question

allswell avatar image
allswell asked

Forced to charge every second day despite having battery bank to last 3 days between charges. HELP!

We have a 24v system on a battery bank that has 705ah. Our daily use is 2.8kw. I have no appliances that draw electricity. I have no heat generating things here. Based on that useage that's about 117ah a day from our bank. At a 50% DOD we should have 352ah available to use to use before we should need to do a genset charge (we get very little light in our area this time of year so our solar isn't what we are relying on in this season for a charge). By my math, we should be able to go 3 days between charges (117ah x 3 days = 351ah used out of the 352ah available). However, every second day I see on the Victron VRM dashboard, the voltage dropping dangerously close to 24v on our bank so I do a charge to get it up to 28.8v until the amps drop down to at least 14 charging amps.

I'm wasting tons of gas doing this every second day when it should be something I don't have to do until every 3rd. I basically get one day of use when it should be three.

The company that sold me my components keeps asking me if we have electric radiant floors or something else that we are "unaware of" that is making a "mystery draw on the system".

No, there is not. We have propane appliances (furnace, hot water heater, stove, oven, fridge), no heat generating appliances such as toasters, curling irons, etc... and all of our lights are low draw LED's.

Please can anyone help me sort this mystery out? I just can't seem to figure out what I'm doing wrong.

Our Components:

  • 24v system, comprised of twelve Rolls 6v 235ah batteries in series and parallel, flooded acid
  • VE Multiplus 24v 3000w 70a-50a Inverter
  • VE Smartshunt
  • VE Cerbo GX
  • VE Smart Battery Sense
  • VE 100/50 MPPT Charge Controller
  • 11,500 starting watts, 9,200 running watts generator (also brand new)
  • 4 Longi 370w 75 cell panels

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6 Answers
techie4hire avatar image
techie4hire answered ·

@Allswell

Well, after 10 years of full-time sailboat cruising, and another 10 years full-time in a motorhome -- I have a few thoughts on batteries. :-)

First, when it comes to flooded, gel and agm batteries and charging by generator -- living between about 50% and 85% charged is where you'll end up. This allows you to bulk charge the entire time you're running your genset, giving you maximum output for the fewest generator hours.

With these battery technologies, it just takes too many additional hours to get your batteries to float. Temperature is also a factor in battery performance, and since you get little light this time of year, I suspect it's cold there too -- with the corresponding fall-off in battery performance.

The equation changes when you have solar available of course, as getting to float is generally a daily goal. Lithium-ion changes the situation too, as you can bulk charge most of the way to 100% with a genset.

So I don't think you're doing anything wrong per se. However, I think you'll find this time of year you'd be better off with more frequent, shorter generator runs. Bulk charging and the early part of the Absorption cycle are the most productive. You will want to charge all the way to Float occasionally, but if you're like me that'll happen when you have other needs to run the generator.

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Alexandra avatar image
Alexandra answered ·

@Allswell

Not a long enough Absorption charge means the chemical reaction is not fully reversed to get to the true 100% SOC.

We ran on lead acids for over five years... Killed the first set with bad practices such as not fully charging every day (and the real full charge, including a good absorption phase).

Lead based batteries do not last long being kept in partial states of charge. You might benefit from reading this book. Wish we had it when we first started.

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allswell avatar image allswell commented ·
@Alexandra that looks like a GREAT read! Thanks for the link!!! I will devour it as we killed our first set too and just purchased a second so I really want to get it right this time! Thank you!!!!!
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techie4hire avatar image
techie4hire answered ·

@Allswell @Alexandra

Everybody's situation and use case is a bit different, but in our case anyway, the amount of time and fuel it would have taken to get our 800Ah battery bank to Float everyday would have been prohibitive. When under sail, out on the open ocean, maybe a 3 or 4 hour generator run would have been OK. But at anchor, I doubt we would have been very popular with our neighbors.

We found, that for us, a couple of 45 minute generator runs at the beginning and end of the day worked best. We could pull of fridge and freezer holding plates down, make water at 25 gph, and put 100Ah or so back into our batteries with each run.

We replaced the gell cell batteries the original owners of our boat had installed with exactly the same batteries after 2 years, and got 8 more years out of the new ones before we sold the boat -- and it wasn't like they needed to be replaced immediately at that point.

I appreciate that some books, and manufacturers may say different, but we found that those sources often fell short for those of us really putting this stuff to the test.

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kevgermany avatar image
kevgermany answered ·

+1 for @Alexandra comments about partial charging.

Some thoughts.

What's your ac input current limit set to? Could be you're limiting the generator input to the batteries. Certainly the multi isn't capable of pumping all the generator output into the batteries, this will waste fuel.

Your charge efficiency seems high, Probably more like 90 or 85% would be more appropriate.

Charged voltage/absorption voltage seem high, what are the battery specs?

What C rating is the battery capacity stated? If you have shorter periods of heavy current, this will reduce the effective battery capacity. But from your load description, doesn't look likely.

What are temperatures in the battery housing? Lead capacity falls a lot with low temps. The battery data sheet will, if you're lucky, have a factor for capacity change with temperature. Guessing this is the cause. But worth checking the rest.

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allswell avatar image allswell commented ·

Thanks for your contribution. My AC Input Current is 50a though my inverter is the Multiplus 24/3000/70-50a so it should be able to take 70a as the AC Input Current but it seems I'm limited for some reason to a ceiling of 50a. Please see below:

screen-shot-2022-11-24-at-60907-pm.png

I contacted the battery supplier and they confirmed the 85% charge efficiency so I've changed that, thank you for mentioning it.


Charge Voltage and Absorption Voltage: taken right from the Rolls Manual for fla's:

screen-shot-2022-11-24-at-62148-pm.png

For the life of me, I'm still trying to understand what a c/20 rating is. Our bank is twelve 6v 235ah batteries wired in series and parallel to make a 705ah bank. The Rolls manual details for our bank starts on the fla page at page 10. I am really struggling to find the c/20 rating. I'm sorry I can't help more there.

As for temperatures in the power house, I added a Ruuvi sensor for that. Here are the temperatures over the last week:

screen-shot-2022-11-24-at-63639-pm.png

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klim8skeptic avatar image klim8skeptic ♦ allswell commented ·

For the life of me, I'm still trying to understand what a c/20 rating is.

Have a look at the bottom left of the specification sheet for your model battery. Different discharge current rates (current @ time) results in different battery Ah capacity.

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allswell avatar image allswell klim8skeptic ♦ commented ·

Thanks so much for that @Klim8skeptic. So just so I'm clear, you're saying the c/20 rate for my bank is the 11.75a? screen-shot-2022-11-24-at-75858-pm.png

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allswell avatar image allswell allswell commented ·

@kevgermany looks like my c/20 rating is 11.75a. Where should I be entering that in the settings? I'm worried I've done something really wrong. I haven't entered that anywhere! Which device, which screen please? Any direction you could provide would be amazing. Thank you.

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kevgermany avatar image kevgermany ♦♦ allswell commented ·
This thread is getting messy, hope you see this.

11.75A is the current at the 20 hour rate. I.e. if you draw a constant 11.75A, it will take 20 hours to completely flatten the battery. In the specifications referred to by @klim8skeptic these currents are shown in the first column after the capacity You'll see that capacity decreases as you increase the current and reduce the discharge time.

You'll need to work out the current you're drawing from the batteries to decide which capacity to use. Or continue with the C20 (20 hour…) rating for now. This figure is used to calculate state of charge and time to go in the shunt/multiplus.

I mentioned it because if it's wrong, you may think there's more battery capacity than there really is with your usage patterns. From the screen shots, the 705Ah is correctly entered. (3*235).



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techie4hire avatar image techie4hire allswell commented ·

@Allswell

Actually you have used your 20 hour rate in your configuration already. 12x 6v batteries with a 235Ah rating (capable of delivering 11.75A for 20 hours) means that each group of 4x 6v batteries wired in series will also deliver 235Ah, but now at 24v. Multiply that times the 3x 24v equivalent batteries you have and you get 705Ah -- which is the number you correctly entered as your battery capacity.

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kevgermany avatar image kevgermany ♦♦ allswell commented ·
A bit confusing, but the 70 in the model number is the max charge current to the batteries. The 50 is the max AC input current. 50*120 is 6kW.

Calculating charging, 70*24 is about 1.7kW. So enough to fully charge your batteries in 10 hours, (ignoring current fall off as the batteries become charged). Generalising, this is a good rate for battery life in your system.

But you're running a big generator at low load to charge the batteries. This wastes fuel.


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techie4hire avatar image
techie4hire answered ·

@Allswell

Don't get me wrong -- I'm not saying there's anything wrong with getting your batteries to Float. It's desirable, and is very doable in a solar panel and Lithium-ion battery oriented world.

Having said that, when you're dealing with more "traditional" chemistry batteries and a generator, long charging runs have multiple things against them. Noise, inefficient fossil fuel usage and increased maintenance just to name a few.

I agree with @kevgermany that your charge efficiency and absorption numbers seem high but I don't think either are going to be the magic answer.

As I said before, I think you're battling cold conditions for your batteries along with trying to get them to Float. Trying to get that last 15-20% into your batteries can and will increase your generator run times and fuel usage by a multiple.

How about adding a wind generator or two into the equation? I'm guessing you have some wind? Yes, there's some noise, but it might be just the ticket during your long dark winters. Bulk charge with the genset and let Mother Nature get you to Float. :-)

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allswell avatar image allswell commented ·
Thanks for your contribution. Yes, we are planning to add a micro hydro trickle charge into our system as we get a lot of rain where we are and we have a creek that is in flood during the entire rainy season. Thoughts were this would offset our useage with a trickle charge and then we'd just handle a deep charge once a ?
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techie4hire avatar image techie4hire allswell commented ·
This micro hydro setup sounds interesting, and not something I'm familiar with. What kind of power output are you hoping to see from it?
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techie4hire avatar image
techie4hire answered ·

@Allswell

After looking at your profile, and assuming you're still roughly near Vancouver, I have a couple of additional thoughts:

At 50 degrees N more-or-less, you should be getting some production from your panels. This assumes decent placement, and angle for Winter solar. And as such, the time of day you run your genset becomes important.

Ideally, you want your more limited Winter panel production to address the gap between the efficiency of Bulk charging and inefficiency of later in the Absorption cycle -- while at the same time unlocking that battery capacity between 85% and 100% charged. Best for your batteries, as others have pointed out, and better for you to go into the evening starting with a full charge.

I think you'll find, as we did, this means running your generator first thing in the AM. Get that bulk charging done before the sun starts contributing. And then, let your more limited panel output contribute when your lead-acid batteries can't take as much amperage anyway, in the later stages of the Absorption charging phase.

A wind generator could help your cause too, but given your latitude, it seems like your panels should be able to do the job when properly combined with your generator.

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allswell avatar image allswell commented ·
Thanks for your contribution. yes, we are just outside Vancouver. Our panels are angled for the season at 45 degrees... will move to 50 degrees next month. Understanding the time of day to run the genset is something that is new to me because we only just added the solar a few weeks ago. Before that, we were just dependent on our genset. So you're suggesting we run the genset first thing and complete the bulk and then let the day take care of absorb if it's a sunny one? We are hoping to add some micro hydro to solve our rainy day trickle charge when the panels are not producing much. We get a bunch of rain where we are up Indian Arm. We do get wind here sometimes, but not as reliably as rain!


Anything else you can help me understand about times of day for genset running would be amazing, and thank you.

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