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Author Topic: AC coupled battery performance - how good should it be?  (Read 620 times)
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« on: October 19, 2020, 02:45:00 PM »

I've just had a 5.8kW PV system + 4.8kW of Pylontechs with a Solis battery inverter installed. It's all running well but the performance of the battery system is very disappointing. I've contacted the installer but they don't seem very interested, they say they're raised the issues with Solis but I've heard nothing. There are 2 issues:

1) The Solis inverter is not accurately recording the grid current - its typically about 70W out, but can be as much as 250W out, this is resulting in over/under discharge of the batteries by the amount of the inaccuracy. I have no experience of other systems so I don't know if this is normal or not, but I do know my SolarEdge PV inverter is very accurate at reading the grid current. I'm wondering what other AC coupled systems are out there and how accurate they are in terms of charging/discharging based on the grid current.

2) The other problem is that I'm only getting about 72% efficiency out of the batteries in terms of discharge power v. charge power. I'm thinking it should be more like 90% - what do other systems deliver? I think the problem might be that its impossible to stop the battery inverter using up power for the backup system, even though there is no backup system, I've turned the option off, but it looks like the backup circuitry is still draining 30Wh from the batteries. I don't know if anyone else has one of these battery inverters (SOLIS RAI-3K-48ES), the installer recommended it, but I'm now wishing I'd opted for the Sofar ME3000SP....

I'd be grateful for any perspective on this as I don't have a reference point. Thanks.
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« Reply #1 on: October 19, 2020, 08:31:51 PM »

If its measuring the grid current via a cable clamp they can be quite inaccurate especially so at low power. I had a Growatt battery system which was terrible, as bad as your experiencing. The Sofar ME3000SP was much better and my Sunny Island is good too.

In terms of battery efficiency, the datasheet for your inverter says, Max Battery charge efficiency 94.0% and Max Battery discharge efficiency 94.5%. But it does not give you an efficiency curve like my Sunny Island, you see my Sunny Island has a max output efficiency of over 94% like yours, but look at when the output is very low, it falls off a cliff. I find my real life Sunny Island round trip efficiency is about 80%, so your 72% may not be that bad depending upon its usage.

So for a full round trip efficiency calculation, you have the charging losses from AC to DC, then battery charging losses from DC to Chemical, then battery discharge losses from Chemical to DC, then discharge losses from DC to AC. They all add up.

« Last Edit: October 20, 2020, 10:44:58 AM by nowty » Logged

12kW+ of PV installed and 65+ MWh's generated.
Useable home battery storage of 50+ kWh's.
Hot water storage of 15+ kWh's.
Storage heaters of 15+ kWh's.
EV BMW i3 (another 30+ kWh's of storage).
6kW+ Ground source heatpump.
310,000+ litres of water harvested from underground river.
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