Charging the last 5% of our Growatt battery takes forever - Energy Storage

RE: Charging the last 5%

ChandyKris — Fri, 16 May 2025 13:14:12 +0000

@allyfish That's impressive. We are on a similar journey, we are almost all electric with heat pump and an EV. We still have the gas hob that needs replacing at some point. We were on Cosy as well after we got the heat pump commissioned towards end of January. Even with the 6.5 kWh battery, we would hardly touch the grid during the peak time as we could top up the battery three times a day. Hopefully with the added capacity we can stick to Octopus Go throughout the year.

We have a 6 kW heat pump and if my calculations are correct, we should spend about £750 per annum including the standing charges for electricity and gas. Based on my modelling, we should consume about 6400 kWh from the grid and another 2000 kWh coming from solar panels. Last year was all over the place as I was using immersion heater for DHW, infrared heaters and electric oil filled radiators until winter kicked in and switched to ASHP only in the last week of January. So, don't have real consumption numbers yet. But, looking at how efficient the Aira heat pump has been during a very cold February, I am pretty confident of my consumption model.

The house has never been more warmer, all rooms are heated throughout the day. The best part is that we will reduce our carbon emissions by 4 metric tonnes from the heat pump, EV and solar.

RE: Charging the last 5%

AllyFish — Fri, 16 May 2025 10:42:02 +0000

@chandykris our experience is that the BESS added to the solar retrospectively is a financial game changer.

During the heating season we schedule charge the batteries 3 times a day on Octopus Cosy, using the 8hrs of low rate tariff and maximising the low rate import. If we get any solar PV generation that's just a bonus. This alone shunts 85% of our energy import to low rate, with about 10% standard rate and 5% high rate. That saves us around £600 a year just in the heating season, even with a 90% round trip efficiency of charging/discharging the batteries = 10% more energy import. It utilises the batteries to our advantage when they would otherwise sit there in a discharged condition doing nothing.

During the non-heating season, i.e.: now, we're self-sufficient, importing almost nothing, paying only for the standing charge, which is largely offset by exporting surplus power at 15p/kWh. We have a 10kW ASHP and an all-electric house, and our monthly DD is £100 on Octopus Cosy, £1,200 a year. The heating is on all day every day in winter. Our annual energy import is around 6,500-7,000kWh, with about 3,500kWh additional generated from our modest 3.6kW solar PV array.

If we were importing 10,500kWh at a typical standard variable rate tariff, we would be paying around £3,000 a year. So with solar PV, BESS & Cosy we're massively benefitting. It justifies the investment in the solar PV and BESS alongside the ASHP. And of course we've massively lowered our carbon footprint compared to our old non-condensing oil boiler. We would never have dared have the heating on all day with oil, it would have been prohibitively expensive.

RE: Charging the last 5%

ChandyKris — Fri, 16 May 2025 09:53:57 +0000

@allyfish Thanks, that's good to hear from other Growatt owners that this behaviour is expected. On adding the second battery, yes absolutely no brainer, especially during the winter months for running the heat pump. Whether it's a good decision from a financial perspective is a different question as future energy prices are always changing. But, the ability to avoid importing from the grid during the peak times and reduced cycling was good enough for me. Especially given how low the battery prices have dropped.

RE: Charging the last 5%

AllyFish — Fri, 16 May 2025 09:30:33 +0000

Hi @chandykris, we have two 6.5kW Growatt batteries, they do take a long time to charge the last 5% or so. It's a reduction in charging power as the batteries approach full. You may notice this more with two batteries than when you just had one. Remember the maximum charging & discharging power is 3kW, and this is now charging two batteries, so each battery is getting half the maximum charging power it used to, so it takes twice as long to charge them in parallel. That's good as the amps in the DC cabling is half what it used to be, so the cabling doesn't get as warm! We had one 6.5kW battery initially installed than got another a couple of months later when we realised how effective the batteries are on Cosy tariff and timed charging.

RE: Charging the last 5%

ChandyKris — Fri, 16 May 2025 07:58:10 +0000

@judith I assume Growatt is doing the soft finish as well. Just that the effects can be seen more clearly with a bigger battery capacity.

The app shows the values. But given that it just charges from 90% to 96% in around 40 minutes, the graph is not very clear.

RE: Charging the last 5%

Anonymous 5011 — Fri, 16 May 2025 06:33:05 +0000

This was my charging last night. Only charged to 70% but there is a slope of reduced charge rate towards the end.

11387

RE: Charging the last 5%

Judith — Thu, 15 May 2025 21:53:44 +0000

Certainly our Givenergy battery shows the same effect with the newest software. It is deliberately designed to reduce the rate of charge so it has a “soft finish”. I assume it is expected to improve battery life, by either reducing thermal stress from full current to nothing then start discharging OR because the battery calibration is not accurate. The latter point is the degradation internally of over-charging and writing the software to reduce the likelihood of full current over-charging. Having said all of that on the Givenergy the soft finish is from mains charging, there’s no obvious such code effect on the solar charging.

Do you have software that displays the effect? Perhaps someone might be able to help more with the details to look at?

Charging the last 5% of our Growatt battery takes forever

ChandyKris — Thu, 15 May 2025 08:26:09 +0000

Ever since we installed additional capacity to our battery storage (added a second Growatt 6.5kWh battery), the last 5% seems to take for ever and in fact a lot more than what the 5% capacity should be. For example, it's been charging since 7 am as there's excess solar, and has added 1 kWh to top up like 3% SoC. But, the battery capacity is definitely not that much. I would assume it should have reached 100% by now. Experts here, any ideas what the problem could be please?

Even if I set the maximum SoC to be 95%, that works only when charging from the grid. When it is charging from solar, it seems to be ignoring the setting. The Battery First setting on Growatt is time bound (00:30 am to 05:30 am), so I assume now when it's charging from solar, it sees as the battery needs to be topped up to a 100%.