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What Batteries Should Power My Arduino Robot?

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Caleb Kraft
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a mix
Caleb Kraft   11/21/2013 1:57:03 PM
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I would look to the world of radio controlled vehicles for battery packs. My first inclination would be to power the electronics from small li-po cells for weight and space, but I'm not sure how well mixing batteries does.

Max The Magnificent
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Re: a mix
Max The Magnificent   11/21/2013 2:05:29 PM
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@Caleb: I'm not sure how well mixing batteries does.

My knee-jerk reaction is that there shouldn't be a problem if all they do is share a common ground ... but I learn something new every day...

Duane Benson
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Re: a mix
Duane Benson   11/21/2013 2:13:37 PM
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Max - I'd try to avoid separate batteries for the different systems. Pololu, whom you mentioned, has some innenxpensive DC/DC converters. This one here (http://www.pololu.com/product/2110) will give you 3.3V or 5V at 3.5 Amps.

It's bigger brother (http://www.pololu.com/product/2111) does the same at 7 Amps. The also sell a number of smaller DC/DC converters that will fit into a standard three terminal "7805" regulator foot print. Use one of those with a 12 V main battery and you'll be all set.

I use 12V SLA (sealed lead acid) for any of my larger robots - anything that might be drawing an amp or more - because they're pretty safe and simple. They are heavy and bulky though.

For my smaller bots, I tend to use LiPoly, like this one from Adafruit (http://www.adafruit.com/products/328). I've desined a charger chip into some of my robot boards so it's pretty convenient. But they're 3ish Volts, so won't do for 12 Volt motors.

Those are pretty much my two selections.

Max The Magnificent
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Re: a mix
Max The Magnificent   11/21/2013 3:09:10 PM
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@Duane: For my smaller bots, I tend to use LiPoly, like this one from Adafruit (http://www.adafruit.com/products/328). I've desined a charger chip into some of my robot boards so it's pretty convenient. But they're 3ish Volts, so won't do for 12 Volt motors.

These look very tasty -- certainly small enough to fit in my "engine room". The Adafruit page says "The output ranges from 4.2V when completely charged to 3.7V" -- so I could strap three in series to give me the 12V required. So I could have theee sets of three -- one for each motor.

The thing that scares me is how much I've forgotten over the years. For example, the Adafruit pages says the battery has a capacity of 2500mAh. My motors draw ~400mA. Does this mean that three batteries (a set of three stacked in series) can power one motor for 2500/400 = ~6 hours? That seems to be pretty impressive.

Actually, the Adafruit page says "This battery has a capacity of 2500mAh for a total of about 10 Wh." Now Power (in Watts) = Current x Voltage, so what does "2500mAh for a total of about 10 Wh" actually mean?

 

Max The Magnificent
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Re: a mix
Max The Magnificent   11/21/2013 4:04:34 PM
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@Duane: How about stacking a set of three in series to get 12V, then connecting three of these stacks in parallel to give me some "oomph" -- all of these could be located in the "engine room" -- then use this "super pack" to power everything?

Rcurl
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Re: a mix
Rcurl   11/21/2013 6:02:24 PM
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@MAX:"How about stacking a set of three in series to get 12V, then connecting three of these stacks in parallel to give me some "oomph"

That could work, but the charger would get to be pretty complicated.

Personally, I'd go for Sealed lead acid.  Use one big battery. For the lower voltages, try these: http://www.murata-ps.com/data/power/oki-78sr.pdf

 

Duane Benson
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Re: a mix
Duane Benson   11/21/2013 6:44:08 PM
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Max - My biggest fear with putting LiPoly batteries in series is the risk of fire if they aren't charged or discharged evenly. It may not be a rational fear, but none the less, it makes me nervous.

The charger chip I've been sing is a 3mm X 3mm part with almost no external components needed. I should look at building a triple charger to mitigate the uneven charging risk.

I'm not exactly sure how to put it all together though. The power in to the charger chips would have to be parallel, but the Vout from the battery would need to be in series. Has anyone reading here done such a thing?

Duane Benson
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Re: a mix
Duane Benson   11/21/2013 6:59:18 PM
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Max - re Wh / mAh:

In theory, "one motor for 2500/400 = ~6 hours" would be correct. In practice, it's not quite that easy due to factors such as the battery's discharge curve, rate of discharge and such. But's close enough and for the most part gives a relative picture.

I just rule of thumb it and shoot for about 50% head room. I'm going to see if I can design a multi-charger like I described in my prior comment. It could have nine battery connections to charge all nine individually, but discharge them in series/parallel for 12V out.

DA_100
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Re: a mix
DA_100   11/22/2013 3:28:13 AM
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Actually, the Adafruit page says "This battery has a capacity of 2500mAh for a total of about 10 Wh." Now Power (in Watts) = Current x Voltage, so what does "2500mAh for a total of about 10 Wh" actually mean

LiPo batteries are built in increments of 3.7v per cell. So 3.7V * 2.5A = 9.25 Wh

Electric model aircraft commonly use three cells in series giving 11.1V and in capacities of 500mA / 1000mA and 2200mA. These cells have common chargers which cost about £20 whilst the cells cost about £10 to £20 each set. The LiPo cells can be loaded up to 20 times thier rated capacity without damege. I have used them for years without any problems : just don't damage them! Your local model shop will show you what is available.

Hope that helps.

 

 

DA_100
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Re: a mix
DA_100   11/22/2013 4:52:08 AM
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Commercial LiPo multi cell battery packs have balancing connectors incorporated in them. These are monitored by the charger for each cell and the amount of current charging each cell is varied according to the state of charge of each cell. In this way each cell is optimally charged. The better cell packs are built from matched cells hence optimising the charging / discharging system.

Nickel Metal Hydride or Nickel Cadmium cells as used for model car racing are another option and again are available ready assembled. Automatic chargers are also cheap and both readily available from your local model shop.

I see that the free run current is about 300mA but the stall current is 5000mA. The LiPo battery being considered is rated at a maximum discharge of 2000mA so one could anticipate problems in the system voltage collapsing under stall or high acceleration conditions.  This is not a high discharge capable battery, it has built in self protection unlike the aircraft types which are still stable at 20C.

As an aside at 200rpm and 4" diameter wheels the speed would be (200 * Pi * 4) / 60 inches per sec = 42 ins/sec = 3.5 ft/sec = 2.4 mph which seems reasonable.

 

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