Need help on choosing battery

HI!
Noob here, so I need to power two continuous servos with an external battery and I don't know how to pick the right battery based on the servos' specifications. I will be using DF15RSMG which has 5-7.4v working voltage and 3A stall current. I might need to use the full potential of the servos so do I need to use a battery that is 7.4v and has a 3AH capacity? I might not able to find a battery with those parameters so what alternatives do you suggest? I need help on this so every comment or suggestion is much appreciated. Thank you!

It is usually not safe to power devices at the very top end of the voltage range. A 7.4 V LiPo battery can have voltage over 8 V when freshly charged. Plus you need special chargers for those batteries.

I would stick with 6V NiMH battery packs, like these. The larger the Ah rating, the longer the servos will run per charge.

Also, it is a common misconception that a battery with a given amp/hour Rating (such as 3AH) can provide that amount of amps at any given Point. They often provide much less at any given Point but of course for longer than an hour. So, a 3AH battery may be too small to cover a 3 Amp load.

LiPo cells usually have C rating which indicates the maximum current by reference to the capacity. Foe example I have some 240mAh cells with a 20C rating. That means they can produce 20x240 = 4800mA (4.8 amps). Not for very long, of course. At 4800mA the whole 240 would be gone in 3 minutes if my maths is correct. And discharging at the maximum rate will greatly shorten the life of the cell. Also, it is bad for LiPo cells to discharage them below a certain voltage (sorry, can't remember the figure and don't want to quote the wrong one).

New AA alkaline cells seem able to produce about 5 or 6 amps briefly - I have never done a long test. And rechargeable NiMh cells should be able to produce a much higher current.

...R

Thanks for the replies. How about Sealed Lead Acid Batteries? Do you think it will do the job?

A fully charged (6V nominal) lead acid battery is about 6.9 V.

baddarung:
Thanks for the replies. How about Sealed Lead Acid Batteries? Do you think it will do the job?

Possibly.

But you have not told us how long the system needs to work before the battery has to be recharged. And you have not told us what is the average current drawn by the system.

In fact you have not told us what the system is. If you do that you may get more useful advice.

...R

Robin2:
In fact you have not told us what the system is. If you do that you may get more useful advice.

The system I am working on is a Patient Follower IV stand using 3 ultrasonic sensors and 2 continuous servos. Human tagging is not involved since that is complex for me at the moment. The project is for patients that need to have a short walk eg. a trip to the WC so the battery needs to last for up to 10 minutes at most.

baddarung:
so the battery needs to last for up to 10 minutes at most.

Who is responsible for reconnecting the battery to the charger?

From your brief description I suspect the answer would be "not me".

If that is true then you need a sophisticated system that does not need to be charged after 10 minutes of use - once a day or once a week maybe - and which gives a clear notification that it needs to be charged before it is discharged so low as to damage the battery.

On the other hand, the weight of a substantial lead-acid battery may not be a problem as I presume the IV stand has wheels.

...R

The idea was to have the nurse charge the system or the battery at night when the patient is sleeping The system is expected to perform until it is scheduled "charging time". The weight of the lead acid battery is not an issue for I will use a standard mobile IV stand.

baddarung:
The system is expected to perform until it is scheduled "charging time".

That is what I suspected. Your suggestion of "10 minutes at most" in Reply #7 is very far wide of the mark.

IMHO you should be designing the system so that a normal day with heavy usage only discharges the battery about 15%. That way it can comfortably go for 2 days if someone forgets to plug it in at night. Also keep in mind that the practical capacity of most batteries (especially lead-acid) is about 50% of the sticker value.

You need to figure out approximately the average amps required when your system is in use and how many hours per day it is in use. Then you can calculate the amp-hr requirement.

If you don't already have a 6v lead acid battery you could probably do some tests with a pack of 4 new AA alkaline cells and measure the current with a multimeter. Most of the cheap multimeters can measure up to 10 amps. Your battery obviously needs to be able to supply the peak current required but that is usually only for a very short period. The average is what is needed to work out the required battery capacity.

...R