Storage battery question

[Northspoon]

I have recently had 12 PV panels installed (Aiko 445W), a Fox Hi-5-E G2 inverter and two Fox 4800 batteries (nominally 9.32kW).

The other evening I noticed that the load was around 6kW, the battery was only giving 3.5kW and the rest taken from the grid. With the inverter of that size, I expected more output from the battery. Have I misunderstood how the system should work?

[Dave Foster]

It's not the inverter, it's the battery pack size.

The inverter has a maximum charge/discharge current of 40A, the 2 batteries you have ECS4800-H2 has a nominal voltage of 89.6V and so the max power it can charge / discharge at is 40 * 89.6 = 3,584W

If you add another battery to take it to an ECS4800-H3 the nominal voltage will increase to 134.3V and the max usable power to 5,376W

[Northspoon]

Thank you for explaining it so well. My installer never mentioned that limitation.

I have the space for two more batteries. Would adding two make the system more efficient, or just add the one to get a higher output when the load is high?

[calum]

The more packs you have for a given load, the less hard any of them need to work. Given you've got a high capacity inverter, I'd say you want as much battery as you can afford / for into the space.

Having each pack work less hard should also mean somewhat longer overall life for the bank as a whole, although these lithium iron phosphate batteries are pretty tough in any configuration.

[Northspoon]

Thank you for the help.

My installer has previously said that if I wanted another battery it would cost about £1800 including installation. Does that sound about right?

[Dave Foster]

Thank you for the help.

My installer has previously said that if I wanted another battery it would cost about £1800 including installation. Does that sound about right?

The installed price will be vat free and you'll pay near enough that (inc VAT ~£1,700 delivered) if you install it yourself - it's an easy enough install but at that price I would bite your installers hand off.

[Northspoon]

It's not the inverter, it's the battery pack size.

The inverter has a maximum charge/discharge current of 40A, the 2 batteries you have ECS4800-H2 has a nominal voltage of 89.6V and so the max power it can charge / discharge at is 40 * 89.6 = 3,584W

If you add another battery to take it to an ECS4800-H3 the nominal voltage will increase to 134.3V and the max usable power to 5,376W

Just checking.....my installer says that adding an extra battery would only give 4.85kW as the 5kW inverter is only 97% efficient. I am told that to get 5.376kW, I would need a 6kW inverter. Is that correct?

[Dave Foster]

It's not the inverter, it's the battery pack size.

The inverter has a maximum charge/discharge current of 40A, the 2 batteries you have ECS4800-H2 has a nominal voltage of 89.6V and so the max power it can charge / discharge at is 40 * 89.6 = 3,584W

If you add another battery to take it to an ECS4800-H3 the nominal voltage will increase to 134.3V and the max usable power to 5,376W

Just checking.....my installer says that adding an extra battery would only give 4.85kW as the 5kW inverter is only 97% efficient. I am told that to get 5.376kW, I would need a 6kW inverter.

No that’s not correct the H1-5 has a maximum AC output of 5kW and to achieve that because of losses in the DC>AC conversion it will need your batteries to be discharging at just over 5.2kWh which they will do to meet the 5kW maximum output of your inverter.

When I force discharge at full power on my H1-6 (6kW AC output) my battery discharge is nearly 6,300 watts to achieve my max output was 6kW.

The other thing to remember is that whilst the max discharge current is 40A, I chose the ‘nominal’ voltage to do the power sum - 3 batteries actually have a voltage range of between 121.8-154.5V (between 10%-100% SoC) so the actual maximum power they can provide will be between 4,872W and 6,180W depending on their state of charge and temperature.

The main thing to note is that you will not achieve the maximum 5kW power output of your inverter on battery only with just 2 batteries, but with 3 batteries you will be able to (most of the time) and 4 batteries would be able to all of the time.

[Northspoon]

Hi Dave,

Thank you very much for the reply. I am not very technical, so rely on advice of others.....like you and my installer.

[Dave Foster]

Hi Dave,

Thank you very much for the reply. I am not very technical, so rely on advice of others.....like you and my installer.

No problem, when I start to do power maths and explain voltage ranges I know the subject has got too complex, which is why I summarised it with 3 batteries most of the time and 4 batteries all of the time.

Calum hit the nail on the head, more batteries is better for a number of reasons, they are not subject to temperature or voltage variations and the higher voltage means lower currents to achieve your maximum power so they don't work as hard.
The downside of more batteries is the extra cost- are they worth it?, you have to balance your need versus the cost of taking a bit of power from the grid occasionally, if you only need to top up the battery power with 2 kW's for 5 minutes each day you will only have used 166wh and even at peak that's only 5p of power at 30p/kWh

[Northspoon]

Well at the moment, at the end of the day, the batttery (9kW) is usually down to about 30%. Although we have 12 panels on a south facing roof, the weather has not been that sunny for a week or so. The battery is at 100% by the afternoon though.

For example now at around 6.30 pm, the battery is at 76% after using the immersion heater and doing some cooking.

The forced charge period is set for 0200-0500 as we are on Economy 7.

Before getting the solar panels, we were paying around £6 a day for electricity in the winter.

[calum]

(I wrote this yesterday and forgot to press send!)

There is a distinction to be made between the maximum DC power the battery can provide, and the maximum AC output of the inverter.

By way of example, I have x6 HV2600 modules here, giving a nominal voltage of 307.2V and a recommended discharge current of 25A (maximum of 50A).

25 x 307.2 is 7680, ie the batteries could supply ~7.7kW DC, but my 3.6kW inverter will never draw that much power. I am looking into the possibilty of upgrading the inverter as we are an all-electric household and the existing inverter is limiting more often than I'd like.

Depending on the configuration you can be in a situation where either your batteries or your inverter are the limiting factor in the peak power the system can provide. For you, right now the batteries are limiting you. For me, it's the inverter.

As to the cost - if you're paying £6/day in winter, and you (say) double your battery capacity, would you get through a day without needing to draw from the grid? If so, the additional cost might be worth it. But as Dave points out, there is a point where the additional cost of the batteries has a debateable payoff.