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Old 28-06-2011, 05:22   #16
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Re: Voltage drop under load, amps read 99% ??

Voltage drop?? It is a strange way of asking if your battery bank is large enough for your ship's load or on the other hand if your batteries are too old. All in all, it is not a good way of defining anything.
- - There are tables in the marine electrical wiring books like Nigel Caulder's that will give you an idea of what size wire you need to use to minimize "voltage drop" due to wire size, load, and length of wire. Too small a size of wire and you will get a "voltage drop" when the electrical load is turned on.
- - Assuming that is not what is being asked about then there is the observed decrease in voltage on the boat's voltmeter whenever a significant electrical load is turned on or comes on line. This "voltage drop" is due to the inadequate size of the battery bank (and/or wire size). Think of it as if your have a skinny garden hose and you are trying to get a lot of water quickly. The pressure drops (voltage is akin to electrical pressure). If you had a fireman's size hose and needed the same volume of water, the pressure would remain much higher. In this case once the electrical load is turned off the observed voltage will "recover" back to near what it was before the load was turned on.
- - Likewise, if the batteries are near the end of their life they will not supply the quantity of electrical power that your electrical load is requesting. So the observed voltage drops as you try to use more than is available. In this case the observed voltage remains low after the electrical load is turned off.
- - So there are several reasons for voltage drops/decreases and you will need to analyse why the voltage is dropping. But it is normal to see a voltage drop whenever an electrical load is turned on - it is just that too deep a drop or to slow a recovery indicates a problem.
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Old 28-06-2011, 06:19   #17
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Re: Voltage drop under load, amps read 99% ??

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Originally Posted by VVD View Post
I have three Power-Tech Gel batteries each 200amp, 12V batteries of which one tested 90% under a lead test done months ago, the others tested 100%. The yard at that time told me to continue to use it and monitor it.

Now I am about to take the suspected one out and see what happens to the voltage.

After running the Generator or engine the battery meter reads 12.45 (loading under 14.2), it remains there until I switch on the fridge or invert, or anything else that takes a bit of load. The voltage meter drops quickly, in a matter of 4 hours, to 11.3 or lower.

Any suggestions what to do other than taking this one out and check if this is the culprit. If so, should I replace all three batteries?
So what did you do and what did you find?

My battreies did the same a couple of weeks ago (2 4D wet cell batteries). Even after I disconnected the bad battery the good one didn't have much capacity and had to be recharged too offen. If you have run them all together a while the odds that all are now at the minimum semi-bad.

In the end I replaced both 4Ds with 4 6V golf cart batteries. And they fit in the 4D boxes (they are a little taller so measure the clearance if under your sole). And the 6V batteries were a lot easier to move around at half the weight.
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Old 28-06-2011, 11:37   #18
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Re: Voltage drop under load, amps read 99% ??

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Originally Posted by osirissail View Post
- - This "voltage drop" is due to the inadequate size of the battery bank (and/or wire size). Think of it as if your have a skinny garden hose and you are trying to get a lot of water quickly. The pressure drops (voltage is akin to electrical pressure). If you had a fireman's size hose and needed the same volume of water, the pressure would remain much higher. In this case once the electrical load is turned off the observed voltage will "recover" back to near what it was before the load was turned on.
Hi Osirissail,

Thanks for the reply, yes I'm seeing a the voltage drop on the boats voltmeter when appliances are turned on. The voltage does quickly recover after the load stops. For example; when a fridge using 6 amps turns on 12.3V drops to 12.0V. I have a battery bank of 720amp hours. I suspect that the voltage drop I'm observing is greater than normal and possibly evidence of damaged batteries (second hand boat). I was hoping I might be able to find some info on what normal voltage drop might be?
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Old 28-06-2011, 11:41   #19
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Re: Voltage drop under load, amps read 99% ??

I think a healthy battery bank would not have a voltage drop like that It I leave my hot water heater on and turn on the inverter is the onlt tome I see a voltage drop of courser that is a big load
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Old 28-06-2011, 11:46   #20
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Re: Voltage drop under load, amps read 99% ??

I have had terrible luck with 8D batteries. Not sure why but wouldnt advise them. I guess they are basically truck starting batteries and commercial trucks often run constantly ... onthe other hand had good luck with 4D batteries... even used gel ones mixed with a wet cell! If the dimensions work out, the best bang for amps is likely golf cart batts.
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Old 28-06-2011, 12:00   #21
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Re: Voltage drop under load, amps read 99% ??

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I have had terrible luck with 8D batteries. Not sure why but wouldnt advise them. I guess they are basically truck starting batteries and commercial trucks often run constantly ... onthe other hand had good luck with 4D batteries... even used gel ones mixed with a wet cell! If the dimensions work out, the best bang for amps is likely golf cart batts.
There are different types and brands of 8D and 4D batteries. I've had great luck with my Lifeline 8D AGM batteries (four of them in parallel, using an appropriate charging system.)

As for Voltage drop, have we determined if the OP is measuring at the battery terminals, or at the end of a cable run? Where is his monitor connected?
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Old 28-06-2011, 12:42   #22
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Re: Voltage drop under load, amps read 99% ??

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There are different types and brands of 8D and 4D batteries. I've had great luck with my Lifeline 8D AGM batteries (four of them in parallel, using an appropriate charging system.)

As for Voltage drop, have we determined if the OP is measuring at the battery terminals, or at the end of a cable run? Where is his monitor connected?
I'm watching my battery monitor, and with a 500 amp-hour bank of 5 year-old gels I see 12.70 volts with a 6 amp refer draw on, and 12.76 volts with it off. The voltage is measured at the battery terminal.
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Old 28-06-2011, 13:12   #23
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Re: Voltage drop under load, amps read 99% ??

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I'm watching my battery monitor, and with a 500 amp-hour bank of 5 year-old gels I see 12.70 volts with a 6 amp refer draw on, and 12.76 volts with it off. The voltage is measured at the battery terminal.
This 60-millivolt drop sounds pretty normal. I'm looking at the discharge curves for a 180Ah AGM battery (not the same as yours, but the only usable discharge graph I could find in a hurry), and for a 9A change in current the output drops by about 200 millivolts. I think your 500Ah gellcell bank is operating in the right ballpark.
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Old 28-06-2011, 14:27   #24
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Re: Voltage drop under load, amps read 99% ??

"My regulator maxes at 14.1 "

It might be noted that if the entire charging system is not performing properly, the new batteries will be RUINED IN SHORT ORDER AS WELL.

If the regulator really maxes out at 14.1 it may have been designed for gel batteries. It may be defective, and "14.1" may indicate a bad alternator diode and some reverse current. Or the voltmeter may be off.

In any case, that entire SYSTEM needs to be looked over and what each component is actually doing, or should be doing, confirmed.
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Old 28-06-2011, 15:48   #25
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Re: Voltage drop under load, amps read 99% ??

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Originally Posted by Hugh Walker View Post
Hi Osirissail,
Thanks for the reply, yes I'm seeing a the voltage drop on the boats voltmeter when appliances are turned on. The voltage does quickly recover after the load stops. For example; when a fridge using 6 amps turns on 12.3V drops to 12.0V. I have a battery bank of 720amp hours. I suspect that the voltage drop I'm observing is greater than normal and possibly evidence of damaged batteries (second hand boat). I was hoping I might be able to find some info on what normal voltage drop might be?
- - First off, if your "resting" voltage of your batteries is 12.3V those batteries are already discharged about 1/3rd from full. Below is a Voltage versus % of charge table. Resting batteries should have a voltage of 12.6V or higher if your charging system is working correctly.
- - I don't think the drop from 12.3V to 12.0 is abnormal as it could be caused by corroded wire terminals/connections and inadequate wire size in the system. But more than likely you have batteries that are weak to start with and they do not have the "Umph" needed to supply the load for very long.
- - Also as others have asked - what and where are you measuring these voltages? What are you using to measure them?
- - A panel mounted analog voltmeter is rather inaccurate beyond telling you that - yes there is some power left in the batteries - or no, they are flat. Also analog meters can indicate if you are actually charging the batteries or not.
- - You need a 3 digit digital voltmeter/VOM that can display voltages to 2 decimal places. Half a volt can make the difference between a full battery and a depleted one.
- - Likewise, measuring the voltages at the boat's distribution panel instead of at the actual battery terminals introduces all the losses due to wire size and corroded connections. State of Charge Resting voltages need to be taken with nothing connected to the battery.
- - There is crude test to see the condition of your batteries using only a digital voltmeter. It involves charging the battery until you see 13V or more at the battery terminals for a sustained period of time. Most charging systems go to a 13.1 to 13.6 "maintenance" voltage after they sense the battery is full.
- - Then you turn off the charger and physically disconnect the battery cables from the battery. Measure and record the digital voltmeter reading every 2 hours for half a day or more. You will see the 13V decrease to 12.6 or so rapidly and then continue to drop over time. A good battery will stay near 12.5V for a very long time while an old or bad battery will continue to exhibit a decreasing voltage all the way down to 10V or so if the battery is really bad. You can plot the time versus voltage readings and see a curve where after the initial steep drop the voltage drop levels off. Where it levels off tells you roughly the condition of the battery.
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Old 28-06-2011, 16:25   #26
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Re: Voltage drop under load, amps read 99% ??

One thing to check is to make sure the volt meter wiring and ground is good. I would measure the battery voltage at the battery with a good volt meter first under load before spending cash on new batteries. I know that a bad connection in the ground, or if the volt meter ground is connected to to the DC panel ground, It could be voltage drop in the ground wiring. Just a thought.

Under load my panel voltmeter reads .2 to .3 volts lower then the resting battery voltage. It could be a long shot, but maybe worth checking. The thing is to check the battery voltage after 4 hours on Refrig load (After) giving the battery time to rest.
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