question

lazza avatar image
lazza asked

Implications of AC-coupled Off-Grid with Lithium-ion battery bank

Hi

We're currently sizing up systems for Off-Grid AC-coupled Fronius with Victron Quattros. For example:

  • 20kVA of Victron
  • 20kWp PV (obeying Factor 1.0 rule)
  • 16 x 2.5kWh modules of LiFeP04, eg BYD 2.5

I undestand that you need to have the Victron inverters scaled-up to be able to take the full power of the Fronius grid-tie inverter during frequency shift adjustment delay, but I'm not sure if this also affects Li-ion battery bank size.

In general i'd like to know:

How does the AC-coupling combine with Lithium ion batteries. Are they more or less robust than their Lead-acid counter-parts?

What precautions should I take when sizing up an AC-coupled Off-Grid systems with a Lithium-ion battery bank?

Thanks

Lazza

AC PV Coupling
2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

3 Answers
boekel avatar image
boekel answered ·

You have to make sure the batteries can accept the peak charge current from your PV installation, and than I'd add some extra to make sure you're not always on the maximum.

So look for 'continuous charge current' and also if it can accept this during the complete SOC range (some batteries cannot handle this when low or high in the SOC range)

Anyway, it usually doesn't make a lot of sense to have a battery that can only handle two hours of solar input, so most of the systems aren't to harsh on the batteries.

2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Guy Stewart (Victron Community Manager) avatar image
Guy Stewart (Victron Community Manager) answered ·

This is not the answer to the questions you have asked, but you have inspired me to address another issue with AC-coupled solar on lithium batteries.

Please read the updated BYD-Victron manual "Known Issues" section, about

10.4 Self-Startup from Low voltage shutdown modes


2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Paul B avatar image
Paul B answered ·

yes the battery output is quite important. example for Lithium Phosphate style Battery

I have seen batteries that are suppose to be 12v 200A/h but continues output is 100amps or 0.5C and charge rate 50 amps or 0.25C Peak current 250 5 sec


and another top brand brand same size 12v 200A/h but continues output is 400amps or 2C and charge rate is also 200 amps. or 1C

peak current 1000 amps 10sec


big difference,



2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.