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Batteries

An Open Letter To Designers & Manufacturers Of Portable Power Stations

In recent years there has been a huge rise in the demand and types of portable battery storage systems available to consumers. This has brought an industry that practically did not exist 10 years ago to the forefront. Customers can now buy portable batteries that can run appliances, with sizes ranging from 85Wh to almost 20kWh! Not to mention the smaller powerbanks that are USB-powered to charge smartphones/tablets and laptops, with even smaller versions can be found as low as 2000mAh and can fit in our pockets.

This open letter is about the features I think power stations should have that would increase their utility and make them more consumer friendly. Which also increases sales.


Sizing:

There are many sizes of power stations on the market, and customers don’t always understand what size they need. Also many consumer appliances don’t tell you their wattage prominently on the package (and most people don’t realize they can read the fine print on the label on the back to find peak draw) and many devices are power hungry. Hence careful sizing of your offerings is paramount.

Frankly anything under 250W for a plug-in appliance power station is a gimmick. You can get 85W USB/120V power stations, but they don’t handle much more than charging phones/tablets and maybe a partial charge of a laptop. With 250Wh minimum you can charge almost any laptop and run very small appliances for a reasonable amount of time, LED light bulbs, radios, some CPAPs without the humidifier, and so forth.

500Wh is the next size up. At this size you can run everything from most CPAP machines (with humidifiers) to laptops to charging cordless power tools, a few hours for most TVs, a portable fridge for most of the day, and so forth.

The benefits of 500W are lower relative cost and low weight/portability. This is about the smallest size most people should aim for when buying a portable power station.

1kWh is great for day trips and passable for emergencies. You can run many appliances off it, it can double as emergency power for the home, it would be very tight, but it can keep a fridge running for up to a day or power many LED lights and it can be recharged with solar. It is still portable, but is getting into heavy territory.

2kWh is good for emergencies and off-grid with solar. It is quite heavy and may be technically portable but can be unwieldy. Some manufacturers have been equipping 2kWh+ power stations with a frame on wheels as you find on many gasoline generators, which is a good idea. Combined with solar, this can handle power outages for a few days or even longer if the customer is very smart with how they use power and has sufficient panels. Of course the price for this size power station is very high.

Above 2kWh you are getting into generator replacement territory. At this size you can run almost any plug-in appliance (though depending on wattage, maybe only for short periods), and you can certainly use it for backups during power outages. Heck, many fridges and freezers can run for 12 hours to 2 days (depending on their efficiency) off a 2kWh power station before even needing recharging. Also, at this size, the power station should be able to put out surge currents to start appliances with motors (fridges, window air conditioners, washing machines, etc.).

And EV charging becomes realistic at this power level. As high discharge rates are not ideal for lithium batteries, the EV charging rate may not much higher than Level One charging, but the portability is amazing.

The downsides of 2kWh+ power stations are weight and cost.

When you get to 2kWh+, having the feature to daisy-chain them would be ideal so customers can continue to add to their storage as their finances permit. Having them work in tandem means they can power an entire home when combined with connections to the main panel, especially with solar/wind recharging on site. The hardware to connect to the breaker panel to power a whole home off the generator is already sold for gasoline generators.

Thus this essentially becomes a DIY (Tesla) powerwall for off-grid use or during power outages.

Charging:

Power stations should be able to fully charge in 4-6 hours — this is reasonably quick but preserves battery longevity. Longer charging intervals make it hard to ensure the battery can be recharged in a day of solar charging. While quick charging reduces battery life, it is a feature consumers like. Thus it can be worth having a selectable feature for fast charging, but in general 4-hour charging is a good balance. If you can only design one option, choose the 4-hour charging for battery longevity.

Have several charging options, virtually all power stations can charge from the grid and most can already charge from solar, which is excellent. Ensure yours can charge from solar/wind. Many small banks/stations charge from USB, PD charging is a good idea, but once you get over 500Wh it gets slow. So it’s a nice feature if adding it does not add much cost. Also if your power station can charge from more than one source simultaneously, make sure to mention this in the user manual, as some power stations have the ability but don’t tell the customer.

Add the option for the user to terminate charging at either 50%, 80%, or 100%. No power station I am aware of currently has this feature.

The idea behind this is that 50% is an excellent storage charge for when the power station is not in use, which will extend its longevity. 80% is an excellent charge level for standard use, which also extends battery life. Storing at this charge balances power stored for the day or emergencies vs battery longevity. 100% of course for when the user needs maximum storage.

Pass-through charging (able to plug appliances into the power station while its charging) is a very useful feature, especially if the power station is being used off-grid or as backup during an outage. Make sure the power being used comes from the incoming power and not the batteries to prevent more cycles being used up. This also allows the power station to be used as an Uninterruptible Power Supply (UPS) which increases the consumer base. Combined with a 50% and 80% charge termination ability, it makes it a better UPS than the lead acid battery units on the market today, as it would have higher power density and longer longevity. Make sure you advertise this ability.

If the power station is stored at 100% charge, then design it to intentionally drain itself to 80% after a week of non-use in order to maximize battery life. Explain this feature in the owners manual.

Measure the phantom drain rate when the power station is not in use and tell the user what this is. For example, if your power station uses 0.75Wh a day in storage, the user can calculate that at 80% charge on a 500Wh station it will drop to 40% in 266 days. And to 0% in 532 days. Thus the customer can make sure to top up as needed. A chart explaining this is a good idea.

Built-in Features:

The power station should have a numerical percent indicator so that the user knows exactly how much power is remaining. Many stations have bars or dots which are not very accurate, sometimes being 20-30% off in addition to having a wide range — on a station with 4 dots, two dots could mean anywhere from 30-70% on some stations. This is not precise enough for consumers.

It is also a good idea to numerically show the wattage draw rate when something is plugged in and using power (some stations even project time remaining).

Measure and tell the consumer the efficiency of the 120/230V inverter and the USB outlets and 12V outlet (if applicable). This way consumers can calculate how much energy they ultimately get from the power station and can plan accordingly, which helps reduce prematurely running out of power complaints.

User Information (Manuals):

Give the users a detailed owners manual and a simplified (even laminated) user manual for everyday reference.

The simplified user manual should explain:

  • The surge and continuous power rates the station can provide
  • The 50%-80%-100% charging feature and how to activate it
  • The charge methods available and how long they take to charge to 80% and 100% from empty (if not identical)
  • The efficiency of inverters for plug in appliances, USB and/or 12V outputs
  • The brand/model of battery inside the power station and how many cycles it is rated for

Miscellaneous:

If the power station has a built-in fan, then run it as needed instead of continuously. Low draws should not require it at all, reducing noise and extending battery life.

Tell consumers what brand and model of battery is used in the power station and the number of cycles those batteries are rated for.

Educate customers on best practices for battery/power station longevity, including the 50% storage charge and 80% charging features. Or you can link to my Lithium-Ion Battery Care Guide. Please don’t steal it without permission, though.

Be sure to state the peak wattage and constant wattage the power station is able to produce. This is common already, but not universal. Most lithium can sustain 1C and produce 2-3C in surge power. Size your inverters for this.

Consider adding a battery health monitor. Laptops and cell phones from Apple and Android (using Accubattery) can calculate battery health (capacity). Some powerbanks such as this one tell you what percent of original battery capacity is remaining. You might even consider adding cycle count to the displayed health data.

Of course, competitive pricing is important. And it is always a quality vs pricing trade-off, but producing a product that has a short lifetime will give your company and the industry a bad reputation, so make sure you design your power station for longevity while selling for a competitive price.

And finally, strong warranties bolster consumer trust and increase your Amazon ratings and repeat sales, bringing in new customers. 5 years or more of warranty should be achievable since the electronics are not expensive to make durably and there are many battery options with 1000+ cycle lives. Heck, with LiFePO4, over 3000 cycles is achievable.

 
 
 
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Written By

I've had an interest in renewable energy and EVs since the days of deep cycle lead acid conversions and repurposed drive motors (and $10/watt solar panels). How things have changed. Also I have an interest in systems thinking (or first principles as some call it), digging into how things work from the ground up. Did you know that 97% of all Wikipedia articles link to Philosophy? A very small percentage link to Pragmatism. And in order to put my money where my mouth is I own one (3x split) Tesla share.   A link to all my articles

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