7,4 V

[Asko]

Hi

I wonder, if 7,4 V would be dangerous for Robonova? 8O

[bauermech]

I wonder, if 7,4 V would be dangerous for Robonova?

Yes it would. You NEED a 6.0v regulator.

[subpilot]

I think you can get away with it for awhile but you will be operating near the limits and will no doubt see servos fail eventually. I don't beleive the servos have over temp or over current protection so there's nothing to prevent them from burning themselves up.

[i-Bot]

I think that while death will not be immediate for the C3024 (regulated), or the servo processors (processors regulated). The fate will come in the servos. The servo motors and H bridges are rated to 6V and not 7.4 V. If you are talking LiPo then even maybe over 8V. The Surgeon General takes a pretty dim view of smoking Bipeds.

The RoboNova is actually an very nice balance between weight, speed, and the capabilities of the servos at 6.0V. Ignore this at your peril.

A well charged LiPo can almost double the power dissapation in the servo motor and drivers, which it cannot handle.

Bauermech has recommended a suitable 6.0V regulator elsewhere, so use it !

A good regulator lets you use the advantages of LiPo for longer operation between charges.
Look after your servos, that is where your $ are.

[Bullit]

I checked the spec on the vishay h-bridge used for the hsr 8498hb, vishay Si9958 is actually a 20V part. Most likely you will start to see h-bridges or motors fail with the higher current. I personally haven't tried running mine at 7.4V because of the fear of damaging very expensive servos. I have on other models run other 6V servos well above 7V without problem. I know some of my servos on my RN1 get warm after running for while at 6V. I can only imagaine at 7.4 they would get very warm.

[Asko]

Hi

Yes, Im trying to take away those 110 g NI-MH batteries from the up in the chest and put LI-POs under feets like Matt did.

What kind regulator you Matt used, will 5 A be enough? And did uoy use any under voltage cut off in you'r Robonova with LI-POs.

[Droidling]

I suggested the idead of putting 7.4V packs in my RN-1 to the HiTech tech support manager. He said he tried it and the servos didn't last long. he did mention that he had already suggested to their engineers that they had to support at 7.4V in the next generation. That won't help any of us, unless you feel like replaceing all of your servos.

For now I wonder if anyone has found a 6V regulator that will handle the current for 16 servos.

Terry

[Asko]

Hi
At least bauermech has done it.

[subpilot]

I'm running a 3Cell 200mAh LiPoly pack which takes up most of the body cavity but weighs about the same as the NiMH pack. It runs at >11.1V and I regulate it down to 6.5V using an International Rectifier iP1203 Buck converter. The converter is good for 15A continuous so I have plenty of power to spare. I manage the battery with a TI bq2060 Gas gauge IC. I am eventually shooting for having the RN find his charger when the battery is nearing empty. There are two levels of voltage cutoff. The first threshold causes the RN to sit down and de-energize servos. The second cutoff disables the regulator and drops the current draw to <2mA.
This is an expensive solution which is probably not for everybody but gives me nearly four times the power and optimum battery management. I may go with two 2cell LiPo packs in the feet (like Matt) but connected in series. The regulator can handle the >14.8V and I should get at least as much, if not more, power than what I have now and free up some space in the body for more goodies while potentially improving the stability.
I charge my LiPo cells individually rather than as a complete pack. That's the way to do it in order to get max power (and life) from them but requires a more sophisticated charger.

[Bullit]

subpilot, did you use the eval board for that buck converter? or did you make your own. I just noticed the eval board is rather large and expensive. The part it itself looks somewhat difficult to mount.

[subpilot]

I hacked the eval board with a dremel. It's pretty small once you cut away the un-necessary bits. I had it for a another project so it didn't really cost me anything but it is an expensive eval board. TI practically gives their stuff away but not IR.

[subpilot]

I'd give one of these a try if you want a cheaper alternative. http://focus.ti.com/lit/ds/symlink/tps54550.pdf

[bauermech]

What kind regulator you Matt used, will 5 A be enough? And did uoy use any under voltage cut off in you'r Robonova with LI-POs.

I'm using a slightly modified "MPI Miracle Switch"... minus the charging switch. 10A continous/20A peak. And, soldered on new leads (slightly heavier ga. wires).

And yes, my RN-1 is wired up in parallel... well, lets say it has that option. I know there's a lot if info stating that that is a bad idea... using Li-Po's in parallel, but I've been consulting several RC hobbyists who do it.

The danger w/ parallel is when the cells become unbalanced from one another. The newer Li-Po's have an additional feature where they have charging/monitor leads separated from the discharge/main leads coming out the pack. What this does, is allow you to charge and balance each cell individually. I'm looking into a couple different balancers. I have already purchased a "Blinky Balancer" (as kindly recommended). This checks the voltage of each cell, discharges all the cells w/ a higher voltage... down to the lowest cell's voltage. Then, (while charging) senses when one cell reaches its max charge, brings the remaining cells to an equal level - either by discharging the higher ones, or continuing to charge the lower ones. All-in-all, I really don't need to have them wired in parallel. I get plenty of time using only one pack at a time and terminating the other lead. Of course, both packs remain in the feet (for C.O.G./balancing reasons). When one gets too low, I can plug in the fresh pack and disconnect & terminate the other. Regardless of how you wire up your packs, you should balance your muli-cell'd Li-Po's anyway.

I have two active on-board battery checkers (one for each pack). They have an array of 10 LED's to verify voltages in real-time. As long as I keep a good eye on them, it really decreases the chance of over-discharging. Of course, I also have the Critically-low voltage sensor (as diagrammed in the manual), but isn't the most reliable. It's simply there as a "you better physically turn me off and get me to a charger NOW!" thing - incase my wondering eyes, too often, look away from the voltage monitor. This ~woks for now, but subpilot's method would be much more secure in ensuring good battery health....

I manage the battery with a TI bq2060 Gas gauge IC. I am eventually shooting for having the RN find his charger when the battery is nearing empty. There are two levels of voltage cutoff. The first threshold causes the RN to sit down and de-energize servos. The second cutoff disables the regulator and drops the current draw to <2mA.

[limor]

pictures pretty please..

[subpilot]

I am very suspect of the "miracle" switch. By the description (and price) it appears to be just a linear regulator. If so it is very inefficient and the regulator dissipates the excess power as heat.
The heat is the product of the dropped voltage x current so if you are regulating a 7.4V pack to 6V@10A then 14W of power are wasted as heat.
Not only are they inefficent but they are prone to failure if you don't get rid of that heat somewhere and it doesn't appear to have much heatsinking. The switchers are far more efficient (>85%).

If you over discharge a LiPo pack even slightly one time it will never be able to deliver a good charge again so it is critical to have a good automatic cutoff.
Here's a good LiPo discussion http://www.rcgroups.com/forums/showthread.php?t=209187