How Can I Use A Low C-Rate Battery To Charge A High C-Rate Battery?

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Is the C rate of a pack adjustable relying on the design of the cell connections inside a pack? No.

Now you have to marvel, how can this be?

I can describe C charge this manner. When you’ve got a 1A hr cell, and it could discharge all of its cost in half an hour, it has a C charge of two. When you’ve got one other 1A hr cell, and it has a C charge of 0.5, it could be discharged in 2 hrs. C charge is at all times in reference to time, often hours. C charge is simply the utmost charge a cell can discharge. Its a attribute of a selected cell chemistry, and it doesn’t change for a given cell, except that cell adjustments.

Simply as an apart. We’re going to use models of amp hours, abbreviated A hr. This can be a most well-liked methodology of describing batteries and its used a terrific deal. Right here is an easy instance: Let’s select a 1A hr cell with a C of 1.

There are the ideas of parallel and collection in electrical engineering. For voltage sources in collection, the voltage provides, and the present is similar. For voltage sources in parallel, the voltage stays the identical, and the present turns into the sum of the currents. You could find this in basic electrical engineering texts.

What occurs if we put 4 cells in collection? We nonetheless have 1A, however we’ve got 4x the voltage. What’s the C charge? One. It will possibly solely discharge in 1 hour.

What if we put all 4 cells in parallel? We then have a 4A hr grouping, however solely 1x the voltage. What’s the C charge? One. It, too, can solely discharge its full output in 1 hour. All 4 of the cells can solely discharge in a single 1 hour.

You see this? C charge is barely a measure of the speed a cell will be absolutely discharged. It’s actually about time and nothing else, so it’s impartial of V and A.

In our examples, we fluctuate A and V, however the outcome nonetheless has the identical product, VxA, or energy ranking.

This is among the fascinating issues about C charge. You are able to do no matter you need with the cells — parallel, collection, and as many instances and with any form of capability. The C charge is at all times the identical because the C charge of the cell.

How do I get a low C-rate battery to cost a excessive C-rate battery?

You don’t actually get a special C charge while you clump extra cells collectively, however you are able to do one thing else about it.

A much bigger pack with a decrease C charge can ship the identical charge of cost as a smaller pack with a better C charge this manner.

Say we have to ship 100 kWh in 40 minutes. Now we have two sorts of chemistries. Chemistry A has a C charge of 1.5, and Chemistry B has a C charge of 0.33. With Chemistry A, we will use a 100 kWh pack to ship 100 kWh in 40 minutes, as a result of with a C charge of 1.5, which means it could ship its full cost in 1/1.5 hr = 0.666 or ⅔ of an hour, precisely 40 minutes.

How can we do it with Chemistry B? With Chemistry B, we will solely ship the total pack cost in 3 hours, reasonably than 2/3 hour. We are able to use a bigger capability pack in kWh to ship the cost. What measurement pack do we’d like? Nicely, we need to ship the cost in ⅔ of an hour and the pack discharges full on the charge of three hours. The ratio between them is 3/(2/3) or 9/2, 4.5. This implies a 450 kWh pack is required.

Let’s examine the maths.

450 kWh/3 hrs is 150 kWh/hr. Its can also be 100 kWh/0.66 hr or 100 kWh/(2/3)hr.

A battery pack with decrease C charge should be made bigger in energy or VxA so as to match the charging charge, or C charge, of a better C-rate battery pack. The voltage should be the identical, however the present has to extend. It is going to be an even bigger battery pack or extra cells in parallel. It requires extra cells.