Choosing a battery for a small first robot

[Warhawk87]

Hey guys,

I just read the tutorial for building a first robot and I just wanted som advice for choosing a battery.  I've being looking around All-Battery.com and I noticed some decent batteries that say they are for airsoft.  Is there anyway I can use them for my purposes instead of airsoft? If not what types of batteries are good for my purposes?
*FYI* my purposes are a small uncomplicated robot that I can easily build and expand later on to make a decently complicated robot

Thanks in advance!

[Ro-Bot-X]

Find a 6V 1000-2000 mAh battery or a 7.2V 1000-2000 mAh battery. Make sure you also get a proper charger for the chemistry you choose. NiMH or NiCd batteries are more common and do not need expensive chargers. You can also get 6 AA NiMH cells and charger from Walmart and install them in a battery holder.

[Warhawk87]

Alright thanks! Is there any major difference between the 6 and 7.2v batteries?

[SmAsH]

Besides the voltage? No.

[chelmi]

Alright thanks! Is there any major difference between the 6 and 7.2v batteries?

with a 6V battery, you will need to use a low dropout voltage regulator, not a "regular" 7805.
The advantage is that you can use the same battery to power the microcontroller and the servo.

[Warhawk87]

Alright thats good to know i would have used the battery to power both regardless.... Now that i know not to do that, what is a low dropout voltage regulator?

[SmAsH]

It is a voltage regulator that requires less than a normal one to produce the same voltage eg.
A lm7805 needs about 7v to make 5v whereas an ldo regulator only needs, say 5.5v to make 5v.

[Warhawk87]

And i need a regulater if im going to connect a battery to a microcontroller or a motor?

[chelmi]

Microcontrollers need regulated voltage. For motors it's better to use unregulated voltage. This is because of the way linear voltage regulators (either standard or low dropout) work. To regulate the voltage they "waste" the excess voltage by transforming it into heat. This is ok for low power components like microcontrollers, but not very efficient for motors. You battery will last a lot less if you plug your motor on regulated current.

Chelmi.

[SmAsH]

And if those motors are servos i would recommend using a 6v battery and a 5v ldo regulator.

[Ro-Bot-X]

I have used 7.2V with GWS servos directly. Most servos do not get damaged and higher end servos, designed for robotics, work with voltages up to 12V. However, cheap servos DO get damaged fast at 7.2V. I have burned a few HXT900 when the servo tried to move from one end to the other and encountered an obstacle in the way. I like the fact that a higher voltage makes my DC motors run faster and keeps the microcontroller from reseting (I am using a LDO regulator). So, to protect my cheap servos I had to use an expensive solution: an adjustable switching regulator that I could adjust to output 6V and it works until the battery drops dead and no other electronics work anymore. Now I have to keep an eye (read analog pin) on the battery voltage...

However, DO NOT power the servos with 9V like I did by mistake. When you apply power to the servos, they will twitch, at 9V, a servo that was close to the end stop twitched and broke a few teeth and burned the electronics.

[Warhawk87]

Ok, so to paraphrase, i should buy a 6 volt battery and a 5 volt low dropout regulator to use for the microcontroller, but plug motors in straight to battery correct?

[Warhawk87]

Also, i wanted to have 2 dc motors to run the wheels but after a quick glance i only found 12v and higher dc motors.... If i go with a 6v battery, does that mean im stuck with servos?

[SmAsH]

Well, you could have two battery packs?

[Joker94]

I use 2 battery packs for my systems if i am using DC motors. The reason this is depending on the current draw by the motors the servos can become jittery and many other things dont work as smooth.

For This reason all of the systems i run with DC motors have a motor battery pack and a 6v battery for the servos and micro controller.

Hope this helps

Joker94

[Ro-Bot-X]

Joker94,

With an adjustable switching regulator like this one, you can easily use only one battery on your robot. You adjust the regulator to output the voltage you want the servos to run at and it doesn't matter how low the battery will go when the motors draw a lot of current. Most of the times, a 2 battery system is harder to charge, if you want to build a charging station for your robot. And the price of the regulator is about the same as the price of a 6V battery plus charger. Plus, your servos will never be jittery because of power problems. Instead of monitoring 2 batteries, your robot needs to worry only about one and if this one gets too low to drive, the robot will still be able to wave his hands or look around for help...

Also, a 5V switching regulator would be nice if your battery is 12V or more...

[Warhawk87]

Thanks for all the advice everyone!

From the comments posted, I think I am going to go with a 6 volt NiMh battery with an adjustable regulator(For the purposes of being able to expand) to control voltage to the microcontroller.  For my first robot, I think I will just use continuous servos to power the wheels, as the dc motors seem to be more heavy duty.  (I'm going to make a new topic about motors and torque and what I need to make the robot move if you want to help with that) Im going to need help with choosing a microcontroller too since I know nothing about them either... But thats all for another thread!

If you have any more suggestions on powering my robot (I've decided it will be about five pounds, three servos and some photoresisters), I will continue to look at this post.

Thanks for all the help!

[voyager2]

I have been searching for an appropriate battery for my robot fish.
Are there any that you recommend.
Needs to be small.
Needs to be light.
High energy density.
Thanks
 

[waltr]

I have been searching for an appropriate battery for my robot fish.
Are there any that you recommend.
Needs to be small.
Needs to be light.
High energy density.
Thanks
 

LiPo
I have been using a pair of these:
http://www.sparkfun.com/commerce/product_info.php?products_id=8483
wired in series for my latest robot. They last much longer than equivalent physical sized NiCads and their self discharge is almost nil.
Just get or build a charger designed for LiPo and fixed to the number and size of the cells you use and there wouldn't be a problem. I'm using an MCP73842-840I/UN from Microchip set to 275mA for the charge circuit.

[Warhawk87]

Hey I have had an interesting thought, Most of these batteries come with a plastic piece on the end for you to plug it into things, and I thought you could just cut it off and work with the bare wires...  But if you do that, how do you charge the battery?  The charger and the battery connect via those plastic things i mean to cut off....

[waltr]

On my bot I have a board that the two LiPo's plug into to make the series connection. The board also has the charger chip and a few linear regulators (the Sharp IR sensors like have their own regulators). I use a SPDT center off toggle switch. Center is all off, one direction is Bot on, the other is charger connected. The board then has a coax power connector for 12V in to charge the battery.

[Warhawk87]

So you have the charger wired in with the rest of your electronics and u just toggle to charging with a switch?

[Razor Concepts]

There are also ICs that enable you to power the electronics with the battery and charge it at the same time also.

MAX1555 for example.

[Warhawk87]

Putting the charger on the robot is an interesting thought... I asume that the charger would work through a connection via breadboard correct?

[Warhawk87]

Also, if I get a battery like this http://www.all-battery.com/12v2600mahnimhbatterypack11620.aspx do I get the charger with it? It doesnt mention it and since you guys do this all the time i figure you'll know.

[waltr]

No, a charger must be purchased or built separately.

Putting the charger on the robot is an interesting thought

For my dual LiPos I use the Microchip MCP73842 chip and a MOFET. These are pretty small so the charge circuit is less than 1/2 x 1/2 inch.
One reason I did this is because I always seem to misplace the charger. This way any 12V DC power source can be plugged in. And since the newer charger chips are so small having the charge circuit built into the Bot is very convenient.

There are also ICs that enable you to power the electronics with the battery and charge it at the same time also.

The charge chip I use could be left connected to the battery but I like playing it safe (left too much smoke out of parts). Having the circuits power on a Bot while its plugged into the wall (12V AC-DC adapter) is a none issue at the moment since the Bot can't go exploring while tethered.

A future plan is to have the Bot find the power source and connect itself. I'll then modify the circuit to allow a power connection and battery charging to take place with the Bot powered. But there's just too many ideas and not enough time to try them all.

[Warhawk87]

No, a charger must be purchased or built separately.

Well thats a bummer....

For my dual LiPos I use the Microchip MCP73842 chip and a MOFET. These are pretty small so the charge circuit is less than 1/2 x 1/2 inch.
One reason I did this is because I always seem to misplace the charger. This way any 12V DC power source can be plugged in. And since the newer charger chips are so small having the charge circuit built into the Bot is very convenient.

So you built your own charger like that?  What about battery specifications? will that arrangement work with a 6 volt the same as with a 7.2 or a 12 volt battery? and what about current?  Or does the microchip take care of all that?

[waltr]

So you built your own charger like that?  What about battery specifications? will that arrangement work with a 6 volt the same as with a 7.2 or a 12 volt battery? and what about current?  Or does the microchip take care of all that?

No.

The charger needs to be designed for the battery chemistry (LIPo, NiCd, etc) and the number of cells. Many manufacturers make battery charge chips. Search for one by battery type (chemistry).
Either the external components or the charge chip itself is set-up for the number of cells: different battery types have different voltages per cell nominal as well as minimum voltage without damage and the recommended constant voltage during charge.
The charge chip, controller, will take care of the various conditions to proper charge the battery.
I did give the Microchip part number so anyone reading the post can go look up the data sheet.

Also go the Linear Tech, Analog Devices, Texas Instruments for their offerings in battery charge controllers.

[Warhawk87]

I dont quite understand how all this works, but after a curory search I found this (http://www.maxim-ic.com/datasheet/index.mvp/id/1666) and it looks like what your talking about for a NiMH battery like the one im looking to use.  Problem is, I dont know if its any good, and it says it needs external components like capacitors and resistors to work.  I'm not sure if I really need them or if thats for something else, but all in all Im just looking for an opinion... Thanks!

[chelmi]

My 2 cents:

Buy a charger, it's cheap, guaranteed to work and safe.

http://www.all-battery.com/smartuniversalchargerfornimhbatterypack24v-72v.aspx

Just make sure the connector is compatible with your battery pack. Buy an adapter if this is not the case.

Chelmi.