question

thomasbru avatar image
thomasbru asked

large difference in batt energy in / out on MODBUS

Hello,

I do have a MPII/5000/48 and a Polyntech 5000/48 battery. Running since 2.5 months.

Reading out the registers via Modbus I do get the following values:

Reg 76 ACin -> Batt 9.99 kWh

Reg 92 ACout -> Batt 65.41 kWh

Reg 86 Batt -> ACin 34.15 kWh

Reg 90 Batt -> ACout 17.82 kWh

Current SOC is 20% which means ~1 kWh is in the batt

That would sum up to 75.4 kWh going into the batt and 51.97 kWh going out of the Batt.

That means I'm missing 22.43 kWh (~30%).

I know there is batt efficency and consumption of the MPII, but this missing amount is by far larger than expected. Do I miss something here? Is there be another value that should be counted?

Any other idea?

Thx for your help.

Multiplus-IIbattery charging
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3 Answers
Alexandra avatar image
Alexandra answered ·

@ThomasBru

I assume you are running ESS?

Even on a keep battery charged systen you will see this happen. Each day the battery will rise up to absorption/bulk value but then when solar production stops, the battery goes back down to the float level. So not technically feeding loads is consumption from the battery.

Then there is the amount lost for balancing and system consumption and heat losses, conversion losses etc.

If 2.5 months represents about 75 days, then 22.45kwh is about 0.299kwh per day. Not so bad looking at it as a per day value.

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andy156 avatar image
andy156 answered ·

Depending on how fast you are charging/inverting then worst case can be around 85% I think. That is each way. So 85% of 85% is about 72%. So that's before the other losses that Alexandra mentioned. From memory and the graphs I saw I think it was around 1kw output/input for the most efficient settings. You want to charge from DC MPPT to skip one step from AC to DC .

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thomasbru avatar image
thomasbru answered ·

Yes I'm running ESS and thank you for your answers so far, so I shouldn't worry...

Unfortunately this 30% loss is eating up all my moneytary benefit from storing and self-consuming :-(

Are there any golden rules for an ESS system to minimize these losses?

E.g. limit the balancing losses by limiting the SOC to a max value? Or limiting the load current to limit heat losses?

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andy156 avatar image andy156 commented ·
You have two options, use it or store it. So anything stored is better than sending it to the grid or not storing it. That is unless you make more money from sending it to the grid I guess.
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