2 Batteries and 3 switch setup - SAIL magazine

[Bobby T.-26X #4767]

good info.
glad we finally got it cleared up.

[Moe]

There's nothing wrong with the 3 switch setup, but I personally prefer the Off-1-Both-2 switches, especially those that separate those positions by 90 degrees. That's just my personal preference. Others may not agree.

If I were running a two battery bank for everything, I'd use an Off-1-Both-2 switch, connect the motor starting and wire to the master DC panel to the common switch terminal, and leave that switch in the Both position forever unless it became necessary to disconnect one (i.e. a shorted cell in one) or both (i.e. an electrical fire). No combiner required.

If I were running a separate start and house bank, I'd use an Off-1-Both-2 Switch, with the start battery on the 2 position (or the furthest position from Off), the cable going to the house bank on 1 (or the closest to Off) and the cable going to the starter on the common terminal. If the house bank were a single battery, I'd use an On-Off master cutoff switch as close to it as possible, with the battery on one side, and the wires going to the master DC panel and starting Off-1-Both-2 Switch on the other. If the house bank was two batteries, I'd use another Off-1-Both-2 switch for them, as close as possible, with the wires going to the master DC panel and starting Off-1-Both-2 switch on the common terminal. The starting Off-1-Both-2 switch would stay in 2 forever, and the house Off-1-Both-2 switch would stay in Both forever, both "forevers" excepted by emergencies.

In the case of a separate start and house bank, I'd use a "combiner" (which includes BEP's "voltage sensing relay" and Blue Sea's "automatic charging relay") between the 1 and 2 terminals on the Start Off-1-Both-2 switch.

When possible, most boat owners connect the charging line from the alternator to the house bank and charge it first. That usually isn't possible with small outboards without modifying them, since they hook the charging line to the starter cable. There are good reasons for charging the house bank first, if it is possible. One is that with a fully charged start and deeply discharged large house bank, relay chatter can develop because it turns on when it sees the high voltage of the charged start battery, but can turn off if the house bank is heavily discharged and pulls the combined voltage of the start and house batteries down far enough... at which it turns on again and the cycle repeats. Another reason is that a near fully-charged battery (greater than about 80%) charges VERY slowly compared to a deeply discharged one, so charging the barely discharged starting battery first can make for poor utilization of the alternator run time. A third reason is that you aren't putting a higher charging voltage (especially with a 3-stage regulator) on the nearly-charged starting battery while the house bank is being charged.

--
Moe

[Frank C]

Thanks a lot, Moe.

Even if just one approach to these scenarios, it's nice to see it all wrapped.

[Frank C]

... And I'm convinced that it's far greater folly not to keep your two battery banks fully isloated for safety reasons.

But, you also agreed (with my opening premise) that it's wasteful to dedicate a full-sized battery (80 ah) to starting these little motors (700 to 1400 cc). Upon reflection here, and considering Moe's suggestions, I've decided to parallel a couple of batteries with a switch. If ever I happen to forget my jump starter, I can still just segregate the House into two parts with the switch.

I read that article too. In the second part, he recommends the "Duane approach" of a dual-battery house bank, parallel for 12-volt batts of course. Our small outboards just don't justify dedication of 60 pounds of battery, and the benefit is that doubling the size of the house reserve can potentially triple battery life.

Final consideration ... I must replace the factory battery in a week or so. Due to a wiring snafu by dealer, the factory battery has been used for all duty past six years, both house & starter, ample confirmation that the Suzuki's starting duty is deminimus. This factory battery's 'partner' has been an EverStart 115ah, five years old but kept at full charge and rarely used (only for very rare switch 'parallel' when the factory battery went low). Otherwise it has been kept up by the Suzuki's alternator, plus a very few, occasional pumps from a 110 AC charger.

The replacement will be either an EverStart twin, or maybe an Optima-red. In spite of being pricey, the Lifeline pages reveal some nice benefits of the AGMs. Can my existing EverStart wet cell be paralleled with a red Optima (55 ah)? The Optima is $106 at Costco. Duplicating the EverStart-115ah would cost about $65. (It's still a bit tough to swallow 50% of the reserve for 150% in cost.)

[Moe]

All AGMs are rugged, and the upside to the Optima spiral-wrapped design is that if anything, they are more so. The problem with that design is that it is inefficient when it comes to amp-hours. The "six-pack" design doesn't use all of the length x width x height volume so there's less room for lead. Compare their specs to those of other similar sized batteries. But wait... you can't do that for the standard Group 24 sized marine battery, because Optima only offers the smaller (mainly shorter) Group 34, which has even less room for lead. That being said, Optimas produce a lot of cranking amps. One of their dual-purpose Group 34s would be ideal in our 15' Whaler, which gets airborn a lot on Lake Erie, slamming down hard. I don't see that need in a Mac. For amp-hours, I much prefer conventional form-factor AGMs, such as Lifeline and other brands.

Keep in mind that with parallel batteries, if one gets a shorted cell, it will pull the other down to 10-10.5 volts in a very short time, so I'd bite the bullet and put two new ones in. Sure, with the switch, you could isolate them and troubleshoot to find which is the bad one, but I'd rather avoid that in the first place. You also run into the difference in charge rates between AGM and flooded cells, so I'd rather have two of the same for that reason also.

--
Moe

[Bobby T.-26X #4767]

if i were doing what you are proposing...
i'd suck it up and get two "new", same type batteries.
you'll be good for 3-5 years.

[blanton]

For compact lightweight starting batteries check out aircraft batteries.

http://www.odysseybatteries.com/battery/pc925mj.htm

I need to find the specs. for the one I used to use on my Continental IO-550
(550 c.i., 300+ hp) back when I could afford aviation as it was smaller and lighter than the one pictured here. Not much larger than a motorcycle battery.

[Night Sailor]

I believe if you will check the specs and factory service recommendations you will see the red top is designed for frequent heavy loads of short duration like starting cars and trucks on a daily basis. I have used an Optima red in my tow truck for the last 9 years and it's still like new.

The blue top is much better for sail or power boat use because it is designed for starting and longer draw downs over a long period of time.

I have used 6 Optima blues in my boats and RVs for the last 10 years and have had absolutely no problem. I chose them for their smaller case size and ligher weight. The cell exposure of the spiral design and the much purer lead used makes it much more efficient per group size when drawing down on long cycles than any conventional flate plate battery. They charge very fast and can take any amperage over 10 amps up to 100 amps without damage.