1.0 rule battery capacity workaround?

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slaute asked Aug 28, '22

1.0 rule battery capacity workaround?

Hi,

Sorry if my question has already been answered elsewhere. I’ve thoroughly read Victron’s 1.0 rule document and MultiPlus II specs and manual and tried to find an answer to my question in this forum without luck.

According to the 1.0 rule, the battery capacity of an AC coupled ESS should be 4.8 kWh per 1.5 kWp solar installed on the AC out side.

My question is, could a smaller battery capacity be used if the charged current is appropriately configured on the MultiPlus II? If not, why?

For example, Pylontech’s US2000C accepts 25A at 48V continuously (1.2kW), 89A (5.3kW for 1min) and 200A (9.6kW) for 15s. I don’t understand why I couldn’t use it with my 3.8kW PV system if the charge current is well set to 25A. Even in the event of a surge because of abrupt load change during off-grid operation, the battery would accept up to 5kW, giving enough time to the MultiPlus II 48/5000 to raise the frequency and reduce the PV inverted output.

Thanks in advance

battery capacity

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Answer 1 · 2022-08-29T00:06:08Z

marekp answered · Aug 29, 2022 at 12:06 AM

The "1 to 1" rule applies to the power of AC coupled PV inverter and MultiPlus power.

Battery has no part in it.

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Answer 2 · 2022-08-29T00:55:09Z

Guy Stewart (Victron Community Manager) answered · Aug 29, 2022 at 12:55 AM

Hi @slaute,

The rules outlined in the AC-Coupling guide (https://www.victronenergy.com/live/ac_coupling:start) are in place to give the most certainty possible for a reliable system.

A system designer is technically able to override them, but must do so with the EXPLICIT understanding that they are outside the tested specifications, and ANY system instability must first be assumed to be a consequence of their design choices.

This is obviously a terrible situation and one that almost all system designers will avoid at all costs - because they are then fully responsible for potentially redesigning the system if there is any instability.

From my own experience, the software controls are responsive enough while within those limits.

Output from a PV array can increase 100X in a few seconds in the right conditions, and those documented limits factor in an overshoot buffer to account for that. When the consequences are a system shut down and a power outage, the risks of installing outside specification are not worth it.

Please stay within the limits specified for known reliable system design.

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