>
>     No idea where you would find 800 V capacitors?
>
>     EV City (Waltham Street) have a Nissan Leaf cut in half
>     (diagonally, so it is still has 4 wheels and is drivable) showing
>     the internal components. I think they also have an open drive unit
>     and control unit. You might be able to see what they use.
>
>     Stephen Irons
>
>     On Thu, Oct 1, 2020 at 09:16, Peter Harris
>     <petes.username at gmail.com <mailto:petes.username at gmail.com>> wrote:
>>     General question: What is the rule of thumb for selecting the
>>     voltage of electrolytic capacitors?
>>
>>     Specific question: I am working with a Nissan Leaf battery, the
>>     absolute maximum pack voltage is 405V. Are 450V electrolytics
>>     likely to explode used at 405V?
>>
>>       P
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