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Author Topic: What battery to use ?  (Read 509 times)

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Offline AtussTopic starter

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What battery to use ?
« on: December 29, 2011, 05:18:59 PM »
how would you power this motor ? http://www.greenweld.co.uk/acatalog/Shop_LowVoltage_347.html#aQ0636 , if it uses 9V and 2A than a single 9v 280mah battery would last less than 14 minutes  ? (2A = 2000ma, 280ma / 2000ma = 0.14 = 14 minutes) ? Am i right with my calculations ? I thought that it would last longer. The other option in this case would be 8 x 1.2v 3000mah AA batteries but they would weigh a lot more. How would you go about this ?

Offline Soeren

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Re: What battery to use ?
« Reply #1 on: December 29, 2011, 07:11:23 PM »
Hi,

how would you power this motor ? http://www.greenweld.co.uk/acatalog/Shop_LowVoltage_347.html#aQ0636 , if it uses 9V and 2A than a single 9v 280mah battery would last less than 14 minutes  ? (2A = 2000ma, 280ma / 2000ma = 0.14 = 14 minutes) ? Am i right with my calculations ? I thought that it would last longer.

This motor can draw a whole lot more, when loaded, I measured a draw of around 8..9A IIRC and the inrush and stall current is nasty as well.

A 9V PP3 won't be able to drive this motor, as its internal resistance is way too high.
Besides that, you cannot just calculate a 14 minute discharge from the nominal (5 hour) discharge capacity, as I seem to have written on too many occasions in the last few weeks - use forum search if you want the explanation.


The other option in this case would be 8 x 1.2v 3000mah AA batteries but they would weigh a lot more. How would you go about this ?

8 NiMH AA-cells would be able to cope for a while, but no need to use 16 cells for 2 motors as you asked (no reason for PM's - that's just keeping answers from helping others), as they can both use the same supply.

However, the 6V motors I linked you to would be a much better option for you at this moment. For some odd reason, some newbies seem to think that bigger is better and want to make an extremely complicated huge robot for their first try, even if they have to be led through each and every step of the way, rather than making something smaller, which they could actually build with a minimum of guidance AND learn from - I'll never understand this ignorance towards advice and common sense - After all...
It's better to succeed with something small than fail at a biggie!
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline AtussTopic starter

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Re: What battery to use ?
« Reply #2 on: December 30, 2011, 08:39:11 AM »
You got me completely wrong, the only reason why I asked how to power this motor is so that I could buy all the batteries that I will need at the same time (therefore save money) rather than buy some to power the smaller motor and once I want to go on to the stronger motor buy more batteries (same goes for the charger, if I will need 9V batteries in future than I will buy a battery charger that can charge both 9v batteries and AA batteries (and save money) rather than buying two separate one ). Also the only reason I am buying this motor is so that I can save some money on postage and buy it together with the smaller motors and other things. I am still going to use the smaller motor for my first experiments and bots - learn from it and than go onto something bigger. Hope I have cleared this up. So what batteries should I buy ? :-\

Quote
(no reason for PM's - that's just keeping answers from helping others)


That is what I thought and that's why I have created this topic on the forum  :)

Offline Soeren

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Re: What battery to use ?
« Reply #3 on: December 30, 2011, 02:24:49 PM »
Hi,

[...] so that I can save some money on postage and buy it together with the smaller motors and other things.
OK.
I don't know how shipping within England is priced, but they had to know the total weight of what I bought before they could tell me the shipping price (and they sent it in two separate boxes :-/). So there may or may not be a saving in getting it all at once (depending on national rates).

I just tested one of the motors and it won't start running at anything less than a setting of around 1..2A, but that may be the foldback current limiting of the PSU I used, as it dumps the voltage to close to 0V on over current draw.

It starts reliable at around 0.7V, as long as it's allowed the current it needs - stall current is around 2.5A to 3A at 0.7V.
At 3V, you cannot stop the axle with your fingers without getting a friction burn and it can take 12V just fine (but I'd add a temperature sensor if I was to constantly overdrive it).


I am still going to use the smaller motor for my first experiments and bots - learn from it and than go onto something bigger. Hope I have cleared this up. So what batteries should I buy ? :-\
All cleared up :)
Since the larger motor accepts a higher voltage and the controlling element (transistor switch or whatever) will have some small voltage drop, not forgetting the discharge voltage range of the battery, I'd go for 10 AA's, which would be from ~14.5V fresh out of the charger, down to ~9V (at that high a current, ~11V on low current draw).
This will ensure full speed until the batteries are discharged and using PWM, you would get 9V from the fully charged battery by using a duty cycle of 62%

For the 6V motor, 6 NiMH AA-cells will give you from ~8.7V down to around 6V over the discharge life of the cells (again depending on discharge rates), with a duty cycle of 69% to get you 6V initially.

So, 10 cells for the large motor and 6 cells for the 6V motor is my best recommendation.

If you search a couple of R/C-sites, you might find battery packs with an equivalent number of cell at lower prices and/or higher capacity, but stay off Lithium based batteries (anything starting with "Li") until you know enough about them to stay safe.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline AtussTopic starter

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Re: What battery to use ?
« Reply #4 on: December 30, 2011, 03:05:02 PM »
From a quick search I can say that battery packs seem to be quite a bit more expensive, I can get 8 1.2V 2900mah batteries for £11 and a battery pack that gives me the similar  power seems to cost twice as much .

Example :

 http://www.storacingproducts.com/hpi-plazma-7-2v-2400mah.html - battery pack

normal batteries

 http://www.ebay.co.uk/itm/AA-8-x-2900-mAh-NiMh-Rechargeable-Batteries-Vapextech-/220368454050?pt=UK_ConsumerElectronics_Batteries_SM&hash=item334efbc1a2

« Last Edit: December 30, 2011, 03:07:13 PM by Atuss »

 


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