When I mix a lead acid system and a lithium it often is done to use up the life of the lead acid system. In this case, the lithium is just left off and hopefully gets used and tested monthly or so. In winter offgrid, the LFP gets used in bad weather and increases the days you can run in bad weather without a generator.
Doing this is a challenge as both batteries have different set-points and I have never been able to use the same DC buss for my clients. This DC bus is where the solar is. What I do is use a seperate inverter for the lithium and charge it from the AC bus of the lead acid inverter. It sounds complicated but it is just a flick of the AC bypass in the power distribution panel of an XW or Radian. Another breaker turns on the charger in the lithium inv/chg. Eventually the old lead acid will go away. The clients love this as it gets them a completely independent power system. They can do the same thing later and just have two LFP banks/inverters. Dave Angelini Offgrid Solar "we go where powerlines don't" http://members.sti.net/offgridsolar/ [1] e-mail offgridso...@sti.net [2] text 209 813 0060 On Sun, 24 May 2020 07:15:53 -0700, Jay wrote: One question I have for those more versed than me in lithium is, I'm under the impression that having a lithium battery in float for years isn't good for it. Or at the very least it has to be cycled with some regularity. I'd like to hear data on that Thx Jay Peltz power. On May 23, 2020, at 10:08 PM, Ray wrote: I suggest starting with a very simple look at the continuous output rating of the battery and match that to the inverter continuous rating. This ends up with quite a bit more battery than you might think at first. However, if the customer paid the big bucks for a battery backup system, they should at least get enough battery to operate the inverter at its rated power. Anything less, I would consider poor design. The same goes for lead acid batteries, they just don't work well, even on grid, when the battery is undersized. Even lead acid batteries have maximum current output ratings that should at least be matched to the inverter rating. If you WERE going to under size the battery, then you should keep it safe by reducing the size of the inverter battery breaker as well. I know this "micro battery for GTB" is becoming a trend, but it doesn't mean its right. Ray Walters Remote Solar 303 505-8760 On 5/23/20 5:48 PM, Mick Abraham wrote: Greetings, All~ Mark Frye had asked: "...with a 4000w Outback Radian, AC coupled with 2000w of PV, what is the smallest Li battery I would want for stable operation?" Mick's reply: In the lithium arena, one often finds the ratings in kilowatt-hours rather than in amp-hours. This is an easy conversion for us energy nerds to make, so below I'm mainly using watts & watt-hours with occasional conversions to amp-hours. I suggest that you start with the spec sheet (of whichever brand lithium battery is on your radar), & see what the manufacturer says about "allowable rate of recharge". You may find that C/2 is an acceptable rate with several manufacturers, Mark--much faster than what we're accustomed to with typical lead-acid batteries. Let's assume: * ...that your AC coupled PV array crests around 1800 watts to the battery when there are no loads * ...that the battery is below 100% state of charge, and * ...that the solar conditions are optimal. Let's further assume that those conditions represent your battery's highest energy rate in either direction. 1800w X two hours = 3,600 watt-hours dividing by 48v nominal = 75 amp-hours in the above hypothetical situation. ++++++++++++++++++++++++++++++ The rate at which a battery can receive energy is (usually) also the rate at which it can comfortably deplete, so let's briefly look at the flip side: Assuming the battery mfr. wants the watt-hours to be no less than two times the maximum wattage, a 3600 watt-hour (C/2) battery should not be depleted faster than 1800 watts. Your 4kW Radian, Mark, could exceed that if the loads demand it--& maybe your loads don't. Motor starting surges should probably be part of the thinking & it wouldn't hurt to ask the mfr.'s opinion about short term surges which briefly deplete the battery faster than the hypothetical two hour rate. By approaching this question based on the battery manufacturer do's ">ᐧ _______________________________________________ List sponsored by Redwood Alliance List Address: RE-wrenches@lists.re-wrenches.org [4] Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org [5] List-Archive: http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html [6] List rules & etiquette: www.re-wrenches.org/etiquette.htm [7] Check out or update participant bios: www.members.re-wrenches.org [8] _______________________________________________ List sponsored by Redwood Alliance List Address: RE-wrenches@lists.re-wrenches.org Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out or update participant bios: www.members.re-wrenches.org Links: ------ [1] http://members.sti.net/offgridsolar/ [2] mailto:offgridso...@sti.net [3] http://www.abrahamsolar.com [4] mailto:RE-wrenches@lists.re-wrenches.org [5] http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org [6] http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html [7] http://www.re-wrenches.org/etiquette.htm [8] http://www.members.re-wrenches.org
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