Robot Batteries and Chargers

How to know which is the best battery to select? There are many options which at a glance can be a bit confusions.

What to consider when selecting a battery? 

• Chemistry
• Is vs Not Rechargable
• Discharge Rate
• Cost
• Voltage
• If rechargeable consider memory effect – means how well the battery is charged to full capacity.
• Weight
• Delivery Time
• mAHr (milliamp hours) – measures how many hours a battery will run when fully charged. For example, If you have a 1Amp load and a 10Ahr (10,000mAhr) battery, the battery will run for 10 hours in theory.

Create a spreadsheet that compares batteries on these dimensions. 

Batteries are one of the most expensive features of a robot. 

The one main consideration to determine if one battery or multiple batteries should be used for the robot.

Multiple batteries require less design time and can be more efficient. The drawbacks are that each function has a different battery supply different parts of the robot may stop working at different times and could take longer to charge multiple batteries. 

You will know if you need multiple batteries based on the voltage requirements. 

Your electronics, sensors, and actuators could operate at different voltages which would necessitate a different volt battery. 

Most microcontrollers have a built regulator to 5V to the I/O pins. If you pick a normal microcontroller its volt range is likely 9 to 12. So, if you pick a 12V battery and power both the micro controller and servo (rated 4V – 6V), you’ll burn the servo quickly. Therefore, that would be a scenario where different battery voltages would be required. 

One battery may be lighter weight and there’s only one battery to charge. However, it could be a bit more difficult to wire and may require a voltage regulator. 

Having different voltage requirements for your components requires a voltage regulator. The regulator may step up the voltage – for instance it could take an input of 6V and step up to 12V – meaning that we take a 6V battery and step it up to power the 12V. 

Voltage dividers allow you to power devices at different voltages. Most electronics are powered at 6 to 9 volts while motors may be 12 volts or higher. 

In my case I selected a 24 volt geared DC motor. 

The other option is to use one battery for your motor and one for your electronics. 

Always review voltage specifications for your electronics.

If you use a battery with voltage less than listed on your motor then the RPM will be changed fractionally. Meaning – if your motor voltage requirements are 24V and you use a 12V battery then your RPM will be 50% of listed. More contextually if your motor is 42 RPM and list listed as 24V but you use a 12V battery then your motor will perform at 21 RPM.

For quick reference >

12V: 10x 1.2V cells (always configured as one NiMh battery pack) or one 12V rechargeable lead acid battery pack. Many smaller DC gear motors and most microcontrollers work well at this voltage.

Large robots use 12V or 24V from one or more led acid battery packs.

Anything above 12V is usually reserved for very large robots. If you have a 14.4V LiPo or 18V NiMh pack from a cordless drill, keep in mind that finding motors which operate at these voltages is not easy.

Battery Chemistry Options

NiMh is the most common type of battery used in robots. It is rechargeable and has good price / capacity / weight. 

NiCd don’t have a good memory effect. If you don’t discharge the battery properly and then recharge it to its full capacity then you lose part of the capacity each time. 

Alkaline are the least expensive in the short term and provide a higher voltage than Nihm. However, you have to keep replacing them and they are not good for the environment. 

Lead acid is the cheapest option for high voltages but are typically used for medium sized robots due to their weight.

LiPo becoming popular due to light weight, high discharge rates, and relatively good capacity. However, they increase voltage in increments of 3.7v so you must plan ahead to use LiPo before selecting electronics. 

Types: 

• Battery Holders

Pros:

Power electronic components

Cons:

Will not be enough voltage to power a motor for larger robot

• Lead Acid and SLA Batteries

Pros:

Cheapest option and most widely used

Do not require circuit protection. Only require a fuse

Cons: 

Heaviest Battery

• LiFePO4 and Li-ion Batteries

Pros: 

Provide the best weight and size to power ratio

Transformers to charge your batteries along with an automatic battery tenders that will monitor your batteries charging progress and stop charging when the battery reaches peak voltage

LifePO4 is more stable than Li-On

Cons: 

LiFePO4 is more stable than Li-On but more expensive

Require specific under-voltage circuit protection to prevent the battery from dropping too low. If this happens the battery is unrecoverable. 

Lithium-ion require extreme care so they do not overheat, etc resulting in a fire or explosion.

Where to buy? 

SuperDroid Robots: https://www.superdroidrobots.com/electrical-parts/batteries

Amazon

How to connect the battery to the motor?

When using two 12V batteries you will need to wire them in parallel to power 12V motors and in series to power 24V motors.

If the motors receive insufficient voltage, say 12V to a 24V motor, the motor will either move very slow or not at all.

Sending 24V to a 12V motor will cause permanent damage to the motor and possibly start a fire.

Consider leveraging a voltage regulator to either step up or step down the voltage from battery to motor. 

Consider using hookup kit from SuperDroid Robots: https://www.superdroidrobots.com/electrical-parts/Wire-Cable-Hook-Up-Kits/Electrical-Hook-Up-Kits

As a test build a simple hookup between a motor and two 12V batteries relayed together

Reference this basic wiring guide: https://wiki.sdrobots.com/index.php/Basic_Wiring_Guide

Chargers and Transformers

When picking out your own chargers, it is absolutely critical that you pick the charger with the correct voltage and battery type. Not selecting the appropriate charger will be a fire hazard and cause permanent damage to the battery, charger and your property.

SuperDroid Robots Chargers: https://www.superdroidrobots.com/electrical-parts/Chargers-Transformers

Looks like for all battery types there are chargers. So, that is good.