Asking for help choosing ECU cap replacement

Hello, I am fairly new to PCB components but have been doing a fair bit of research lately getting ready to re-cap my 1997 truck ECU. The old ones were fine but I am keeping the vehicle long term and I’m not about to wait for the old ones to leak and ruin the board, so doing some preventative maintenance. My goal is to replace them with capacitors of about the same dimensions, do the same job (or better), that will hopefully far outlast the old ones which lasted 25 years (and many miles).

The markings on the old ones are Nichicon H9633, 105C, 25V, 220uF. The dimensions are ~8.25mm diameter and ~16.5mm tall. They are radial/through hole.

I’ve read a person could replace with higher voltage and higher temp rating for longer life. Is that true? I’ve also been reading about hybrid polymer-electrolytic caps being the new best thing for vehicle ECU. Would it be a no no to switch types of capacitor? The old ones are most certainly classic aluminum electrolytic so in my mind it wouldn’t be a huge difference to switch to hybrid polymer-electrolytic but I’d like input on this because I’ve never recapped anything although I have some basic soldering experience.

Another question I have is, I see many different caps that would likely work but there’s the temp vs endurance rating and many times there’s lower temps with higher endurance ratings and higher temps with lower endurance ratings. I’m sure there are many “good” options for me. But knowing this is a vehicle I will be keeping for many many years, what in your opinion is the “best” option if I wanted the new ones to far outlast the old ones without creating any new issues I didn’t have before?
Would it be better to get a cap that’s rated at 105C and 8000-10,000hours or one that’s rated at 125-145C and 2000-4000hours?
I’m planning probably to at least bump the temp rating up from 105 to at least 125 and the voltage maybe from 25v to 35v. Also it probably needs to be AEC-Q200 certified.

Please let me know what you advise! Some may say I’m overthinking this but it’s actually kinda fun to learn and plan.
Thank you for your time and input!

Hello bgkh.sail and welcome to the forum.

To your first question, yes you can go over on voltage, but that does not necessarily guarantee a longer life for the cap. Through Hole Aluminum Electrolytic Capacitors | Electronic Components Distributor DigiKey shows caps responsive to your stated spec.

To your second question, I would simply minimize the allowable temperature you are willing to have the cap function at and maximize the lifetime at said temperature. If you want to go in depth then page 7 of Chemi-Con’s appnote on cap lifetimes may prove interesting. If you don’t want to read just note that the voltage rating contributes very little to the degradation of the cap.

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Ok, thank you for your input. It sounds like the liquid of the electrolytic caps is the main point of wear/failure. Can I switch to solid polymer or ceramic of the same voltage, capacitance, temp? I’ve found a couple solid polymers and 1 dipped ceramic (very expensive) but none of them are AEC-Q200 rated. Would it be better to get those or go with a liquid cap rated for AEC-Q200? Again my goal is to keep the vehicle forever and not have to worry about the capacitors ever again.

bgkh.sail ,

How strict are you on the physical dimensions? A rule of thumb is to not go larger since components in closer proximity on a board will generate more heat, however in some cases can be beneficial to go larger to gain better specs from a part. Polymer is generally better, however the ol’ electrolytics have been around for much longer and typically have more options available which may provide better specifications.

As Michael_Rudi suggests, if you are staying within the specifications rating whether that be temperature, voltage, current etc… choosing the part with better specifications would typically be best. However be sure to analyze the datasheet testing parameters as this will give specific details of their tests, and sometimes the tests can be done at lower ratings which may be stemmed from a marketing point of view to prop up the part. It is a good idea to leave head-room as well on these specifications, since sometimes a board may have unknown issues such as noise/voltage spikes or other design or age related problems that cause stress on other components.

Here are some DigiKey part offerings:
Polymer - 227ALG050MGBJ-ND
Electrolytic - Click Here

Notes to consider:

  • Stay with automotive rated / AEC-Q200, these are designed/tested for temperature fluctuations and approved for automotive use where regular parts are not.
  • Don’t allow things that are designed to move, “sit”. There is a great article regarding capacitors and their shelf life in This post - Why do electronic components have a shelf life?. Clicking on the Capacitors section, teaches the importance of not letting a capacitor be unused for long periods of time which would include receiving “new” product of which may have an expired date code from an untrustworthy distributor.
  • If the capacitors are puffed, you most likely need higher voltage rating. If the capacitor is dried out, this is most likely a temperature issue but of course can be an amalgamation of various issues.
  • “Lithium Capacitors”, or LICs are not recommended unless your capacitor replacement calls for this type specifically since it contains a lithium-like battery structure. More on that type of capacitor/battery is found here.
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Hi bgkh.sail,

It would be unlikely to be harmful to switch to hybrid polymer aluminum electrolytic capacitors from standard aluminum electrolytic caps in most applications. There are a few exceptions however, which include some DC-DC converter circuits which depend on the internal equivalent series resistance (ESR) of the capacitor to be within a specified range in order for the circuit to remain stable. Since hybrid polymer caps have much lower ESR, it’s conceivable that its ESR could be below the minimum required to maintain stability.

However, because the ESR of standard aluminum electrolytic caps can change very significantly with temperature, it seems unlikely that a reputable car manufacturer would use DC-DC circuits which have this special ESR requirement, as it would likely not be capable of operating reliably over the full expected automotive temperature range. Therefore, hybrid polymer aluminum electrolytic capacitors should be acceptable in any automotive circuit I would expect them to be placed into.

I would lean toward hybrid polymer types over solid polymer, as hybrids have a few key advantages in high-reliability automotive applications. Specifically, they have lower leakage current, they are less susceptible to high humidity problems, they are better at self-healing, and they will much more likely fail open rather than short. Solid polymer caps tend to have longer lifetimes, assuming optimal conditions, but humidity, and high vibration conditions can, in some cases, shorten that significantly, and the nature of their structure means that there’s a somewhat greater possibility of short-circuit failure, according to some references I have read.

Regarding finding the longest lifetime caps, their ratings are always given at maximum temperature and voltage, and for liquid electrolyte type capacitors, the lifetime roughly doubles for every 10 degree C drop in temperature. This means that a 2000hr 125°C part should last as long as a 105°C 8000hr part.

With this in mind, I would take a look at these parts for your consideration.

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Thank you for your input. I have been thinking about my project. The ECU I am working on is not designed well for easy maintenance and I’m not sure it will even survive a second recapping. Without taking the time to describe it, I would just say I really don’t want to have to open this up ever again. Its not designed like most ECUs which are very easy to open and recap. I have my doubts about liquid caps no matter how highly rated, lasting as long as solid. It seems the general consensus is if they dont leak and ruin the board then at best they will dry out in 25 years or less.
Solid polymer is a maybe? What about ceramic or something else? What if I put in dipped ceramic? Setting aside the tremendous cost, would it work? Thank you again for your time.

Hi bgkh.sail,

The reason aluminum electrolytic caps are used is because they have much higher capacitance for a given volume and price.

Ceramic capacitors rated well into the micro-Farad range, such as the 220uF you would need, use Class II materials, which have some non-ideal characteristics. These include susceptibility to capacitance change with change in temperature (+/-15% for X7R material) and change in capacitance with applied voltage (capacitance will typically drop by more than 50% if 25V is applied to 25V rated part). So, for example, the KTD250B227M90A0B00 25V 220uF X7R ceramic capacitor would drop from 220uF to less than 110uF if 25V were applied. This makes them not suitable.

About the only parts other than aluminum electrolytic type capacitors that could potentially be used would be polymer tantalum capacitors. However, at the required voltage (one must derate polymer tantalum capacitors by 20% for those rated above 10V), the only viable option we stock is the T54E4197M035VSB014, which is rated for 35V and 190uF. The somewhat lower capacitance may not be a problem because of it’s very low ESR (14mΩ), which can help make up for lower capacitance by allowing a very high ripple current. Unfortunately, the T54E4197M035VSB014 is not an inexpensive part, so one would really need its capabilities to justify the cost.

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