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Go Back   SailNet Community > On Board > Gear & Maintenance > Electrical Systems
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Old 07-23-2009
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System voltage

I have searched for similar thread but cannot find anything on this topic.

My boat (new to me) has two house banks, each T105s, one bank is new and the other is about 6 years old. The older bank is reported by a battery shop to be toast, but seems to run the system for 24hrs no problem with a little help from solar. The batteries are charges by an older Guest 50 amp charger. Arriving at the boat following a week of shorepower, both banks read 12.8 with no load. Application of 7 amps, (reefer/lights) and the system voltage drops to anywhere from 11.5 to 12.2. Remove this load and system voltage increases back to original value.
Is this normal? If so, and realising accurate battery voltage requires 24 rest period, how can I determine when each bank has reached 12.2 (50%) charge?
Next purchase is a smart charger and two more 105s. At that time I will leave the house battery switch on both to essentially have one bank of 450 ah (225 usable)
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Old 07-23-2009
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I would replace the 6-year old T-105's without further ado. Then, I would run both banks as one single bank, giving you 450AH @ 20hr rate.

When fully charged and rested overnite with no load and no charge, you should measure 12.6-12.7V at the battery terminals (the only way to measure accurately in a new-to-you boat). Use a good digital multimeter, calibrated if possible.

For a new smart charger, I'd recommend you consider the Iota DLS series. They are IMHO the best value on the market today...well built, smart, reliable, and with a PWM charging technology which helps your batteries to last longer. They are a bargain at the discount prices offered by, e.g., Arizona Wind Sun. For your 450AH bank, I'd recommend the 75A model (DLS-75/IQ4).

New batteries need to be exercised a few times times (med-deep discharge followed by full charge) in order to reach their full capacity potential. If you have reason to believe they've been sitting awhile at the dealer's, it might be a good idea to equalize them, too.

Bill

Last edited by btrayfors; 07-23-2009 at 09:51 PM.
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Old 07-23-2009
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" Arriving at the boat following a week of shorepower, both banks read 12.8 with no load. Application of 7 amps, (reefer/lights) and the system voltage drops to anywhere from 11.5 to 12.2. Remove this load and system voltage increases back to original value.
Is this normal? "
For a dead battery bank, it is perfectly normal.

The float charge on good batteries should read about 13.6 when you get to the boat and drop to 12.6-12.7 when you knock down the float charge.

The fact that one of yours drops down to 12.2 means that bank is only holding a 60% charge, while the bank showing 11.5 volts is quite dead.

Doesn't matter what you can shake out of it--that bank is dead for any real load use. If you are charing both banks at the same time with the same controller, the dead bank might be the drain that is preventing the other bank from getting more than a 60% charge. GET RID OF IT.
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Old 07-24-2009
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Thanks for the good advice.......however the question not addressed is what voltage should I expect to be reading on a fully charged, 225 ah batery bank with a 7amp constant load applied?
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Old 07-24-2009
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7 amps is a small load for a T-105 bank of two 6V golf carts in series.

When the batteries are really "fully charged", you will see a float voltage for a short time...above 12.7VDC.

Thereafter, for quite a while, you should be seeing 12.6VDC. How long? Depends on several factors, some cited above.

If the batteries are in very good condition, have recently been exercised a few times, and have been fully charged at 14.4VDC, you'll see 12.6VDC for awhile. You may see 12.5VDC sooner than you'd think, but it will hang in there for a long time if only 7 amps is applied as a load.

There's no way to know the true capacity of your battery bank unless you do a constant load test, in this case applying about an 11 amp load for 20 hours, or until the bank reaches 10.5VDC.

By the way, voltages should be measured at the battery terminals with a calibrated digital voltmeter. You can't rely on other readouts, unless they are known to be accurate.

Bill
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Old 07-24-2009
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Pior - system voltage (as this thread is called) is of fairly limited usefulness in determining the health and charge state of a battery bank. It's better than nothing, but a battery monitor will give you much better information.
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Old 07-27-2009
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Hello: How much can one read into a float charge of 13.3 vice 13.6?
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Nothing. without further information. Both are appropriate, depending on the situation. What's your question? What are the details of your setup? Why are you concerned?

Bill
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Quote:
Originally Posted by bhcva View Post
Hello: How much can one read into a float charge of 13.3 vice 13.6?
Well, I almost agree with BT, I'd say "nothing significant" rather than just nothing. In general, a lower float charge would indicate that your battery is weaker to begin with, or there are wiring losses higher than there should be, or the charging system isn't putting out as high a charge as it might/should be.

If the battery is load tested (or at least, tested with the float burned off and after allowing 24 hours for the chemistry to equalize internally) and the charging system is tested in operation, and both test out OK, then whatever the float charge measures, is normal for your setup.

13.3 versus 13.6 could just reflect the different between two meters. Rule of thumb, unless you have a recently calibrated high quality meter, is that the rightmost digit may be off by 2-3 numbers from meter to meter, and that's just because cheap meters aren't intended for high precision.
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I'd just stick with the plain old nothing. Without knowing if the batteries are wet, AGM, or gel and if the charger has a temp. sensor the 13.3 vs 13.6 means nothing at all.
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