Li-ion storage capacity vs C-rating


Please note that this is not an exhaustive discussion of battery charging rates and is intended more for the average buyer to understand the power limitations of many Li-ion batteries on the market today.

Lead acid batteries and their C-ratings is outside the scope of our narrow discussion here. This discussion is purely with respect to Li-ion batteries.

The C rating of a battery is a measure of how fast the battery can be charged and/or discharged.
The storage capacity of a battery is measured in kWh (sometimes in Ah, but in the Li-ion world we tend to talk more around kWh's). A kWH is the same as 1 Unit of Eskom electricity.

A rating of 1C means that the battery can completely discharge its stored energy (from fully charged) over a period of 1 hour.
A rating of 0.5C means that the battery will need two hours to completely discharge its stored energy from fully charged.

In practice, this means that if you have a battery like the well-regarded Pylontech US3000C, which has 3.5kWh of storage capacity and is a 0.5C battery, the most power you can draw from it continuously will be 1.75kW. This is because the 0.5C rating will not allow you to discharge it any faster. Limiting the discharge power like this is generally done to protect the battery cells and ensure a longer battery lifespan.

The downside of this situation is that if this battery is paired with an inverter capable of delivering say, 5kW output to the load, the inverter will effectively be limited to the 1.75kW that the battery can offer, when the system is operating under battery power alone.

This may not be a problem if your load never exceeds 1.75kW, but if it does you will need another of the same battery in parallel with your first battery in order to boost the output power to 3.5kW and then a 3rd battery to get the total to a theoretical 5.25kW of output power if you need the full 5kW on the load side. Only when you have 3 of these batteries in parallel, will the inverter be able to deliver its full 5kW of rated power. At this point your stored energy in the battery will have gone up from 3.5kWh to three times that amount, 10.5kWh

In the case of a 1C battery, for example our Hubble AM-2 battery, the discharge power is higher. Since the battery stores 5kWh of energy and is rated at 1C, a single battery can discharge at 5kW continuously, so it will work very nicely with a typical 5kW residential inverter. There is no need to add additional batteries just to up the power delivery. In the case of the Hubble AM-2, you would add more batteries purely for the energy storage capacity, not because of the C rating limitation.

So, when sizing a system for yourself, you need to take both the number of hours you want the battery to run the load for, as well as the highest continuous (more than 1 minute for example) power draw of the load.


Example 1:

I just want to run my TV and a few lights for 3 hours during load shedding plus say another 3 hours just in case there are longer lasting blackouts. I have calculated that my TV draws 350W and the lights total 50W.
In terms of storage capacity, I therefore need (350W +50W)for 6hours, which is 400W x 6 = 2400Wh, or 2.4kWh of storage capacity.
This means that a 0.5C Pylontech US2000C is pretty much ideal since the storage capacity of this battery is 2.4kWh (actually only 0.95 x 2.4kWh, since it discharges to a depth of only 95%, but close enough).

But will one battery have enough discharge power to run these items? The answer in this case is an easy yes.
Since the US2000C is rated at 0.5C, the inverter can continuously pull 0.5 x 2.4kW - 1.2kW from the battery.
The load only require a constant 350W + 50W in this case, which is only 400W, so we are we'll within the C rating limit.

Example 2:

I want to run a large coffee machine and a microwave at my coffee shop, but I have calculated that altogether, they will only operate for about an hour during load shedding, since they are not on all the time.
The coffee machine needs 2.5kW of power to run and the microwave needs another 1.5W's for a total of 4kW continuous power draw.
Over one hour of use, this means that I need a battery that stores at leat 4kWh of energy.

The Pylontech UP5000 battery looks like a contender as it stores 4.8kWh of energy so that will do the job.
But what about the continuous power draw required? Will this battery be able to handle it? The UP5000 is a 0.5C battery so it maximum continuous power delivery is then 0.5C x 4.8kW = 2.4kW.
This is too low to run both my coffee machine and my microwave!
I can add another UP5000 to get the power delivery of the 2 batteries up to 4.8kW which will cover both appliances, but then I am actually paying for a whole lot of additional storage capacity that I don't need.
So, in this case, I would be better off looking at the 1C Hubble AM-2 which has a storage capacity of 5kWh's and a continuous power delivery of 5kW (5kWh x 1C).
The Am-2 in this example meets both my capacity and power draw requirements on it's own and I do not need an additional battery in this scenario.