For the electrical geeks among us, here's a really well done page on specifically AGM state of charge (SOC) measurement etc. This guy knows his stuff. And as he says there's a lot of poor quality info floating around. https://workshoppist.com/agm-battery-state-of-charge/ TL;DR - don't run your lead-acid down below 70% of capacity, or, get a lithium-ion. He does mention waaaaay down the page something that would better be at the top. Namely, that the battery, whether resting or under load, needs to stabilize before measurement or the reading will not give accurate info. Also he only says well after the part about "under load" measurement that the load needs to be steady and consistent for a while before measuring or that will also give poor indication of SOC. He does say fairly up front that an under load reading is relative to the load. If you can't measure your load with a meter and keep the load steady for the times he indicates, don't bother with the under load method because it will not give nearly accurate enough info. He says the quick & good enough is to wait 10 minutes, for resting battery. More is better though, as he says, some manufacturers say 12 hours. But that is when you need accuracy greater than we typically do. My experience with true deep cycle batteries (wet cell not AGM, same chemistry, different construction) indicates that at least 20-30 minutes is "good enough" for seat of pants practical purposes. AND, if they have been charging heavily, or discharging heavily, longer is needed for the chemical reaction to complete and settle down. Batteries are chemical storage devices. Incoming electricity is converted to chemical reactions; chemical reactions provide outgoing electricity. These reactions take time, and have inertia. Meaning simply that the chemical reaction doesn't just stop dead when you disconnect the load or charge current. It keeps going for a while - 'coasting' as it were. That's why the need to let a battery rest before you test. His comments indicate best to keep AGMs at 70% capacity or higher to avoid life-shortening damage. This is reasonably consistent with most RV/Marine specs I've seen. 80% is a bit safer but 70% not unreasonable. If you want to *use* 80% of capacity consistently, you need a lithium-ion battery. Different chemistry and construction, typically rated to cycle down to 20% capacity for rated number of cycles. They cost a bunch more up front, but if you do the math on usable capacity vs. rated capacity and life expectancy it comes out not that different in dollars per unit of usable energy. Plus much lighter than lead acid for a given capacity. cheers, John On 11/17/22 15:27, John Schinnerer via montgomery_boats wrote:
Some battery info:
Draining a starting/cranking lead acid battery ("car battery") more than 10% repeatedly, shortens battery life. The more it's done to it, the shorter the life.
For a "marine/RV" battery, it's usually 20%. They get called "deep cycle" but only compared to starting/cranking batteries, they are "deep cycle lite" in fact with only a 20% duty cycle rating.
"Duty cycle rating" means depth of discharge allowed if you want the rated life of the battery. For example a battery rated for 2000 cycles at 20% duty cycle means that if you don't abuse it by repeatedly using more than 20% of capacity, you ought to get 2000 discharge-charge cycles out of it.
True "deep cycle" batteries are rated at 50% discharge, meaning repeated discharge to half capacity is OK, won't shorten their life. These are the kind used for off grid power storage, golf carts, forklifts, pallet haulers, backup power systems, etc.
Example: to calculate no-abuse-to-battery available power for a marine/RV "deep cycle lite" battery, take 20% of the rated AH capacity.
If it's a 60AH marine/RV battery rated fir 20% duty cycle, you can use up to 12AH without taking it below 20%.
Or you can use more, and shorten the life of the battery by some amount. If you run it down more a time or two and recharge (slowly) promptly it's not a big deal. If you consistently run it down 30%, 40% or more then it will hit the battery harder.
Fast charging is also hard on lead-acid batteries. Never fast charge unless you absolutely have to.
cheers, John
On 11/17/22 11:40, casioqv@usermail.com wrote:
Hi Rusty,
I completely agree it's no replacement for a gas outboard, but is a viable replacement for a paddle or oars for entering and exiting a protected windless harbor if you are otherwise prepared for, and 100% comfortable with engineless sailing for the conditions and trip you are planning, once out of the harbor. I personally do not trust the reliability of gasoline outboards. I know this is controversial, but if there is any chance of needing a "kicker to get [me] out of trouble" I would not be doing the sail in the first place. I always plan a sail as an engineless sail, and I also leave the motor at home unless I'm bringing children along or are on a tight schedule. My M15 sails noticeably better without a motor. I keep storm sails aboard, lightwind sails including a nylon drifter and spinnaker, a parachute style sea anchor, and two anchors.
This simple little motor has no voltmeter built in, I used an accurate voltmeter to measure the remaining charge at the end of the day and looked it up in a charge level table for AGM batteries, 12.8v at the end of the day.
If the motor draws 20 amps at full speed, that would be about 2.75 hours of runtime on my 55ah battery. Each run out of the harbor is about 0.2 nautical miles. At 2 knots speed, each run would take 0.1 hours or 6 minutes. So I would expect to be able to do about 2.75*60/6= 27.5 runs on a full battery.
So 6 harbor exits should have used about 20% of the battery, not 10%. I didn't actually measure the speed, so I would guess some combination of: (1) I was probably going a bit faster than 2 knots; (2) the motor, once up to speed, actually draws less than the rated max amperage (which may be a stall/starting amperage); (3) these charge level charts are probably not 100% universal and depend on the specific battery manufacturer and design.
Overall, I'd expect to actually only motor an hour and a half max with this setup, so probably about 3nm total with no wind or current, to leave a reserve as fully draining a lead acid battery permanently damages it.
Sincerely, Tyler
----- Original Message ----- From: "For and about Montgomery Sailboats" <montgomery_boats@mailman.xmission.com> To: "For and about Montgomery Sailboats" <montgomery_boats@mailman.xmission.com> Cc: "Rusty Knorr" <rustyinafrica@yahoo.com> Sent: Thursday, November 17, 2022 11:05:31 AM Subject: M_Boats: Trolling motor
These are currently $91 on Amazon.
But…forgive me, but I find that kind of performance on 10% of battery usage hard to believe based on the manufacturers expected run time. How did you calculate the 90% remaining charge? Was this the motor’s battery meter? If so, there’s no way I would trust that. On flat water with zero tidal flow I suppose it would be useful for getting in and out of a slip, but in situations where you need your kicker to get you out of trouble you’d be in deep sh$t expecting that to save your hide. No thanks, I’ll stick with a proper outboard as designed for boats the size of an M15. YMMV!
Rusty
www.rustyknorr.weebly.com
On Nov 17, 2022, at 8:19 AM, casioqv@usermail.com wrote:
Recently, my 5 year old son and I got to participate in the Sail a small boat day at Richmond Yacht Club (San Francisco Bay). It was really fun, we got to take people from the community on rides all day long and teach them about sailing and sailboats. I can say I was a bit annoyed that almost everyone familiar with boats assumed my M15 was a Potter, even to the point of telling *me* I was confused about what kind of boat I had! We even got to take out two blind sailors, as well as two young kids (age 5-7 ish). One of the blind sailors was actually a Montgomery enthusiast and had owned M boats before! It was raining all day long, but that didn't turn people away at all.
Also, for the first time ever, I tried using a small electric trolling motor instead of a gas outboard. I have a Watersnake Asp T24, which is a tiny 24lb thrust trolling motor designed for kayak use, that only weighs 6.7lbs, is highly corrosion resistant/salt water grade, and costs only $150 new. With a 55ah sealed lead acid battery, we were able to exit the harbor under power, directly into a 12 knot wind with the main up, about 6 times that day using the motor. It could motor directly into the wind about 2 knots, and at the end of the day had 90% of the battery remaining. I have decided to start using this in place of a gas motor, and include a large flexible solar panel to recharge while at anchor/dock.
Sincerely, Tyler '81 M15 #157
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-- John Schinnerer - M.A., Whole Systems Design -------------------------------------------- - Eco-Living - Whole Systems Design Services People - Place - Learning - Integration john@eco-living.net - 510.982.1334 http://eco-living.net http://sociocracyconsulting.com