H1-3.7 & unnecessary battery charging

[ml]

Hi,

I have an H1-3.7 and 2xHV2.5 batteries. At night, they're being charged from the mains prior to being discharged (i.e. the battery level is >20% minimum that's currently set). I guess this is due to the temperature, and the inverter protecting the batteries (loft is ~5C and the batteries seem to start charging at ~10C).

There is a draw from the batteries overnight of ~170W. Charging seems to increase this to ~450W (i.e. the batteries are costing me 450W of grid electricity).

Either charging or discharging the batteries should generate heat, so I have a couple of questions:

a) How much energy do I need to draw?
b) how do I send energy in the batteries to the grid (i.e. to force the batteries to discharge without wasting the energy)?
c) would I be better off running a 300W heater driven from the batteries to warm them?

as an aside, I seem to be getting clipping of the solar array, even in these short days (5.6kW system size). Will it charge the batteries in parallel with pushing the 3.7kW to the grid or is there no way to use this PV energy?

thanks,

Mark.

ps. my intention has always been to increase the number of batteries, with the expectation I'll need a draw of zero from the grid, and the any spare is pushed to the grid.

[Dave Foster]

Hi,

I have an H1-3.7 and 2xHV2.5 batteries. At night, they're being charged from the mains prior to being discharged (i.e. the battery level is >20% minimum that's currently set). I guess this is due to the temperature, and the inverter protecting the batteries (loft is ~5C and the batteries seem to start charging at ~10C).

There is a draw from the batteries overnight of ~170W. Charging seems to increase this to ~450W (i.e. the batteries are costing me 450W of grid electricity).

Hi Mark,
Yes you are correct at cold temperatures LFP batteries exhibit a few odd characteristics, the BMS will charge randomly to maintain charge and battery health, the capacity won't be as much as normal so they will take less charge and be able to discharge less, they may not reach 99% charge - when it is cold <10C the BMS will turn down/off the charge when the battery voltages rapidly rise to protect them from damage, and if it is very cold <0C they will stop charging altogether.

All of these affects are temporary during cold spells and do not cause battery damage.

a) How much energy do I need to draw?

It will depend very much on what the external temperature is, at or below 0C they will not charge at all (the BMS will protect them) - from my experience it normally needs a 1kW load or charge to make any meaningful change, probably more, as the battery nears full the batteries will rapidly rise in temperature which helps to warm them.

b) how do I send energy in the batteries to the grid (i.e. to force the batteries to discharge without wasting the energy)?

You can't send battery power to the grid (not without some clever trickery wired to the CT clamp and that isn't recommended) - The batteries don't waste energy, any power they take will be sent back to the inverter with small losses (<10%), unfortunately the inverter needs DC power which comes from either solar or batteries to stay awake, it will draw a small amount of power (~70W) to keep itself awake which is what you are seeing as waste.

At this time of year when there isn't a lot of spare solar energy to charge them, the only way to counter this unless you are going to take advantage of economy or EV tariffs to charge fully overnight is to charge them to 50% and switch them off. The inverter will go to sleep overnight until there is enough solar power to wake up, it will then cover the house loads and feed back to the grid if there is any spare.

c) would I be better off running a 300W heater driven from the batteries to warm them?

Quite a few people have built insulated enclosures to be used during the cold months, 50mm Kingspan/Recticel etc.. is a popular choice - It is best to keep an air gap around the batteries for air to circulate.
Any heat generated by the batteries helps to keep them warm and it will make a difference, some people have also added small 50W tubular heaters attached to a thermostat (Inkbird make some very good good value temperature controllers). You really don't need much heat in an insulated enclosure, it is better to warm batteries slowly than shocking them with a large heater.

Foxess don't recommend insulated enclosures in writing (unless you speak to them, then they will) the reason for this is that insulated enclosures do work well when it is cold, but can cause the batteries to heat excessively during summer, and overheating of the batteries >50C will age them more quickly and reduce their operating life, so you should remove them when the weather really starts to warm up.

as an aside, I seem to be getting clipping of the solar array, even in these short days (5.6kW system size). Will it charge the batteries in parallel with pushing the 3.7kW to the grid or is there no way to use this PV energy?

If you only have a small battery pack, this time of the year is very frustrating as there isn't enough solar to charge them, and because of the temperature effects and the batteries being used to keep the inverter awake overnight you tend to use more power from the grid than you did before. Foxess recommend that in the colder months you charge the batteries to 50% and turn them off to avoid this.

If you get more batteries you would be able to take advantage of the economy 7 type tariffs to charge your batteries overnight (which keeps them warm) and to discharge them during the day to run the house when it is peak tariff - it's a much better way to run them, the batteries have an extremely high lifecycle and you could charge them everyday and even after 10 years they would still be near their original capacity.

This all becomes largely irrelevant once the solar increases, in summer months the batteries would get everything they need from solar and then any spare solar would go back to the grid so nothing lost.

[ml]

Thanks for the very comprehensive reply - much appreciated. the CT hack can wait until the summer months.

r.e. the cold issues -

You've suggested that the batteries can/should be turned off - that's via "enable force charge on & enable charge from grid off" options, rather than the physical switch? (significant as the former I should be able to automate).

I'll look into the enclosure option - I hadn't really considered it as it'll need removing in the summer months, but it's minimum energy, and given current data it.

[Dave Foster]

You've suggested that the batteries can/should be turned off - that's via "enable force charge on & enable charge from grid off" options, rather than the physical switch? (significant as the former I should be able to automate).

I'll look into the enclosure option - I hadn't really considered it as it'll need removing in the summer months, but it's minimum energy, and given current data it.

Turning the batteries off is a Foxess recommendation for when it's cold and solar is low; it's a physical turn off (power/isolater off), sadly you can't stop them charging with any of the app switches, it's the BMS that takes control.

It seems a bit harsh to me but I can understand if you've only got a few batteries and rely on solar to charge them, it must be frustrating at this time year when it's cold and they keep cycle charging and using power to keep the inverter running overnight. The only answer to that is to 'winterise' them by physically turning them off. The inverter will go into standby when there is no solar and wake up when there is enough to power it again.

I can't remember whether I included the Foxess battery information that comes up on the app when it's cold (courtesy of William Eccles from his test system) but attached it now.

[ml]

really useful - thanks.