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battery back up

Discussion about Tx, Rx, Servo's, Batteries, Chargers, and all the other things we like to talk about..
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jack
Posts: 91
Joined: 20 Mar 2015, 21:55
Location: chepstow,south wales

battery back up

Post by jack »

Ok,l intend using a 5 cell sub c nimh pack as a primary battery in the Pilatus,but due to space considerations,l am planning on using two 5 cell double "A" packs as back-ups....these will be linked in parallel via a "Y"lead,reason for using these packs is that l can get them well forward to help with the cg,both packs are 2500mah and from the same manufacturer,so should be fairly well matched,however l am a bit concerned about one pack trying to "charge" the other,l know l can include a Schottky diode in the lead to each pack to prevent this,but l am not convinced its necessary....do any of you electronics buffs have any thoughts on this :?: :? ,both the primary and back ups will have their own switches,and will be feeding an frsky rb-20.....if l do fit diodes it may be a problem to charge them insitu,though a charge lead between the pack and diode would cure that l think....well,hope you all had a good xmas heres hoping for great flying weather in 2019...jack
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Peter Balcombe
Posts: 1399
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: battery back up

Post by Peter Balcombe »

Jack,
I would strongly advise using diodes between each pack and the receiver.
The diodes will prevent a single battery failure from pulling the other one down - thus giving you NO receiver power, particularly as Murphy’s law ensures that the model will be in the air when a problem occurs.

Schottky diodes give the lowest voltage drop & as you say, you can charge each battery in situ by adding a charging lead to each battery/diode link.
Each diode must be capable of supplying the maximum PEAK system current, plus a safety margin, else one will blow when servos stall etc., leaving you without the expected dual battery redundancy feature.

Note that when diodes are used, the battery with the highest voltage at any point in time will be connected to the receiver, thus BOTH batteries should be used at various times during the flying session if they start off at the same voltage.
Peter
Elliot Howells
Posts: 333
Joined: 18 Mar 2015, 08:18
Location: West Wales
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Re: battery back up

Post by Elliot Howells »

A couple of 2S lipos and a Jeti Max Bec - zero failures in umpteen models in 10 or more years.

regard it as insurance ;)

Ell.
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RobbieB
Posts: 547
Joined: 07 Mar 2015, 22:22
Location: North West

Re: battery back up

Post by RobbieB »

Jack, hope this helps with the wiring, very cheap, very reliable.

The 5A rating of the diodes was for an average 6/7 standard servo model and was drawn up many years ago. If you are are talking about higher servo/high power installations you will need to ramp up those diode specs accordingly.
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battery_backer.pdf
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jack
Posts: 91
Joined: 20 Mar 2015, 21:55
Location: chepstow,south wales

Re: battery back up

Post by jack »

thanks for your help and advice guys-particularly you Robbie,your schematic is just the job and will do for me :D :D …l have a boatload of diodes looking for a home plus l got a snazzy new soldering station for crimbo courtesy of my lovely lady wife,so l can put the big iron away and get busy making up a couple of harnesses :D :D ,enjoy the rest of the festivities guys,see you on the field!!
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