Author Topic: the bigger the battery the better??  (Read 3441 times)

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Offline Carter05Topic starter

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the bigger the battery the better??
« on: August 20, 2008, 10:41:52 AM »
Is a 10000mAH battery better to buy than a 3800mAH battery? I'm not too concerned on cost difference, just usuability. I'm fairly new to robotics and I want a battery to last me many projects and many hours. I don't know if it would be better to buy say, two 3600mAH batteries or just one 10000mAH battery.

I plan on buying the Axon, would a 6v battery be a good choice?

Thanks!

10000mAH
http://www.all-battery.com/index.asp?PageAction=VIEWPROD&ProdID=2106

3800mAh
http://www.all-battery.com/index.asp?PageAction=VIEWPROD&ProdID=2198
« Last Edit: August 20, 2008, 10:45:06 AM by Carter05 »

Offline ArcMan

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Re: the bigger the battery the better??
« Reply #1 on: August 20, 2008, 11:21:30 AM »
Bigger doesn't necessarily mean better, just bigger.  More capacity means higher cost, larger size and more weight.  So what you need to do is estimate your robot's load and how long you want it to run before recharging.  For example, if your average load is 1A and you want it to run for 2 hours, then you will need a 2000 mAH battery.  If your load is 2A and you want it to run for 5 hours, then you could use that 10,000 mAH pack.

Offline MarkBrown

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Re: the bigger the battery the better??
« Reply #2 on: August 20, 2008, 12:37:38 PM »
If you  can handle 7.4 volts, I would recommend looking into Lithium Polymer, LiPo, batteries.  They range is size and wieght depending on the amperage you need but they are generally much lighter than a Nihm battery of the same rating would be.  Also, they are generally able to handle larger current draws.  I am currently using one to power my $50 robot MCU while I have another 4.8v pack powering the servos.  once I figure out what best way to regulate this power for servos and such I will be using one for my servos too.

Mark
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Offline Ro-Bot-X

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Re: the bigger the battery the better??
« Reply #3 on: August 20, 2008, 12:53:57 PM »
If you  can handle 7.4 volts, I would recommend looking into Lithium Polymer, LiPo, batteries.  They range is size and wieght depending on the amperage you need but they are generally much lighter than a Nihm battery of the same rating would be.  Also, they are generally able to handle larger current draws.  I am currently using one to power my $50 robot MCU while I have another 4.8v pack powering the servos.  once I figure out what best way to regulate this power for servos and such I will be using one for my servos too.

Mark

I am using Futaba modified servos by myself. I run them at 7.2V from a 6 AA pack of NiMH. They work fine, no overheating. 7.4V will also work. I want to get the whole robot running from a single battery of LiPo (2 cells in series), but I need to be able to recharge them together, not separate. I hate to have to take out batteries to put them to charge and then back. I just want to plug the robot to the charger, and when that will be possible, have it autocharge.
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Offline Carter05Topic starter

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Re: the bigger the battery the better??
« Reply #4 on: August 20, 2008, 01:30:34 PM »
I am using Futaba modified servos by myself. I run them at 7.2V from a 6 AA pack of NiMH. They work fine, no overheating. 7.4V will also work. I want to get the whole robot running from a single battery of LiPo (2 cells in series), but I need to be able to recharge them together, not separate. I hate to have to take out batteries to put them to charge and then back. I just want to plug the robot to the charger, and when that will be possible, have it autocharge.

That's what I'm aiming for, one battery to power everything. From the datasheet file, the axon can't handle 7.2v so I guess Lithium batteries won't work for me.

Offline MarkBrown

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Re: the bigger the battery the better??
« Reply #5 on: August 20, 2008, 01:45:38 PM »
I am using Futaba modified servos by myself. I run them at 7.2V from a 6 AA pack of NiMH. They work fine, no overheating. 7.4V will also work. I want to get the whole robot running from a single battery of LiPo (2 cells in series), but I need to be able to recharge them together, not separate. I hate to have to take out batteries to put them to charge and then back. I just want to plug the robot to the charger, and when that will be possible, have it autocharge.

There are tons of LiPo battery chargers out there.  In fact, for my helicopter batteries, all LiPos, I use an Astro Flight charger and a cell balancer.  The charger hooks up to the balancer and the balancer hooks up to the balancing wire on the LiPos.  The balancer controls the amps going to each cell of the battery making sure that all cells are charged to the same voltage.  All in all it takes about an hour, sometimes less, to charge a LiPo battery with a decent charger.  No tearing apart battery packs or taking them out of a robot, assuming you have access to the balancing wire or make some custom interface.

That's what I'm aiming for, one battery to power everything. From the datasheet file, the axon can't handle 7.2v so I guess Lithium batteries won't work for me.

If I am not mistaken, the Axon already has a 5v regulator.  If not, or if unsure, you can make a small little circuit that would accept the 7.4v from the LiPo.  Then send the regulated 5v to the axon and the unregulated to the servos.  I am not intimately familiar with the Axon and how it handles power or whether it allows multiple power supplies, but if it allows a power supply just for the controller chip and another for the output power bus then this would be a way to do it with one battery that is light and can handle high discharge rates.

Mark Brown
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http://marksproject.blogspot.com/

Offline Ro-Bot-X

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Re: the bigger the battery the better??
« Reply #6 on: August 20, 2008, 08:57:01 PM »
Quote from the Axon Datasheet:
Quote
Absolute minimum required voltage is 5.35V, while the recommended battery voltage is 6V to 7.2V. Maximum voltage at 20V, however most servos cannot handle above 7.2V before being damaged.

As you can see, it will handle LiPo cells with no problem, I'd say up to 4 in series (14.8V), so you can use a laptop battery to power some powerfull motors and the Axon's voltage regulator will take care of the 5V for the microcontroller and whatever sensors you plug into it. However, the greater the difference from 5V to the battery voltage, the higher the power waste (in disipated heat) by the voltage regulator. That means the voltage regulator will get at least warm if not hot. In this case, it is advisable to use a DC-DC switching voltage regulator from the battery voltage to the Axon (or any other microcontroller).

Edit: Axon is using a Low Dropout voltage regulator so it works with low voltages, like the datasheet says. To use it with higher voltages you better change the voltage regulator to a regular 7805, because this one will disipate less heat (greater voltage drop on it and different arhitecture inside), but you will still need a heat sink.
« Last Edit: August 20, 2008, 09:02:01 PM by Ro-Bot-X »
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Offline Admin

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Re: the bigger the battery the better??
« Reply #7 on: August 22, 2008, 08:38:15 AM »
Quote
Axon is using a Low Dropout voltage regulator so it works with low voltages, like the datasheet says. To use it with higher voltages you better change the voltage regulator to a regular 7805, because this one will disipate less heat (greater voltage drop on it and different arhitecture inside), but you will still need a heat sink.
That depends on how much current you take from the regulated power bus (not servo power buses).
« Last Edit: August 22, 2008, 08:39:47 AM by Admin »

Offline Ro-Bot-X

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Re: the bigger the battery the better??
« Reply #8 on: August 22, 2008, 05:54:34 PM »
Quote
Axon is using a Low Dropout voltage regulator so it works with low voltages, like the datasheet says. To use it with higher voltages you better change the voltage regulator to a regular 7805, because this one will disipate less heat (greater voltage drop on it and different arhitecture inside), but you will still need a heat sink.
That depends on how much current you take from the regulated power bus (not servo power buses).

Right! Forgot to add that... Thanks!
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