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Old 10-11-2012
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Switching batteries to start the motor.

Both batteries on my simple boat perform the same function (starting and bank), though I will switch from one or the other when on the hook overnight (for example) to ensure one will start the motor in the morning, while the other runs lights, etc..
I assume this simply assures that I have a full charge on at least one battery to start the motor, though usually the one I have used overnight will still start the motor. Should I switch back to both batteries to start in the morning or switch back to the fully charged battery?
By switching to both, I assume I bring the charged battery to the same voltage as the partially discharged battery (splitting the difference).
Does it make a difference to either battery or the starter, or is potential - potential, any way you get it?
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Old 10-11-2012
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It depends on the size of the batteries meaning the amp hours of each and the amount of discharge you have incurred during the night. You probably will not lose too much voltage if it is only overnight so both batteries together should have no problems at all in starting the engine.

I have 600 amp hours of battery and I use about 90 amp per 24hr period. We are a power hungry boat. My solar panels would recharge this loss

I probably would recommend that you use only 1 battery for starting then turn to both to charge both.
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Last edited by St Anna; 10-11-2012 at 04:35 PM.
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Re: Switching batteries to start the motor.

It all depends on how it's wired. In most cases, the charging source goes to the C post of the switch, which determines two things: when charging is present, which battery gets charged, and when being used, which battery supplies the power. Don't combine two banks unless charging is present.

Want to learn more? Read the post above or below this one.

Reply #4, here: http://forums.catalina.sailboatowner...ighlight=honda
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Old 10-12-2012
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Re: Switching batteries to start the motor.

For people who have matched battery banks (that is, both "A" and "B" are of identical size a frequent strategy is to rename them "odd" and "even" (in your mind.) You use bank "odd" as your house battery (to run lights and stuff) on odd days, "even" on even days. The idea is that this way you distribute usage cycles over both banks evenly. Batteries "age" depending upon the number of times they have been charged and discharged, not how long they have been around.

Why do you care about aging them equally? One reason is that you don't want to discover that your second starting battery (the one you didn't use overnight) is worn out and can't start the engine. But a more important reason is that currently flows from higher voltage to lower voltage. A fully charged battery is at a higher voltage than partially charged battery. When you put your "A" "B" "ALL" switch in the "ALL: position the fully charged battery will start to charge the partially charged battery. Presuming that your alternator is charging this is not a big deal. That is, there is no reason to only charge the partially charged battery until the voltages are the same. But until the alternator charge has brought both batteries up to the same voltage there will be a cross current between the two battery banks that is in effect discharging your fully charged battery. Should you stop the engine and forget to switch back to "A" or "B" you would end up with two partially charged batteries rather than one that is "full."

With respect to which battery to use to start the engine it doesn't matter unless the battery you have used over night is down to less that 50% charge. In general batteries lose more life if they are being used when the voltage is in the 0% to 50% range than when it is in the 50% to 100% charged range. I don't know why but every battery manufacturer suggests you try not to discharge the batteries below 50%. Otherwise you will just use up a little more of the life of the more discharged battery. The next time you do this (on the other battery) things will even out.

Hope this helps.
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Re: Switching batteries to start the motor.

The only advantage to using the "both" position is the extra cranking amps as amps are additive and volts divide.

It's very possible that a battery can show 12.6v and yet have no amps to crank.

Therefore, use the designated starting battery to start - unless it won't, then switch to both, start and leave it there to charge (i.e., leave it on both to charge because something is wrong with the start - that way if the start is dead you can at least get something into the house while trying to charge the start).
Only turn the switch while the engine is running if you have a "make before break" type switch.
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Re: Switching batteries to start the motor.

Don't use BOTH unless a charging source is present. The lower battery will drain down the higher battery.
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Re: Switching batteries to start the motor.

Quote:
Originally Posted by svzephyr44 View Post
For people who have matched battery banks (that is, both "A" and "B" are of identical size a frequent strategy is to rename them "odd" and "even" (in your mind.)... (section deleted)... Otherwise you will just use up a little more of the life of the more discharged battery. The next time you do this (on the other battery) things will even out.Hope this helps.
Thanks, a thoughtful response and it does help. But your post brings up another issue for me. I just installed a battery monitor. The dedicated shore power charger has always maintained both batteries at around 13.3 volts (when I would happen to check them with a voltmeter for any reason). Just had a big weekend of sailing, and noticed that one of the batteries was at 12.6 for a couple of days on the charger, though, it finally came up to the 13 volt norm. Does a battery taking longer to charge indicate it's weak? I'll check the specific gravity asap. Seems like it should have charged overnight, if not motoring back into the Marina. Maybe it's always done this, but I didn't realize it until I had the new toy. Sometimes ignorance IS bliss!

Last edited by L124C; 10-12-2012 at 04:05 PM.
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Re: Switching batteries to start the motor.

Quote:
Originally Posted by chucklesR View Post
The only advantage to using the "both" position is the extra cranking amps as amps are additive and volts divide. It's very possible that a battery can show 12.6v and yet have no amps to crank.
Why, because the battery is deficient? I thought Volts are a indication of potential. Not necessarily so?

Quote:
Originally Posted by chucklesR View Post
Therefore, use the designated starting battery to start - unless it won't, then switch to both, start and leave it there to charge (i.e., leave it on both to charge because something is wrong with the start - that way if the start is dead you can at least get something into the house while trying to charge the start).
Only turn the switch while the engine is running if you have a "make before break" type switch.
What is a "mbb switch"? Mine is a "Guest" and rotates from 1 to "both" to 2.
I assume the charging circuit is closed unless I turn it to the bottom position which is "off".? I would think the harm would be to the alternator if the switch opened the circuit while the motor WAS running.
YIKES....the more I learn, the less I know! Please explain.
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Re: Switching batteries to start the motor.

MBB, I assume you mean make before break - it makes the next connection before it breaks the old connection. This prevents a voltage spike and frying of diodes.

Some older switches didn't do that.

a sick (sulphated I think) battery can THINK it's full and hold 12.6v as long and nothing is drawing from it, but in fact be unable to hold any useable amp hours. My AGM's showed 12.6 - 13.1v until a load was placed on them when they dropped to 7v - 10.2v (the range is because I had a bank of 3 and all were different).
Thus, as soon as I turned the key I got one "rrr" then not even a clicking solenoid.

Hindsight being 20/20, I think I had a dead starter battery - and had the switch in 'both' so the house bank continuously drained to the start. Meanwhile the house never got fully charged (which AGM's require) - and that in turn killed them.
They were so dead a honda 2000 couldn't get me started even after 4 hours of 'trying' to charge a single battery.
Fortunately the dinghy is a pull start - and a open marina willing to sell overpriced batteries was near by.
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Re: Switching batteries to start the motor.

Battery capacity and battery "ability to hold a charge" are two very different things.

All lead-acid batteries (flooded, gelled, AGMs, TPPLs, etc.) begin losing capacity shortly after leaving the factory. While there are many reasons for this, the principal one is the reduction of available plate surface due to sulfation. Sulfation is the formation of PbSO4 crystals on the surface of the plates and, over time, the embedding of these crystals in the plates themselves, thereby reducing available surface for storage of energy.

The more time a battery is left undercharged, and the warmer the ambient temperature, the faster sulfation will occur. Full charging at proper voltages and periodic "controlled overcharging" are ways to slow down the sulfation process and to prolong battery life.

Consider the lowly battery in your automobile. It begins its life will something near "full capacity", i.e., it can deliver the rated CCA or CA needed for engine starting. Throughout it's miserable life, being tucked under the hood next to a hot engine in sometimes exceedingly high or low temperatures, its capacity decreases.

Maybe when new it can deliver 1,000 CCA. A year later it may only be able to deliver 500CCA. One more year and we're down to, e.g., 250CCA.

But, you don't notice this, because it still cranks your engine. After all, it only takes LESS THAN 0.5 AMP-HOURS to start your engine, and a few minutes after the engine starts the alternator has replaced the energy used for starting.

Finally, after three or four or five years there comes that morning when the battery won't crank your engine. Voltage may be good...12.6VDC resting...but it simply doesn't have the capacity needed to start your engine.

House batteries in boats lose capacity, too, and you're more likely to notice the loss because things don't run as long without charging, and because voltage drops faster than it used to.

How much loss of capacity is tolerable? How much is too much?

That depends on the application, and your preferences and your cruising style. If you're mostly at the dock or do overnites or weekend sailing, with a bunch of motoring to boot, you'll be able to tolerate large reductions in capacity. After a few years your spiffy 400AH battery bank may only have about 200AH capacity, or even less. But, because of the way you use your boat, that's OK. And, you can continue along until, presto, you decide that the batteries are just too old or "too weak" and need replacement.

For one sailor this point may come when the batteries have lost half their capacity. For another it might be 75% loss. For still another it might be 90% loss is needed before replacement is considered.

And that, in a nutshell, is why statements like "I got 12 years out of my batteries" are virtually meaningless, since they say nothing about real capacity of the batteries or cruising style.

How do you know how much capacity is left? Only a 20-hour controlled discharge or measurement with a good (and expensive) inductance/conductance tester like the Midtronics series will tell you. The difficulty in measuring true residual capacity is why so many of us are willing to seek proxies, like voltage measurements, which in reality tell you almost nothing about residual capacity.

Bill
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Last edited by btrayfors; 10-12-2012 at 06:55 PM.
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