# How safe are pre-charged supercaps compared to pre-charged batteries?

How safe is it to transport and sell pre-charged supercaps with a system?

I'm designing a solar-powered system and I'd like to use something like a
Maxwell's BCAP0005 supercap (2.7V, 5F) to store the energy.

Even with a supercap charger circuit, it could take 80 secs to charge up. It's been suggested that I could package the system with the supercap pre-charged to save time on the first use. This sounds reasonable because there are systems that come with pre-charged LiPo's; but I've heard anecdotally about safety concerns with large capacitors.

Added:

Thanks to people who pointed out that a supercap may self discharge in a period of under a day, making the question about shipping charged caps somewhat moot.

Please address matters relating to self discharge in my new follow up question
How to calculate self-discharge time of capacitors given the leakage current?

• Besides safety, won't capacitors self-discharge in storage pretty fast? – Chris Laplante Jul 11 '12 at 18:33
• The datasheet tells that it has a short-circuit current of 16A at 65 degrees Celsius. – abdullah kahraman Jul 11 '12 at 18:58
• You're looking to save someone 80 seconds of their time? – gbarry Jul 11 '12 at 19:32
• This question should be split into two questions. You started off askingabout safety, and that is still the subject line, but have added an essentially unrelated question re discharge rate. As you suggest in a comment that the application is just a made up phantasm to put people off the scent (maybe not quite how you put it :-) ) then sticking to the original question is even more important than usual as a question with no real underlying basis can wander randomly and carry its pseudo random 'back story' with it. – Russell McMahon Jul 12 '12 at 8:03
• You do NOT need to make up stories to keep people happy. As long as you can clearly explain the rationale behind your question it does not have to immediate specific application. The usual insistence on 'knowing the story' is because a large proportion of people DO have a specific application in mind but their question does not convey their true requirement and they waste much time in to and fro refining Q&A sessions with initial answers being answers not very relevant. – Russell McMahon Jul 12 '12 at 8:06

## 3 Answers

How safe are pre-charged supercaps compared to pre-charged batteries?
I'm designing a solar-powered system and I'd like to use something like a Maxwell's BCAP0005 supercap (2.7V, 5F) to store a lot of energy.

• This supercap is far less energetic and far less dangerous than a battery pack of 2500 mAh AA cells or even than an individual AA cell.

• The supercap that you have chosen would produce a burst of power for a fraction of a second that slightly exceeded what you would usually get from a modern 2500 mAh NimH AA cell - but it would then be fully discharged.

• A single AA cell would however produce somewhat less power but for many minutes. A single cell or a few of them in a battery would be quite capable of creating very high temperatures and starting a fire. The supercap could be used to start a fire only with great difficulty.

• Some supercaps or ultracaps are far more capable than this one. Some can be used for eg automobile starting.

" .... pants on fire" - almost: On "a few occasions" I have come close to setting my trouser pocket on fire or of burning myself with the temperatures generated by (stupidly) carrying a number of AA NimH cells (under 2000 mAh capacity) plus coins and keys in the same pocket and having a conductive path form. Removal of the cells from the pocket suddenly becomes one's sole and overwhelming priority with removal of trousers a close second choice. This is not an experience that I intend to replicate in future :-). On such occasions some coins or keys achieve temperatures well above their safe handling level in a few seconds. The supercap chosen as an example could not achieve this result.

First, let's look at the energy capacity aspects: By any normal meaning of the term, a 2.7V, 5F capacitor will not store "a lot of energy". If all the energy in the capacitor was available it would provide E = 0.5 x C x V^2 Joule. For a 2.7V, 5F capacitor E = 0.5 x 5 x 2.7^2 =~ 18 Joule.
By comparison a 2500 mAh AA battery will provide about
E = V x Ah x 3600 Joule
= 1.2 x 2.5 x 3600 = 10,800 Joule.
So the 5F supercap will store about 18/10,800 =~ 0.17% of the energy in the battery. Also, whereas the battery will be able to deliver almost all this energy in a typical application, the supercap will need extra effort to recover energy as the voltage approaches zero.

Discharge safety:

Where a supercap is useful compared to a battery is in it's ability to charge rapidly and to discharge rapidly, and to do so over many more cycles than a battery can with little loss in capacity.
in SOME cases this rapid charge & discharge capability is large compared to that of a typical battery, and is extremely large compared to that of a battery when expressed in terms of their total capacity.

However, the example that you have chosen, and all the other members of its family, are far "wimpier" than some super/ultra caps. The data sheet available here shows internal resistance of 170 milliohms suggesting a short circuit current of around 2.7/0.17 =~ 16A when fully charged, and the typical short circuit current is shown as 16A at 65C and 14A at 85C. As temperature would rise rapidly and equivalent voltage (once de-shorted) would fall very rapidly under short circuit (as at 18 J gross capacity the s/c discharge time would be well under 1 second) this capacitor would not produce vast amounts of energy (maybe 20 to 30W peak) and only for well under 1 second.

A typical modern NimH AA cell contains much more energy and will deliver much much more power for much longer.

For example a 2.5 Ah Nimh AA cell may be typically discharged at up to about 5A and may have a loaded discharge capacity in the 10A to 20A range for short periods at very reduced voltage. So it's output Wattage almost matches what the supercap will produce for a fraction of a second, BUT the battery will produce 10+ Watts for many seconds and 5+ Watts for many minutes.

• I've done the old battery + keys = hot_pocket trick a few times too :-) Being a guitarist that used to gig a lot, I always used to carry 9V batteries around (for effects pedals, tuners, DI boxes, etc), which due to both terminals being one the same side are much easier to short. – Oli Glaser Jul 12 '12 at 10:46
• Once I was at a church lunch with suitably august guests in attendance. Russell strolls into hall with camera, then suddenly drops all his equipment, starts jumping about as he tries to hold pocket away from leg while digging into it and throwing contents to floor helter skelter as they come to hand. Batteries, keys, batteries, coins, batteries, coins batteries ... . Ah! Then the explanations :-). Twice perhaps in a car as passenger. Equally frenetic but less embarrassing. I'll really try to have learned that lesson. – Russell McMahon Jul 12 '12 at 11:41
• Hehe, that paints an amusing mental picture... Luckily for me it never happened during a performance, would have given the phrase "wow, he's on fire tonight" a whole new meaning :-) – Oli Glaser Jul 12 '12 at 12:43
• Many years ago I read of a cop who made the mistake of carrying a spare walkie-talkie battery in the same pocket as some loose revolver rounds....... – WhatRoughBeast Mar 31 '15 at 22:30

From the actual datasheet, that part has leakage current maximum of 0.015 mA. SimpleCoder is right, it will self-discharge.

• Putting an @ in an answer does not alert the user and is generally considered to clutter the answer. – Kellenjb Jul 11 '12 at 20:58
• How to calculate self-discharge time of capacitors given the leakage current? - follow up question haha – Atav32 Jul 11 '12 at 21:03
• Since this is more related to the discussion in the comments rather than the answer to the question, do you think you could move this to the comments instead? Thanks a lot! – Atav32 Jul 11 '12 at 21:30

It will discharge, but not quickly - at 15uA constant leakage (very rough figure) it would take 2.7 * (5 / 15e-6) = 900000 seconds or 250 hours. So it's likely to hold significant charge for a few days.

As far as safety goes, at that voltage it poses no risk of electric shock, but with a short circuit capability of 16A I imagine it could quite easily start a fire if shorted. Then again so can batteries, and they get delivered already charged. If it was me, unless there was a real incentive for doing this I'd err on the side of caution.

• Thanks for taking the time to do the calculations! Since this is more related to the discussion in the comments rather than the answer to the question, do you think you could move this to the comments instead? Thanks a lot! – Atav32 Jul 11 '12 at 21:30