the-salmon avatar image
the-salmon asked

Battery voltage drop when high load is connected


I have installed the following off-grid system, which has been working pretty good for one year.

  • 4 300W solar panels
  • SMARTSOLAR MPPT 150/35 48V
  • 8 Acid Lead batteries of 6V each (Deka GC15)

However I'm wondering if the behavior I see when I plug a 1200W water heater during the central hours of the day is expected.

As you can see from following graphs the battery voltage drops when load is active and jumps back to an higher value afterwards.

Reading other posts on this subject I've got this is expected indeed, but my question is: do I have to worry about this voltage drop? In other words: should I power on the AC generator if the battery voltage goes down to 46V like in the picture below?

Thanks a lot,




MPPT Controllersbatteryoffgridvoltagelead-acid
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3 Answers
michelg avatar image
michelg answered ·

Hi The-Salmon ! I also use a 1500W water heater , but I have performed several tests to avoid using too much power to heat the water. It runs usually 20-30 minutes, to heat the water at a temp of 80C, 10L, which is more than enough for my needs.

So, seeing a drop in Voltage is absolutely normal, as you are using around 30A for a long period. What is missing is the power provided by your array of panels. As far I can see, it seems that your battery bank is being loaded by the mppt, but the missing power to heat the water is coming out of it.

I am afraid you will discharge them too much and this will create sulfation. Any chance to reduce the hot water consumption ? so it will equally reduce the need to use too much power from the battery bank.


PS: according to the 2nd graph, you are taking up to 24A from the batteries, so this means not enough power is provided by the PV array, and the batts are compensating it.

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the-salmon avatar image the-salmon commented ·

Thanks for your answer @MichelG. See below the graph showing the power output from the panels (4 x 300W). You've got the point: as the weather was cloudy that day I had to take a lot of current from the batteries.

Decrease hot water consumption isn't an option (we are a family of 5...) so my idea is to add another panel array to double the power.

As far as you know, during a Voltage drop (like the one in the figures above) what is the minimum voltage to which it can become dangerous for batteries? 46V?


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michelg avatar image
michelg answered ·

Hi Matteo, I can tell you about my situation : I have 4 arrays of PVs (4x5x140Wp) south orientated, and 2 arrays of PVs (2x6x140wp) west orientated, using in both configuration an angle of 60 degrees (optimized for winter should be 62 degrees), which brings me more hours of sun during the winter. Consider doubling your PV installation in order to get enough power and not killing your batteries...

My setup has also vacuum tubes heating water system with a 250L reservoir. Even in winter the water is not "so cold or cold as it should be", which means you will use less energy from the battery bank to heat the water, as it will be already pre-heated. Here is the system during its installation. Using solar electricity to heat water is the worst scenario, every Watt you store in your batteries has a high cost. "ith vacuum tubes, you have a great solution, even if in winter you will have to use the water heating system in conjunction, but much less than before !


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the-salmon avatar image the-salmon commented ·

Hello Michel.

Yes you're right, I think your system is the best solution, I thought about it too.

Unfortunately I have some constraints in my case:

  • Space constraint to place heating water panels
  • My house is used only during the weekend, so I'm afraid I could have problems with overheating in the summer and freezing in the winter

Right now my idea is to double the PV installation adding another MPPT to better support electric water heater during the summer. For next winter I plan to install a wood-fired water heater to be used instead of the electric one.



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raulag avatar image
raulag answered ·

Hi. 3 weeks ago I installed a PV system. I have 9 x 390W solar panels (3 series of 3 panels), an inverter Easysolar 5000VA with an MPPT 150/100 inside, a smartshunt and 2 x 100Ah Lifepo4 batteries connected in series. During these weeks I learned a lot from this community, as the installer didn't know that much, so thank you for that!

My problem/concern is related to the power drops that appear when a (medium I would say) load of 1100W appears. You can see from the graph below how the battery voltage drops a bit (not much I think, as they are brand new), but the current drop is significant and most of the time my lights are flickering and my UPS is intervening.
What can I do? Do you think that 100Ah is not enough to coupe with a load of 1100W? Could be something wrong in my installation? The electrical connections seem good, all the cables from the inverter to the house electrical panel are new.


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nickdb avatar image nickdb ♦ commented ·
What batteries are these? What are their specs - especially constant discharge current?

A 5kVA (4000W) system can pull a fair amount of power 80A or more and that is probably way above what your batteries are capable of.

It isn't the capacity rating (though 100AH isn't a lot) that is the issue, it is the ability to deliver that power.

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raulag avatar image raulag nickdb ♦ commented ·

Hi nickdb. Sorry for my late reply, but I didn't receive any notification about your answer to my post.

I'm using Victron Energy batteries (LiFePo- Smart 25,6V/100Ah) . They have the following specs:
-> Maximum continuous discharge current: 200A
-> Recommended continuous discharge current: <100A
It appears I'm way under these values, as I saw current drops no bigger than 25A, but still, when a load bigger than 800W starts to consume, the light bulbs are flickering, even if the batteries are fully charged.

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nickdb avatar image nickdb ♦ raulag commented ·
Flickering lights can be something else. Some LED's are just sensitive, the cheaper the driver the worse it is, this is not unusual on inverter systems. That is not a major voltage drop, I would suggest you experiment with different LEDs and/or dimmers, chances are you will find a combination that works.
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