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eLim Energy Limiter Discussion

 

Comparing CAPACITY to ENERGY

Everyone has become used to the Naviga M Class battery limits and naturally think in terms of battery CAPACITY and the number of cells a pack has.

This may lead some to think that the eLim limits the battery CAPACITY. This is NOT the case. The eLim limits ENERGY.

So what is the difference?

Well, some simply math is required here but in order to keep it simple I will just consider LiPo batteries for Mono1/Hydro1/ECO Expert for now.

First of all, I will assume that the voltage of a LiPo cell when being discharged at typical race currents is 3.7V.

A typical 3S pack that meets the current weight requirements (as of Dec 2016) is about 4800mAh

So the pack CAPACITY is 4800mAh or 4.8Ah

The amount of ENERGY that this pack contains is:-
3 x 3.7V x 4.8Ah = 53.28 WattHrs or 3196.8 WattMins

If you were using a 2S pack, to give the same amount of energy you would need a 7200mAh pack:-
2 x 3.7V x 7.2Ah = 53.28 WattHrs or 3196.8WattMins

At any point in time, it may not be possible to buy a 2S pack that had a capacity of 7200mAh and so the 3S user would have an advantage. At other times it may be the other way around.

With the eLim, it is only concerned about energy and will give the same whether you are using 2S or 3S (or any other "S"), thus giving a level playing field for all.

If we now look at the other classes:-

  Mini Mono/Hydro ECO, M1, H1 M2, H2 FSRE
Cells 2 3 2 3 4 6 9
Cell capacity (Ah) 2.7 1.8 7.2 4.8 7.2 4.8 4.8
Energy (WattHr) 19.98 19.98 53.28 53.28 106.6 106.56 159.84
Energy (WattMins) 1198.8 1198.8 3196.8 3196.8 6393.6 6393.6 9590.4

 

As can be seen from the above table, there are four different energy bands which is why the eLim allows the selection of four energy levels. The default values of the eLim are as follows:-

Class WattMins
Mini 1200
ECO Expert, M1, H1 3200
M2, H2 6400
FSRE 9600

Note: As of Dec 2016, Naviga have not approved energy limiters. However, individual countries have already approved the use of energy limiters for the 2017 season and mlm Solutions suggest these values to be adopted until Naviga makes an announcement.(Update: On the 26th Dec 2016, Naviga proposed the values above on the M Section Forum)

These values were chosen to be as close as possible to typical racing pack energy levels at the moment (Dec 2016). These values can be changed with the associated programmer.

Some useful formulas for conversion between Capacity and Energy are:-

WattHr = Cells * VperCell * Ah
WattMin = Cells * VperCell * Ah * 60
Ah = WattHr / (Cells * VperCell)
Ah = WattMin / (Cells * VperCell * 60)

Nominal Cell Voltage

You may well ask, why did we chose the nominal value of 3.7V in the calculations above.

If you have ever measured the voltage of a LiPo cell when it is drawing its typical race average current, I think you will find that it is not far off.

Everyone will be aware that a new pack should perform better than an old pack. This is largely because the internal resistance (IR) of a new pack is lower. It is the IR that causes the voltage drop when drawing current.

Before the energy limiter, racers who could afford a new pack for every meeting or even every race, were at an advantage. The energy limiter removes this advantage because it monitors voltage and current and therefore the higher power being consumed would use up the energy limit more quickly. Again, it levels the playing field.

Lowering the Cost of Batteries

Battery packs that are Naviga weight legal and competitive are not common and tend to be expensive. By using a limiter and having no weight limit means that the choice of packs becomes much wider and cheaper. Also by selecting a higher capacity pack means that the racer will not have exhausted the energy in the pack at the end of the race and so the life of the pack will be much greater.

Imagine using one pack for a whole season.

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