Technical - Rechargeable Batteries and Y-Leads

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If you are new to radio sailing then you have probably started off by using 12 ordinary alkaline cells: 8 in the transmitter and 4 in the boat pack. To continue in this way is of course the expensive option, to both your pocket and the environment, and once you have converted to rechargeable batteries you wont spend anything on powering your winches from one year to the next!

 

Differences between  Alkaline Cells and Rechargeables.

The key difference between alkaline and rechargeable AA cells is that alkaline cells provide 1.5 volts whereas the rechargeables only provide 1.2 volts.

This difference in voltage does not affect the transmitter; this will work just as well on 8 x 1.2 volt rechargeable AA cells as 8 x 1.5 volt alkaline AA cells. When it comes to the boat pack however, the difference in voltage will affect the speed at which the winch is able to sheet in the mainsail. Only using 4 x 1.2 volt rechargeable batteries will mean that the sail takes longer to sheet in, thereby wasting precious seconds in a close racing situation.

The solution to this is to add an extra cell to make a 5 x AA boat pack. As can be seen from fig.1, this takes the form of a specially made unit where all 5 batteries are connected together underneath a water resistant covering with one single lead protruding for connection to the boat.

NB From a safety aspect rechargeable cells have MUCH lower internal resistance so can deliver the stored energy very quickly if shorted out and WILL get very hot in seconds. Keeping an eye on leads and connectors for signs of wear or corrosion is an important and necessary maintenance job.

 

 

Fig.1 Rechargeable Boat Packs

How long will Rechargeable Batteries last?

The capacity of rechargeable batteries is rated in milliamp hours (mah). Typically cells used in the RC Laser will range from 700mah to 2000 mah. The higher the number the longer they will last before a recharge. For a 3 hour session 700mah will be enough for the transmitter batteries, although as they get older they may not quite have the capacity to stretch this far; 2000 mah should easily see you through a whole days sailing. The batteries used in boat packs typically range from 1000mah to 2400mah, again the higher capacity pack will easily last all day whereas the smaller pack will be better suited to a 3 hour session.

Charging

Besides a boat pack and a set of transmitter batteries you will also need a charger. The charger pictured in fig.2 is supplied by K-Bits who are the UKs leading supplier of all things electrical in model yachting. The charger has three outputs to which leads can be connected. The leads then plug into the boat pack and transmitter as shown in fig.3 (transmitter cells are charged whilst in the transmitter itself).

The outputs on this charger are 80ma, 140 ma and 180 ma which are roughly 10 percent of the capacity of the cells: as the charge rate is only 10 per cent of the battery capacity, the batteries are not damaged if the charge time is accidentally exceeded. In order to fully charge a battery the following calculation is used:

Capacity(mah) /charge rate(ma) x 1.4 = charge time in hours.

E.g. for a set of 1800mah transmitter batteries on the 180ma setting:

 1800/180 x 1.4 = 14 hours charge time.

With the charger pictured in figs.2 and 3, if a 2100mah boat pack is used with a set of 1700mah transmitter batteries then both can be charged from flat to full in 17 hours.

When fully charged the battery pack will start to feel slightly warm indicating that the charging current has finished its chemical conversion and is starting to dissipate the current as heat.

Fig.2 Charger with three charge rates

Fig.3 Charging Transmitter and Boat Pack

Y-leads

As mentioned above, the speed of the winch is of paramount importance when racing the RC Laser. The winch itself is standard and under the one-design rules cannot be modified. However, many skippers use a Y-lead in order to move a bit more power from the battery pack to the winch. As can be seen from fig.4, the conventional set up is for the battery and winches to be connected to one another via the receiver. However, when using the battery input on the receiver the BEC (battery eliminator circuit) prevents anything more than 5 volts moving from the battery to the winch. This limits the current that the winch can use when it really needs it.

A Y-lead allows you to plug both the winch and the battery into the winch slot on the receiver (fig.5), thereby bypassing the BEC and allowing the winch to receive more power.

 

 

 

Fig.4 Receiver with battery, winch servo and rudder servo all plugged in to separate sockets.

Fig.5  Y-lead takes both the winch  lead and battery lead into the channel 2 slot on the receiver. Rudder servo still goes to channel 1 slot as usual.