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post #1 of 11 Old 09-18-2013 Thread Starter
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Battery keeps charge

The popular wisdom is that an alternator can only bring a battery up to about 80% of charge.

The Allmond 30 I've been messing with the last couple of weeks has a diesel with alternator and two new batteries.

While the engine is running I get a pretty good charging voltage for battery 2 but only 12.2 for battery 1.

I'm assuming that for some reason battery 1 is not being charged.

The puzzle for me is that battery two shows a voltage of 12.6 after it has sat for 20 hours even though there has been only engine charging, no shore side cable since the boat went in the water a month and a half ago.

The engine runs maybe an hour a week.

There are almost no other demands on the battery other than engine starting. Once we ran nav lights for an hour, most of the gauges don't work.

The engine is hard to start and needs to be cranked a bit before it starts, maybe 30 to 60 seconds total over three tries.

So my question is why isn't the battery being depleted more than it is.
I would think that just from sitting losses and engine starting the battery would run down a bit.

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post #2 of 11 Old 09-18-2013
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Re: Battery keeps charge

On the charging question : do you have an old-fashioned 1-2-both-off switch, that is set to 2? Mine is wired so that it will also only charge the selected battery, which is quite common.

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post #3 of 11 Old 09-18-2013
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Re: Battery keeps charge

You must be replacing the amps lost during the starting. Looks like you're at 80-90% charged on the charged battery. A good multimeter is a handy tool to have.
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post #4 of 11 Old 09-18-2013
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Re: Battery keeps charge

Quote:
Originally Posted by davidpm View Post
The popular wisdom is that an alternator can only bring a battery up to about 80% of charge.
This is not true. An alternator certainly can fully charge a battery to 100%. However on a sailboat we are very often UNWILLING to run the motor for the 4-5+ hours it would take to go from 50% to 100% so the alternator often gets blamed.. Getting from 50% to 80% is easy and relatively fast because you are in bulk mode. Once you hit the absorption voltage, usually around 80% SOC, less and less and less current flows into the battery and the last 20% takes a lot longer than the 30% you just replaced.

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Originally Posted by davidpm View Post
The Allmond 30 I've been messing with the last couple of weeks has a diesel with alternator and two new batteries.

While the engine is running I get a pretty good charging voltage for battery 2 but only 12.2 for battery 1.
Because you are likely not charging battery 2. If the batteries were in parallel they would be at the same voltage. They are not in parallel so they are at differing voltages. Your battery switch is like being used as a "charge directing switch"...

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Originally Posted by davidpm View Post
I'm assuming that for some reason battery 1 is not being charged.

The puzzle for me is that battery two shows a voltage of 12.6 after it has sat for 20 hours even though there has been only engine charging, no shore side cable since the boat went in the water a month and a half ago.
This is pretty good when you consider most flooded LA batteries will be at 12.72V+ when fully charged.

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Originally Posted by davidpm View Post
The engine runs maybe an hour a week.

There are almost no other demands on the battery other than engine starting. Once we ran nav lights for an hour, most of the gauges don't work.

The engine is hard to start and needs to be cranked a bit before it starts, maybe 30 to 60 seconds total over three tries.
An hour a week, with only light loads, should be adequate to push you into the mid 90's if you only started at 90% SOC to begin with...

He will want to address these starting issues. 30-60 seconds of cranking is a LOT. It is bordering on hydrolocking the engine if the sea **** is not closed when cranking for that long.

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So my question is why isn't the battery being depleted more than it is.
I would think that just from sitting losses and engine starting the battery would run down a bit.
Even 30 seconds of cranking (which is WAY more than an engine should eve crank) removes very little capacity. Lets assume the average cranking load is 150A and then see how many Ah's 30 seconds at 150A consumes....

150A X 0.0083 Hours (30 seconds) = 1.245Ah's consumed.


Not a heck of a lot of capacity. The average starting duration for the engines and boats I work on from "loaded" to "unloaded is under 3 seconds with most being under 2 seconds.. This is how a properly running engine starts. Ours starts loaded starter to unloaded starter in .75 seconds.

Lets figure a 3 second start at an average of 400A, just for grins:

400A X 0.00083 Hours (3 seconds) = 0.332 Ah's consumed during starting.

Starting an aux diesel engine uses very little Ah capacity due to the short duration. IF we factor in for Peukert it uses a little more but still very little overall capacity in Ah's...

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Last edited by Maine Sail; 09-18-2013 at 04:51 PM.
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post #5 of 11 Old 09-18-2013 Thread Starter
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Re: Battery keeps charge

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Originally Posted by MarkSF View Post
On the charging question : do you have an old-fashioned 1-2-both-off switch, that is set to 2? Mine is wired so that it will also only charge the selected battery, which is quite common.
Rats, I think I knew that but obviously forgot. Thanks.

Got used to the setups where they charged one battery then automatically charged the second.

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Re: Battery keeps charge

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Originally Posted by Maine Sail View Post
Because you are likely not charging battery 2. If the batteries were in parallel they would be at the same voltage. They are not in parallel so they are at differing voltages. Your battery switch is like being used as a "charge directing switch"...

Lets figure a 3 second start at an average of 400A, just for grins:

400A X 0.00083 Hours (3 seconds) = 0.332 Ah's consumed during starting.

Starting an aux diesel engine uses very little Ah capacity due to the short duration. IF we factor in for Peukert it uses a little more but still very little overall capacity in Ah's...
Forgot about the charge directing concept. Knew it at one time but forgot.

Didn't know that starting was so gentle on the batteries.
Thanks.

So if we are motoring I should put the switch on both or maybe to the lower voltage battery?

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It is a lesson about the limitations of wax as an adhesive.
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Re: Battery keeps charge

If I am daysailing I alternate between 1 only (by that, I mean starting and running on 1), and 2 - according to the date. Odd dates, 1, even dates, 2. This has the effect of evening out the battery life, and keeping both charged.

If I'm overnighting I keep one for starting, the other for house use. Start the engine on the starting battery, give it 10 mins to replace the charge from starting, then switch to house.

"Both" should be reserved for emergency starting, due to the risk of leaving it on both and flattening both batteries.

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Re: Battery keeps charge

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Originally Posted by MarkSF View Post

"Both" should be reserved for emergency starting, due to the risk of leaving it on both and flattening both batteries.

There shouldn't be any "risk" if you are playing attention.

My current boat has a start battery, but my last just had a battery bank that supplied everything. It was my responsibility to ensure I didn't sit around sucking down power to the point of not having enough to start the engine. Since I normally didn't run the frig on that boat the battery bank could go days of anchorage use.

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post #9 of 11 Old 09-18-2013
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Re: Battery keeps charge

Quote:
Originally Posted by MarkSF View Post
If I am daysailing I alternate between 1 only (by that, I mean starting and running on 1), and 2 - according to the date. Odd dates, 1, even dates, 2. This has the effect of evening out the battery life, and keeping both charged.

If I'm overnighting I keep one for starting, the other for house use. Start the engine on the starting battery, give it 10 mins to replace the charge from starting, then switch to house.

"Both" should be reserved for emergency starting, due to the risk of leaving it on both and flattening both batteries.
"Both" [without the engine running] wouldn't flatten both batteries, connecting batteries in parallel is a common practice after all.

The issue is that because the different banks might start at different voltages you cause a little wear and tear as one bank charges the other. It will lose a bit of power in losses but not much else.

But if you don't put the batteries on both when running the engine you're wasting an opportunity to charge them which means your average charge level will go down. This will also reduce the life of the batteries.

The best thing by far is to get an ACR relay or equivalent that takes care of this for you.
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Quote:
Originally Posted by asdf38:1090587

"Both" [without the engine running] wouldn't flatten both batteries, connecting batteries in parallel is a common practice after all.

HUH?

The issue is that because the different banks might start at different voltages you cause a little wear and tear as one bank charges the other. It will lose a bit of power in losses but not much else.


HUH?
Maybe I'm misunderstanding these comments?
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