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svzephyr44 09-20-2004 07:12 PM

Battery Voltages vs Capacity
I have a classic 1 x 2 battery set up in my Catalina. The starter battery charges up to 13+ volts and holds there rather well. The house batteries charge up to a slightly lower voltage. (BTW - these are big batteries - I forget the group but they are approximately 16" long by 12" wide - they are also less than 3 months old.)

Here is my problem. Tonight we went over from our dock to the fuel dock for fuel. The only draw on the house batteries was from our Raymarine ST50 gear - maybe 2 to 3 amps according to the ammeter. We ran the boat about .3 hours - based upon the engine hour meter. When we returned to our dock the voltage reading with all circuts off on the house bank was about 12.2 or 12.3 volts. This seems very low to me. I checked to see if something was drawing on the batteries but the ammeter said no - 0 AMPS.

My question - what voltages are considered to be 75%, 50%, and 25% charged presuming that there is no load on the battery and it has had a few minutes to recover from the previous load.

GordMay 09-21-2004 01:05 AM

Battery Voltages vs Capacity
Open Circuit Voltage is a poor indicator of state of charge - but here’s a rule of thumb:
>12.7 V (rested) = 100%
12.5 V = 75%
12.2 V = 50%
11.9 V = -0-

Note: Recently charged batteries must have Surface Charge removed prior to OCV test. Requires a LARGE load (up to 200A) for 10 seconds or so.

WHOOSH 09-21-2004 02:02 AM

Battery Voltages vs Capacity

I''d encourage you to do two tests using Calder''s Boatowner''s Mech/Elec Manual. You''ll find both of them in the 12V section of the book (which I don''t have in front of me, and so can''t be sure of the terminology).

One is a 5-sec pull, using the house bank to start the engine but with the engine start feature disabled. This will require a bit of temporary rewiring. You''ll be viewing the voltage drop on the housebank (which, yes, I did notice is ''new''). Just how ''new'' and also healthy that house bank is will be reflected in the test.

The second test is measures the voltage drop across the circuit from battery bank to load (engine). The goal here is to look at things you might/might not be able to see, but which produce resistance in the circuit and therefore reduce battery performance even tho'' the battery(ies) may be potentially healthy. Corrosion, poor crimps and other gremlins will all show up; I''m sorry I don''t recall Caldern''s term for this second test.

I''ve used both these tests to diagnose battery and charging circuit performance and think they can be very helpful. Whether they reveal the particular problems in your set-up or not, they will tell you thinks about your 12V system that you''ll appreciate knowing.

Good luck!


svzephyr44 09-21-2004 05:24 AM

Battery Voltages vs Capacity
With your additional information I can get to my next question:

OK - 30 minutes of ST50 load - Wind, Speed, Depth, GPS only. Started with 2 fully charged (I think Group 8D) batteries wired in parallel. End voltage about 12.2 or 12.3 volts. Seems to me like I have a problem. Would you agree?

Sailmc 09-21-2004 05:34 AM

Battery Voltages vs Capacity
Full charge under a light load would be about 12.75 volts. My St 60 suite of instruments draws less than 1 amp per hour. You should be able to run those instruments for days.

fer@fer 09-23-2004 05:18 PM

Battery Voltages vs Capacity
Yes you do. But the key is knowing which problem exactly. And after knowing that, you will have the correct answer.

As Jack expalined, conections are usually the main source of problems. Check wether the alternator is providing at least 13.8/13.9 v at the battery terminal when the motor running. You should also check connectors at the battery, looking for any loose bolt or corroded or sulphated terminal, as well in the alternator, thru contact keys, and isolators and fuses in the cables running from alternator to battery bank, if any.

We do assume that electrolyte level is OK, and never happen the level to be below the plates.

However, and sorry for this, according to the symptoms, I guess your batteries are about to die. Probably due either to a low electrolyte level, or undercharging.

If they are less than 18/24 months old, probably they can be equalized and probably more acid be added in a work shop.

Some Ac chargers have a feature by which you can equalize them. This consisting in charging the batteries with 15.5 volts during one or two hours. This will make an acceleration of the chemical process allowing the convertion of the salts adhered to plates. Accumulation of this sulphates is the natural process of living and dying of chemical batteries. Calder''s book ilustrates this from square one.

Even if you buy brand new batteries, I believe one should understand what happened.

Good luck, and keep on trying


WHOOSH 09-24-2004 03:56 AM

Battery Voltages vs Capacity
Zeph, you clearly have a problem. Putting an electronic ''load'' on a large housebank is almost nothing. You need to follow Calder''s instructions, using the engine (start disabled) to put a REAL load on the bank, while checking both circuit integrity and also capacity of the bank. No short cuts, please - read to understand those tests and give them a try.

BUT let me add one suggestion. A day or two before doing so, run a Equalizing Charge on the bank, trying to shake whatever sulfation has occurred. Then run your tests.


svzephyr44 09-24-2004 05:52 AM

Battery Voltages vs Capacity
Thanks to all for your suggestions. I am going to run the equalzation charge today. Since the battery bank has an inverter attached I can easily add a 100 amp load (its a freedom 25, so 2500 watts at 12 volts equals about 200 amps) by turning on the coffee pot and the microwave.

I guess I am going to have to purchase Calder''s book (or get it from the library) to understand his testing procedure.

Thanks for all the advice, more to come


svzephyr44 09-27-2004 08:49 AM

Battery Voltages vs Capacity
Well - long story - try to condense:

Spent the better part of a day tracing wires on the boat. Discovered that someone had attached the CD Player to the starter battery bank - no fuse, no nothing. So that was the source of the continuous drain on the starter battery. Of course, the CD player is attached to the radio/stereo which was attached to the house batteries. Thus, power was going from the starter battery through the CD Player through the stereo and bact to the house bank. No good. Interesting voltage readings on the various wires.

Then, in the process of debugging, found that the ST-50 seatalk power line was attached (and jumpered) between the wind, depth, and speed on off switches on the 12 volt panel. Of course, each of these switches was on a separate circuit breaker. So in essense I had three circuit breakers in parallel if I turned on all of the switches at the same time. Not good. But the best was, the autopilot end of the seatalk cable was attached to the course computer with about 12 AWG cable, the other end of the same run was attached to the depth gauge with about 22 AWG cable. (think on an extention cord with a male plug at each end, and the outlets in the middle) So, if you turned on the instruments by turning on any one or more of the depth/wind/speed instruments you got a .4 Volt voltage drop at the autopilot. If you turned on the autopilot end, you read near battery voltage. So, net net, we were trying to run the autopilot on a 12 ton 42 foot sailboat through 22 AWG wire. No wonder the autopilot complained "low battery" all of the time!!!!! Disconnected the wind/speed/depth end, will give it a sea trial tonight and see if the autopilot works for more than 30 seconds!
And ... (yes, there''s more - it slices, it dices!) I discovered that the reason that the chart plotter autopilot mode did not work was that there was no NEMA circuit to send sentences from the chart plotter to the autopilot. There was only the connection in the other direction - e.g. to send navigation data to the chart plotter. Ran out of time - but one more pair of wires and I should be able to point to a destination on the chart plotter and have the autopilot steer to it. That would be "all good."
Of course, after spending the better part of the weekend debugging the electrical systems we tried to clean up after dinner and could get no fresh water in the sink. Discovered that the fresh water pump had thrown its belt as the pullies were out of adjustment. As they say, win some, lose some. Went down and fixed the water pump this morning. But, what a bummer after the high of getting a handle on the electrical problems to have yet another problem exhibit itself.
Thanks to all who contributed ideas. The lesson learned is - don''t trust anything on a boat you have purchased to be done correctly. Work you way through each system until you are sure they are OK.


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