First off I think you need to understand ohm's law first before going any further. And know the difference between amps as it pertains to current vs capacity.... You need a power supply not a battery charger.... Be aware you need one that has a minimum of 20amps for running multiple servos. I use a 6v 50amp power supply to run my inMoov. 6V because it uses 6v Hitec servos not 7.4v ez robot servos.....

Thanks so much for your help. I'm running 8 servos. I will search for 7.4V 20amp power supply. I can then connect it directly to the power input cable of EZ-B V4?

It's hard to say what is going on with your battery. Not enough information. What kind of batteries? What size? How many servos are you using and are they all from the JD? Are you using all of them?

If I had to bet I'd say you have to much amperage being drawn from the batteries and perhaps the wrong type of batteries.

You do not want to just hook up a battery charger to your robot to keep the batteries charged. Charge the battery with the robot powered down. You do not want to use a battery charger to power your robot. That's not what the battery charger is intended to do.

You can use either the right type and sized battery or an external power supply. However without a relay switching system to change from one source to the other it's hard to use both. There are simple ways to manually switch from one to the other but it sounds like you are not at that level yet.

Lets talk power supply, load and distribution. First you need to know that voltage and current (amps) are not the same thing. Amps are drawn by the devices in your circuit. Devices like your servos. If your devices are trying to draw too much current then the power supply or wires and connectors can handle a number of bad things can happen. Moistly heat. a device only knows it wants current so it can run as designed. It will keep pulling that current till it gets it or something in the chain breaks. The worst case is things will get hot, burn and catch fire. Best case is your power supply will shut down (Power converters) or drain (batteries). You must have your power supply, wires and connectors sized large enough to handle the current (amps) that all your motors, lights and devices will draw.

To insure that you have a properly sized power circuit you first need to add up how many amps all your motors, lights and devices pull at peek load. Once you know that you can decide how big the wire and connectors need to be to transmit your power and how big of a power supply you need.

Now that you have that knowledge you can decide if you want to use external power or battery power. You can certainly use either or both as I said earlier. If you go with a power converter and know how many amps you need to draw there are many choices. If you have a small load (maybe less then 2 amps) you may be able to go with a little wall wart. They say right on them how many amps they are designed for and the voltage they will put out if plugged into the wall. Forget using them for motors. Lets say your robot can pull up to 20 amps. You will need a quality AC to DC power converter. Get one that is able to handle at least 25% more load then what your peek amps are going to be.  So in this case I'd get a 30 amp power supply (Because that's the next larger one available). Feed it with power from your home outlet and attach your devices to the lower voltage DC side of the converter. One more thing; Make sure you get the proper voltage power converter your devices will need. They usually come in different voltages from 3.3vdc to 48vdc. Here's a 12V 30A Switching Power Supply that may work with our scenario above if your devices use 12vdc:

AC to DC power supply (I do not have an opinion on the PS i have linked to. I't just an example).

Now Batteries? Mostly the same applies as far as designing your circuit and power feed. However what does differ is the type and size of battery. A lot of people use Lipo batteries now because they will handle a motor's amp draw better then most other type's of batteries. I'm not a good person to give advice on batteries. However I do know that if you are just using a AA battery pack on your full sized robot with a ton of servos then that is your problem. You need the proper size and type of battery.

I hope this helps in some way. Good luck and have fun.

What ever voltage you power the EZB v4 with, you will also have coming out of the powered digital ports. The EZB can handle between 4 and 17 vdc.

You are not going to find a power supply that will give you 7.4 vdc. You are going to have to get a higher voltage PS and buck it down with a converter. They have very affordable and adjustable power converters available. Just make sure you get one rated for the amps you are going to draw through it.

Thank you Dave for your time and in depth explanation. It's very much appreciated! I'm using 8 servos from the JD robot. I'm using the Lipo 1300 mAh EZ ezrobot battery that comes with the JD.

The battery that comes with jd should last an hour or even much longer - entirely depending on how much activity the servos are moving.

Perhaps the battery isn’t wired correctly to the ezb? Which is hard to imagine but you never know. Or what are the servos lifting? If it’s lots of weight? Or is the charger given enough time to complete the charge?

is the battery puffy?

these are things you might want to check the ezrobot learn section about. Check the learn section on ezrobot website about proper battery care of their product. It would be unsafe to use a battery that has been neglected and puffy. It could explode - so they provide warnings on the website and in the packaging to properly review the tutorials and manual.

The op did say he bought a new battery and charged it for 1.5 hours. That seems quick.

Yeah - 1.5 hours is not long enough, unless the battery was already mostly charged. Once the EZ-B says "my battery is low", it would usually take a number of hours to charge (4-8 hours)

Thanks everyone for your help, The old battery was not puffy when I removed it from the EZ_B. It's almost the same battery that I use for my drone so I'm familiar with charging LIPO batteries and their danger signs.

6 of 8 servos are under load The "shoulder" servos are attached 20 CM long arm sections that weight about 1 KG so I'm near the limit of the HDD servos that come with the JD. Is there some kind of protection mechanism that shuts down the servos if are overloaded or overheat? I will try charging the battery again.

Here is the skeleton.

Very cool skeleton!

If the servos read hdd on the label, they have a shutdown feature if overloaded.

the weight that a servo can lift is calculated by not only the weight, but distance from the shaft. There’s information on our how a servo works page.  But it’s like you would have learned in school with lifting weight with your arm out. 5 pound weights at the end of your arm is way heavier than when it’s closer to you

You may want to think about ways to relieve some of the work of the servos, like using elastic bands that hold the arms in their "rest" position without any work fro the servos, so they are only working when you are in a different position.

Alan

Hey guys I've done the tests. A 1300mAhr ez-robot battery can go from drained to fully charged in 1.5hrs or less (usually 1hr 20mins). You have to remember that the ez-robot v3 charger charges the battery at 1A/hr.

@bmarkert1985 I would definitely recommend using a high current power supply as other have suggested for your build, you can use 5V (you just have to disable the battery monitor in the connection control settings) and it can even be an old computer ATX power supply if you have one.

Yes. You can lower the voltage feed to the 7.5 volt servos but will that lower the available torque? If so will it be a lot? The Op is probably pushing the HDD servos pretty close to the limit as is. Alan gave an excellent suggestion,  give the servos a helper like a band or a spring.

Have you seen your EZB reboot at all? If you are feeding servos directly from the ezb and your servos pull a lot of current away from the ezb it will reboot. To solve this you feed the servos directly from a property sized power supply and not through the ezb. If you are not having this happen that's a good thing. Just something to keep in mind as you move forward.

@Dave Yes, lowering the voltage does lower the available torque but not by much, a 2.5V difference isn't huge. The important thing is to have a power supply that can deliver the current.

The biggest strain will be on the shoulder servos (in the chest area), I wish I had a good idea to mechanically assist them but sadly I do not.

*Edit: Maybe a 3D printed worm gear design? Like inMoov.

2.5v difference from 7.5v is a 32% difference in energy potential. So it will be quite noticeable, but I’m with Alan and Jeremie on the assisted design as well. The shoulder servos are holding A TON of weight. The fulcrum calculation is gonna be off the charts for the length of the arms that the shoulder is holding.

A worm gear is a real good solution. Does the original poster have access to a 3D printer to make one?

A change in voltage will effect the speed of the servos, which in turn does effect the torque, but without the available current needed the motors aren't going anywhere. I guess my advice really is that a 7.5V high current supply is much harder to find than a 5V one, and it would be better to find a supply with the highest current rating possible rather than focusing on keeping everything at a higher voltage. 5V high current supplies are plentiful (ATX, Meanwell, Xbox 360 brick, etc). ;)

Thanks everyone for your support.

I will add the rubber bands (like tendons) great suggestion. I will also purchase a 5V 20 amp power supply. I redesigned the shoulders with a the 3D printed "strap" that now takes the load off the servo and transfers to the black exoskelton above still allowing the shoulder to rotate.  I agree that I'm pushing the limit of these HDD servos. The rotational inertia is high but I have built the skeleton to be very light but strong. See the pic. 

Keep us posted on your progress. This is very interesting.