Battery selection for large robot - understanding Amp @ voltages...

Good morning all,

The end goal of my questions is to determine the best battery to use for my robot.

I have totaled up all the current draw for all the electrical components and it comes out at 8.3 amps.

The battery voltage needs to be 24 volts in order to have enough power to drive the Nema 34 stepper motors effectively.

The 24 Volts are stepped down to 12 volts using a DC-DC converter which I believe is for automotive or marine use. That is further stepped down to either 5 volts or 3.3 volts for the various sensors, arduinos, and Raspberry Pis.

At 24 volts, the two stepper motors, while powered, should draw about 1.2 Amps each (they are 3.5 amp steppers). At 12 volts there will be less than 1 amp of draw.

All the rest of the current consumption is in the 3.3 to 5 volt range.

2.4 Amps consumed at 24 volts
1.0 Amps consumed at 12 volts
4.9 Amps consumed at 5 volts or less

Obviously, not everything is running all the time, however, it is possible. Now comes all the questions....

As far as discharge rate is concerned on the battery, ultimately all current draw is at the 24V level. I am guessing that my 4.9 amps of draw at 5 volts is probably a different amount at 24 volts - guessing it would be less than one amp? What is the conversion or how is this calculated?

Regarding Amp Hour rating, it is my understanding that this rating is the amount of Amp that the battery will provide in one hour of time after which it is drained. So, a 10 Amp hour battery can only provide 10 Amps at the batteries voltage for 1 hour at which point it should be charge again. Is this correct?

Finally, what battery chemistry would be best for this type of application. I have SLA on a similar robot and it works, but I would like something that weighs less and has a better discharge rate.

Thanks in advance!

Chris D

Chris_D:
As far as discharge rate is concerned on the battery, ultimately all current draw is at the 24V level. I am guessing that my 4.9 amps of draw at 5 volts is probably a different amount at 24 volts - guessing it would be less than one amp? What is the conversion or how is this calculated?

5V*4.9A = 24.5Watts

Current draw at 24Volts =24.5W/24V = 1.02A

This would be true only if your buck converter was 100% efficient. So you have to measure the current draw using your multimeter to determine how much current the buck converter actually draws in order to determine it's conversion efficiency.

Chris_D:
Regarding Amp Hour rating, it is my understanding that this rating is the amount of Amp that the battery will provide in one hour of time after which it is drained. So, a 10 Amp hour battery can only provide 10 Amps at the batteries voltage for 1 hour at which point it should be charge again. Is this correct?

Depends on the battery chemistry. A 10Ah battery may actually provide only 7 or 8 Ah if your current draw is at 10A. Just look for their discharge curve at different current draws.

Also the discharge current capability of batteries depends. Some batteries can only provide a small current output compared to their Ah rating. For example a CR2032 battery may have 220mAh capacity but may only provide less than ~10mA of max discharge current on continuous use.

Chris_D:
Finally, what battery chemistry would be best for this type of application. I have SLA on a similar robot and it works, but I would like something that weighs less and has a better discharge rate.

I would recommend a LiMn2O4 (Lithium Manganese) or LiFePO4(Lithium Iron Phosphate) in either 18650 or 26650 size used in series as they can provide very good discharge current and are lightweight compared to Lead Acid batteries.

But you're not mentioning the most important criteria when selecting a battery - For how long do you want to run your project on your batteries.

Update: If you have difficulty determining which type of Lithium ion cell you need then you can check this site.
BU-205: Types of Lithium-ion - Battery University - Look for the term"Discharge (C-rate)"

The normal thing to do that is to add up the power consumed not the currents:
2.4A @ 24V = 57.6W
1A @ 12V = 12W
4.9A @5V =24.5W

Total power 94.1W, allow a bit for inefficiency in the converters so call it 105W

105W @ 24V = 4.375A

Your understanding of capacity is basically correct, though it's usually wise not to use the WHOLE capacity before recharging, more like about 80%. So 10Ah battery will deliver about 8Ah, so 8A for an hour or 4.375A for about 1.8 hours.

I would use Lipo batteries for lower weight than an SLA but you need a proper Lipo charger for them and some care when using them. A 6S lipo gives around 24V.

Steve

Thank you very much for your quick responses and clear explanations!

I am off the the battery university to investigate the types of battery to get.

Chris D

Chris_D:
Finally, what battery chemistry would be best for this type of application. I have SLA on a similar robot and it works, but I would like something that weighs less and has a better discharge rate.

SLA is probably the cheapest.

LIPO should be the lightest but is quite expensive, also has safety implications.
Used where size / power to weight is important , as in RC aircraft, phones , laptops.

LiFePO4 batteries have similar performance from a power to weight ratio but are bulkier..

These are often used in power tools and electric bikes ETC.

"Big" is a relative term. How big is big?