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Sep 07, 2019, 08:47 AM
Thermal in Peace.... :)
Chuck Glider's Avatar
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Discussion

Voltage Regulators, UBECs, etc


I fly gliders. At the moment, they are all off 4.8V or 6.0V (NiMH) receiver batteries, I want to migrate to higher capacity Lipo batteries in the gliders. Main drivers are longer flying time and lower weight. Servos are all standard, not high voltage. Rx is FrSky X4r, which takes 4-10V no problem.

So, what are my options and what is the best answer?

1. I've heard of "smart" 1S batteries that step up the 3.7V nominal to 5V, but I can't find any online. Are they any good? Recommended or not?

2. 2S batteries are 7.4V, so although the Rx will be fine, the servos would be pushed to outside their ratings and may die younger than they should. However, I've heard of some people just doing it anyway.

3. Put some kind of step down unit between the 2S battery and the Rx pack. Now this is where I get most confused. Some say don't do this as it creates another potential point of failure. However, others use a LM7805 ic - see this article from Peterborough Model FLying Club. From my experience of that chip, I think it may run too hot, I don't know for a 2-4 channel glider. LM2597 switching regulator type units seem a good option too, but are a tad heavier at 10g. Then there's the Matek micro BEC, that I don't know much about.

4. I guess another option is to have spare charged NiMH pack(s) ready, and be better on my battery management. During longer flying sessions, switch them over at appropriate times.

Any words of wisdom on this subject? Thanks for your help!
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Sep 07, 2019, 09:11 AM
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1 creates higher load on the battery. It's good for extremely space restricted small motor models, like 1:87 cars or such. A "step down" is always a better option, as it uses lower currents.

2 is nice if you have servos that can withstand 8,4V. I you do not, you're pushing your luck. Don't.

3 is best solution, although not on a 7805, that one can only supply one amp. There are plenty voltage regulators on ebay that have 3A max rating and selectable voltage. I'd go for those, and have the model separated into different domains so that one failed BEC will not interfere with a landing.
The type i'm talking about looks like this https://www.ebay.co.uk/itm/DC-Buck-S...QAAOSw5Oxa0IRk
It has a on/off input and can be solder bridged to various voltages (cut the adj jumper!)
I use them everywhere, and if you look for the pic you can get them for as low as about 1$/€ per piece.

4 is second best (or 3 & 4 switch places, that is arguable). It is more "Ni against Li", not "change them or not".

Failure-point wise 1 &3 are the same, and a 2S Lipo needs monitoring, either by connecting a lipo alarm (that may be hard to notice flying miles away), telemetry (needs equipment) or strict flight time limiting.
Weight and capacity is better on Lipo, tho.
Sep 07, 2019, 03:36 PM
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GeoffS's Avatar
A quick note on switching DC-DC converters...

Check the current specs. carefully. The one mentioned above is 3A max, 2.1A continuous.

That's pretty typical for the small LM2596-based boards (ex. https://www.ebay.com/itm/DC-DC-LM259...V/173319084685 ).

I don't know what the limiting component is in any of these circuits. The LM2596 chip current-limit is probably from heat dissipation.
Based on the spec. sheet ( https://www.ti.com/lit/ds/symlink/lm2596.pdf ) 3A max. is pretty conservative.

I use these boards all the time, but I've never used one anywhere near 3A, so I don't have any personal experience with their behavior.

BTW, some boards have even larger max/continuous current difference.
For example, this one is listed 3A max, 1A continous: https://www.ebay.com/itm/Mini-DC-6-5...C/311982075567
Sep 07, 2019, 04:18 PM
Thermal in Peace.... :)
Chuck Glider's Avatar
Thread OP
Thanks, that's really helpful guys. I will try to get hold of the ones that learningarduino mentions.
Sep 07, 2019, 11:08 PM
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vollrathd's Avatar
Quote:
Originally Posted by Chuck Glider
Thanks, that's really helpful guys. I will try to get hold of the ones that learningarduino mentions.
How much room do you have for battery space inside your glider? And, have you considered those LiFe receiver batteries for your models? LiFe's and higher performance A123's have a very flat voltage discharge curve, and are a direct drop in replacement for any receiver/servos that can handle a 5 cell NiMH receiver battery pack. No fancy regulator is required, they put out 6.6 Volts DC in the model. Just plug them in.

For me, I've run across four LiFe receiver batteries in my RC club in the past couple of years with a defective cell. A123's are a big improvement. A123's come in both 1100 Mah and 2500 Mah sizes. Both the 1100';s and the 2500's can be topped off at the field in 15 minutes with a charger capable of a 10 Ampere charging rate. (Ten Amps is the specified maximum charge rate on the 2500 mah cells. And 4 Amps is the maximum charge rate on the 1100's.)

The A123 2500 mah cells measure 26 mm by 65 mm in size, and weigh 74 grams. The A123 1100 mah cells measure 19 mm by 65mm and weigh in at 40 grams. Both the 1100 and 2500 Mah cells have current capability far in excess of what the receiver and its servos can ever draw. Last year, one of my RC club mates shorted out the receiver switch on his 2500 Mah A123 pack.

That battery pack heated the #20 wires red hot, finally melting them. The A123 pack was undamaged, I ran a discharge test on it with my West Mountain CBA battery analyzer, it checked out just fine. I rewired it, the modeler is still using it today.

Oh yes, the A123's last many years with little loss of performance. And, they are rated for 1000 plus cycles at 100% discharge testing. I've got some 200 A123 cells in my various electric models, routinely pulling 40 Amps per cell out of them.
Last edited by vollrathd; Sep 07, 2019 at 11:19 PM.
Sep 08, 2019, 03:56 AM
Thermal in Peace.... :)
Chuck Glider's Avatar
Thread OP
Vollrathd, that is super interesting. I've never heard of A123 until now and I know diddly squat about Li-Fe batteries. How do you charge them? Do you need a special unit or can you use a LiPo charger? In some of my gliders I have space. In others, it's cramped.
Sep 08, 2019, 08:04 AM
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vollrathd's Avatar
Quote:
Originally Posted by Chuck Glider
Vollrathd, that is super interesting. I've never heard of A123 until now and I know diddly squat about Li-Fe batteries. How do you charge them? Do you need a special unit or can you use a LiPo charger? In some of my gliders I have space. In others, it's cramped.
Most newer Lipo chargers have a charge routine for the Life battery packs. And the same Life setting is used for the A123's.

I'll post A123 receiver pack sources tonight. Or, you can solder up your own A123 packs for under $20 each.

No one in my RC club is using NiMh receiver batteries. Most are using 2500 Mah packs in their gasser, along with a couple of LiFe's.
Sep 12, 2019, 12:59 AM
Registered User
vollrathd's Avatar

A123 cell sources


Here is where I buy my A123 cells. Note on Ebay that some A123 cells are "Pre-owned" and are not brand new. Also, the latest A123 is the 26650 "Type B" cells with a 2500 mah capacity, versus the 5 year old Type A 2300 mah cells. CRA123 is NOT a rechargeable cell.

2500 mah cells (These are all type "B" 2500 mah cells)
http://a123batteries.com/anr26650m1-...phate-battery/

1100 Mah Cells
http://a123batteries.com/apr18650m1-...indrical-cell/

Prebuilt A123 battery packs (One of many sources)
https://www.radicalrc.com/category/A...ells-Packs-199

Ebay
https://www.ebay.com/sch/i.html?_fro...cells&_sacat=0

Type "B"
https://www.ebay.com/itm/1-pc-A123-S...0AAOSwl-ddI6zN
Last edited by vollrathd; Sep 12, 2019 at 01:07 AM.


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