The robot has around 10 servos with each potentially using about 9A at 7.4 volts each.
Where can I find a good NiMh battery that can meet the high current demand of these servos?
I saw the small battery cells online but I'm afraid they may not be able to provide so much current to even one motor without heating up.
I'm currently using a lead acid car battery although its far too heavy.
I basically need the equivalent of a light car battery.
Thanks in advance.
kolleamm:
The robot has around 10 servos with each potentially using about 9A at 7.4 volts each.Where can I find a good NiMh battery that can meet the high current demand of these servos?
I saw the small battery cells online but I'm afraid they may not be able to provide so much current to even one motor without heating up.
I'm currently using a lead acid car battery although its far too heavy.
I basically need the equivalent of a light car battery.
Thanks in advance.
I'm guessing these are the same servos you were using in this thread?
It would likely help people make better batty suggestions if you provided a link to the servos you're using.
I've done a lot of projects with lots of servos. I use Lipo packs to power the servos. I generally purchase my LiPo packs from HobbyKing.
A lot of servos can't be powered directly from a 2S pack. If I need to drop the voltage I use a switching regulator to drop the voltage down to somewhere between 5V and 6V depending on the servos. I used two 15A regulators on my 22 servo hexapod.
I know I've posted this photo a bunch of times but it seems like the topic of powering servos comes up a lot.
This is the regulator I used. I'm not sure if it can really provide 15 amps the two I'm using seem to work fine. I'm sure there are smaller options.
If you wanted to use NiMH cells, you could use "C" or "D" sized cells. Another option would be use use multiple AA packs. You could have each AA pack powering a different set of servos. You'd want to make sure all the grounds were connected together.
It's hard to beat LiPo packs for being able to deliver a lot of current.
These are the same servos you are correct. Here is the link to the servos.
The suggestion for the regulator is a great one! I will definitely use it.
Would LiPo still work for these servos?
kolleamm:
The suggestion for the regulator is a great one! I will definitely use it.Would LiPo still work for these servos?
Those servos look great.
You should be able to power those with a 2S LiPo without a regulator. You don't want to use a regulator is you don't need to. They can introduce problems so if you can get away without them it's generally a good idea to do so.
A 2S LiPo has just about the save voltage as 6 NiMH cells.
You'll want a battery that can source 90A of current. If you get a 20C LiPo pack, you want it to be a 4.5Ah or larger. If you use a higher C rating, you could use a pack with less capacity.
This pack should power your servos without any trouble.
if I were to purchase another LiPo charger, I'd get this one. I've heard good things about it.
Get a couple of these meter/alarms. They quickly pay for themselves in saved batteries.
Wow! That battery is simply amazing! The weight, the size, the price, all perfect. The charger is great as well. I will definitely be getting all 3.
Just a quick question on the 15A regulator. How do you adjust the voltage?
Thanks for all of your help, you made my project a lot more feasible.
kolleamm:
Just a quick question on the 15A regulator. How do you adjust the voltage?
There is a trimpot to adjust. Power the Vin side and monitor the voltage on the Vout side as you adjust the screw.
As I mentioned earlier, I don't think you need a regulator for those servos.
kolleamm:
Wow! That battery is simply amazing! The weight, the size, the price, all perfect. The charger is great as well. I will definitely be getting all 3.
Keep in mind shipping from HobbyKing takes about three weeks. If you can find the items in a local warehouse the shipping won't take as long. They do offer fast shipping but it costs extra (I've used it many times myself).
Is it okay though to use a voltage that is right on the servo's limit? 7.4 at a 7.4 limit?
kolleamm:
Is it okay though to use a voltage that is right on the servo's limit? 7.4 at a 7.4 limit?
To tell you the truth, I'm not sure.
I think it's a safe bet to assume a "7.4V" servo should be able to be powered from six NiMH cells. Six NiMH cells are nominally 7.4V but when fully charged they can be a bit over 8.25V. So can the servo be used with a 8.25V supply? I think the answer is yes, but I'm not positive.
A 2S LiPo pack is also nominally referred to as a "7.4V" pack but it can have a voltage of over 8.4V when freshly charged. Is this okay? Again, I think it is but I'm not the one with $60 servos at risk.
You could ask in Pololu's forum. I've always had very positive interactions with the people at Pololu.
I once tried to increase the power to some 9g servos by powering them directly from a 2S LiPo. I saw several puffs of smoke coming from the servos immediately after I connected them to this power. Needless to say, the servos which released their magic smoke never worked again.
I've tried to power other servos which were supposed to be able to tolerate a 2S LiPo with limited success. These other servos didn't go up in smoke but the servos twitched at the higher voltage. When I reduced the voltage the twitching went away. I doubt your servos will have these problems. The servos you're using look like they're much better quality than the ones I had been using.
Thanks for the advice. I will definitely contact Pololu about this. Worst case scenario I just buy a few regulators.
I remember using a standard servo once and accidentally turning up the voltage from the limit of 6v to 12v and seeing smoke coming out of it.
I'll probably end up playing it safe just in case.
Thanks for the help
kolleamm:
I'll probably end up playing it safe just in case.
I strongly suggest you find out if the servos can be safely used with a 2S LiPo. You will save yourself from a lot of hassle if you can get away without using a regulator.
It's very very common for voltage regulators to go up in smoke when powering a bunch of servos.
I agree. In the meantime I did find this online
"Running a 2S lipo would be a different story because the rated 7.4V would be upwards of 8.2V fully charged which is way too much and that is when you'd want a voltage regulator."
He doesn't exactly say why though.
kolleamm:
I agree. In the meantime I did find this online"Running a 2S lipo would be a different story because the rated 7.4V would be upwards of 8.2V fully charged which is way too much and that is when you'd want a voltage regulator."
Maximum voltage for Servos - RC Groups
He doesn't exactly say why though.
The same thread contains this quote:
"6 volts" doesn't mean 6.0 volts. And "4.8 volts" doesn't mean exactly 4.8 volts.
What they really mean are 4 and 5 cell NiCd or NiMH packs. Which are NOMINALLY 4.8 and 6 volts.
Just as "6V" means five NiMH cells, "7.4V" is often used to indicate the servo can be used with two LiPo cells.
I'd be very surprised, if the servos you have aren't safe to use with a 2S LiPo.
Here's an example of a servo rate at "7.2V" which is supposed to be safe to use with a 2S LiPo.
I asked in the comments of this very irritating YouTube video, if the servos could be used with a 2S LiPo. Their answer wasn't entirely clear but I think the following quote suggests the servos can be used with a 2S LiPo.
Hi, this robot can be powered by 7.4V lithium battery
The servos didn't smoke when connected to a 2S LiPo but they didn't behave well. There was a lot of jitter when powered from the higher voltage. This jitter went away when the voltage was reduced.
I'll run some tests on one servo once I order the batteries etc...and they arrive. I agree in avoiding the voltage regulators and at least seeing if it works purely off the battery. If it does work it would really remove a big hassle off my back.
I also contacted Pololu.
Will report back with what they say, and again once I receive everything to do a test.
Although I do have a 3A power supply with adjustable voltage. I would like to run a test with it but the supply is fairly unreliable since it's amps would drop significantly down to around 1A at 8.2V even though its specs would say otherwise. So not really sure if it would prove anything by using it.
Here is what Pololu wrote
"Generally, hobby servos like
these are designed to be used with batteries and their voltage ratings
are for nominal battery pack voltages rather than absolute voltage.
Since that servo you mentioned has an operating range up to 7.4V, which
is the nominal voltage for a 2S LiPo, we expect it to work. This also
means that we expect it to work with a fully charged 2S LiPo, which can
be up to 8.4V."
I went ahead and ordered the battery,charger, and beeper you recommended without the regulator.
Thanks for your help 
Thanks for letting us know what you learned.