Large Model Association Forum

Hi,
Are there any makers of battery backers in the UK?
I used to use SM Services units but now they have stopped trading.

Sean.

Try Tower Hobbies
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXAFM9

Cannot find any in UK.

Robin

Just plug one battery into each end of the receiver, job done & much cheaper.

If I recall from technical articals in the mags, you need a diode in one of the lines to prevent the low battery feeding of the high battery. Thats what the battery backer does for you.

Robin

Not being entirely trusting of what appears in magazines, I did a test on this a little while ago. It went something like this...

As a test I’ve just plugged a fully charged 4 cell NiMh battery into a completely flat 4 cell NiMh battery (with a watt meter between). The flow of power between the two started at 0.13amps, and after a minute has fallen to 0.05amps (and still falling). That would flatten the charged battery in about 500 hours or so (or never), probably enough to complete a 10 minute slot (see photo below. By the time I'd taken a quick photo, the current had dropped to 0.11A).
I’m going to put on a beret & a stick on ‘tache, talk in an American accent and declare that myth busted!

(Charged battery 5.12v off load, 2500MAh AA cells. Flat battery 3.34v off load, 3300MAh Sub-C cells)

Think i'll go with a PowerBox 12 switch when KLMS has them in stock again.

If you have two battery packs of the same VOLTAGE regardless of AMPAGE, one flat and the other fully charged, and you couple them in parallel the fully charged pack will see a battery of the same voltage capacity as itself so it will not try to seriously charge the flat one. It will charge at very low current until it sees its own voltage, which will take only a very short time and then stop. This is why we charge batteries at a voltage level which is greater than the pack. QED

What would happen if :-

A) One battery went short circuit
B) One battery went open circuit

??????????????????????????

john

Hi John, then you have a fault situation which is not what we were talking about. You don,t normally connect batteries in parallel in real life without diode protection! In fact I would not suggest batteries in parallel anyway. Dave

a) Depends on what and where the short circuit is. A short of a cell in the battery would generally show itself as a reduction in charge capacity, rather than a spontaneous turning of the cell effectively into a length of wire. Nobody would fly with a battery pack they knew wasn't working properly would they?

A mechanical dead short somewhere in the airborne wiring (be it battery, switch, servo or extension lead) would melt something. No diode in the world will help the receiver if there’s a working battery connected to it and somewhere, positive and negative are connected together. There will most likely be a little ‘pfffft’ noise, a little puff of smoke, and a receiver board track will disappear.

b) Not a lot. One battery doesn't have a complete circuit, so no current can flow through it. The receiver is still happily powered by the non-open circuit battery.

Hello Rob, I don't advocate parallel batteries but on the subject of sudden short circuit, whilst in flight, of a battery (insulation breakdown etc etc) then a diode in each of the pos feeds from each battery would provide protection. The diode would prevent the good battery going to ground via the short circuit.

Dave et All

It was a loaded question and the safe answer is to fit Diodes.

we have been doing this on Ghost Squadron Models since the very early days and as a result have saved more than one model.

John

Dave, you’re right, a diode would prevent the other battery from joining the short circuit party, but with the energy stored in even an RX pack, something would melt pretty sharpish if a short occurred.

We need to remember that a diode or battery backer can itself fail open circuit, leaving the model without any power. As with many things, it’s a balance between protecting against a feared event and the fact that something that’s not there can’t go wrong!

Would the advocate of putting two batteries in to one reciever, be kind enough the show me a diagram of where to fit the diode and what size diode to use. Would like to fit this method in my new Byron P51 Mustang
Ignorance is bliss
John 1915

Hello John, you don't need a diagram. In the positive lead from each battery fit the diode in line between the battery and receiver preferably after the switch ( I presume you are using a separate switch for each battery) otherwise you may prevent your charger from working if it connects via the switch. Diodes, of course, are polarity sensitive so you must install them the right way round. The diode usually has a silver ring to identify the anode, this must be on the receiver side of the install, imagine it as the tip of an arrow showing which direction to point it. You will need a high current diode rated at least 6 amps and able to handle whatever voltage batteries you are using. Maplin stock a 6 amp diode, P600D, capable of taking a massive 400 amp surge. Stock number UK60Q at 99 pence.

Hi Dave
Thanks for the information. Will try this on the Byron mustang
Regards
John

For the big models I always use either spektrum dual power rx with safety switch, powerboxes or dual powerbox switch.

The nimrod has two spektrum cockpit powerboxes in with 4 5000mah optipower lipos