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Old 23-08-2012, 03:48   #1
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Voltage Drop From Battery To Main Breaker Panel

Hello. I have a 2003 Beneteau 50 and we're having some undervolt problems. We replaced the three house batteries with new lead-acid filled 135AH batteries a month or so ago. The starter battery is isolated, though it shares a common ground with the house batteries.

If I measure voltage at any one of the the house batteries with no load it reads say 12.3 volts -- but the voltage at the panel is 11.7. As I turn on different loads there is a marked voltage loss at the panel ... If I turn on everything at the same time pulling some 20AH of load on my amp-meter, the voltage drop at the panel is to something like 10.8 or so volts -- I have verified this as well by several means -- the Garmin chartplotter says the same voltage as the voltmeter at the breaker panel ... also my digital voltmeter says the same thing. and my inverter starts an audible alarm below 11.0 volts.

When I measured the volts at the battery instead of losing nearly 2 volts when everything is turned on, the voltage loss at the battery is only 0.1 or 0.2 volts.

We are not particularly careful in terms of battery usage -- we are anchored out about 3/4 of the time but run a generator to charge batteries and our computeres for a few hours at least a couple of times a week. We are often charging computers via the inverter but have a policy that they can only charge when the engine is running.

We have an 80amp Hitachi alternator ... with the battery charger on we get 13.25 volts at the house and starter batteries and about the same with the engine running and alternator charging. At the breaker panel we get about 12.5-12.7 when the engine or charging system is running.

My guess is that we might have some corrosion in the system somewhere -- either that or the wire from the house batteries to the breaker panel is too small.

We haven't added too much in terms of load from the original delivery manifest from Beneteau and recent replacements of RADAR, Chartplotter and going LED for mooring and navigation lights has taken off more for more load than anything we have adding.

So ... what I was planning to do is to chase each connection from the batteries on out to the breaker panel and clean and measure the resistance across each of these wires as power makes its way from the batteries to the breaker panel. (e.g., battery connections -- fusible link, battery switch, breaker panel wire, etc.)

I doubt it is in the ground circult as the starter battery is not affected ... but will probably check there first anyway as that would be a likely problem area.

Any ideas or recommendations would be appreciated. Does anyone else have some direct experience in this or advice that we can use? Thanks for the assistance.
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Old 23-08-2012, 03:58   #2
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Re: Voltage Drop from Battery to Main Breaker Panel

Checking all the connections from the house battery bank to the panel is where I would start. I would look first at the connections on the battery itself. Get a wire brush and clean all the connections both on the terminals and on the cables.

If that doesn't fix it I would next check all the connections to see if any are loose. If no then start checking these.

You might also want to track the voltage through the system to see if you can isolate where the voltage drops, starting at the batteries where the voltage is good.

It also sounds like your charging voltages are on the low side so you may be dropping voltage in the charging circuits as well.
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Old 23-08-2012, 04:01   #3
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Ya. A bad connection somewhere. Probably warm or hot to the touch. Maybe melted. Trace it with the voltmeter. Put one lead on the battery terminal then the other on each terminal between there and the load panel. You'll find it easy.
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Old 23-08-2012, 04:26   #4
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Re: Voltage Drop From Battery To Main Breaker Panel

Charging voltages seem very low - should be getting 14+V to get the batteries fully charged. I would go over all connections, take them off, clean and refix.
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Old 23-08-2012, 04:55   #5
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Re: Voltage Drop From Battery To Main Breaker Panel

As others have noted, your charging voltages are WAY low, and those new batteries aren't going to last long at this rate.

Charge absorption/acceptance levels for flooded batteries should be 14.4VDC at a minimum. Trojan recommends 14.8VDC and my own research confirms that this level is highly desirable.

Your alternator should be putting out 14.4 or higher.

Appropriate float level voltage (when the batteries are nearly full) are 13.2-13.8VDC.

I'd certainly agree that you have bad connections somewhere with the voltage drop you're seeing. And, it's very likely to be more than one problem connection. CHECK ALL OF THEM on both the positive and negative side.

If you have a diode isolator in the circuit, that can account for a modest drop, but nothing like you're seeing.

Also, check voltage drop across battery switches. I've seen some large drops due to bad switch contacts.

It's important to check the end connectors on each wire to be sure they are properly fitted. If there's any doubt, renew them (cut them off and put on new ones, properly, using the right crimper for the terminals you're using).

Sound like a lot of work? Yes, it can be. But your electrical problems are likely to persist if you don't nail this problem now. And, you'll go broke trying to keep your batteries in good condition.

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Old 23-08-2012, 05:15   #6
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Re: Voltage Drop From Battery To Main Breaker Panel

Quote:
... My guess is that we might have some corrosion in the system somewhere -- either that or the wire from the house batteries to the breaker panel is too small ...
Indeed.

To test the voltage drop of any length of wire, do the following:
1. Set the voltmeter to read millivolts or on the lowest volt scale.
2. Put the voltmeter positive lead on the end of the wire closest to the battery.
3. Put the voltmeter negative lead on the other end of the wire.
4. Operate the circuit.
5. If the voltage drop is more than 0.200V* (200mV), the wire, or connections are bad.
6. If excessive, remove the wire at both ends and clean the connections, retest.
7. If still excessive*, replace the wire.

* 0.00V across a connection
Maximum 0.375V across an entire run of wire (3% of 12.5V)
0.30V across a switch
0.10V at a ground

See ➥ Diagnosing Voltage Drops - Electrical Automotive Troubleshooting

And ➥ http://assets.fluke.com/appnotes/aut...e/beatbook.pdf
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Old 23-08-2012, 05:17   #7
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Re: Voltage Drop From Battery To Main Breaker Panel

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Originally Posted by btrayfors View Post
Also, check voltage drop across battery switches. I've seen some large drops due to bad switch contacts.
+1

Instead of checking the resistance at each connection or across each switch check the voltage, at say 20 amps. The larger the current flowing the larger the voltage drop. (Ohms law V=IxR) If you get more than 3% loss = 0.4 volts then you have a problem at that connection. This happened on my master house battery switch which never gets turned off. Turned it on/off 50 times and the volt drop when down to 0.1v. Took it to pieces and cleaned all the corrosion and the volt drop went down to 0.01v.
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Old 23-08-2012, 05:50   #8
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Re: Voltage Drop From Battery To Main Breaker Panel

Another thought. I had battery charging problems on a Jeanneau 45 last year. The output off the alternator was low, 13.5 volts and the electrician I got in reckoned there was a small mod needed to to the regulator to up it to 14.8. Not sure what he did as this was at the end of the delivery but believe he fixed it.
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Old 23-08-2012, 06:44   #9
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Re: Voltage Drop From Battery To Main Breaker Panel

Another way to do this is to rent a thermal camera for checking your electrical system, to find warm or hot spots.

What is your max alternator output?
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Old 26-08-2012, 09:43   #10
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Re: Voltage Drop From Battery To Main Breaker Panel

Thanks for all the replies. I have done some more problem determination. Based on the input I have received so far I think the data shows my big problem is at the battery isolator where there is about a 1.5 volt drop. Then each of the switches seem to lose between 0.1 and 0.3 volts, then further down the line it is fairly consistent throughout the rest of the circuits.
I wasn’t able to get inside the battery charger to figure out its starting voltage, but I measured the voltages across the other elements the same as with the alternator.
My course of action will be to disassemble and clean all battery connections between the isolator, switches and batteries, then re-measure what we have. While I am doing this I will also measure any drop across the switches & depending on time might dismantle and clean them as well.
Any other recommendations would be appreciated …. Thx
Here’s the data
Alternator Batt Charger
Voltage out of Alternator 14.21
Voltage at Starting Battery 12.98 13.98
Voltage at House Battery 12.85 13.39
Voltage at Garmin 12.70 13.00

Battery Isolator middle -- directly from Alternator 14.28 13.03
Batt Isolator Top -- to Starter Batt 13.12 -
Batt Isolator Bot -- to House Batteries 12.94 13.30

Battery Switch 1 (Starter) Top 13.22 13.90
Battery Switch 1 (Starter) Bot 13.10 13.95

Battery Switch 2 (House) Top 12.95 13.36
Battery Switch 2 (House) Bot+A16 12.90 13.37

425 Amp inline Fuse In 12.95
425 Amp inline Fuse Out 12.95

At 70amp DC Circuit Breaker in Panel In 12.98 13.18
At 70amp DC Circuit Breaker in Panel Out 12.98

Volts at largest Red Wire coming into Panel 13.00 13.39
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Old 26-08-2012, 10:33   #11
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Re: Voltage Drop From Battery To Main Breaker Panel

Your voltage out of the alternator is way low, should be 14.8.... what's the shore power voltage showing? Looks like you have dirty/poor contacts through most of system as well so cleaning looks like a good move. I would be concerned to get charging voltage up to right level as otherwise your batteries won't be getting above 85% charge.
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Old 26-08-2012, 11:22   #12
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Re: Voltage Drop From Battery To Main Breaker Panel

Good thoughts from everyone.
The only bit I would add is when measuring voltage drop make sure there is a reasonable power draw in the system. Turn on enough equipment so 10A,or so, are drawn from the batteries.
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