Paul;
Thanks for sending this out. I had composed a draft with similar information, but not sent it out until I had confirmed my pre-retirement recollection that the A*h calculation was based on the 5 amp current draw. (Decades ago I was the marketing guy for batteries for a construction equipment manufacturer and later the aftermarket guy for a fork lift manufacturer, but that seem like an astonishingly long time ago now that I think about it.) I suspect a lot of us don’t really realize where the A*h capacity ratings come from. On a somewhat related note, I noticed that one of the postings in this topic mentioned the reserve capacity of his batteries. Reserve capacity is how long it takes for the battery to discharge 100% at a current draw of 20 (or maybe it is 25) amps. Which, BTW, is what your average car consumes if the alternator or voltage regulator give up the ghost. So it is meant to give you an idea of how long you have to get to a repair shop after the little red light on the dash comes on in your car. It is basically a measurement that is not relevant for boat batteries. Rick Brass Washington, NC From: Dreuge via CnC-List [mailto:cnc-list@cnc-list.com] Sent: Wednesday, October 18, 2023 7:12 AM To: cnc-list@cnc-list.com Cc: Dreuge <dre...@gmail.com> Subject: Stus-List Re: Sizing an inverter Hi Chuck, First off, 12V * 100 A*hr = 1200 W*hr. But keep in mind that A*hr battery ratings for a 100 A*hr battery is tested typical at 5 amps(or 0.05C). That is your 100 A*hr is 5A * 20hr. At this rate the battery is only providing 12.5V * 5A = 63 Watts. At higher current loads, your effective battery capacity will be much much less due to Peuket Effect losses. In fact, a 100 A*hr rated lead-acid battery at 100 amp load would only provide about 20 A*hr of energy(and lots of heat). To add to the insult, lead acid based batteries like AGM should not be depleted below 50% capacity to limit damage, so reserving 1 battery for start leaves only 50 A*hr for your house bank at loads of about 5 amps. At a 20A load, the 100 A*hr (50 A*hr effective) only provides 60 A*hr (30 A*hr effective). That’s about 30 A*hr / 20A = 1.5 hrs of use. A 1000 W inverter could draw a max of 1000W / 12.5V = 80 Amps (but likely higher due to a voltage drop). This would drain your battery in 6 - 10 minutes. I gave a talk to some local Hams about Solar Off-Grid Systems. I posted the slides on my blog (see link below). On slides 5 & 6, I discuss battery discharge capacity and bank sizes. https://svjohannarose.blogspot.com/2021/10/solar-off-grid-system.html On Johanna Rose, I have a 2000W inverter charger. When I run my microwave, it draws about 100A from a 560 A*hr LiFePO4 battery. Like Dennis, I have my inverter connected to my panel powering all AC loads except for my A/C unit and water heater. The inverter has a built in transfer switch which automatically (and smoothly) transfers to shore power when available. - Paul E. 1981 C&C Landfall 38 S/V Johanna Rose Fort Walton Beach, FL http://svjohannarose.blogspot.com/ On Oct 17, 2023, at 7:27 PM, via CnC-List <cnc-list@cnc-list.com <mailto:cnc-list@cnc-list.com> > wrote: OK, an DC/AC Pure Sine Wave inverter looks attractive I've heard some boats have 1000 watt, some 2000watt and some 3000 or more. What size inverter is right for a boat equipped with two 100Ah AGM batteries? One battery provides 12v x 100ah: 1200watts. My system can provide 2400 watts but I usually reserve one battery to restart the engine and run on the other battery. Does the inverter get fed from a bus common to both batteries, or to the selector switch marked, "Off, 1, both, 2"? Thanks, Chuck S
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