Work to Charge Ratio – What it is and Why It Matters

Work to Charge Ratio can be defined one of two ways. 1) Robot working time divided by robot charging time. 2) Number of robots working divided by number of robots charging.

If you've found yourself Googling "what is work to charge ratio?" I'm here to help.

Work to Charge Ratio is the comparison of the time your robots spend working to the time they spend charging.

This ratio helps you track the charging needs of your industrial mobile robot fleet so one day you can hopefully keep 100% of your fleet running 100% of the time.

Never heard of this before?  That's ok!  I wrote this comprehensive guide just for you.

If you want to hop to a specific topic, use the table of contents to get started; otherwise, read on.

Work to Charge Ratio in 60 Seconds or Less

I recommend reading this whole article if it's your first time, but if you just want to hit the highlights, this section is for you.

  • Work to Charge Ratio is the measure of how much time a robot spends working compared to how much time it spends charging.
  • To calculate Work to Charge ratio, divide working time by charging time.Work to Charge Ratio is Defined As Robot Working Time Divided by Robot Charge Time
  • For example, a robot working 4 hours then charging for 1 hour has a work to charge ratio of 4:1.  A robot working 8 minutes then charging for 2 minutes also has a work to charge ratio of 4:1.
  • On a macro level, you calculate Work to Charge Ratio by counting how many robots are working and how many are charging.  Do you usually have four robots working while one charges?  Then that's also a work to charge ratio of 4:1.
  • Sometimes it's helpful to convert the work to charge ratio to a percentage.  For example, a 4:1 work to charge ratio means a robot or robot fleet works 80% of the time and charges 20% of the time.
  • Do you have a fleet of 100 robots with a work to charge ratio of 4:1?  Then at any given time, you have 80 robots working while 20 robots are sitting (to charge)!
  • If your robots cost $50,000 each, that's a million dollars' worth of robots (20 * $50k) sitting there doing nothing but charging right now.
  • There are many causes for robot downtime, but downtime due to charging is the single largest productivity killer in an industrial mobile robot fleet.
  • Downtime due to charging can inflate the overall cost of a mobile robot fleet by as much as 33%.  Eliminating this downtime can cut the costs of a robotics project by 25% or more.
  • There are many ways to improve your Work to Charge ratio, depending on the application.  For simple operations, a battery upgrade might solve your problems.  For more demanding operations, a perpetual power solution like CaPow Genesis can keep mobile robots running non-stop.

What Is Work to Charge Ratio?

As I said in the introduction, Work to Charge Ratio is a measure of the time a robot spends working compared to the time a robot spends charging.

This metric can be applied to fleets of industrial mobile robots as well - it just depends on the application.

To calculate work to charge ratio, just divide working time by charging time.

Work to Charge Ratio Definition

For example, a robot that works for four hours then charges for an hour has a work to charge ratio of 4:1 (four to one).  The following is also true:

  • A robot with a 4:1 work to charge ratio spends 80% of its time working and 20% of its time charging.  The robot could work for 4 minutes and charge for 1 minute, or work 48 minutes and charge for 12.  The work to charge ratio stays the same.
  • Work to charge ratio doesn't take battery size or time between charges into account.  It's best measured as an average over several charging cycles or several robots running the same mission.
  • A work to charge ratio remains true as your robot fleet grows but it's hard to time several different robots running at the same time.  Instead, just count how many robots are working versus how many are charging throughout the day and use that to calculate your work to charge ratio instead.
    • If you have four robots working and 1 charging most of the day, that's a work to charge ratio of 4:1.
    • A fleet of 80 robots working while 20 charge also has a work to charge ratio of 4:1.

In an ideal world, you would have 100% of your fleet running 100% of the time.  Keep reading for recommendations for how to get there.

Why is Work to Charge Ratio Important?

Because optimizing your Work to Charge Ratio increases fleet efficiency and saves you money!

Let's look at a real-world example:  I spoke with a manufacturer last year that was planning a big automation project.  To fully automate all their processes, this company needed 100 AMRs that each cost $40,000 dollars.

100 Robots * $40,000 = $4,000,000 USD

So the proposed cost of this project was $4 million dollars... before factoring in downtime due to charging.

The robotics integrator designing the solution estimated this manufacturer would need an extra thirty robots to account for downtime due to charging.

30 Extra Robots * $40,000 = $1,200,000 Extra USD

So the $4 million dollar robot project turned into a $5.2 million dollar robot project just because of downtime due to charging.

And was just the cost of the robots - don't forget about charging docks, batteries, deployment services, integration services, and all the other overhead that will drive the price up  even further!

Thankfully, this is overage is avoidable.  If you can keep 100% of your fleet running 100% of the time, you'll never have to buy extra robots to account for downtime due to charging again.

YES, you should definitely buy a couple extra robots to account for routine maintenance and miscellaneous failures, but stop buying extra robots just to have them sit on chargers!

How to Improve Work to Charge Ratio

There are lots of ways to improve a robot's work to charge ratio.  Since we can't really slow down the power consumption (working time) of the robot, we have to focus on fixing the power delivery (charging) and power storage (batteries) instead.

We've separated our recommendations into two sections:

  1. High-demand production environments - where downtime is very expensive so robots have to run autonomously 24x7.
  2. Less demanding environments - typically these areas feature collaborative mobile robots or other processes that don't yet fully rely on automated material handling.

Optimizing Work to Charge Ratio in High-Demand Production Environments

Robots are expensive.  Downtime is expensive.  Here's how to avoid overspending on both.

Opportunistic Charging

Remember for a moment the goal of this article is to help you optimize your Work to Charge Ratio so you can eliminate downtime due to charging.

We do that by measuring work time against charge time, but this doesn't show us the whole picture of your robots' actual productivity.

  • What if the robot has to drive away from the production area to get to its charger?
  • Does the robot keep working through lunch, breaks, and shift changes?
  • What if the robot is sitting still, just waiting to be loaded?

These are all cases where the robot looks like it's working because it's discharging the battery, but it's not actually delivering value to your operation.

The way to fix this is opportunistic charging.  Let your robots "take a break" and charge when they're otherwise idle during their normal cycle.  Here are some ideas to get you started:

  • Put chargers as close to the production process as possible to decrease travel time.  Putting chargers in the robot's regular path is best!
  • Design missions so they align with personnel breaks to take advantage of human downtime - use this time to recharge instead of discharge your robots.

Wireless Charging

Xynergy Industrial Wireless Charging
An industrial wireless charger from Xynergy

Opportunistic charging is great, but it's very challenging to install industrial robot chargers along the robots' path unless they're wireless.

Some robot OEMs recommend dedicating over 100 square feet to a single charging dock.  Good luck finding that much open space near a production line!

Industrial wireless chargers like the Xynergy charger pictured above offer wireless fast charging and very flexible mounting options.

Perpetual Power

Mobile Robot with CaPow Genesis Full Solution - Transmitter, Receiver, and Super Capacitor
Fully CaPowered Mobile Robot with CaPow Genesis Perpetual Power - Transmitter, Receiver, and Super Capacitor

When your robots just don't sit still enough for regular wireless charging to keep them running 24x7, then CaPow Genesis is the way to go.

Genesis is like industrial wireless charging on steroids because it charges your robots while they sit still AND it's the only solution I've seen that lets your robots charge while they move.

Ultimately, CaPow Genesis is the only solution that guarantees 100% uptime for 100% of your fleet, so if downtime due to charging is killing your productivity, we should probably talk.

Learn More About CaPow

Want to learn more about CaPow and what perpetual power can do for your robot fleets?

Other Ways to Improve Work to Charge Ratio

If your facility is not yet fully automated and can tolerate some robot downtime  due to charging or operator intervention, here are a few really cost-effective ways to increase the Work to Charge Ratio of your robot fleet.

Hot-Swappable Batteries

Illustration of Thouzer Lite and its Hot-Swappable Batteries
Optional Hot-Swappable Battery for the Thouzer Lite AGV

Hot-swappable batteries are like having a cheat code to the work to charge ratio.

The work to charge ratio formula assumes your robot can either work OR charge, but not do both at the same time.

With Hot-Swappable batteries, the work to charge formula breaks because you can charge several "backup" batteries while the robot works, then quickly change them out as needed.

No more stopping to charge!

All you have to do is stop the robot long enough to swap batteries, which means you can keep running almost 24x7.

Note: Hot-swappable batteries is a very uncommon feature among mobile robots today.  Most robot manufacturers don't want you messing with their power systems but it's worth asking if this feature is supported.

External Power Packs

If your robot doesn't support hot-swappable batteries, maybe you can use an external power pack to extend battery life instead.

A Picture of a Jackery Portable Power Pack
The Jackery Explorer 1000 Plus. ~$1100 on Amazon.

For example, the external power pack above more than doubles the runtime (and work to charge ratio) of a standard Thouzer and you can find them on Amazon for about $1100.

Upgrade Your Batteries

Inventus Power S24V20-U1 Lithium Battery
A 24V 20A Lithium Iron Phosphate (LiFePO4) battery from Inventus Power. Note the RJ45 CANBus connections in the center of the lid.

Remember - battery capacity doesn't help your work to charge ratio, so if you're looking into battery upgrades, you should be looking for:

  1. A high max charge rate first, then
  2. Capacity, and finally
  3. Battery lifespan.

Generally speaking, if you have lead-acid batteries today, upgrading to lithium ion or some other fast-charging technology will drastically improve your work to charge ratio.

Not only do lithium batteries charge faster than lead-acid, but they also offer a better depth-of-discharge as well, which means more of the battery's capacity is available to do actual work.  Just make sure you do your due diligence to make sure your robot can handle different battery types/sizes.

Upgrade Your Battery Chargers

A Delta-Q IC650 Battery Charger
An example of a wired battery charger. This brand, Delta-Q is very popular and can accommodate many different types and sizes of battery.

If you upgrade your batteries you might need to upgrade your battery chargers at the same time.  Just make sure you find a charger that matches your (new) battery type.

Remember - faster is better, but follow all OEM (battery, charger, and robot) recommendations and limitations along the way!  Don't fry your robot!!!

Optimize Your Charging Process

Most robots with a fleet manager take care of this for you, but if you are responsible for configuring your robot charging cycles, you can probably do some tuning to optimize your work to charge ratio.

Why?  Because robots and their batteries generally don't charge in a straight line.Lithium Ion Battery Charging Performance

As you can see from the example above, if we charge this battery at 1C, it takes the same amount of time to charge from 0 to 95% as it does to charge from 95% to 100%.  Don't wait an hour for that extra 5%!

Every battery has its own charging curve so you have to figure out when your batteries charges fastest and then build your charging cycles around that.

Final Notes

There are lots of ways to maximize your work to charge ratio and squeeze every last bit of efficiency out of your mobile robots and I hope this article has given you some ideas for how to optimize your own facility.

If you still need help or think I've missed something here, please leave a comment below or contact me to let me know.  I do check comments and form submissions regularly!

Learn More About CaPow

Want to learn more about CaPow and what perpetual power can do for your robot fleets?

Frequently Asked Questions

Work to Charge Ratio is a measure of the time a robot spends working compared to the time a robot spends charging.

To calculate work to charge ratio, just divide working time by charging time.

Work to Charge Ratio Definition

Optimizing your Work to Charge Ratio increases fleet efficiency and saves you money by helping you right-size your robot fleet.

Each mobile robot OEM handles this differently.  If there's no an easy way to monitor work to charge ration in your fleet management software, submit a feature request for it!

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