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Old 30-08-2010, 12:41   #1
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Assessing the Condition of My Batteries

I have two deep cycle 12 volt batteries and I've been taking readings of their voltages with my voltmeter. To determine how well charged they are I heard that I need to determine the "fully charged rest voltage" of my batteries. Does anyone know how I find that out? I assumed it would be 12.00 volts, but since many of my readings are over 12, perhaps it's higher?? Is it the same for all 12 volt batteries? Or does it vary by make/model? I don't see it on the spec sheet for my battery.

Any advice would be appreciated
George
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Old 30-08-2010, 13:25   #2
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A fully charged battery should read 12.7 volts. A reading of 12.4 volts equals about a 75% charge and is good enough for further testing. But anything less means the battery is low and needs to be recharged.


Battery Voltage and State of Charge:

12.68v . . . . . . . . . . 100%
12.45v . . . . . . . . . . 75%
12.24v . . . . . . . . . . 50%
12.06v . . . . . . . . . . 25%
11.89v . . . . . . . . . . 0%

(NOTE: these readings are at 80 degrees F. Battery voltage readings will drop with temperature roughly 0.01 volts for every 10 degrees F.)
(At 30 degrees F. a fully charged battery will measure about 12.588 volts, and at zero degrees F it will measure about 12.516 volts.)

Battery Testing can be done in more than one way. The most accurate method is measurement of specific gravity and battery voltage. To measure specific gravity buy a temperature compensating hydrometer, to measure voltage use a digital D.C. Voltmeter. A quality load tester may be a good purchase if you need to test sealed batteries.

Charge Level SG Voltage
100% 1.265 12.7
75% 1.225 12.4
50% 1.190 12.2
25% 1.155 12.0
Discarged 1.120 11.9


I charge the battery(ies) until I get a float indication on the charger. I then remove the charger and let the batteries rest 4-8 hours before giving them a light discharge for a few minutes. Then I measure the specific gravity if I'm concerned (it appears to be more accurate than measuring the voltage) or use the DMM to measure voltage.
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Old 31-08-2010, 06:51   #3
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To add to the excellent info by Capt Douglas - the multimeter voltage tests need to be done with a digital meter that displays two decimals places. The "resting voltage" is the voltage of each battery that has finished charging and has been physically disconnected from all the other batteries for about 2 hours. In other words the battery "rests" for two hours and then you take the readings.
- - I found a better way of using voltage readings is to chart the digital readings for the disconnected battery from just after charging is finished and then every hour thereafter for about 6 to 8 hours. You will notice a rapid decrease in voltage from the charger level of normally around 13 volts and then as time increases the voltage curve will "level off" or flatten out. The deeper this average flattened voltage is below 12.6 volts will give a rough idea of how much life is left in the batteries. If the voltage graph does not flatten but keeps on decreasing down below 11 volts then the batteries are pretty much shot.
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Old 31-08-2010, 07:05   #4
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Now you know the propper way to do it, I'll tell you my way

I have solar panels direct into the batteries so it no use using the volt meter during the day... (unless...see below)

At night after the sun has gone I should have 12.7 volts with the Auto Pilot on and the chart plotter at low level scrren brightness.
At 12.2 (i.e. not 12.3 flicking to 12.2) I am hoping sunrise will be very shortly. If it isn't I will recharge with the engine.

Because I know this from months of experience IF I start getting lower readings then I must determain why: Its not charging correctly; the batteries arn't holding the charge as they used to; or I have an electrical problem.

In the last few weeks I have been at anchor and running a fair bit on firigge and computer. The batteries suprised me the other day when they were VERY low. 11.8!!!

I thought the batteris must have been fired!

But I looked at the latitude: 44 north, the days are getting shorter, a bit of thick cloud about and using more elctricity than normal.

So it wasn't the batteries.

I take a volt meter reading several times per day and if you do the same thing you will quickly see what is 'normal' and what is not.





During the day on solar I have between 13.1 and 14.6 volts. It the batteries are flat or something is very wrong the voltmeter will show 12.2 or whatever...

Love thy Volt Meter!


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Old 31-08-2010, 07:35   #5
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thanks- assessing the condition of my batteries

Thank you all very much for your advice. I'll need to read through this info. a few times to make sure I fully understand (I'm still getting my feet wet with this stuff). In the meantime, I'm wondering if you can give me your opinion on the condition of my batteries based on this data from the past weekend.
  • Immediately after charging my batteries for 8 hours I got the reading of 13.1 (battery 1) and 13.0 (battery 2)
  • Then I left them connected to a solar trickle charger for 36 hours
  • Then I returned to the boat and charged them with my battery charger for just 30 minutes
  • 30 minutes after this charging was finished I got the reading of 12.68 (battery 1) and 12.66 (battery 2)
  • 10 minutes later I got the readings of 12.48 and 12.50
  • Then I started up the engine and motored about 4 hours (alternator charging batteries)
  • Immediately after turning the engine off I got a reading of 12.86 and 12.96
  • Then I turned the battery switch to Battery 1, turned on the masthead light, listened to the radio for about 2 hours, used the light for about 30 minutes and went to bed.
  • When I woke up, battery 1 measured 8.28 volts (very low!) and battery 2 measured 12.49 volts.
My batteries are about 5 years old and I've been OK (not great) about keeping them charged. Do you think my batteries are over the hill and it's time to get new ones?
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Old 31-08-2010, 08:04   #6
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Quote:
Originally Posted by Ericsn25Chespk View Post
  • Then I left them connected to a solar trickle charger for 36 hours
  • .
  • When I woke up, battery 1 measured 8.28 volts (very low!) and battery 2 measured 12.49 volts.
?
I'd do that test again, but more simply if you can. Delete the use of the solar trickle charger. Just get the batts up to 100% (or whatever) and then run the radio for 1 hour.

Remember a radio playing music loudly uses quite a bit of electricty in the amplifier.

if then the batteries come oout like thay have I think No 1 is stuffed. No 2 looks fine.


Soome will say you should buy both at the same time. So you could and use the good battery as a 3rd house battery or a spare.


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Old 31-08-2010, 08:11   #7
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One is shot and if they are the same age, I am willing to bet the other not far behind. Put is this way if you don't change both of them now, what is the likely senario going to be, engine won't start so you can't move the boat to go ashore and buy new ones plus no lights, VHF or fridge.

Are they ordinary flooded lead acid? if so 5 years is quite good, you have had your monies worth, time for new ones when it suits you, not on Sunday afternoon when the shops are all closed and you want to go to sea.

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Old 31-08-2010, 08:22   #8
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Reread his post - He was running on Bank/Bat 1 all night.

Bat 2 was disconnected. I would charge the batteries.

Run the test again - same power usage and use bank two and see if results are similar.

Reckoning your cabin light takes 40 watts X 2 hours = 7 a/h
Your anchor light (guess) 40 watts X 8 hours = 26 a/h
Radio .5 amp X 2 hours = 2 a/h

total 35 a/h
Resistance and efficiency adder (30%) = 10 a/h

Call it a total swag at 45 a/h.

I don't know your a/h capacity - one of mine is 70. If that is your overnight usage I would expect the battery to be drained.

I may be totally off on the a/h calcs - I don't know what you equipment draws but to decide if the battery should be discharged you need to know your battery capacity and how much you are drawing out of it.
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Old 31-08-2010, 08:25   #9
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It is always a good idea to purchase all the batteries for "a" battery bank at the same time. But some individual batteries will fail much sooner than the others. So MarkJ suggestion to remove the still good battery 2 and use it as a starting or 3rd battery bank works fine. Just get two new batteries for your main bank.
- - However, don't charge both your existing batteries together - battery switch in both - as the bad battery will drive the charger to overcharge the good battery and possibly cook the still good battery.
- - You can split your batteries into two main banks - two new batteries in parallel attached position #1 on the battery switch and the odd battery attached to position #2. Of course, controlling everything is whether there is space for additional batteries in your boat.
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Old 31-08-2010, 08:37   #10
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Reread his post - He was running on Bank/Bat 1 all night. Bat 2 was disconnected. I would charge the batteries. .
Yes, I know he was running on battery 1, so why charge battery 2? its had 12 hours of charging and down to 12.49v with no load, not brilliant. We don't know the size of battery 1 or the load, but I would have suggested 20w of cabin light and either 10w or 25w of masthead light which a 110 AH battery should run. I am with Osirissail, except its a 25ft boat so as he points out room could be an issue, but two 110 AH house and one smaller engine start battery would be a good set up, actually its what we have.

On one side of the equation is cost, the other the hassle if both fail. Since its autumn might be worth shopping around for some end of season bargins at a good price.

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Old 31-08-2010, 11:41   #11
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Quote:
Originally Posted by Ericsn25Chespk View Post
Snip....
  • Then I turned the battery switch to Battery 1, turned on the masthead light, listened to the radio for about 2 hours, used the light for about 30 minutes and went to bed.
  • When I woke up, battery 1 measured 8.28 volts (very low!) and battery 2 measured 12.49 volts.
My batteries are about 5 years old and I've been OK (not great) about keeping them charged. Do you think my batteries are over the hill and it's time to get new ones?
I'd say battery 1 is shot. Battery 2 seems to show a bit more drop than I'd be happy with, especially after sitting all night with no load on it. If you've got the funds I'd change both (easier to track time wise, and you know you're going to have to change #2 eventually). But I'd definitely change #1.
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Old 31-08-2010, 13:29   #12
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Thank you

Thank you all so much for all of your advice. That makes sense. I think I'll get two new ones (and possibly a separate battery for engine starting) and then be diligent about checking them and keeping them charged.
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