Excessive volt drop

[Paulwr20]

I have a lagoon 380 with 3 brand new 12v 140ah batteries (4 weeks old).
During the night the only power draw comes from my fridge and freezer, which consume 5.5a and 3.5a respectively, whilst running. They run for approx 15mins every hour (on average).
My issue is that I am seeing a large voltage drop whilst they are running overnight. Frequently down to 11.9-12v. When the load stops they recover to approx 12.3. Though this is from a starting voltage at the start of the evening of say 12.9v.
I’m no electrical expert, though from my calculations, my volt drop under a 9a load should only be about 0.04v, not the 0.4v I am seeing. Also I didn’t expect to see such a drop in resting voltage overnight. Particularly with new batteries.

Do I actually have a problem? If so does anyone have any insight into what it is and what I can do to resolve it?

[sailorboy1]

Probably don't have a problem and it is just a function of your new batteries. Your 12.9v evening voltage is probably lower and the "no load" 12.3V is probably higher if allowed to fully rest.

What are you using to measure the voltage with? If it is the panel meter have to checked at the batteries themselves?

[noelex 77]

Where are you measuring this voltage? Is it at the batteries, at the fridge or somewhere else?

[Paulwr20]

The voltages mentioned are simply from ‘domestic on the lcd display of the electrical panel. It can also be switched between domestic, engine (Port engine), and auxiliary (Stbd engine).

[noelex 77]

This is probably measuring the voltage at the electrical panel, but it is worth checking, as this effects the diagnosis.

The voltage drop you are seeing is not normal. A 420 Ahrs battery bank should not drop below 12v with a 9A load, unless the SOC (state of charge) is very low.

The possible causes are:

A poor connection. This will be between the meter reading the voltage and the battery itself or the interconnecting wires between the batteries. Look for any loose connections. Sometimes the defective connection will feel warm.

A low battery SOC. Are the batteries charging correctly?

A defective battery or batteries. Isolate the batteries and check the voltages individually.

A very heavy load on the system somewhere.

A voltmeter that is reading incorrectly.

Defective battery switch (a poor connection inside the switch).

[Shanachie]

Also, consider the quality of the batteries.



I had four newish group 24 Interstate batteries that were billed as deep cycle/85 amp hours each, but actually are dual purpose.


I run a fridge 24/7 and other electronics when I anchor out. From 12.6-7 in the evening, the batteries would be 12.1 or sometimes less when I got up in the morning.


Even at the dock, The batteries would be 12.2 or so just from running the fridge.


I made the switch to Duracell golf cart batteries, 6 volts times 4 for 450 amp hours at 12 volts, after figuring out how to barely squeeze them into the battery compartment.


It made a drastic difference. No matter how many lights are on and electronic gear is plugged in, I never see less than 12.4 volts in the morning.


I'll never buy a dual purpose battery again. In my opinion, they just can't stand up to regular usage.

[Statistical]

You need a multimeter and measure the voltage at the battery. Without that you are just guessing and sailing in the dark.

There are a number of reasons why the voltage could be showing low. Find out what the voltage actually is at the battery both under load and at rest. That will be your first piece of data in solving the "mystery".

[Paulwr20]

Thanks all for your help so far. I shall get on with trying to obtain more accurate readings at the batteries/loads and report back soon.

[smac999]

get a battery monitor like a bvm 712.

then when you wake up you'll know how many ah you used over night.

I bet they are running more then you think. probbaly 100ah a day for both.