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Old 07-06-2011, 23:21   #16
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Re: Help: voltage drops under load

"The voltage drops to 8ish volts with some of the devices on, and 0-2 volts with all of the devices on (obviously nothing runs)."

Precisely 'where' does the voltage drop when you have a load on?
Put on a load that drops the voltage.
Measure between the pos. and neg. posts on the battery bank. OK?
Then measure the voltage at the first connection down the line on the pos. lead, say at the battery switch. Good there?
Next, measure on the other side of the switch.
Keep going until you find the place where the voltage drops. Like others before me, I remind you to not neglect the negative side of the circuit.
Electricity is linear. Think linear.
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Old 07-06-2011, 23:51   #17
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Re: Help: voltage drops under load

Start at the fully charged batteries with a multimeter and trace your way through the 12 volt system until you find the circuit with the big voltage drop. You don't have to guess by doing it this way.
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Old 08-06-2011, 09:06   #18
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Re: Voltage Drops Under Load

We were cranking the engine hard after it died. But the engine still starts. Would a burnt out starter cause a short but still let the engine start? The starter is connected to the engine so I'm guessing this short could be causing the grounding issues?
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Old 08-06-2011, 09:21   #19
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Re: Voltage Drops Under Load

I don't understand the connection? Unless it is the starter drawing current when it is not cranking. Check that circuit for a voltage drop as well.

Are your batteries holding a charge after they are fully charged and then disconnected? Try that to see if it is your batteries are not holding a charge or your DC system drawing down your batteries.

You may also want to buy or borrow a clamp ammeter to see how much current is flowing thorough your DC circuits.... in order to see where those electrons are flowing and how much. This might be easier than tracing with a multimeter since a multimeter can only be used where you have bare wire.
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Old 08-06-2011, 09:34   #20
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Re: Voltage Drops Under Load

Also everything works when AC is on or the battery selector is on combine (the starter batt and both house).
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Old 08-06-2011, 09:46   #21
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Re: Voltage Drops Under Load

If you are looking for an easy answer you won't find it. You need a good multimeter and patience. You need to do a voltage drop test at each connection till you find the drop. This involves not only the main connections but the connection between the terminals and the wire itself. Sometimes you can find it by feel as the increased resistance will cause the wire to heat up but hopefully not to the point where things start to melt.
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Old 08-06-2011, 09:56   #22
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Re: Voltage Drops Under Load

hmmmm, once I was having problems and to make a long story short...it was the battery selector switch , I replaced it, then took apart the switch....the contacts inside the switch were corroded and greenish...working intermittantly and passing low voltage...
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Old 09-06-2011, 10:54   #23
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Re: Voltage Drops Under Load

The details of this problem are not adding up. Way too much hit and miss and partial information. I think you need to go back and analyze the whole problem step by step from the beginning. So:

1. You have two, separated and independent battery systems, one for starting, one for house, correct?

If so, you cannot draw any conclusion about one system based on what happens with the other. So, just because your starter will crank the engine (with or without combiner, battery switch to all or any other factor) does not necessarily tell you anything at all about what is going on with the house system.

2. You purchased two new house batteries and claim they were fully charged, correct? How do you know they were fully charged? Did you use a shore power charger or engine alternator? Did you check the output to confirm it is putting out a full charge? Did you check the before and after voltage at the battery with an accurate, digital meter?

I had a similar problem with a new boat and didn't have all my tools with me so used a cheap analog meter to check the battery. It looked charged and the voltage increased when I turned on the charger so thought it was the battery and bought a new one. No improvement so went out a bought a new digital volt meter and discovered the batter charger was only putting out about 12.5 V, just barely enough to keep the battery alive, but not anywhere close to enough to give it a full charge. You need to check the voltage of the battery and charger with an accurate volt meter to be sure you are getting a charge. You should see around 13.5-14V at the battery terminals when the charger is connected and charging the battery. Wouldn't hurt to get a hydrometer as well.

3. If the batteries are fully charged and you have a short that is draining them to 8V or anything like that in a matter of minutes then you will be seeing hot, melted insulation, smoke and maybe even fire. Absolutely without a doubt. If you are not seeing something really, really, obviously very hot then a short is not the problem (or not the main problem but could be contributing).

4. The starter draws more power than anything else on the boat. Several things about the starter.

a. If it still turns over and cranks the engine then it is not burned out.

b. It will be connected directly or almost directly to the battery with large cables. It should be easy to track and visually confirm if any of those cables were overheated.

c. If a battery has enough power to crank the engine then anything connected directly to that battery should also work unless that thing is broken or shorted. That means directly to the plus and minus terminals on the battery, not through any switches, combiners, grounds, etc.

Try all this, write down the data and results and the exact conditions on how you measured ( for example: with a digital meter at the + & - battery terminals, or from the engine block ground to switch panel postive, or charger on, or engine running, etc) and get back to us.
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