ruben40870 avatar image

Batteries voltage falling rapidly?


I have the following configuration:

  1. Victron Energy Multiplus 12/800/35-16
  2. Two Omnipower OPR120-12, 12V 120Ah Deep Cycle AGM/Gel Hybrid VRLA Batteries (connected in parallel)

I use the system for my desktop and TV set during power outages, they are quite common where I live. I have specced the system to last at least 2 hours as that's usually how long our power outages last. The average power draw for my desktop and Tv set fluctuates from 260W to about 310W. I use the Victron MK3-USB interface on my laptop to monitor the system.

The problem that I have been having is the voltage dips very quickly under this power draw, it typically starts at around 13.5V-13.7V (Float Voltage) and within 20min falls to 11.5V. I have my shut-off setting set for 11.5V but I know the minimum voltage for these batteries is 10.5V. At 11.5V the State of Charge according to the Victron App is still 97%.

The only thing I can think is wrong is one of my settings have to be wrong? I have added screenshots below of all my settings. Any advice will be greatly appreciated.








Thanks again!

MultiPlus Quattro Inverter ChargerAGM Batteryvictron community
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Just to add. Max charge current is 25A according to the specs with 10A recommended, yours is set too high at 35A. Your voltages seem fine.
I set the charging current to 35A since I have two batteries in parallel, so the current splits to 17.5A each. I know it's higher than the recommended but at that rate, it can take too long to recharge the batteries before the next blackout.
2 Answers
kevgermany avatar image
kevgermany answered ·

How many cycles do the batteries have on them? And how deep do you normally doscharge?

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The batteries are around 18 months old but I doubt more than 150 cycles. I only use them occasionally, and I kept the max discharge not below 50%. I only hit 50% a few times, about 20 times.

Without a shunt don’t assume the soc is accurate.

Also, imho, 50% is low to draw lead acid down to with any regulatory.

From the numbers quoted it doesn’t surprise me that they lasted 18 months.

I have seen less.

For most of their lives, I kept the voltage cut off at 11.5V. Looking at the datasheet of the batteries I'm not so sure anymore if that even is 50%.
nickdb avatar image
nickdb answered ·

The most common reason will be that the batteries are on their way out.

If you want the longest life out of them, you normally don't want to draw them down below 70%.

I have a number of inverters with a similar setup where the batteries lasted 1 year. And that was around 150 cycles.

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It would really suck if they were on their way out, they are practically still new. Surely not more than 150 cycles. And they were quite expensive.

Some things to consider.

Looking at battery specs, their capacity drops to 100AH if you discharge over 10 hours, 79Ah over 3 hours and 64AH over 1 hour.

So depending on your outages you have way less capacity than you think.

With rolling outages, batteries may not get to properly recharge - these all count as cycles.

So if you were discharging over say 3 hours, you would have at total of 158AH available at 12V. To discharge 40% of it (to save the battery life) you are left with 63AH or 758WH total which equates to a usable load of 252W (on the DC side, or 226W AC side) for 3 hours.

At 300W on the AC side, you are consuming 330W on the DC side or 990WH/82.5AH.

So you are putting a fair load on those batteries.

When it comes to batteries, especially lead acid. More is more.

I would say the 3-hour discharge is the most accurate for my case. But at the moment these batteries arent even lasting half an hour at 300W or less at the AC side.

From the comments here it seems the batteries have gone?

That is a fair conclusion.

You will probably be better off with a small lithium battery. It will cost more but will last.

I have been looking at a local lithium battery with a capacity of 1400kwh. It might be a bit of a stretch for my energy needs, but the bigger 2800kwh is a bit out of my price range. What do you think?

As well as what Nickdb said, there's extra stress/wear from the high charge rate. Ideally you should charge at 0.2C or less, for your capacity this is 24A. But your charger will put 35A through.

Given your use, might be worth paying out for lithium. Or getting lead carbon, which aren't much more expensive than AGM.

The charge rate is above the recommended 10A but still below the max of 25A if you take into consideration the fact that the current splits as the batteries are connected in parallel. So only 17.5A when bulk charging.