Limiters explained

Energy Limiters explained

If you have been wondering what “limiters” are all about then this document should provide you with the answers.


Back in 2015/6, in fast electric model boat racing to Naviga / MPBA rules, the problem of poor LiPo battery life had become a major problem.

The rules limit the battery weight to 560 gm for the Mono2 and Hydro2 classes, 280 gm for the Mono1, Hydro1 and ECO Expert classes and 110gm for the mini classes. The quest for ever greater performance leads to competitors pushing the LiPo packs to their limits by charging to maximum voltage (4.23 V), discharging to or below their minimum voltage (3.0 V) and drawing very high currents from them.

At the same time, manufactures produced packs with ever higher capacities but they have become less robust with lower C ratings.

The result is that the life of packs can be very short and as the cost is high (>£50 per pack) the cost of racing has become a concern as it was felt that some modellers were leaving the hobby due to the high costs.

Also, despite what it may say on the label, not all batteries deliver their rated capacity and if they do, they don’t do it for long. This means that not all racers are competing with the same amount of energy.

The governing body (Naviga) therefore investigated alternative methods of avoiding overstressing the LiPo packs so that much greater life can be achieved. One method that they investigated was the use of energy limiters. Energy meters were also investigated but were discounted.

The design brief was to produce a device that was small, accurate and relatively low cost. Discussion took place as to what should happen when the allocated amount of energy had been consumed and it was decided that it should reduce the speed of the boat gradually to zero and that it should remain disabled for a period of time before re-enabling.

Alternative designs were produced by three suppliers which were extensively field tested during 2016/17. Much debate occurred within the racing community but at the Naviga presidium in August 2017, the vote was taken to allow the use of limiters alongside the existing Naviga rules batteries. The decision on energy levels and other rules was deferred to allow feedback from all country leaders.

Based on this feedback, a questionnaire was issued in November 2017 which had a deadline of the end of November.

Of the 21 countries eligible to vote, 15 responded with the following result:-

Energy Limits
Mini Classes – 20 WattHr
ECO, Mono1, Hydro1 – 58 WattHr
Mono2, Hydro2 – 116 WattHr
FSRE – 174 WattHr
Frequency of update to limit – Once a year
Calibration in the field – No
Set a Min or Max battery weight – No
Ramp-down Time – 5 secs
Dead time – 60 secs
Check voltage before race – Yes
Check voltage after race – Yes
Do we accept the current types of limiter – Yes
Do we accept new types – Yes

These decisions will be incorporated into the Naviga rules from 1st Jan 2018.

What does an Energy Limiter do?

The energy limiter is an electronic device which is placed between the battery and the rest of the boat electronics (ESC, motor, Rx etc.). It is also connected between the receiver throttle output and the ESC input.The limiter constantly monitors the instantaneous voltage and current so that the power (Watts) can be calculated.

This is logged over time to determine how much energy has been consumed (WattHrs).

The limiter is pre-programmed with the amount of energy that the boat will be allowed to use in the race. When this is consumed, the maximum throttle signal that is passed to the ESC will ramp down from the maximum to zero over a period of time. At the time of writing, this is 5 seconds.

The limiter will keep the throttle demand to zero for the “dead time”. This is currently 60 seconds.

After this time, the throttle restriction is removed so that the racer can return to the pontoon.

How does an Energy Limiter help?

The limiter is connected to a larger more robust battery but only allows a pre-determined amount of energy to be used. Because the battery is not fully discharged, it will now last much longer and they will now deliver exactly the same amount of energy each time to all competitors.

How does Energy (WattHr) relate to Capacity (AmpHr)?

Racers are familiar with battery capacity which is printed on the label of all LiPos, for instance 5000 mAmpHr (ie 5 AmpHr). This means that it could deliver 5 Amps for 1 Hour.

Now this is not the same as energy. Clearly, a 2S 5 AmpHr battery doesn’t contain as much energy as a 3S 5 AmpHr battery. All that is required to calculate the energy in the battery is to multiply the AmpHr number by the nominal battery voltage. Therefore a 3S 5 AmpHr battery contains

3 x 3.7 V x 5 AmpHr = 55.5 WattHr (if we assume the cell nominal voltage is 3.7 volts)

Using this formula, the following table shows some approximate equivalence for the 2019 limits.

One of the major benefits of a limiter is that battery life can be much improved. In order to ensure this, the battery must have a capacity which exceeds that used, by a reasonable amount. 120% is a good starting point but will of course need to be modified depending upon what batteries are available.

Suggested Capacities for use with Limiters

x1.2 AHr
x1.2 AHr
21(1260) 1.892 2.270 2.838 3.406 mini classes
(NADs / MPBA /  Naviga)
60(3600) 5.405 6.486 8.108 9.730 M1 / H1 classes
(NADs / MPBA / Naviga)

Energy Limits (for 2020)

The table below shows the energy levels approved this year for the various organisations and clubs.

1-(MM/MH) 21(1260) 21(1260) 21(1260)
2-(M1/H1/ECO) 60(3600) 60(3600) 60(3600)
3-(M2/H2/CAT) 120(7200) 120(7200) 120(7200)
4-(FSRE) 180(10800) 180(10800) 180(10800)

Note: Values shown are – WattHrs(WattMins)

Author: Martin Marriott
Updated: 24 Nov 2019