Don’t Pass on Pass-Through Charging!

We’ve been promoting the concept of pass-through charging since the early days of portable solar equipment. Primarily with the onset of lithium battery packs. Lately, I’ve seen a lot of buzz about it, which thankfully means that the functionality is now a sought-after feature, and people understand the convenience it provides.

If you aren’t familiar with the concept, it is simply this: pass-through charging is the ability of a battery pack to ‘be charged’ at the same time that it ‘provides charge’.

Essentially, most battery packs you find at the local hardware store, or big box electronics store, can either be charging from a source (wall, USB port, solar panel, etc), or providing power to your device, but not both at the same time. As soon as you plug your iPhone into that expansion battery pack, the expansion battery pack will stop charging itself from the wall & prioritize providing power to your iPhone. But what do you do if you drained the expansion battery pack? You have to wait until it has enough charge stored before you plug in your iPhone.

Imagine for a moment you are out on a wilderness trek. You have a folding solar panel on your backpack, and a solar storage battery to capture all the power. If your camera battery, drone battery, or satellite phone battery suddenly needs to be charged, do you want to take a time-out from solar charging in order to charge your device? Or would you rather be able to keep solar charging, while you charge your device? Have all that solar power flow through to whatever battery in the chain needs the electrons? Of course you would.
So, you need a solar storage battery with pass-through charging capability.

How To Find Pass-Through Battery Packs

Most solar storage batteries offer pass-through charging. It’s the nature of the application. When you don’t know how much power is going to be flowing from the source (solar), or when you will need to connect devices for charging (loads), solar storage batteries must provide that flexibility. It follows the basic design of residential off-grid solar systems. Imagine the inconvenience if you had to unplug from solar to be able to run your fridge at your cabin? Not likely.

So, trust that battery packs from major solar manufacturers offer pass-through. This includes all the USB & universal battery packs on Modern Outpost including Goal Zero, Voltaic Systems, Random Order, EcoXGear, Enerplex, and others. Because if the company doesn’t say it’s pass-through, it won’t be.

Are All Pass-Through Battery Packs The Same?

No. Some battery packs prioritize the charging process and delivery of power to loads a little differently. For the most part, you will be storing & using all the power you generate from solar down the battery chain, but there are a few features you should be aware of…


Some battery packs will disconnect the load momentarily when they lose power from the source. Unplug the solar storage battery from your solar panel, and it will turn off its output port too, and then reconnect it a few seconds later. This is not usually a problem for smart phones and tablets that are designed to reconnect themselves to a source, but it could be a problem for battery chargers and other less sophisticated devices. They may need to be unplugged & reconnected in order to resume charging. Not always a convenient task.

Always On

Some battery packs offer an ‘Always On’ feature. This means that the battery is always waiting in stand-by mode for a load device like a phone or charger to be plugged into it. Having the power available all the time is a definite must-have for many remote monitoring applications where the load devices can’t have the battery pack time-out or turn-off at any time just they don’t need power at a particular time.

We Ran Some Tests…

We took 4 of our most popular USB solar batteries, and ran a quick test using an iPad and a Samsung smartphone to confirm how they operate in a pass-through scenario…

Voltaic V15
Never blinks… always stays connected to the load, regardless of disconnection, reconnection, or low-level solar source power. Voltaic also provides an ‘always-on’ mode, in addition to a power-saving ‘time-out’ mode, so that the battery will never time-out on you if you don’t want it to. The V44 model is designed the same way.

EcoXGear EcoCharge
This small, ruggedized battery also never blinked. connections & disconnections from a source did not affect its delivery of power to the devices. Since it doesn’t have any control buttons, and only a small source power status light, it is safe to assume that it is ‘always-on’, providing power whenever a device is connected & asking for power.

Goal Zero Venture 30
This ruggedized battery remained connected to load devices regardless of what the source power was doing. ‘Always-on’ and ready to provide charge. An excellent battery pack that performed as the best should. Venture 70 is the same.

Goal Zero Flip 20
This popular battery drops the load device momentarily when the source power is disconnected. There is no control button, so the battery is ‘always-on’, and reconnects to devices & chargers automatically when they are plugged-in. An interesting note was that the Flip 20 charged at only 50% of the rate of the other battery packs in this test. The Flip 10 & Flip 30 are similarly designed.

RandomOrder 9K
This battery pack follows the same protocol as the Goal Zero Flip series. It drops the load device momentarily when the source power is disconnected. Note that this battery also has a power button, which means that it will time-out, or shut-down, if your device doesn’t need power. Press the button to reconnect.

Questions? Comments?

If you want to know more, or have any specific questions pertaining to an application you are working on, please let us know.

4 thoughts on “Don’t Pass on Pass-Through Charging!

  1. Tom S Leswing

    I would like a portable battery with the following features:

    A timer to specify when it is charged by being plugged into an outlet (daylight hours when my solar system is producing)
    Outlets to power my appliances and computer
    A charge sensor so if the battery charge drops below a certain level, the power source for the appliances and my computer would automatically switch over to my household electric system.
    Is there a battery or technology with these features?

    1. admin Post author

      Hi Tom, thanks for your note.
      What you are describing is a classic back-up power set-up, and is the exactly how I run my office power system here at Modern Outpost. During daylight hours, solar panels charge batteries via a charge controller that makes sure the batteries are charged/maintained properly. I can take DC power from the battery bank, or household AC power via an inverter. The inverter has a low-battery status programmability which we have set to open-up the AC input connection (to the grid-tied solar in our case, but could also be a gen set) if/when the batteries get low. So essentially, we operate from solar 100% of the time as a default, and then the system uses the grid-tied system from time-to-time, and every few days in winter, to top-off the batteries if the solar is not keeping up.

      Now, if you already have a grid-tied solar system, and want to charge from the AC power produced by that system, and only during the day, then what you could do is simply use a standard load timer (ie like the ones that turn ‘on’ floor lamps while you are on vacation). Put this on the AC input to your inverter/charger, and it will allow your system to only charge from the grid (and your grid-tied solar) during the hours you specify.

      Something like the Socrates 1200 would be the entry-level for this type of system (note that we offer the equipment list in the Socrates 1200… you don’t need to order the crate if you’d rather DIY the system in your own box or on a board).
      To save weight, you could use a lighter-weight inverter like the Victron MultiPlus, but the Samlex have larger surge for motors and pumps.

      Even the pre-built power boxes like Goal Zero Yeti could operate this way.
      You could easily put an AC timer on the input line so that it only charges during daylight hours, &/or plug-in solar panels for off-grid charging. However, they typically lack a ‘low battery’ program. If input is present, they will charge from it. Not a bad thing though to have such a power box always plugged into the wall via a timer… you could easily cruise through the overnight hours on the battery power and charge during the day when your grid-tied solar is producing. Just be sure to calculate your loads with room to spare so that the battery never goes below a desired level overnight.

      I hope this helps!
      Let me know if I can design a system, or recommend a product solution for you.

  2. James

    I purchased a solar generator that advertised it supported pass through charging. However when I received it the instructions said that it did have pass through charging feature but was not recommended because it damages the Lithium battery. Do you know what this means? Thank you

    1. admin Post author

      Hi James, my apologies for missing your question. Which product are you inquiring about?
      The Voltaic solar batteries are all designed for pass-through, and there is no other way of operating them. The circuitry is designed to manage input & output at the same time. This is important for solar applications clearly, since it is an advantage to be able to charge devices while the sun is shining.
      Most standard USB battery packs will not allow this – they are either in ‘charge’ mode, or ‘delivery’ mode, which makes them unsuitable for solar applications.
      Larger lithium batteries that are used in solar applications (ie Bioenno, Discover, SimpliPHI, etc) are inherently pass-through capable. Just like the old lead-acid. Their BMS & system equipment is designed to deal with the voltage ranges involved.

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