Can I model a 100% charged lead acid battery as a store of X coulombs and use a microcontroller to integrate the current out through a shunt or hall sensor to keep track of depth of discharge? Or is there some other tricky factors to consider?
Yes. Technically straightforward but the gods are with you if you can estimate within say 100% of the true charge state. The buggering factor is the current not running pro rata with the remaining charge. Yuasa have a good PDF handbook on their SLA batteries. The graphs show various discharge currents versus resulting time to a flat battery. There's about a 10:1 variation in apparent Ampere hours capacity, dependant on how fast you flatten the battery.
Having said that and if you have the graphs for the particular battery in use and the insane patience to translate them into stored lookup tables + ambient temp'+ battery temp' monitoring. You may see a +/-
25% estimate accuracy. It's much easier and probably as accurate, to just WAG from the battery voltage :)I knew there had to be a catch. Nothing's quite as easy as it seems is it?
Yes, there are tricky factors to consider.
google "Peukert's Equation (Peukert's Law)".
Peukert essentially says that the faster you discharge a battery, the less total energy you can extract from it.
A lead-acid deep-cycle battery is rated in ampere-hours, assuming the battery is discharged over a period of 20 hours. If you discharge at twice the "20 hour rate", you will only get about 89% of the rated energy.
-- Peter Bennett, VE7CEI peterbb4 (at) interchange.ubc.ca new newsgroup users info : http://vancouver-webpages.com/nnq GPS and NMEA info: http://vancouver-webpages.com/peter Vancouver Power Squadron: http://vancouver.powersquadron.ca
if it's a wet battery (liquid acid and not gel) you may do better to measure the density of the electrolyte... yuasa make a filler cap that changes colour dependant on this...
Bye. Jasen
Circa Wed, 08 Aug 2007 19:10:12 -0700 recorded as looks like Peter Bennett sounds like:
And it gets even more complicated than that! Different batteries will have different time frames set for their final amp-hour rating. The lead-acid storage batteries I work with are rated to an 8-hour discharge rate, as opposed to your stated 20-hour rate. (The major difference is probably due to the difference of deep-cylce vs. backup.)
An added complication is that the battery's capacity will change depending upon how it is discharged. I don't have the data at hand, so can't quote the equation, but basically, if the battery discharge is not uniform, and is heavier during one part of the discharge than another, total capacity is changed from the uniform 8-hour rate, and the change differs dependent upon whether a light draw is followed by a heavy draw, or vice-versa.
The data for each particular battery should be obtained from the manufacturer, and as a previous poster noted, the simplest method of determining discharge state is to measure cell voltage and compare to data in hand, and the accuracy of this method is usually good enough.
============================================= Any idea how to read electrolyte density with a microcontroller? PH sensor? Hydrometer rigged up to a gearmotor to squeeze the bulb and a camera and frame grabber and dsp to see where the ball is floating? Or something like that? How to you get this rig over the cells? An x-y table?
A digital refractometer would probably work best (if you could afford it):
I'd start with one of the yuasa magic eye filler caps and look for a way to measure the colour, (RGB LED and a phototransistor maybe)
The cap incorporates a light-pipe and a 2 part floating hygrometer that exploits two parts of differing shape that change alignment in reponse to the density of the electrolyte.
Bye. Jasen
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