Scale Soaring UK

Hi!

I'm new to large, scale RC gliders and was wondering if someone could advise a good option for my RX battery.
I'm building a 5 metre Flamingo with up to 7 servos (and possibly more things like a vario in the future).
I understand having too low a capacity battery can cause various issues, but can too large a capacity also cause issues?
Is it the norm to have a backup battery system for our precious machines?
Surely it's better to replace some of that lead ballast with a bit extra battery capacity?

Thank you!

I'm using a Futaba 6K and haven't chosen a receiver yet.

Hi,
Yes, extra battery capacity is a better alternative to lead ballast & having two batteries with redundancy is a good option just in case one fails in the air.
(Easiest redundancy connection method is via a high current diode in each battery line to the regulator/Rx.)
You need to consider the highest voltage that your Rx/servos can tolerate, else use a voltage regulator to ensure that the voltage is always (especially with freshly charged batteries) is within limits.

Although 4/5 cell NiMn packs were the standard way to get 4.8/6.0v nominal supplies, 2cell LiFe packs often tend to be used nowadays as they hold their voltage very well & the voltage is ok for ‘high voltage’ range gear without needing a regulator. I haven’t looked up Futaba 6K gear, but the spec will show maximum supply voltage. Note that the servos get same voltage as on the Rx battery input.

However, note that any Lithium based technology pack will need to be charged using out of the model (balance lead connection needed) & an appropriate charger.

Re: the battery backup diode -> is that something you can get over the counter or is it something we need to make ourselves?

Jeti makes a battery isolator, available with either normal servo style plugs, or with green Mpx plugs
https://www.esoaringgadgets.co.uk/index ... cts_id=553

You can easily make you own battery isolation device if you can solder wires together & I recall Robbie Bridson putting a connection diagram up on another thread, so probably worth looking through threads to see if you can see another battery query.
The diodes are easy enough to source through eBay. You need something to cope with highest current, voltage will not be an issue.
Something like these should be fine: (connect anode to battery +ve & link cathode/bar symbol ends of both diodes together & connect to Rx battery input).
https://www.ebay.co.uk/itm/18410786720 ... SwKdNeDMbk

Highest voltage battery will then connect through to the Rx & if either fails (extra low volts) then the other (higher voltage one) will automatically power the Rx.
You just need to check both batteries at the end of the day to see if which one(s) have been used.

There you go:

The 5A diodes are quite adequate for a standard seven servo set-up but a greater number or a digital installation will need a higher value. Much bigger than you need is fine - they'll just run cooler.

Also, as there is aprox. 0.7/0.8V drop across the diodes at full load, five cell Nimh packs are essential......or you could go for Schottky diodes with their lower voltage drop but even these can be up to half a volt - give or take.

The high capacity Nimh AA size cells can often have a high internal resistance, so while great for low load application, the voltage can drop significantly under load, the Sub C size cells just shrug off a high load. Must admit I am on the keep it simple side, I use a single good quality 5cell Sub C Nimh or 2s LiFe pack either plugged directly into the receiver or using an electronic switch, with telemetry telling me the receiver battery volts, but my biggest model is only 4.2m.

warning - Really dumb question! - thanks for your patience.
I prefer to use a 5cell NMH packs - nice and simple.

Just to be clear am I to understand that by wiring in a diode into each pack it's simply a matter of plugging in the second pack into a spare RX port? - is it really that simple? won't 2 x packs into the RX be too high a voltage for the RX?

Again sorry for the basic question.

cheers
Stephen

Stephen,
Think of the diodes on each battery pack as a one way valve - so it only passes current in one direction (to the Rx).
Also, think off battery voltage as a water pressure level.
What happens with the diodes is that the voltage (pressure) seen at the Rx is the highest of the connected sources, rather than the sum - so the highest voltage will never be any more than that of either battery.
(In practice, the Rx voltage will be 0.5V or so less than the battery voltage due to the bit lost in the diodes (diode drop).

If you have 2 identical voltage batteries connected then both will power the Rx, but if one discharges faster than the other, or suddenly fails, it will effectively be disconnected from the Rx as current cannot flow backwards through its diode. This feature effectively sums the battery capacities available, but ensures that the Rx is still powered if one battery fails.

Peter

You can get diodes here:
https://www.phoenixmp.com/acatalog/Elec ... ories.html
£1.50 a pair.

Regards

Rob

To simplify things Steven, connecting 2NIMH batteries each with a diode will not increase the voltage, that stays at 5.5 volts, it's the capacity that doubles up I.e. 2300 man to 4600 man.
Hope that helps

Geoff

Thanks Geoff, Peter & Rob,

Really appreciate your help and it now makes perfect sense. Cheap insurance and redundancy I think, given the low cost of NMH these days. I was just
concerned about risking/shorting a new Spectrum RX (AUD145.00) given the cost.
Now just need to worry about pilot error!

cheers and soft landings
Stephen

I really don't understand why anyone would use nimh these days, lipos are a bit of a faff, but they don't sag under load and you know exactly where you are with them.

I have never had power/ rx issues since I started with dual lipos/ jeti maxBec setups, and no black wire either....

Ell.

Jeti do a very nice and cheap solution for 2 separate power supplies to the receiver with diode separation

https://www.esoaringgadgets.co.uk/index ... cts_id=553