Renewable Energy Questions/Discussion > Automation, Controls, Inverters, MPPT, etc

Tracking state of charge

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frackers:
Having just about completed my second generation (sounds good doesn't it!!) controller I'm not convinced that it tracks the inefficiency of the charging process. I'm charge counting but that doesn't allow for losses.

This article sums up the problem - efficiency depends on the SOC and the charge rate.
www.sandia.gov/pv/docs/PDF/batpapsteve.pdf

Has anyone tackled this problem and deduced a reasonable algorithm?

frackers:
Replies to self...

The battery monitor chip I use in my controller tracks total charge in and total out independently (as well as aggregate) such that the difference between them can be used to determine the efficiency.

For example, after 3 years my system has seen 42250 amp-hours go in and 38921 amp-hours come out - thats 92% efficient.

I'll just tweak the aggregate value by that ratio each time I get to the end of a discharge cycle and that tells me how much needs to go back into the battery to get back to where I was.

madlabs:
Frackers,

I do "the usual", meaning I've figgered out what my "average" efficiency is and plugged that into my program. It does ok, but as all of them do, creeps off over time. I'm no math whizz, so let us know if you come up with an algorithm, I'd love to plug it into my system and see what happens!

Jonathan

frackers:
I think my approach will only work for my use case where I have a discrete discharge cycle.

At the end of the cycle I know exactly how much charge I've taken from the batteries (say 45 amp-hours) so I know to put back 45 * 1/efficiency = 48.9 amp-hours. Thats the target I set for the charge cycle and it uses the dump load if required until that amount has gone back in to the battery bank before another discharge cycle is initiated.

Not sure how to sync a continuously variable load (which is why I put a manual sync option into the UI of my controller!!)

rossw:

--- Quote from: frackers on February 09, 2012, 04:24:37 pm ---At the end of the cycle I know exactly how much charge I've taken from the batteries (say 45 amp-hours) so I know to put back 45 * 1/efficiency = 48.9 amp-hours. Thats the target I set for the charge cycle and it uses the dump load if required until that amount has gone back in to the battery bank before another discharge cycle is initiated.

Not sure how to sync a continuously variable load (which is why I put a manual sync option into the UI of my controller!!)

--- End quote ---

Just thinking out loud... but why not "weigh" the charging watt-hours by the efficiency of the bank.

So if you are constantly monitoring the current in and out, for every watt-hour *OUT* you add 1, and for every watt-hour *IN* you add less than 1, depending on efficiency. May be only 0.92 Wh for every Wh in.

Make sense?

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