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Old 03-04-2015, 16:13   #1
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Help Battery Monitor

Hoping for some input on my battery monitor. I seem to be getting mixed signals.
I have two lead acid marine batteries on a house bank. We just moved into the boat yesterday. Batteries were fully charged when we left the marina we purchased the boat at and came here to boot key harbor and took a mooring ball. Since then we have used minimal lighting inside last night. I used the inverter for about an hour to charge my phone and about half Gallo of water pump time at the sinks and a few flushes of the head. The problem I have is the monitor has all four lights at the top green which means a charge remaining of over 80% yet the voltage meter reads 11.95 with no load. I am under the impression that voltage indicates a more discharged battery than 80%. Can someone give me any insight this is all new to me.

thank you.
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Old 03-04-2015, 16:33   #2
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Re: Help Battery Monitor

I cant tell you an answer to your specific problem. But I would go by the voltage. My luck with BM's would pretty match what you are experiencing. You probably need to rezero everything and start over to get some accuracy.
State of Charge12 Volt batteryVolts per Cell
100%12.72.12
90%12.52.08
80%12.422.07
70%12.322.05
60%12.202.03
50%12.062.01
40%11.91.98
30%11.751.96
20%11.581.9310%11.311.89010.51.75
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Old 03-04-2015, 16:34   #3
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Re: Help Battery Monitor

Read the manual for your battery monitor. They have a setup procedure that is required for them to learn the battery capacity. Until then rely on the volt meter for a state of charge approximation.
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Old 03-04-2015, 17:47   #4
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Re: Help Battery Monitor

thank you both. I suspected as much. Running the engine now. Will be getting solar wind and genset soon
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Old 03-04-2015, 18:00   #5
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Re: Help Battery Monitor

Would you share the type of BM with us?
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Old 03-04-2015, 18:44   #6
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Re: Help Battery Monitor

My experience with battery monitors is that the type which counts amps in and out, measured at a shunt, and attempts to calculate a % state of charge, just don't work unless you very carefully adjust the battery capacity, and continually readjust it as your batteries' capacity changes, and frequently zero them out at full charge (hard to do if you're off shore power).

I think their main function is to give a deceptive impression that you know your batteries' state of charge -- false precision. When in fact the reading of state of charge bears little to no resemblance to reality.

I ditched my Victron battery monitory a couple of years ago, and started simply reading voltage at my chart plotter. It works quite well with a large battery bank, big enough that voltage doesn't fluctuate under small loads. The best thing is that any imprecision resulting from loads must be in the direction of underestimating battery charge, you can be sure that if you have 24.6 volts, you have at least 70% charge, and this is really useful.

I have had good luck with this method the last couple of years, and have much more confidence in it, than with the futile exercise of trying to interpret amp/hours in and out.

However, I have a SmartGauge on order and have high hopes for this. This device analyzes voltage without amps, and by all accounts does a superior job of estimating state of charge. We shall see.
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Old 03-04-2015, 18:45   #7
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Re: Help Battery Monitor

How large an inverter to charge phone? I just use a 12 volt plug like a car.

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Old 03-04-2015, 19:44   #8
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Re: Help Battery Monitor

Anything under 12.0v is a pretty dang dead battery. (see SOC chart Cheechako posted)

Unless, it's under a pretty good load. SOC voltages are typically "resting" and doing that for something insane like 24 hours. So if you're powering a TV with an inverter, maybe 11.95 is OK and it'll pop back up after the load is removed. Or maybe not. Depends on your batteries.

You may quite likely find out that the batteries that came with the boat are pretty much toast. Most folks abuse the crap out of their batteries. You'll find out if you find capacity is really poor. That's ok tho, a coupla GC batteries from sams/costco is massive bang for the buck.
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Old 03-04-2015, 20:04   #9
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Re: Help Battery Monitor

I have a smart gauge bat monitor
I give a 5 star rating for bat monitors
Read the panbo report on it.
Normal monitor use peurkets law to
Work out the state of charge & adjusting this
Requires extensive knowledge to get it right,
it also require frequent re adjustment to keep
it right.
This is well beyond the the abilities of most of us.
The smart gauge connects directly to the battery
& reads the bat voltage with no shunts for amps.
It is self calibrating so needs no user input.
It gives a % reading & volt reading it has it's
limits but is far more accurate than the shunt type.
To the best of my knowledge it is the most
Accurate available.
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Old 03-04-2015, 20:54   #10
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Re: Help Battery Monitor

our batteries show a bit different SOC as per the manufacturer so its best to check the type and try find out the SOC % for your batteries.
Ours are SOC should be 12.44@50% 12.52@60% 12.6@70% 12.77@90%
Note that these are generally after sitting idle for 24hrs which isnt going to happen on a boat generally. If you are charging the voltage will read higher. If discharging they will read lower. So if you are seeing 12V at night with the fridge running, they may well be actually 12.3 at rest after 24hrs (after static charge has been removed as well)
Lots to read up on! but generally Id be charging if in your situation of seeing under 12V. Turn off the fridge if you can till you get it sorted out and maybe save the battery.
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Old 03-04-2015, 21:20   #11
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Re: Help Battery Monitor

You say you have two "Marine" batteries. What make and model are they? Most "Marine" batteries are not deep cycle batteries and are not suitable for a house bank on a cruising boat.
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Old 04-04-2015, 05:03   #12
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Re: Help Battery Monitor

wow lots of stuff to absorb. I will pull out the starboard bed and get all that info today and post it.
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Old 06-04-2015, 23:29   #13
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Re: Help Battery Monitor

if the meter is set up and working (most work just fine), then your batteries are probably done. IE you pulled say 40ah out of the 200ah bank. (meter says 80%) but you are under 12v. means your batteries have very little capacity left and need to be replaced.
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Old 07-04-2015, 04:38   #14
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Re: Help Battery Monitor

Quote:
Originally Posted by essej4269 View Post
Can someone give me any insight this is all new to me.

thank you.
So on the BM what was the State of Charge and the amp-hours out?
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Old 07-04-2015, 05:32   #15
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Re: Help Battery Monitor

If you get on the Net in Boot Key, ask to borrow a battery tester. ( channel 68, 9 AM )

Going by your description, the batteries weren't charged or are shot. Anything less then 12 volts is considered discharged.

If you find them bad , the NAPA store delivers to the city dock.

Trade in the 12 volt ones and get 2, 6 volt GC2s .

There is a guy named Alex, there, who gets rave reviews, on electric issues.

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