Low voltage from my batteries.

[hopkinslaw]

My batteries are delivering lower than expected voltage.

My house batteries consist of 2 Rolls AGM. They are 115 Ah for a total of 230 Ah. I bought them about 2 years ago.

When my batteries are fully charged up and I put a 11.5 amp load on them, the voltage steadily drops and after 2-3 minutes settles at about 12.3 volts. After a few hours of a steady 11.5 amp load, when my battery monitor shows I should be at about 70% capacity, the battery voltage has already dipped below 12 volts.

I assumed my batteries were toast. But when I did a 20 hour test on them they performed reasonably well. They lasted over 17 hours before the voltage got to 10.5. My battery seller says Rolls will not warranty replace a battery that performs that we on the 20 hour test.

I'm quite sure I don't have a significant draw on my batteries that is not being recognized by my battery monitor because when I take my batteries off the charger and leave them for 24 hours the resting voltage is about 12.7.

My questions.

1. Am I correct that this is unexpected behaviour? What voltage should I expect a properly working battery to deliver when under load?
2. Do the batteries need to be replaced?
3. What else could be causing this voltage drop?
4. What could I be doing to my batteries to have caused this?

-David Hopkins
Vancouver, BC
1989 Catalina 30

[mark_53]

My questions.

1. Am I correct that this is unexpected behaviour?

No.

What voltage should I expect a properly working battery to deliver when under load?

It depends on the load.  My voltage drops to under 10v when I engage the glow plugs.

2. Do the batteries need to be replaced?

Doen't sound like it if your getting good use out of them.

3. What else could be causing this voltage drop?

It is normal for voltage to drop when you apply a load.

4. What could I be doing to my batteries to have caused this?

Nothing, sounds normal to me.

[hopkinslaw]

Thanks for your answer. But I think things are a bit more complicated.

We know a battery's voltage will drop under load, but the question is by how much?  I'm pretty sure my battery's voltage is dropping too much.

The big problem I am experiencing is that once my house batteries are discharged to about 70% my batteries are at 12.0 volts. As I understand it, this is where you start causing damage to your batteries.

This site, for instance, suggests that under load my batteries should initially be above 12.5 volts and should remain above 12 volts past 50% discharged.
https://marinehowto.com/under-load-battery-voltage-vs-soc/

Chart Resting Voltage              Actual Under-Load Voltage
90% 12.75V                                                      12.55V
80% 12.50V                                                      12.50V
70% 12.30V                                                      12.37V
60% 12.15V                                                       12.24V
50% 12.05V                                                      12.11V

[Dave Spencer]

David,
I agree with Mark that your batteries sound fine based on the information you've provided. 
I'm no battery expert and others will likely jump in with their views but here are a few things to think about:
1.  Rolls is a premium battery.  When you bought them, you paid for technical support so I wouldn't hesitate to give them a phone call to get their views on your observations.
2. You haven't identified your charging regimen.  As Mainesail says in the post that you linked, most batteries don't die a natural death, they are murdered.  Insufficient charging voltage (or too much) will shorten the life of your batteries significantly.
3. The 11.5 amp steady load that you are applying sounds like a lot for a 230AH battery bank.  Is this a test load to conduct the 20 hr test or does this represent your house loads.  If the latter, I think your battery bank is too small.
4. How did you determine the batteries should be at 70% capacity?  If you're counting Ah consumed, it's quite possible that you weren't starting at 100%.

Rather than concentrating on if you're getting the battery's full capacity (or close to it based on the 20 Hr rating), do the batteries meet your house demand needs?  If so, keep focusing on good charging practice and enjoy your boat.  If not, you may need a bigger battery bank because getting 17 hrs at 11.5A instead of the rated 20 hrs doesn't sound like a problem to me.  You are a very patient sailor if you're willing to conduct your own 20 hr test.  I'm sure others have done it but I doubt I could stay on my boat for ~20 hrs drawing against a calibrated load.

Before condemning your batteries, I would recommend a lot of research and reading, particularly on charging protocol... there's lots out there, some is valuable and some is useless.

Let us know how you make out.

[hopkinslaw]

Thanks Dave. Contacting Rolls is a good idea.

[mainesail]

That article is illustrative of that battery at that SoH as tested with known accurate & calibrated instruments. Your batteries are at approx 85% SoH...

The real questions are:

Where are you measuring voltage?
What is the actual accuracy of the volt meter?
How is the bank physically wired?
How are you holding current steady while discharging?
What temp were the batteries during testing?

As batteries age the voltage under load will change too... Also, if your bank is only delivering 85% SoH your 20 hour load would be closer to 10A than 11.5 A...

Thanks for your answer. But I think things are a bit more complicated.

We know a battery's voltage will drop under load, but the question is by how much?  I'm pretty sure my battery's voltage is dropping too much.

The big problem I am experiencing is that once my house batteries are discharged to about 70% my batteries are at 12.0 volts. As I understand it, this is where you start causing damage to your batteries.

This site, for instance, suggests that under load my batteries should initially be above 12.5 volts and should remain above 12 volts past 50% discharged.
https://marinehowto.com/under-load-battery-voltage-vs-soc/

Chart Resting Voltage              Actual Under-Load Voltage
90% 12.75V                                                      12.55V
80% 12.50V                                                      12.50V
70% 12.30V                                                      12.37V
60% 12.15V                                                       12.24V
50% 12.05V                                                      12.11V

[Stu Jackson]

My batteries are delivering lower than expected voltage.

>>>>>>>>>>>>>>>>>>>

When my batteries are fully charged up and I put a 11.5 amp load on them, the voltage steadily drops and after 2-3 minutes settles at about 12.3 volts. After a few hours of a steady 11.5 amp load, when my battery monitor shows I should be at about 70% capacity, the battery voltage has already dipped below 12 volts.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

1. Am I correct that this is unexpected behaviour? What voltage should I expect a properly working battery to deliver when under load?
2. Do the batteries need to be replaced?
3. What else could be causing this voltage drop?
4. What could I be doing to my batteries to have caused this?

-David Hopkins
Vancouver, BC
1989 Catalina 30

David,

After a few hours of a steady 11.5 amp load  Could you detail what loads you have that could possibly be as large as this for "a few hours...?"  Let's say your fridge is running, but it's only 5A for half the time.  I find it hard to be able compute additional loads that could conceivably add up to 11.5A for "a few hours."  Please detail your loads so we can understand how you could possibly have that large a load for that period of time.  I originally thought it was only for testing purposes, but then I reread your OP.  Maybe I misunderstand.

I agree that with that load, your bank is considerably undersized.

The general experience of those of us who like to cruise, rather than marina hop or
just weekend, is that a house bank of ~400 ah works just fine.  With a daily energy budget of 100-150 ah, one could get one or two nights at anchor with that size bank.   24 hours, 100-150 ah is ONLY ~5a average! [5A X 24 hours = 120 ah/day]

Any house bank that is smaller does two things:

1)  reduces the time able to spend at anchor

2)  increases the voltage drop with any larger load on smaller bank

Without knowing your daily energy budget, and how you use your boat, it is difficult to answer your questions.  I think this is what Maine Sail was trying to ask.

[mainesail]

Stu,

The 11.5A is because he was trying to conduct a 20 hour test for SoH....