Discussion Which batteries and why? How about LiFePO4 for DLG?

[tai626]

yeap. Thoses are balance charger for LiFePO4

[kcaldwel]

Although this isn't really what we need for most DLGs, it might be of some use to someone. It appears Spektrum considers switching regs OK for use with their 2.4GHz gear:

http://www.espritmodel.com/index.asp...OD&ProdID=7102

Kevin

[tai626]

Dual rate switch regulator. Interesting... And 7g.

I received an email from K2 and they recommend charge of C/5 or 120 mA constant current up to 3.7V constant voltage. The discharge rate of C/5 or 120 mA to get 600 mAh out of batt. I think 4 analog servos and 72 MHz is the limit for this cells.

[John255]

Hope you guys are clear on the idea that so far the cells shown are individual replacement cells for CR123a primary cells and must be soldered into a 2S pack.

At same time, in addition to the output leads to the switch jack you will need to solder in individual wires to the cells with a connector to allow use of a balancing charger.

You will also need to mount that connector somewhere where it is accessible, or remove the batteries for charging.

Charging cells in series with the normally used charge jack would probably work just as well and relieve the problems associated with the balance charging jack.

Also the 300mA capacity Kevin reported don't seem like much of a gain over NiMH, and no where near the 1550mA capacity of the original CR123a's at 34g.
John255

[tai626]

I am hoping not have to charge balanced everytime. I don't have such small LiFePo4 charger, the only one that I have charges at 10A! I have been charging my A123 without balancing at field and they usually don't go out of balance. I will try the same with this rcr123a and see if they stay in balance. If not, time to shop for a charger! "Which charger and why?"

[pldaniels]

Kevin,

The trouble with 'switchmode' converters is multifold for the one advantage they yield (efficiency at high currents).

1. They require a lot of shielding (around the board)
2. They require a lot of filtering (on the DC output)
3. They aren't as efficient as linear regs at low currents (eg, below 50~100mA)
4. They're heavier as a consequence of #1 and #2

Since most switchmode converter controller chips don't show efficiency graphs down in the lower current ranges it's a bit harder to 'show' what I mean about #3, however if you extrapolate down from typical efficiency curves you see they head quickly below 60% as the current demands drop off.

With larger planes #3 isn't a major factor, however with DLGs the servos spend a lot of time at idle, so you will end up potentially draining a pack faster with a switchmode pack.

Paul.

[John255]

Originally Posted by tai626

I am hoping not have to charge balanced everytime. I don't have such small LiFePo4 charger, the only one that I have charges at 10A! I have been charging my A123 without balancing at field and they usually don't go out of balance. I will try the same with this rcr123a and see if they stay in balance. If not, time to shop for a charger! "Which charger and why?"

Tai,
Are you sure that's right that you are charging at 10A? The max charge rate recommended for RCR's is 550mA? They also recommend charging 1.5~3 hours.
John255

Working Voltage:3.2V and Peak Voltage: 3.6V * Charging cut-off voltage: 3.6V * Discharge cut-off voltage: 2.2V o Please never overdischarge battery below 2.2V/cell o Charge the this battery with LiFePO4 RCR123A smart charger. * Capacity: 750 mAh * Maximum discharging rate:< 550 mA * Maximum charge current: <550 mA * Cell's dimension: 17mm Dia. x 34.5mm H. * Weight: 0.6 oz (18 grams).

[tai626]

10A is for my A123 only not CR123a.

[kcaldwel]

I only tested the Dimension Engineering switching reg down to 200mA. I got about 23% above the measured battery capacity at that current; i.e. 450mAh rated 7.4V nominal LiPo that I measured at about 410mAh with no regulator, gave 500mAh at 5V.

I'll re-test it here at 100 mAh and see what I get, but for digital servos at least, it seems like a reasonable way to go.

I've attached Dimension's efficiency chart.

Kevin

[pldaniels]

Kevin,

Try also at 50mA, since that's about what a modern DSP receiver will idle at with no servo movements.

Paul.

[pldaniels]

What a lot of modern designs are doing now is integrating both linear and switchmode into the design, such as the LT3500 2A buck converter controller. It uses an external transistor to use as a linear regulator for when the switchmode efficiency drops to a point where it's too low.

Designs like the Linear LT3500 are expensive but they do give you the best of both worlds.

The reason why switchmode designs drop off in efficiency so much as you approach lower currents is that switching losses start to dominate the power consumption, there are ways to side step this (using skipping/discontinuous mode etc) but you suffer poor regulation as a result.

In case anyone is wondering, I don't have a vandetta against switchmode designs, I use them a lot in many other areas of my design/production work. Right now however the likes of the LT3500 are too expensive and the gains aren't sufficient enough to warrant moving over from linear regulators on small/low-quiescent current situations such as many DLGs or small gliders.

For things like helicopters and sports planes, I absolutely without hesitation would recommend a well shielded and filtered switchmode design.

Paul.

[kcaldwel]

The average draw in a DLG seems to be about 100 to 150mA from the best data I've seen:

http://www.flyquiet.co.uk/smf/index.php?topic=824.15

With the trend to digital servos, the current will go up.

I get a lot of flight time out of eTec 450 packs with the Dimension switching reg. Works for me. I'd just a soon not have to use a regulator at all, but until more servos will take higher voltages, or there is another rechargeable battery solution, I guess I'll stick with this.

Kevin

[pldaniels]

Kevin,

Yes, ultimately it'd be nice if servos took 7~8V inputs, though the trouble could be that higher voltages will require more copper on the armatures which means more weight and more cost, or finer wire.

That said, the alarm side of the BACs is still very handy (and the on/off/charge jack facility).

My 2 x 5.8g servo 50" DLGs draw about 30mA at idle, so I guess it all depends on the setups.

Would be good to get more real-life idle current data, rather than computed averages.

Paul.

[kcaldwel]

Yep, more data is always good!

Of course the efficiency at low currents doesn't matter nearly as much, since the current is low. As long as the efficiency is good at currents at and above the average, and not horrible below, the overall efficiency will be good. Losing some percentage of a small current doesn't matter nearly as much. Saving at the peak of 500 or 800 mA is far more important than saving at 30mA.

You also get the multiplier effect with the switcher, where the higher voltage is converted to more mA, which doesn't show up in that efficiency chart.

Kevin

[tai626]

The order was placed on Oct 26 almost on Oct 27. the package was shipped on Oct 29 and arrived on Oct 30 from NV to CA.

It is well packad and weight exactly 16.5g each cell.

More tests to come