question

sectorsolar avatar image
sectorsolar asked

Battery Discharge more than actual loads

Good Day

We have come across this issue on our one Victron Installation. The loads that gets used is less than what the battery reports it is giving out.

Is this something to worry about?

how can this be fixed?


BMV Battery Monitorbattery48v battery
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6 Answers
H.W.Oelofse avatar image
H.W.Oelofse answered ·

@SectorSolar

This is not abnormal and is nothing to worry about. All inverters have a difference between what is being drawn from the battery and what is being used to power loads.

If you divide the load value (2169) by battery draw (2843) your current efficiency is 76%. Several factors affect this percentage. One is inverter sizing verses the load on the system, the other is internal efficiency and the another is the power factor of the loads you are running. Some cheap LED bulbs have power factors of 0.5 (not so good), while kettles are usually power factor of 1 (good).

So there are some things you can do to improve the percentage if that is what you are looking for.

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petajoule avatar image
petajoule answered ·

@SectorSolar

While it is not abnormal that you need to draw more energy from the battery than arrives at the AC loads, I would argue with @H.W.Oelofse that it is abnormal to have a loss of almost 700W in your case (plus I see a 69% current efficiency).

Especially as you seem to have a 48V system, such losses are actually abnormal and I personally would worry about it. So the questions are:

* Battery chemistry and capacity?

* DC cable battery<->Inverter: length, cross section?

* What Inverter model is it actually?


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Alexandra avatar image
Alexandra answered ·

@SectorSolar

https://www.victronenergy.com/upload/documents/Output-rating-operating-temperature-and-efficiency.pdf

I do also agree that there will be some differences between what is drawn from battery and load.

You can get losses in the system for various reasons. Inverter type, cable sizing; battery type, condition and age do have an effect on that as well as load type. Temperature in the operating conditions as shown in the whitepaper about inverter efficiency also affect this. As a note 25ºC is where both the battery and inverter are designed to work.

Since we don't have a full picture as to the installation and operating conditions there can only be guesses as to what is really happening there.

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sectorsolar avatar image
sectorsolar answered ·

Thank you everyone so far for the inputs.

@PetaJoule @H.W.Oelofse @Alexandra

We installed a parallel system for our client with the following configuration:

2 x 5Kw Multiplus 2 inverters

1 x 10Kwh 51V lithium battery

1 x 250/100 MPPT Charge controller

1 x Lynx Power in distributor

Because this is a parallel system all our AC and DC Cabling are of equal lenght for each inverter to distributor and DB

Our DC Cable used for the batteries are 35mm² from batteries to breaker and then to lynx distributor. Also 35mm² DC cable from lynx distributor to MPPT 250/100 Contoller all total lenght 3m for both positive and negative.

Our AC cable used is also same lenght and size for both inverters from and to distribution box at 16mm²


20200208-181924.jpg (532.4 KiB)
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vicky avatar image
vicky answered ·

Whilst I agree there are several factors for losses, I am not sure if I would agree to a 76% efficiency in a solar system where you usually fight for each % of efficiency, thus avoiding losses. A Multiplus 48V is rated 95% and the load shown is in a good range where efficiency shall be good too.
.
Do you have a chance to measure real values, i.e. not relying on what is visualized?
I am currently "challenging" my Venus GX as it shows approx 25% more charging current than I see in the register. Is it miscalculation or do I use the wrong register? I'll try to get decent measurement equipment to know the facts. Just a thought.

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H.W.Oelofse avatar image
H.W.Oelofse answered ·

@SectorSolar

Pretty neat install.

https://www.victronenergy.com/upload/documents/Manual-MultiPlus-II-24V-48V-3k-and-5k-230V-EN-NL-FR-DE-ES-SE-IT-.pdf

On page 11 of the manual it gives the recommended cable sizes from the battery to the inverter. A smaller diameter cable than recommended can also cause losses in the system.

Have you tried load testing the system to the max loads and checking DC Ripple. This can cause inaccurate readings in the system.

3 comments
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sectorsolar avatar image sectorsolar commented ·

@H.W.Oelofse

Much appreciated. We also believe now that the battery cables needs some upgrading from 35mm to 70mm cross sections as their load at times are pulling some heavy arsenal and getting DC ripple warnings with system tripping. We hope by upgrading these cross sections most of the issues will be sorted

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sectorsolar avatar image sectorsolar commented ·

This Document is a really useful tool and very informative Victron Wiring Unlimited.

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Alexandra avatar image Alexandra ♦ sectorsolar commented ·

Wiring Unlimited - a book we refer to often. Upgrading the cables will definitely reduce losses in the system and sort out the DC Ripple issue.

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