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Go Back   SailNet Community > On Board > Gear & Maintenance > Battery charging
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Thread: Battery charging Reply to Thread
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Topic Review (Newest First)
11-29-2006 01:06 PM
btrayfors Thanks for that clarification, Dave. Lotsa folks don't discriminate between measured voltages UNDER LOAD vs. NO LOAD or no connections. Also, presumably, the charts for batteries under load refer to a 20-hour load only.

One other item: an hour or so isn't enough for a recently charged battery to lose its "float charge" and get down to 12.6 volts or so. Many batteries will hold a higher voltage for much longer. In my experience, 24 or even 48 hours is a safer interval to give a more accurate indication.

Note also that gells and AGMs exhibit higher voltages; those cited above are for flooded batteries.

Bill
11-29-2006 12:57 PM
dave.verry
Table

Sorry, had to make this smaller.
11-29-2006 12:54 PM
dave.verry
Full battery

If your battery goes below 80% charge, generally when you start charging at the maximum recommended charge current, the voltage at the battery terminals will be below float voltage level (13.3V to 14.7V depending on battery type and manufacturer). At the point where the battery charges sufficiently to allow the terminals to reach the float voltage level, the rule of thumb is that the battery is 80% charged. Usually, with the proper sized charger, this takes 20% of the total charge time (again rule of thumb). The remaining 20% charge usually takes and additional 12 to 18 hours.

If you do not discharge the battery below the 80% level, it will take shorter to charge the battery to full. Do not rely on the “Full” indication on most battery chargers, It is usually better to charge the battery for a prolong time to insure minimizing sulfation. Ideally a battery should be on a maintenance (float) charger continuously unless in use.

Also, do not rely on cell voltages unless you know the temperature of your electrolyte and the battery is clean with no load. The 10.5V level mentioned above is for a battery under load. Open cell voltages for charge state are different.

The attached table is for wet cell, lead/lead or lead/calcium batteries. The voltage is open cell voltages, no connections. As you can see, the voltages vary greatly over temperature. If you must use open cell voltages to monitor state of charge, use this table taking the temperature into consideration.
11-29-2006 10:44 AM
camaraderie "Full" on the charger doesn't mean full in real life. To be truly full...the battery at rest needts to read 12.6V+ when at rest and under no load. Next time you charge up...measure the voltage at the battery terminals after waiting an hour or so with NOTHING else running on the battery. Here's a quick list of various state of charge readings:
Voltage State of Charge
12.6+
100%
12.5
90%
12.42
80%
12.32
70%
12.20
60%
12.06
50%
11.9
40%
11.75 30%
11.58
20%
11.31
10%
10.5
0%
11-29-2006 09:43 AM
drynoc
New Question

Dave (and others),

Thanks for your knowledgeable replies, but now I have a new question. You say that it should take 12 to 18 hours to top off a single battery, but I get a reading of "full" after two to three hours of charging. I charge the batteries every week during the winter, either by taking the boat out, or by hooking up the charger. Does this mean that the batteries haven't drained that much and there really isn't that much charging to do, or is that "full" reading misleading?
11-28-2006 01:15 PM
dave.verry Someone WAS paying attention!

Exactly right! A single battery charger will charge a parallel string just as if it were one battery. Since the capacity is doubled it will take twice as long to get to the 80% charge level (when the battery reaches the float voltage for the battery) but charging the remaining 20% will take an additional 12 to 18 hours, the same for each individual battery alone on the charger.

You should set the charger as you would for a single battery.
11-28-2006 12:58 PM
jones2r Drynoc,

I don't profess to be an expert on batteries, battery types, and recharging. However, I believe your setup is straight forward. You may want to wait for the more knowledgeable to verify or elucidate before acting on what I am about to relate.

You have two wet-cell 12v batteries physically wired together, not through a switch or combiner circuit. With this configuration, you essentially have one 12v battery with twice the amperage capability of a single battery. As such, you should set the charger for one battery. Since you are experiencing difficulties, I would clean all of the terminals and check the electrolyte levels in the individual cells (if you have not already done so). With the batteries separated, you can charge them individually if you choose. My preferred charging arrangement would be with them interconnected, and with the positive charger cable connected to one battery and the negative charger cable connected to the other other battery. If this configuration does not work, disconnect the batteries from on another and try charging individually; you could find that one of the batteries has a problem.

Having reread your posts, it appears that your problem was in setting the charger to "Both". I'm guessing (always a bad idea) that your charger has two sets of cables. If so, you should be able to disconnect the batteries from one another, and charge them individually and simultaneously using the "Both" setting. This discussion may be pedantic, but it could help you to understand your charger.

Good luck.
11-28-2006 09:25 AM
drynoc
Charging

I have two 12v wet cells set up in parallel. Now that I read all this, I am wondering about my charger. I can't remember the brand name, but it is a three stage, portable, smart charger purchased at West Marine. It has separate settings for various battery types and charging levels. I believe it is adequate to the task, but someone more knowledgeable might think otherwise. It makes sense to me that the charger ought to be able to charge both batteries if hooked up to one of them and with the battery control switch set to "both". It also appears that there is a difference of opinion on this. Thanks to everyone for the replies, though.
11-27-2006 11:22 PM
jones2r Drynoc,

There's been a lot of discussion here about configurations and battery types, but I don't know if anything has really been determined. Do you have two identical wet-cell batteries? I believe that has been the assumption herein. Are they both 12v in parallel, or is possible that you have a couple of 6v golf cart batteries hooked in series?
11-27-2006 10:54 PM
camaraderie Good news/// What happens on leap year or months with 31 days??
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