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muzungu avatar image
muzungu asked

Minimum Battery Requirements Again! How is it calculated? A thread to clarify this for good

Hi.

I have looked at the other threads but they are all project specific so I decided to get some information on the minimum battery sizing requirements as listed by Victron for AC coupled systems. The 1.0 factor rule is well explained but the battery storage is not well explained.

In section 3 here: https://www.victronenergy.com/live/ac_coupling:start they say that you need a PV:Storage ratio of about 1kW PV: 5 kWh or 1.5kW PV:5kWh


For Lead Acid:

1 kWp installed PV power requires approximately 5kWh of battery:

  • 100 Ah accubank 48 Vdc

and for Lithium Ion:

1,5 kWp installed PV power requires:

  • 100 Ah accubank 48 Vdc


Could I please get a detailed response to this maybe with an example (also for the future readers of this thread) so that the reasoning can be understood? In some places it says 'for system stability' in others it says 'for battery lifetime'. Neither is a very good answer for an otherwise detailed knowledge platform.

My thinking on this:

Assuming the batteries could get charged by full power in an uncontrolled manner through the AC coupled PV inverter, for a 1 kW PV to a ~5kWh Battery@100Ah/48V, you would be charging at about 0.2C which is low enough. And for Lithium Ion at about 0.3C which is also quite low.

What would happen if we only had for e.g. 5kWh Lead Acid for 5kW PV output? The charge rate would be 1C, which is high for lead acid but not dangerous.

So what am I missing? Why such a high minimum battery capacity? If the PV power is high enough and the battery is only used to form the grid (but not discharging much since the load is met by PV), then it would be good to save money on storage and instead invest it on PV.

Thanks for spending your time on the answer and for sharing your knowledge.

Stay safe!

M


AC PV Coupling
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2 Answers
Daniël Boekel (Victron Energy Staff) avatar image
Daniël Boekel (Victron Energy Staff) answered ·

Hi @Muzungu

The system needs that battery size to be able to run well, a too small battery will cause overshoot in voltage and therefor can damage the batteries and inverters.

1C charging will damage any lead-acid battery, and when the battery becomes more charged, it will not be able to absorb any peaks in charge current.

These peaks happen when loads suddenly drop, if the load drops while PV production is 100%, it will take quite some time for the system to increase the frequency and throttle back PV production, all this time the batteries have to take in the excess current, or the system will fail with overvoltage on DC and or AC.

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muzungu avatar image muzungu commented ·

Ok. So its battery power capacity then. Got it! Thanks,

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energy avatar image
energy answered ·

Thanks Daniel. I want to ensure I get this. The answer therefore is the battery capacity must be able to absorb transient conditions while the MPPT chargers cut back. I don't really care if they are slow on the take-up.

I can limit the battery charge current, so if I set the charge current limit very low, say 10A on a 48v system with a 4.8kWh battery 1C charge rate capacity, then that current can increase to 100A during the transient while the charge controllers are being cut back.

That is 90A x 48v = Call it 4 kWp PV power. So then the ratio between PV and battery works out to 120% battery power to PV power (in this case).

I understand that you have to set a safe general rule, but understanding it rather than follow a rule is more in our South African nature. The reason for wanting to push my luck is because I need a system that does not need batteries for now. I am putting in batteries because the end-game system will have batteries but the Multi must have batteries connected from the start. Its about the available budget for now. Else I have to start with a grid tied system, throw it out and then go to a Victron architecture in 2 years from now. That is a waste.

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Daniël Boekel (Victron Energy Staff) avatar image Daniël Boekel (Victron Energy Staff) ♦♦ commented ·

Hi @energy@greenpro.co.za

We're talking about AC coupled PV here, not DC coupled.

Please note you'll still need batteries capable of handling charge and discharge currents, but overvoltage risk is much much smaller with DC coupled systems.

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