Maxwell 16.2 V SuperCapacitor cascade, Teardown - Page 1
Forum section: electronics / projects

author: HighVoltage
author_rank: Super Contributor
timestamp: June 05, 2015, 09:24:06 am
content: I just got a hold of a original Maxwell cascaded super capacitor box.This is Maxwell part number: BMOD0058 E016 B026 x 350 F and 2.7V Supercaps in seriesTotal of rated 16.2VTotal capacitance of 58 FSomehow I expected a real great balancing circuit inside.What I found is just one little tiny 1/4 Watt resistor per cell for balancing.What a disappointment.Somehow I expected a small sensing circuit with good size transistor.But this seems to be the low end version of the Maxwell system.

author: Kjelt
author_rank: Super Contributor
timestamp: June 05, 2015, 09:36:02 am
content: Well if a resistor is good enough.....I saw the same thing once with a lampdriver pcb that could be configured for 120VAC and 230VAC.They used two elco's rated at 200VDC. For the 120VAC configuration they were set in parallel and for the 230VAC version they were set in series with 3 or 4 balancing smd resistors between them.

author: Ian.M
author_rank: Super Contributor
timestamp: June 05, 2015, 09:55:17 am
content: If the capacitors are well matched and good quality, any imbalance will be small.The last photo has the colour code visible so we know the balance resistors are 10K. 16.2V/6 is 2.7, so the max balance resistor current is 0.27mA. As long as that's an order of magnitude greater than the maximum leakage current for an individual capacitor, it will keep them balanced effectively.If the application can tolerate up to 0.3mA leakage current there is no point in having anything more complex and many reasons not to, including cost and reliability.

author: Psi
author_rank: Super Contributor
timestamp: June 05, 2015, 10:28:49 am
content: The self discharge of maxwell super caps it pretty damn low but noticeably more when charged to max verses when charged to say 70%.So any cap that gets higher than the others will self discharge faster, so they tend to naturally balance without too much effort.

author: moffy
author_rank: Super Contributor
timestamp: June 05, 2015, 11:29:38 pm
content: Quote from: Ian.M on June 05, 2015, 09:55:17 amI am not so sure, you have to take into account long term aging effects, which will differ from cap to cap.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 06, 2015, 04:15:17 am
content: How much ESR does the PCB add?

author: HighVoltage
author_rank: Super Contributor
timestamp: June 06, 2015, 03:31:26 pm
content: Thank you all for your feedback.Interesting question about the ESR of the PCB, I think I will measure that next.Today I ran some test, charging the 6 capacitors with up to 100.00A and up to 6.000VSo, in theory they all should have 1.000 Volt at the end.I measured each capacitors voltage with an Agilent 34470A Multiplexer.Here is the result of the measurements with a charge current of 60AIt seems that the one Capacitor is not fully charging to 1.000V, because it has a higher internal resistance.Does anyone here have experience with these Maxwell capacitors in regards to balancing?Will they get more stable over time or can I expect this behavior to stay like this or get worse?And one more question, is there a good balancing circuit, other than the one proposed by Maxwell, that is patent protected, it seems.

author: Marco
author_rank: Super Contributor
timestamp: June 06, 2015, 07:14:52 pm
content: Hmm? Am I reading it wrong or wasn't the offset there from the very first measurement and didn't really change during?Using leakage balancing resistors for series capacitors is not patentable, you'll be able to find prior art going back a century.

author: Zero999
author_rank: Super Contributor
timestamp: June 06, 2015, 07:48:11 pm
content: Quote from: HighVoltage on June 06, 2015, 03:31:26 pmThat's odd. The ESR shouldn't make any difference to the steady state voltage which will be determined by the leakage current. If one of the capacitors is charging to a lower voltage than the others, it means it's leaky. In this case the equivalent parallel resistance will be significant with respect to 10k, indicating it's bad.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 06, 2015, 07:58:41 pm
content: You can use TL431s and 33 ohm or so resistors to make an active balancer.