Electronic – Leakage Current of Battery in Parallel with Capacitor

batteriescapacitorelectrolytic-capacitorleakage-current

I am creating a microprocessor design that will have a battery backup in case the mains power goes down.

My plan is to have 2 3.6 V batteries in parallel, so that either may be swapped in/out while maintaining battery power to the CPU.

The batteries are rated 2.4 Ah, and the CPU should only draw about 8 uA when on battery backup; the CPU should survive for a very long time on battery power.

I want to also include a cap in parallel with the batteries, that would keep the CPU powered for minutes / hours. My question is, if I used something like this http://media.digikey.com/pdf/Data%20Sheets/United%20Chemi-Con%20PDFs/KMH%20Series.pdf which has a leakage current of approximately 3 mA, am I going to start burning the batteries through the Cap's leakage current?

Reference schematic below. Assume SW1 only closes once main's power has been disconnected.

schematic

simulate this circuit – Schematic created using CircuitLab

So here are my questions:

  1. Is the 3 mA leakage current in the datasheet realistic? Or is this a drastic over-estimate?

  2. Are there alternative cap's which have leakage currents levels of magnitudes lower?

  3. Will the batteries leak through the cap even when "SW1" is open?

  4. Are there any widely-used ways to implement my idea (cap in parallel) that won't drain the batteries?

Best Answer

  1. That cap has a leakage quoted as

0.02CV (uA) or 3mA, whichever is smaller, after 5 minutes at20C. Where I = Max. leakage current (uA), C = Nominal capacitance (uF) and V= Rated voltage (V)

So for a 6.3V 4700uF cap, the leakage would be 592uA, which is much smaller than 3mA, but still roughly 100x the circuit's standby power consumption, which does seem a bit sad.

  1. Yes, there are lots of caps specifically made as 'low leakage' - obviously that can be a bit like 'low cost', in that it's relative to someone else's expectations, but you can do much, much better than a generic electrolytic. There are also caps made specifically for what you're trying to do here, called 'supercaps' or 'ultracaps' - they're so big and give such long retention times at your low microamp currents that you could probably get rid of your second battery. Here are some supercaps which have very low leakage: http://www.maxwell.com/images/documents/hcseries_ds_1013793-9.pdf

  2. Yes.

  3. Use a more suitable cap, see above.

A general note about doing low microamp-level microprocessor projects - you have to be really very careful about everything to get down to that sort of level, and even trivial mistakes with a single pull-up turned on in a micro, or a carelessly chosen component can leave you absolutely orders of magnitude away from where you need to be. You need to make sure you have the kit to make trustworthy measurements of low current.