Hitec 815bb power


Yesturday tested my 815 with a arm 2 3/4 long in my IOM, it was sitting on its stand on the driveway as there is still ice on the pond. I installed 4 rechargeable batteries = 4.8v and the servo had problems pulling in the sails from a simulated run. How much would it help to put 4 regular batteries = 6v in? or do I have to by a 6volt pack? I am not racing but bought the sail arm because I thought it would have enough torque. I have the hitec sail winch install in my other IOM and wanted to try somthing else.

Stall Torque (4.8V): 274.96 oz/in. (19.8kg.cm)
Stall Torque (6.0V): 343.01 oz/in. (24.7kg.cm)



It is better to go to a 6v pack, as the connections between cells is better than in a battery box. For maximum current flow use Nicad or LiPo rather than NiMH batteries. You could also use a Y cable to reduce the current flowing through the Rx, but its not ‘required’.

Also, make sure to install the winch arm so that the sheet lies along the axis of the arm at closehauled. This increases the leverage in favour of the servo and should reduce any ‘pull back’ by the pressure on the sails.

If the line is attached to the hull and then through the end of the arm ( a 1:2 system), then it is important to minimise friction, so using a ball bearing block on the arm is recommended.

Has anyone ever tied a 9v battery (the square one with snap on attachments)?

9 volts through the Rx may damage it. I don’t know if you can run 9v through that servo.

I use an RMG winch that runs on 7.2v (can run up to 9v) and it has a voltage regulator that provides 5v for the Rx and other servos.

I thing there are voltage regulators that you can use.

Iw as thinking of just the little square 9v. I do not know much about batteries so I guess 9v is 9v no matter the package it is in?


Listen to John. It’s good advice.


Most of us use a pack of 5 = 6v. I always keep a standard battery holder and some new non-rechargeable AAs in my tool box for emergencies - they also add to 6v.

A nine volt battery( the small one with snaps) cant provide anywhere near the current needed.

Depending on where you live, NiCad batteries are being phased out of production. NiMH batteries have over twice the capacity/size as the NiCads and current draw, at least for servo currents, isn’t really critical.
A 5-cell pack of AA size like those on: http://www.onlybatterypacks.com/items.asp?db=7 should be fine.

In case anyone is as cheap as me I dicovered something. When battery packs(for power tools,cell phones, etc) fail it is usually only one cell that dies, You can take them apart and still use the individual cells or take out the dead one and solder in a replacement. There is a battery recycle bin at our garbage dump and there is always three or four packs laying there. Granted, the remaining cells are a bit diminished but there is no need for them to be wasted before they are used up. I recently picked up two 18v packs. One had 1 bad cell and the other had two. That leaves 27 usable cells. At $5 each—. These are ‘C’ cells which are great for my power boats. For ‘AA’ you would have to find smaller packs. Sorry to hijack this thread but I just can’t stand waste and this was and opportunity to stand on my soapbox.
We now return to regular programming

I am with you on the re use ideas, both of my IOM (not official) are mostly put together on parts scrounged from anywhere possible. What do you use to test the cells? I had not thought of power packs for rechargable tools. I will now have to keep my eye out for them. One of my greates recources is Princess Auto, they have a surplus section.


I use an old analog voltmeter. A good cell will show + volts one way and -volts the other. A bad cell will show nothing. There are probably other ways, just compare the readings of all the cells in both directions and the one or few that differ are probably bad. I have also recovered bad cells by flashing them numerous times with reverse polarity at twice the rated voltage. Don’t hold it on, just tap it. Sometimes it takes 50 taps. They don’t recover for that long though.

With an IOM sail plan you need all of the available torque from the servo. Which means you need that nominally 6V value available all the time. If you use a 4 pack of 1.5V cells the theory is that you have 6V. And you probably do but just for a short period of time when they are absolutely new. But the power drops off to under 6V nearly immediately and now you are below optimum voltage.

A 5 cell, nominally 6V Nimh pack will peak at just below 8V and then shortly afterwards sit at just under 7V but that is right in the sweet spot for these cells and you will find that the batteries stay above 6V for probably a few hours. They will continue on in the high 5V range for even longer.

Arm winch setups are a bit harder on battery capacity as they are hardly ever in a position where they are locked so there is pull on teh servo allthe time and it is always drawing some current to get itself back to the signalled position.

The other thing that suffers is the speed of travel. Although the speed on 4.8V is still pretty darn good it is oputstanding on 6+ volts.

Whether you need a Y cable or not is dependent on your receiver. Some receivers have a voltage regulator built in and those usually cut the voltage to the servos to around 5 V regardless of battery condition. If you have that design of receiver then you need a Y cable to make sure the sail servo is getting all the voltage available.

As John mentioned, it is an excellent idea to have the sheet line lay right along the length of the arm at close hauled so that the servo has a position, often during some of the higher loading, where the line and sails have less mechanical advantage over the servo.