Power Department

Application Notes:
Lithium Batteries... Quirks & Solutions

Lately I have been doing a lot of testing of portable solar power systems for laptop computers specifically, and I thought I would take a moment to share the results.

We've been eagerly anticipating the release of lithium battery packs that can be charged from a variety of sources, including solar, but have yet to find anything appropriate. The main reason for this is that lithium battery packs have traditionally been designed to be external laptop back-up batteries, and as a result, seem to have the same quirks as laptop power systems do.
Unfortunately, external laptop batteries & laptops themselves, are not well suited for use with solar panels.

Here's why...
A laptop's power system is designed to maintain a steady level of power to the system. If the grid fails, the laptop will switch over in the blink of an eye to use its internal battery pack. But what happens in the case of having a solar panel attached to the computer? Well, it turns out that unless you have a large panel that is easily capable of producing the 20-30 watts needed for basic computing, reliably, the computer will ignore the panel and use its own battery. We find the same behaviour with lithium battery packs & external laptop batteries.

Will the lithium battery still use the power produced by the solar panel to help slow down its discharge when it is running equipment?

NO.

Lithium batteries & laptop power supplies are designed to work in an 'either/or' scenario. They will not "pass-through" power to the load and make-up the difference if needed. ie. if the solar panel is producing 10 watts, and the computer needs 30 watts, the lithium battery will not provide the additional 20 watts. Instead, it cuts off the solar panel, and provides the full 30 watts itself.

If the lithium battery pack is not providing power to any devices, then any size solar panel should, in theory, be able to provide charge to the battery pack. But, what we have found is that most lithium products have a minimum input threshold, below which no charge will be stored by the battery pack.

For example, the Tekkeon myPower All 3450 & 3300 are excellent back-up & external laptop battery packs, but they require a minimum input voltage of 8V, and a minimum input current of 1A. Since most solar panels are designed to charge 12V batteries, they have outputs of 16-20V, and it would take a 25W solar panel to exceed the 1Amp minimum current to charge the myPower All products.

Of course, it is also true that a 25 Watt solar panel is only putting out 25 Watts in summer months on clear, sunny days. At all other times, it will be a percentage of this output, and therefore will not have the power to charge the myPower All batteries. If it takes roughly 16 Watts to charge these batteries, a 25 Watt panel will put no power into the battery when the panel's output is below 16 Watts, and only 9 Watts will be stored if the panel is operating at max output. As you can see, these batteries do not take advantage of the investment in solar that you made.

So, all the solar gadgetry out there that promotes itself with phrases like "run your laptop anywhere" are not giving you the whole truth. Yes, you can use their panel to "charge" your laptop battery, but 'NO', you cannot "run" your laptop directly from their solar panel.

Brunton has released two new portable lithium battery in late 2008 that are able to support laptops.The new Solo 7.5, & Solo 15 kits are able to run a 65 Watt AC inverter, and/or run power directly to laptops, even when the battery packs are connected to a solar panel smaller than the power draw of the equipment connected to it. Brunton is solving the "either/or" problem, as well as the "minimum input" & "max discharge rate" issues. The Voltaic Generator solar power storage battery pack is another unit that supports laptops with input from solar panels, but does it completely via direct DC connection.

So, if you need to operate equipment from lithium batteries, like a laptop computer, the best solution is to use an intermediary battery that can act as a float battery. Something that stores the power from the solar panel, and is able to provide all the power the laptop needs. Unfortunately, we have yet to find a lithium battery that will provide pass-through power. ie: act like a float battery. A lithium battery will cut off the solar power and run the load itself if the solar is not at least as much power as the laptop is asking for. So the best solution is still the lowly, tried & tested, SLA (sealed lead acid) battery. These allow pass-through power. A simple stand-by battery will combine with the solar panel to provide the power needed to 'RUN' your laptop & charge its internal battery at the same time.

So, the only thing left to decide, is how much power is needed for the work you need to perform. This requires sizing the solar panel & reserve battery to meet your needs. Check the Modern Outpost website for more information about sizing your solar power system, and contact us if you have questions or would like us to help build your system.

Read the Solar Laptop application note

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