Battery size

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Rory

(@rory)

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Topic starter 19/12/2024 4:00 pm

What size of battery?

Here are a few posts that you should take a look at:

How big should my battery be?

Battery Cycle Life

As you now should have an informed view you will want to share the load between batteries and also keep a higher state of charge, not use it all.
So if your daily use is 10Kwh of power, then a 10kwh battery is going to have a 100% DOD. that is 2000 cycles or 5.5 years.
Now lets say you have a 20Kwh battery, your DOD drops to 50% that becomes around 6000 cycles or 16.4 years.

it is important to work out the life and cost and value from the battery that you are thinking of buying.
going back to Kwh over the life you can then work out if solar is value for money.
lets look at what they may provide you in terms of £ per KWH.

10kwh - 5.5 years 20,000 kWh £4,800
20kwh - 16.4 years 120,000 kWh £28,800

its seems a no brainer that having more battery makes more money savings, and if we use the £10kwh is £1500, then we can see what our costs will be and also the returns we can expect. There are some caveats to this, such as temperature control, and charge and discharge rates which will effect the battery life.
For the math I did use a unit price of £0.24. but I would assume that the cost of power will rise over the next few years so that saving will increase.

The other consideration is that is just the cycles, we have to consider charging costs, this may be solar, and if you are in the UK, you may find you are grid charging - so that could add a cost of between £0.05kwh and £0.11 depending on your provider. if you want to work the numbers yourself, solar should be able to provide power for 275 days of the year, the remaining 90 days should be considered as grid charged.

The next consideration is current in and out.

Loading, or the current in and out of the battery will effect its life. we have posted up research into the charge and discharge power of the batteries and how the battery life is effected, needless to say a lower charge and discharge current is good.

The current out will be set by the inverter power and your loads.
Doing the math, is rather simple. You take the watts and divide this by the voltage of the battery.
for example 3kw is 62.5amps on a 48v system. (3000w \ 48v)

We apply the load current to the battery Ah rating, so our 200ah 10kwh battery would be discharged at .32c.
The C rate for batteries varies and this also concludes the rating (AH) of the cells, but the variable is 0.2c and 0.5c
That's just under a quarter, and a half. Going beyond these will lower the AH of the cells.
That means lowers the battery capacity.

The .2C rate of a battery means it can discharge in 5 hours. The C-rate, or discharge rate, of a battery is a measure of how quickly it can release its stored energy. The C-rate is expressed as a multiple of the battery's capacity, and the higher the C-rate, the faster the battery can charge or discharge.

Here are some other C-rates and their corresponding discharge times:

1C: The battery can discharge in 1 hour
0.5C: The battery can discharge in 2 hours

The discharge time of a battery is also affected by the load size. For example, if a 48V 200Ah battery is discharged at a 2C rate, it could last for 10 hours. However, if the battery is powering a 2000W air conditioner, the discharge rate will increase and the battery may only last for 5 hours.

When charging.
If you have access to cheap rate power from the grid and are grid charging, you should Lower the current to fit the window.
You would need to know the discharge to make a accurate plan, as most batteries should not be fully discharged and therefore you are not having to cover a full cycle.

You should also be aware that a 80% charge is quick, and the last 20% takes longer as the battery chemistry slows things down.

lets use a more wide charge requirement.
20 kWh battery is discharged 14 kWh over the day; so we know we need to add 14kwh and our charge window is 6 hours long.
14000\6 is 2.33 kW charge. which is 48.6 amps (DC)

A lot of people get confused here as there is DC and AC currents. you would not need a AC current of 48.6 amps as that is on the DC side, the AC for the same power is 10.3 amps (AC).

Solar charging.

Charging by solar is a little more complex as you would need to do power factoring from the array to the battery voltages.
The current from solar varies, so how much you are Charging also varies.

lets say you have 8 panels, they are 37 volts and 13 amps (STC) it is a single string array.
Array VOC 296v and our system voltage is 48v.

The Power factoring is 6.17. This means that the current from the panels would be 6.17 times more.
for example 6 amps of current from the array would give you a 37 amp charge current to the battery. 13 amps of current from the array would give you a charge current of 80 amps.
In winter you may see charge currents of just 3 to 7 amps

Charging with solar is variable but we can use certain hours of windows though the year. 8 being summer peak, 6 being general, and 2 being winter.

Sun Hours and Seasons: How to Angle the Panels — RenewSolar

OFF SET

Probably one of the more complex math's and planning is off setting as the solar charges rather than the grid you would need to carefully balance this to ensure your lower grid charge is covered by the solar.

We always ask customers for various loads in time frames though a 24 hour period, during solar time we want to power the loads ( home power use) from solar but we also want to charge the battery. this means that we need to have the solar match the requirements.
lets say you have a 500w load through the day from 9 to 5 that's 8 hours and our charge window... we will them say we want the 14 kWh of charge back into the battery, and we know we need 2.3kwh of solar per hour. We then add the load making the power requirement 2.7kw per hour. we also want to add 35% to the array size. This gives us an array size of 3.82kw

If you absorbed this information you would have noticed that we have taken away the day loads. so if you got a reading of 14kwh of power without solar, then you would see that we put this to our array calculation and took away 4kwh of power use. We do need to keep this in the battery size as solar changes and we still will have the load, but in summer you will not be using that power from the battery. So you save £1 per day! But you will use the power to do washing or something else.

GOING BIG.
The bigger the battery the better it will be, that is a myth, you are better to have a number of smaller batteries grouped together.
its all about load sharing and a little about redundancy. if you have 4, 5kWh batteries. you could have 400amps of current which is 19.2kw of inverter, though 5.5kw by 2 or 8kw by 2 is more practical over a massive single inverter.

Lets say you have a 8 kw inverter which is 166 amps, each battery takes 41.6 amps of current You would likely want to have 8 packs to drop the max current to .2c but equally you could have 6 which is 27 amps per pack. 0.27c rate

with 6 packs you get 30kwh and we also have a single 15kwh battery. 15kWh Battery 48v — RenewSolar

You would save money buying the bigger packs and you get the same output of 400amps (being limited from 580amp) and each battery load is 83 amps, but the C rate 0.32 and therefore within spec.

So the size of the battery should be greater than the user load per day with consideration to the DOD. The pack size or number should be considered for both the I (amps) in and out to the specification of the battery along with the costs per option. further consideration will be based on DOD through the day based on the sun hours and load off set, but winter should also be considered, if grid charge is available and off sets though the year as to give some estimates to the life of the battery and stresses on the systems in respect of use over the year.

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