Battery volt drop from inverter.

[mvweebles]

I have a small solar system with a pair of Interstate GC2 6v FLA batteries (220ah @ 12v capacity) . When fully charged to 12.7-12.8 volts and I pull 60 amps for a 700w electric tea kettle, battery voltage drops to 11.7v. I realize some battery volt drop is expected, but this seems high for just 60 amps pull. Issue is that I have another 5% drop to the inverter so is under 11v at the inverter.

Is this drop normal for FLA? Costco/interstate batteries don't have the best reputation- wonder if it's that or something else? Is there a formula similar to distance/wire gauge table based on current? Suggestions?

Thanks in advance.

Peter

[Stu Jackson]

It's normal. Higher load in amps drops voltage.

[mvweebles]

Originally Posted by Stu Jackson

It's normal. Higher load in amps drops voltage.

Thanks for quick response. Is there anyway to predict loss based on battery bank capacity vs current load? How much would this change (if any) with lithium?

[Stu Jackson]

It's not based particularly on the type of batteries but rather on the bank capacity, although some chemistries can sustain heavier loads with less voltage sag.


Think of it as %C. 60A/225 ah bank is X%. Doulble your bank capcity to 450ah and the 60A load is less.

[Stu Jackson]

Here's a link that came up on another battery thread just today:


https://www.victronenergy.com/upload...limited-EN.pdf

[secessus]

Originally Posted by mvweebles

How much would this change (if any) with lithium?

Flooded banks have relatively high internal resistance and will exhibit greater voltage sag under load (and quicker voltage rise during charging), ceteris paribus. Lithium has extremely low internal resistance and will show less sag/rise.

Some crude generalisations for comparison:

• flooded - 1.0 mOhm
• agm - 0.5 mOhm
• lithium - 0.1 mOhm

Mainesail refers to the effect of internal resistance in this article where he is talking about charging (and therefore voltage rise):

Because the LiFePO4 battery is a very stubborn source to move or increase the voltage on, the solar controller remains in bulk mode for much longer than it does with lead acid batteries... In contrast a typical AGM or flooded lead battery will come up to 14.4V significantly easier and significantly faster with the same current source.

[waterman46]

That's a reasonable number for voltage drop for that high a load and the small size of your battery. Your bank is undersized for your size boat if you have typical appliance usage on the inverter. If you want to draw 60A for even a few seconds you better have a 400AH minimum bank.
Our microwave oven draws probably about 700W and we run it from a 440AH (nominal) bank. If not really fully charged the voltage drops to 12.0 when the microwave is on.

[mvweebles]

Thanks for all the replies. Very helpful. There is always so much chatter about voltage drop across cables that it didn't occur to me that there would be a significant drop across the battery bank.

Regards

Peter

[Boatwright]

"I realize some battery volt drop is expected, but this seems high for just 60 amps pull.":

Actually, 60 amps is a large current for your battery setup.

Folks, you can't expect to plug in heat producing household appliances: coffee makers, hair dryers, microwaves, toasters, etc. into a 12v system without a big battery bank and a high amperage charging system, along with the large gauge wires required.

[thomm225]

Originally Posted by mvweebles

I have a small solar system with a pair of Interstate GC2 6v FLA batteries (220ah @ 12v capacity) . When fully charged to 12.7-12.8 volts and I pull 60 amps for a 700w electric tea kettle, battery voltage drops to 11.7v. I realize some battery volt drop is expected, but this seems high for just 60 amps pull. Issue is that I have another 5% drop to the inverter so is under 11v at the inverter.

Is this drop normal for FLA? Costco/interstate batteries don't have the best reputation- wonder if it's that or something else? Is there a formula similar to distance/wire gauge table based on current? Suggestions?

Thanks in advance.

Peter

Yes, that's a normal voltage drop for that much current.

My batteries are usually at 11.7 or 11.8 volts in the am after running the fan all night with that low current draw, and they stay at that voltage for the first few hours of daylight if I'm sailing and running the autopilot and VHF.

You might consider a propane or butane hotplate to heat up water with.

I use a propane hotplate that the PO left on the boat for making coffee and cooking.

I have two 12 volt 90 ah batteries in parallel that I keep charged with about 135 watts of solar.

They power everything on the boat which includes 400 watt inverter, autopilot, lights, Depth, VHF/AIS/GPS, fans, computers, HDTV, chargers, etc

[rslifkin]

That seems fairly normal to me with an FLA bank of that size. AGMs will handle high loads better, but in general, with lead acid, the best answer is a bigger bank relative to your load.

My 415ah (2x L16 AGM 6v) house bank dropped to about 11.8 volts when I ran the vacuum cleaner from the inverter at one point recently. And that was with the bank at something like 95% SoC. Total draw was ~115 amps, so similar to yours relative to battery bank size. It held at that voltage for the 2 minutes I had the vacuum running, then climbed back up when I turned it off.

[Stu Jackson]

Originally Posted by mvweebles

Thanks for all the replies. Very helpful. There is always so much chatter about voltage drop across cables that it didn't occur to me that there would be a significant drop across the battery bank.

Regards

Peter

Peter, it is not necessarily "a significant drop across the battery bank." While you may think of the battery as just a large wire, that's not what is happening in a battery. Rather, you are extracting a huge amount of energy from the storage medium, i.e., the battery, which is extremely small compared to that load. That's why many of us are saying your bank capacity is too small for that load.

I'm sure there are detailed chemical "reasons" for this, and that there are relationships between loads and bank capacity based on this as well as rules of thumb and experience that could guide you, if you chose to do some more research on batteries.

[mvweebles]

Originally Posted by Stu Jackson

Peter, it is not necessarily "a significant drop across the battery bank." While you may think of the battery as just a large wire, that's not what is happening in a battery. Rather, you are extracting a huge amount of energy from the storage medium, i.e., the battery, which is extremely small compared to that load. That's why many of us are saying your bank capacity is too small for that load.

I'm sure there are detailed chemical "reasons" for this, and that there are relationships between loads and bank capacity based on this as well as rules of thumb and experience that could guide you, if you chose to do some more research on batteries.

Thanks Stu, and thanks for your early replies including the link to the Victron white paper on batteries.

I guess I was hoping for some quantitative sizing guidance vs qualitative observations of "draw is big" or "battery bank is small." For example, for cable sizing, I routinely go to Blue Sea's cable-size chart to size wire/cable for a given circuit. I was hoping there might be something similar for battery drop. From my single data point, if you pull 25%-30% of the battery capacity in amps, you can expect a 8%-10% voltage drop at the posts, at least for a FLA battery. Armed with some sizing data, I could make some intelligent decisions. Again, this is in addition to loss due to cable size/length. The two losses really add-up and result in 10.7V at my inverter, a loss of 2V from fully charged batteries. I can easily upgrade the cabling which is what I will do. Was wondering if there was some science to sizing the battery bank beyond "make it bigger."

Thanks again to all -

Peter

[rslifkin]

It's not necessarily a linear relationship between load and how badly the voltage drops. In general, I'd try to keep any load more than a brief peak to no more than 15 - 20% of the C20 amp-hour capacity for FLA, maybe 25% for AGM. There will still be a noticeable voltage dip at that load, but it should be manageable.

Just how much you can get away with will depend on voltage drop in the cables as well as how well various equipment you have running will tolerate a dip in voltage.

[Stu Jackson]

Peter, there isn't. While part of that is because different boats can have drastically different daily ah loads, there have been some rules of thumb that I've seen rather consistently over the years, based on book-stuff and real life input from this and other boating forums.

For mid-sized cruising boats, say in the mid-30 foot size, daily loads range from 100 to 150 ah. Cruising boats could be "real" cruisers who are truly "out there" to weekenders who spend a weekend at anchor (not marina hoppers) or a week or two cruising locally on vacation. These folks tend to have house banks in the nominal 400 ah range, either with 3 or 4 12V 130 ah batteries or four 6V golf carts for 425-450 ah.

Whether they have solar or not is immaterial to this sizing trend it seems.

These are the patterns I have seen and, of course, there are outliers in all cases. rslifkin is also correct.

I have a 1500W inverter (Freedom 15 I/C) and when I run my small 700W microwave, I see the voltage sag to 11.9V but it rapidly returns. I have a 390 ah house bank of wet cells and have for the past 20 years.


I am sure that here are scientific answers to your specific question, but the other approach is the one of "What seems to work?" based on the use of the available. The pattern emerged...it's what works for folks. Without getting too deep...