question

allswell avatar image
allswell asked

Trying to understand how much my genset can output

I have this 9000w genset and I'm having a hard time figuring out how many amps it is able to output while charging my system. It can output 120v or 240v.

What math I'm using is this so far but it's such a big genset, it must be able to output more than 60a???

9000w - 20% inefficiency = 7200w
My multiplus 24v/3000w/70-50 is 120v so the math should be 7200w/120v = 60a output.

Is that correct?

Right now I'm charging and I can see that my system seems to only be accepting 20a from the genset while charging! What? How can I fix this so I can get more charge into my bank in less time and gas spent?

Help!

STEPS I HAVE TAKEN:

Following this document by Victron, I have:

Deactivated Power Assist in the VE.Config
Changed the AC Input Current Limit to 31a based on their equation below:

(a 5kVA generator typically provides about 4 kW
80% of 4 kW is 3200 W
3200 W / 230 V volts = 13.9 A
Set the AC Current Input Limit to 13 A)

Mine looks like 9000w - 20% as above = 7200w / 120v = 31a

But I'm not seeing anything different at all.


PS: Please ignore the "stopped" in the genset info. I don't have it set to auto start/stop yet so everytime I manually start a charge, it still reports it as stopped on the VRM.

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Generatorcharge current limitinput currentNot sure - Admin please assign or create
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3 Answers
Alexandra avatar image
Alexandra answered ·

@Allswell

Check the charge current in the charger tab. But i think for your lead acids 60A is max anyway?

60a x 27v (battery) is the 1700 is watts you are seeing.

If your loads arent higher then it wont draw more from the generator. Remember what the system draws from the generator is battey charging and loads total.

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allswell avatar image allswell commented ·

Thank you @Alexandra for commenting. How can I find that max amperage for my batteries? Here is the spec sheet for mine. I have 235a batteries totalling a 705a bank. This is what I have in my VE Config at the moment:

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Alexandra avatar image Alexandra ♦ allswell commented ·

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allswell avatar image allswell commented ·

Hey @Alexandra, I'm needing clarification on this one. Our genset is capable of outputting 90a and the batteries are capable of accepting 70a but the multiplus has an ac input limit of 50a. The load (house use) that we are using is very low from what I understand. What I'm still stuck on is why when charging is the genset showing that it's only outputting about 20a? If I can get that number to go up, my charge time would reduce and thus would save us gas. At this rate, we are charging each and every day for about 6 hours a day and it's costing us a fortune in gas.


This is the info I'm referencing that I'm confused by:

screen-shot-2022-11-30-at-92836-am.png

If it can go up to 50a (says the multiplus and it's limit), then does this mean we are using 30a in our house while charging? I assure you, we aren't. That would be 3600w of use (30a x 120v). Our max useage for 24h is tops 2800w but we aren't running the genset for a full 24hrs. We are running for about 6hrs which at the rate of 2.8kwh would be 700w = 5.8a

So where are the missing amps and how can I get them devoted to powering the charge of the batteries while the genset is running? By my math, there are about 30a missing?

What am I getting wrong here?

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Alexandra avatar image Alexandra ♦ allswell commented ·
The short of the story is that the inverter cannot use the tull power of the generator. And it is already charging at the maximum it can to the batteries.


Your systen will never be able to use or pass through the full power your generator can provide. It can only use or pass through 50a of the amps the generator can provide.



I think the main confusion here for you it the 70A going into the battery does not translate to 70A from the generator.

Why I was explaining using Watts, it is it is the unit of power that is useable (or can be understood) across AC (normal electricity) and DC (battery and solar) power.

So you have to think of everything in Watts. Watts is Volts x Amps. W=V x A or V*A

9000w is what the generator can provide. (9000W ÷ 120V is 75A)

6000w is the max the inverter can transfer it use. (50A [input limit] x 120V is 6000W) so it already cannit use the full power of the generator.

The max the inverter can charge at is 70A x 24Volts that is 1680W (or 1890W at 27V)

So 6000w [max input] minus 1680w [max charging] leaves 4320W for normal loads such as water heating or other. If you aren't using that power on other loads then it won't draw it from the generator.

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allswell avatar image
allswell answered ·

Thanks again @Alexandra. So what you're saying is that I should actually be charging my system at an AC input rate at 11.75a? I'm getting super confused. Our generator is capable of 70a, and the inverter is capable to take 70a, but the charge setting regarding the charging amperage should be 11.75a? At that rate, wouldn't it take me more than 24 hours to get anywhere charging it? I'm honestly trying here so hard. I'm sorry for being so taxing. I keep working on this situation day after day and I feel like I'm getting nowhere at all.


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Alexandra avatar image Alexandra ♦ commented ·

Your batteries at the DC level can only charge at 70A max (limited by the inverter) but it is using 24v system. So we need to translate it to AC.

24vdc x70A = 1680Watts / 120v ac = 14A so if there are no AC loads (house loads) then the most you will see the inverter pull from the generator is 1680watts.

A very simplified approach you batteies to charge up to 28v so there will be a bit more watts drawn at that voltage. (70a x 28v= 1960W) although by then the current should be dropping off to move to absorption charge.

If you are now running a 2400W load in the house loads plus charging at max you will see the system pull 4080W from the generator.

The battery charging amps on the datasheet is for one unit, you multiply that by the number of batteries in parallel.

You said you have 705AH so at C10 that is 70A per hour. Or C20 that is 35A charge per hour.

Ag ain a very simplified explanation as battery resistance adds to the mix and a few other factors.

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allswell avatar image allswell Alexandra ♦ commented ·

Thank you so so much @Alexandra. You have a talent for explaining things and making them make sense for sure. So much appreciated. So you're saying that the AC input current limit should then be 35a?screen-shot-2022-11-30-at-91819-pm.png

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Alexandra avatar image Alexandra ♦ allswell commented ·
@Allswell

You can leave it at the 35A. It should more than cover your loads and charging at the same time.

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allswell avatar image allswell Alexandra ♦ commented ·

I have three sets in series to get the 6v batteries to 24v, and then those three sets are in parallel. How does this affect your statement, or does it @Alexandra? "The battery charging amps on the datasheet is for one unit, you multiply that by the number of batteries in parallel."

In this case then I'm seeing I should multiply that 11.75 c/20 rate by 3 because I have three in parallel so that would put me at a c/20 charge rate of the 35a that you advise for best longevity in my new bank. @JohnC has mentioned that I can go up to 70a without any problems, and then @kevgermany has clarified along with you that the multiplus I have cannot go higher on ac in than 50a so the question is, what is the difference between your 35a recommendation and maxing out the ac in at 50a as @JohnC suggests to do (he actually suggests 70a is fine but the system is maxed at 50a, hence why I've adjusted his 70a to 50a)? Am I understanding correctly that it means a faster charge, but at the price of the life of the bank being shortened by a faster charging when they prefer it slower?

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kevgermany avatar image kevgermany ♦♦ commented ·
To add to @Alexandra excellent answers, the AC in is also limited by design. The last figure in the model is 50, max 50 Amps AC in.
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Alexandra avatar image Alexandra ♦ kevgermany ♦♦ commented ·

100% the 50 on the end of 24/3000/70-50 means that the max input is 50A.

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allswell avatar image allswell Alexandra ♦ commented ·
Ugh. When I bought the multiplus they said the max ac in was the 70 number, not the 50. So, trusting the community, thanks for clarifying they were wrong. That sucks. Thanks for educating me.
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kevgermany avatar image kevgermany ♦♦ allswell commented ·
You can check this in the datasheet or manual for you unit.
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R avatar image
R answered ·

Do you know what the C rate (max charge current) for your batteries are?

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allswell avatar image allswell commented ·
No, nor do I know how to find it.
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JohnC avatar image JohnC ♦ allswell commented ·

Hi @Allswell (again, sick of me yet?).

Follow the links in the one you posted. It's here.. https://rollsbattery.com/wp-content/uploads/2019/11/Rolls-Battery-User-Manual.pdf

10% (A per Ah of capacity), but you can go higher, so 70 A is fine. While there you'll see Temp Comp at -5mV/cell/degC, so your -32.4 should be -60. Take some more time and check what they say about multiple parallel strings voiding warranty. We've been there talking Absorb V, Rolls know too.



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allswell avatar image allswell JohnC ♦ commented ·

: ) Never sick of you @JohnC and always grateful for everyone's help. I read the fla section again and for the life of me I cannot find any statement where it says the c/20 rate for my batteries is this... or that... It refers to it, but it keeps it a mystery: "For Flooded models, the recommended Bulk charge current is 10% of the AH capacity of the battery bank, based on the 20 Hr AH rate (C/20) (min 5%, max 20%)."

That's great, but what is the information that this is referring to to make that sentence make sense to me: "based on the 20 Hr AH rate (C/20)"???

Here is the spec sheet for my particular battery. It states the rate there (I think?):

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So given what @Alexandra said about the max ac in that the Multiplus can take is 50a, then trying to charge at 70a seems an impossibility with my current inverter.

I'm really not understanding the math on the 11.75 c/20 rate and how to get that to the correct number to charge the system at. Because I have a 24v system should I just multiply the 11.75 by 12? That's 141a which can't be right.

Can you help me understand the math so I can get it right in future with a new bank?

Thank you!



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kevgermany avatar image kevgermany ♦♦ allswell commented ·

Again, adding to @Alexandra excellent answer.


You need to do the voltage conversion before comparing currents.

50A input at 120V is equivalent to 250A at 24V, ignoring conversion losses etc. This is why output to the batteries can be at a higher current than the input current.

On current through batteries, the same current goes through each battery in your installation. A bit like water in a pipe. If you add another length of pipe (another battery) no more or less water goes through the extra pipe. But because of the extra pipe, it's harder to push the water through. Volts are like water pressure. Amps are like water flow or current.



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JohnC avatar image JohnC ♦ allswell commented ·

@Allswell

That 11.75A is just the amount required to apply for 20hrs to get 235Ah. It's irrelevant otherwise, as you can discharge/charge higher, so it's not a limit. The rating is applied down to some unholy V that you (hopefully) will never see anyway.

The other guys have pointed out that you're confusing the Amps at different V's, so I won't go there.

To get the most from your genset, schedule your loads to run when you plan to run the genset. Good time to run things like dishwashers, clotheswashers, water heaters, even a radiator. No, offgridders don't need to skimp.

But that's not an expensive genset. Call me cynical if you may, but I'd be putting it to the real-world test before complaining you can't use it's full output. Watch it's holding up V and Hz, which are often adjustable, but just as often not. The quality of an AVR doesn't sell gensets, and can fall short, limiting usefulness. I hope yours lives up to it's promises.

Back to your batts. Fix that Temp Comp figure, and please lower that Absorb V. Rolls don't recommend parallel strings, so they don't offer recommendations for dealing with them. I say go down to like 28.4-28.6V Abs and be careful how long you apply it. They'll still work fine. Imbalance is the issue, and you'll never know it's happening unless you measure that or get into the habit of touch-testing for their Temp after a serious charge session. This is your eventual destiny, as that's how they'll likely fail. I'd just like to see you to get some decent time in before you experience that.




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Alexandra avatar image Alexandra ♦ allswell commented ·

Your bank capacity divided by 10 hours is C10. Rate. Your bank is 705AH so 70A is the C10 rate of charge.

So capcity divided by hours ( 10 or 20) for the C10 or C20

Your inverter can accept 50A AC power and charge at 70A DC power (battery Voltage) max.

The 64 to 69A you see at battery level is pretty decent.

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allswell avatar image allswell Alexandra ♦ commented ·
@Alexandra, yet again my mind is blow with the ease of understanding I have at what you've written. Thank you for making that make sense for me! Major breakthrough!
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