Why Slow Charged Swap is Better Than Buffered Fast Charge – CleanTechnica

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One can analyze the differences between charging cases by examining the smallest unit of charging, the charging transaction, and comparing the cases. The charging transaction is a vehicle entering a station charge depleted and leaving with more charge. It is possible to use this transaction as a unit transaction in the same way the concept is used generally for all normalized per unit mathematics. In this case, the same rules apply to every transaction, and all effects can be modeled per unit. This gives the advantage of being able to easily determine a particular result by using the unit case and modifying it by the number of unit transactions in the particular case. This per unit concept is a well known concept.

In this case, it is applied to charging transactions, numbers of packs used, pack lifetimes, and so forth.

Cycle Life, Miles Driven, Range, Pack Size, and Pack Lifetime

All battery packs have a cycle life. Normally, cycle life is on the order of 1,000 to 4,000 cycles for packs used for electric vehicles.

Maximum range is determined by vehicle characteristics and pack size. A vehicle can be rated according to the amount of miles/kWh. If the pack size is given, the range is determined as:

Range (miles) = Pack size (kWh) x miles/kWh.

For a given vehicle, range and pack size are a given by a fixed constant proportion.

For a given chemistry and formulation, or for identical cells, and identically constructed packs, performance is equal. 

The pack cycle life is a constant. This can be referred to as cycle life.

The distance traveled for one full charge cycle is the range. In order to determine lifetime miles, we may use the formula:

Lifetime Miles (miles) = Range (miles/cycle) x Cycle Life (cycle) 

Comparing Two Cases

Let us compare two cases on a unit transaction basis. The unit transaction is described this way:

Case 1, Buffered Fast Charge

A truck pulls into a station with Pack 1 on board in a given state of charge. This station has a fast charger with a battery buffer connected to the grid. In order to reduce power sourced from the grid, the buffer storage, Pack 2 was slow charged at a given rate. In order to be certain to fully charge an incoming pack from the buffer pack, the buffer pack must be the same size or greater. 

Let us use the same pack size for Pack 1 and Pack 2. 

The truck connects to the fast charger and energy stored in Pack 2 is transferred to Pack 1. Pack 1 is charged, and Pack 2 is depleted. The truck leaves the station with its original Pack 1 now charged.

Case 2, Slow Charged Swap

The truck pulls into the station with Pack 1 in a given state of charge. Pack 1 is exchanged with Pack 2, a slow charged pack in the swap station. The truck leaves with charged Pack 2 on board. Pack 1 is depleted and remains at the station and is slow charged.

Comparing the Two Cases

Notice how at the moment the two cases arrive at the station, the onboard pack and the off-board pack can be identical in state of charge and in every meaningful way. Because of this, many observations can be made of the two packs equally and identically. It makes no difference what the remaining charge is on the entering truck pack or the off-board pack at the station. Those can be identical situations. Whatever those states are, the results will be substantially the same between Case 1 and Case 2 after a transaction, within constraints of efficiency and so forth. The dropped off pack in the swap case behaves similarly to the storage buffer pack. Both are depleted by similar amounts. The amount of charge with the onboard pack is also similar. Their results are proportional to both the initial state of charge of the off-board pack and the onboard packs at the time of the transaction start. Any differences owe to particular characteristics of the way they are charged, such as the efficiency of the charge states. Otherwise, they are the same.

In particular, we can draw some important conclusions that extend from the per unit transaction to all transactions and any multiple of the unit transaction and can base our comparisons solely on the unit transaction without need for a particular case, with no loss of accuracy.

Miles driven only depends on mobile packs (i.e., packs that are onboard a truck).

Case 1 uses only Pack 1 for mobile use. Pack 2 contributes no miles.

Case 2 uses Pack 1 and Pack 2 for mobile use.

At the point of transaction, we could take the buffer storage Pack 2 and mount it on the truck, the same as Case 1, and use Pack 1 as the new storage pack. Other than specific charging efficiencies, Pack 1 and Pack 2 would assume similar charge states for either case. There are compelling reasons to do so.

Comment: It is at this point one can see how there is no advantage to buffered storage. If both packs are there and the initial and final states of charge are virtually identical, there is no loss in swapping the stored pack for the one onboard and no gain in leaving the storage pack in place. This last part has far reaching implications. 

From these observations we can draw some important conclusions. 

First, there is no substantial difference in the initial number of packs used between buffered fast charge and slow charge swap when both mobile and stationary packs are considered. The cases both use identical Packs 1 and Packs 2.

Second, any given initial state of charge for Pack 1 and Pack 2 results in substantially the same effects from initial state of charge to final state of charge, with specific results different by different efficiencies between the cases.

Third, the charging swap pack acts the same as the storage buffer pack in Case 1, the buffered storage case. If there are other uses, like utility storage for a buffer being slowly charged, the swap case has exactly the same features.

Conclusions

The unit transaction cases prove there is no real benefit to buffering fast charge compared to slow charged swap.

Both cases start with two packs at initial operation.

Slow charge from the grid may be the same without loss of generality.

Both of the swap packs contribute to miles traveled.

Only one of the buffered fast charge packs contributes to miles traveled. Because of this, over time, buffered fast charge uses 2× packs compared to slow charged swap.

Over full operation, both use the same number of packs for miles traveled, because miles traveled is only proportional to pack size × cycle life, identical for both packs.

The buffer storage pack has substantially the same cycle life and longevity as the mobile pack, because it has the same number of cycles, one per charge transaction.

There is no utility storage function advantage for buffered fast charge. Swap has the same capability. For use as utility storage, the pack charging at the swap station behaves substantially the same as the buffered fast charge pack.

Initial conditions for Pack 1 and Pack 2 result in no difference in post-charge transaction results.

General conclusions apply also to the case where a buffered storage pack is of a different chemistry. No matter how much it costs, it is an extra pack beyond the packs necessary for mobile use, because it does not contribute to miles traveled. It is always an extra expense that swap does not have, regardless of characteristics. It must also be approximately 25% larger due to higher fast charge losses.

The loss analysis is as follows: Swap case, one slow charge. Buffered fast charge case, one slow charge to buffer pack, one fast charge to truck mobile pack. There is a 10% loss in the storage pack and a 10% loss in the truck mobile pack. Comparing the two, the slow charge cancels and the fast charge is responsible for an approximate 20% loss.

Fast charge is less efficient, primarily because of internal losses in the pack, independent of external losses in the charger. Swap losses in slow charging electronics may also be lower, or the cost of slow charger electronics may be less than fast charger electronics.

The AC/DC conversion before delivery to either the slow charger or the buffer storage is the same.