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[Raen]

Hi All,
I am wiring up a new/old build (started long ago) and have a weird issue as follows:When trying to send the motor in reverse the indicator light on the RX changes from green to red and the motor steps instead of drives in reverse.All seems to work perfectly when operating the motor in forward direction.When the motor is stepping the rudder servo also does not function, or it functions sporadically.
The battery is a 6V SLA, ecu is Action P80 with BEC and motor is a 540.

Any ideas on what is happening here?

I have sketched up a diagram of the setup:

[Shipmate60]

What radio set are you using.


Bob

[Raen]

Hi Bob,
I am using a Futaba 6EX 2.4ghz

[malcolmfrary]

Reverse with a P80 happens when the relay operates.  If the battery voltage is at all low, it might not be operating properly.  At the same time, the BEC might not work if its supply voltage dips, cutting off the radio and any control signals that should be coming from it.  Not having the diagram to hand, I can't remember if the relay operates off the main supply or the radio battery side. 
Servo testers are great for eliminating sources of problems.  They work ESCs as well as servos.  A voltmeter is useful as well.
Futaba used to like to have their pulse logic reversed - everybody else used "longer pulse, more forward, shorter pulse, reverse".  I don't know if they grew out of that with the 6EX, but operating the channel reverse switch used to cure a lot of problems.

[chas]

The most likely cause is a poor / nearly flat battery. It's easy to check, either use a voltmeter on the battery whilst it's on load ( in use) it should be reading around 6.3 volts +. Or you could temporarily substitute the bec for a receiver battery, if the glitches go, problem found.
Charles

[malcolmfrary]

The most likely cause is a poor / nearly flat battery. It's easy to check, either use a voltmeter on the battery whilst it's on load ( in use) it should be reading around 6.3 volts +. Or you could temporarily substitute the bec for a receiver battery, if the glitches go, problem found.
Charles

Yes, thinking further, and hoping that memory is right, I think that the relay runs off the logic supply to ensure that it doesn't give the coil too much heat to get rid of.  If the BEC is only getting marginally more than it needs to regulate from, under load the voltage out might dip, putting the relay into the "not sure" area of operation.
Any ESC with a relay must be set up so that under normal (forward) running, the relay is not operated, as it imposes an extra load on the receiver supply.  A separate RX battery can be run down quite quickly if the relay is permanently operated.
If the glitches are cured by having a reliable power source, the misbehaviour becomes a warning that the sailing session is almost over.

[john44]

Hi I must agree with the above, most likely a faulty/ undercharged battery.
Have you tried reversing the motor whilst measuring the voltage across the battery
with a multimeter to see if the voltage drops below 6V


John

[Fastelectrics]

The minimum input voltage for the BEC regulator is 6v, so you will be running close to the minimum voltage for the device. In addition to checking the battery voltage, also check the connectors for anything that could cause some volt drop under load. Over-long connecting wires could also cause problems.

[Raen]

Hi All,
Thank you for the replies, i popped the SLA Battery on charge and the problem went away.Good to know if/when it happens again in the future that I am low on juice.

[malcolmfrary]

Remember to check that the relay only operates for reverse.  I don't remember the exact figure, but it puts an extra 100mA on the load.  Doesn't sound much, but it is an extra load and does cut into the run time.  And then you get left with reverse and no forward.

[chas]

I'm glad it was a simple solution, I have a couple of suggestions. If the sla is a few years old, it might not be holding charge so well, so check again in a couple of weeks. If it needs replacing, I would change it for a 7.2 volt nimh battery, these work differently, and even a lower ah pack will run the model longer without the risk of losing control.
  When sailing with a dodgy sla, you will most likely have a problem accelerating from stop to full speed, this wants more amps from the battery than it can give, so the voltage drops and control disappears. If this happens on the water, stop, wait a minute, then come to the bank on minimum throttle..
  I hope this helps, I've lost count of the number of supposedly faulty radio installations I've seen that were down to a shortage of volts.
Charles

[Raen]

It is an old SLA I am using for testing, I was considering either NIMH's or a 6.4v lifepo battery. Are the Lifepo batteries suitable for use in a scale tug model?
https://www.jaycar.com.au/6-4v-4-5ah-lithium-deep-cycle-battery/p/SB2200

[chas]

Electrically, the lipo or nimh battery will work perfectly well, but there are other considerations. Firstly both are lighter, especially the lipo, so you will need to re ballast the model. Choosing between the two it's worth bearing in mind how these batteries are used. Lipos are great for light weight and delivering high currents, important in a very fast boat, but not an issue in a tug. Couple that with the need for a strict charging and maintainence regime in order to be safe, a lipo wouldn't be my choice in your model. A nimh ( and suitable charger) will have a longer life, won't need a safety storage bag, and is easy to use on a grab and go basis.
    Basically, lipos are great for fast models but need the user to be very aware of proper care, nimhs more forgiving and would suit your tug very well. 
    Whatever you choose, safety first, with good neat wiring is important, modern batteries hold a lot of energy.
Charles

[malcolmfrary]

What chas said.
It does not matter what type of battery is used to drive a load, as long as the battery supplies the required voltage and can sustain the current needed.
The load does not care what chemistry is going on inside the box that is supplying the volts and current, as long as it gets enough of both.
A consideration that LiPo batteries introduce is the need for either ESCs that give early warning of imminent discharge or an extra warning circuit.  That's apart from the extra care in charging needed.  LiPo batteries of whatever type have the characteristic that they give full power until they don't, when everything stops very suddenly.  Older battery types give plenty of warning because performance drops fairly gently as the battery discharges.
Like chas said, the best option for capacity, weight and ease of looking after is NiMH.

[john44]

I totally agree with the above advice, go NIMH again with a suitable (charger).
Weigh the old battery, weigh the new battery install the difference in ballast.


John

[roycv]

Hi to perhaps expand on the Chas comment a bit about SLA and Nimh batteries.  The SLA (lead acid) battery has an Amp hour rating based on its ability to deliver the current over a 20 hour period.

So a 7 Ah SLA will give the full amount if it discharges at 0.35 amps (350 m amps) over a 20 hour period.  If you want more current it will deliver but not as effectively and some of the battery power will go in heating up the battery itself.  You will not get a constant 2 amps over 3 and a half hours as you might expect.

The NiMh and Nicad battery is rated at the 5 hour rate, so a 7 Ah battery will deliver its capacity in 5 hours so could run at 1.4 Amps for 5 hours and deliver full capacity.  Please note that lead acid Cyclon cells are also rated at the 5 hour rate.

This gives the very real difference in current delivery of the 2 types of battery and why SLA batteries appear cheaper.
REgards
Roy

[Colin Bishop]

Roy, you don't mention that if you deep discharge a SLA battery then there is a very real risk of damaging it. I always regard SLAs has having 50% of their nominal capacity in practice as deep cycling is not good for them although some lead acid batteries can be deep cycled more than others. NiMH cells can deliver almost all their nominal capacity and be recharged without damage. And, as you say, they are much better able to cope with high discharge currents.

Colin

[roycv]

Hi Collin take your point my arithmetic bit was for making the point.  I no longer use SLA, but when I did I ran them until the boat limped back and then immediate re-charge on returning home.  I do not use LiPos because I forget and NiMhs batteries are very forgiving in this respect.

I have a couple of 6 volt packs of 4Ah 'D' Nimh batteries that seem to go on forever they must be at least 20 years old.  I also have a Watt meter which registers the amount of charge a battery receives or discharges, which points at dodgy cells in a battery pack.
Regards
Roy