actually this is not the problem i have with amp hour meters on grid tie with battery systems.
a floated battery does not need an occasional bulk charge. it is being float charged which compensates for any internal self discharge. the only thing needed is an occasional eq to stir the electrolyte. here is how the meter inaccuracy problem happens: amp hour meters treat charge and discharge differently. normally this is a good thing, because batteries are not 100% efficient. on a grid tie system, the batteries are floated nearly the whole time... but there are still small charge and discharge currents going in and out of the battery. with an amp hour meter set at 99% efficiency, it will still take 101 amp hours of charging to off set 100 amp hour of discharging. this is what leads to the inaccuracy (where the meter progressively shows an increasing discharge to the battery). if there was the ability to set an amp hour meters efficiency at 100% the inaccuracy would not accumulate... but when the grid is down, the meter will falsely show faster recharge because the batteries are probably around 90% efficient... so that is not a great solution either. this is why my idea to leave the amp hour meter set to automatically determine battery efficiency, and use a time delay relay to short out the shunt when the grid is up. when the grid is down the shunt would be back in the circuit and the meter would perform accurately. the time delay feature on the relay would allow for a couple of hours of recharging after the grid is restored before the shunt was taken out of the circuit again. it seems strange to have to come up with hackey solutions like this for more and more common battery back-up grid tie systems. todd On Friday, August 10, 2012 7:51am, "Mick Abraham" <m...@abrahamsolar.com> said: Happy Friday, all~ Allan & Todd were discussing the issue where a "reset type" amp hour battery monitor (such as TriMetric) never gets to "reset to full" when operating on a GTWBB "grid tie with battery backup" system...because the battery stays in float and the monitor never sees a bulk charge voltage. The problem here seems to reside more with the chicken than with the egg. A battery in "float" for long intervals will actually still undergo some self discharge, so it needs an occasional "rebulk" charge to restore full state of charge. Inverter manufacturers please take note that a simple addition to your code base could cause a GTWBB inverter to rev up the battery charge voltage back through the bulk/absorption phase on a calendar basis such as once per month. Victron Energy already does this with their inverter/chargers but Victron is not certified for US terrestrial and it's not set up for grid tie. If the inverter system would do an occasional "rebulk", that would also cause the battery capacity monitor to reset. Until that function becomes automatic within the inverter(s), the next best suggestion is for the client to manually simulate a power company blackout once per month by cycling the input AC breaker off, then on. This kicks up a bulk cycle, ensures good SOC on the battery, stirs up the electrolyte if the batteries are floodies, and resets the battery capacity monitor all with a single intervention. The same manual intervention might also serve as a good time for the owner to check water levels on a flooded pack, near the end of the charge cycle. The Wrench List is the bomb! Jolliness, Mick Abraham, Proprietor [http://www.abrahamsolar.com] www.abrahamsolar.com Voice: 970-731-4675 Sent from Finest Planet WebMail.
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