What's that nasty niff?
Oh... That's the unmistakeble smell of battery gas. Not good when you're running a bunch of "sealed for life" gel and AGM batteries.
Closer inspection revealed two of the AGM Marathon batteries were way out of balance under charge. I'd been cutting back on usage to get the bank fully charged (as you never know when it will be able to charge again in these gloomy and now snowy days). The pair in question have some kind of lost capacity problem. One is getting full or has a higher internal resistance than the other and so the two 12V batteries are out of whack, with one sitting at 14.4V; boiling up and gurgling (never a good sound from an AGM pack), and the other is languishing at 13.2V; not even making the float Voltage... When operating at partial charge, they bumble along without one gassing, but the partial charge will soon destroy all the batteries in the bank; not just these misbehaving twins.
Probably the only thing for it is to take them out of service (dropping my bank capacity from 495Ah to 395Ah), and use them as spares for a 12V system, where they can run in parallel (and not suffer the series Voltage imbalance stress), or be used just one at a time. I took them out of service for one afternoon to load test and, individually, they seem ok. They just can't "get along" as a series pair.
The only other alternative would be to invent some kind of active pack balancing electronics to bypass some of the current on one or t'other battery when charging; a-la lithium ion packs that have individual cell balancing. Lead acid batteries (especially sealed ones) would last a lot longer if the makers introduced similar Battery Management Systems (BMS) that makers of more flamable cells are forced to.
Heaven only knows how you manage to keep a 48V AGM pack from self-destructing from imbalance. Most fork lift packs suvive because the passive balancing they employ is exactly allowing some cells to overcharge and "blow off a little steam", so to speak.
On the upside, I've just started making enquiries about a new 24V 160Ah lithium iron phosphate battery module from an American outfit that makes large format lithium cells. Other makers put together packs using small (3-10Ah) cylindrical cells in big blocks, with a BMS to keep them in check. International Battery make these new HUGE 160Ah-200Ah cells, so the BMS is simpler and the construction is simpler (and cheaper), as it only has 8 cells connected in series to give a 24V pack. A lithium pack should offer much more usable power, as you can run it in partial charge mode all the time, and it won't die like a lead acid pack. In fact, lithium chemistry prefers not to be kept fully charged for long periods. Ideal for opportunist solar charging. Operating in partial charge mode also means not wasting solar power when your battery has gotten full and can't accept any more charge. If your battery gets full every day, it's too small for the array. But lead acid chemistry demands that you keep the battery as close to fully charged as possible all the time.
The specs for the pack look promising. Min working Voltage is 20.0V (the inverter cuts out at 21.0V). Max working Voltage during charge is 29.0V and the inverter can work up to 30.5V before cutting out.
I'm hoping they'll say that I can use my existing Morningstar charge controllers as they can be programmed to provide Voltage phases (as in bulk, absorption and float phases with different Voltage limits) or even a single Voltage limited output (float only) with a current limit. Over-current shouldn't be an issue though as the cells are rated for 1C discharge (160A) and C/2 charge (80A). The inverter load at 3kW tops out at 125A and the most I've ever seen from my solar array was 73A charge.
Check it out here:
http://www.internationalbattery.com/news_nov_01_2010.php
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