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Battery choice!

Discussion about Tx, Rx, Servo's, Batteries, Chargers, and all the other things we like to talk about..
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Jason Leech
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Joined: 14 May 2021, 21:35
Location: Bristol
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Battery choice!

Post by Jason Leech »

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.

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Peter Balcombe
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Location: Clevedon, North Somerset, U.K.

Re: Battery choice!

Post by Peter Balcombe »

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.

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Jason Leech
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Re: Battery choice!

Post by Jason Leech »

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

harry curzon
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Joined: 21 Jul 2018, 09:32
Location: Gloucestershire

Re: Battery choice!

Post by harry curzon »

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

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Peter Balcombe
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Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: Battery choice!

Post by Peter Balcombe »

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.

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RobbieB
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Re: Battery choice!

Post by RobbieB »

There you go:
battery_backer.pdf
(45.39 KiB) Downloaded 23 times

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.

FrankS
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Joined: 18 Mar 2015, 14:29
Location: Gloucestershire

Re: Battery choice!

Post by FrankS »

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.

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