There are three leading rechargeable battery technologies on the market today: Lithium-Ion, Nickel Metal Hydride, & Sealed Lead Acid (SLA). Each of these technologies has its own unique strengths & weaknesses which dictate its appropriateness for certain applications.

Lithium-Ion & Lithium Polymer

One of the newest battery technologies, and the one that is developing at the fastest rate. They come is several different chemical variations, but we will avoid that whole discussion. Lithium is also the technology receiving the most attention lately in the press. Lithium-Ion batteries are widely used in smaller electronic devices such as cell phones, iPods & digital audio players, PDAs, & watches where they excel at providing low power over a long period of time. They have a very high charge density, which means that they can pack a lot of electrical potential in a small space. However, with this concentration of power comes volatility.

The chemistry of a Lithium-Ion cell requires strict adherence to charging & discharging limitations, and impurities in the manufacturing of these cells can often result in problems & massive product recalls. Many people will remember the recall of thousands of Dell & Apple computers in the summer of 2006 when their batteries, manufactured by Sony, were found to contain a flaw that could result in overheating & fire. Lithium batteries do not tolerate overcharging, and so have built-in safety features like integrated circuits that prevent the battery from being charged further once it has reached a 'full' voltage during the charging process.

Lithium Polymer cells are a very recent development, and essentially are a 'dry cell' version of Lithium-Ion. They perform better at higher temperatures (ie over 25C/70F), and offer manufacturing flexibility in extremely flat shapes as thin as a credit card. Due to their customized nature in manufacturing, they tend to be very expensive, and only rarely provide advantages over more common Lithium-Ion cells.

Nickel Metal Hydride

NiMh batteries came on the scene commercially in the 1980's, and were thought to be a more ecologically sound alternative to the much older Nickel Cadmium (NiCd) batteries. NiCd batteries use the highly toxic element 'Cadmium' in their construction, and as households are not as good at recycling batteries as corporations are, a Nickel-based battery that was not nearly as toxic was exciting news for consumers. NiMh drawbacks include a relatively high rate of self-discharge that sees the battery lose approximately 30% of its power within 1 month. They also take approximately twice as long to charge as Lithium or NiCd cells do.

Although not as versatile a performer electrically as NiCd, NiMh batteries are stable, and do not suffer from the same 'memory effects' as NiCd's do. They do not need to be discharged completely (full cycle) before charging on a regular basis like NiCd's do in order to prevent internal crystal growth that cripples the battery. Rechargeable NiMh batteries in the 'AA' size format have a similar voltage to ordinary Alkaline batteries, and therefore are the most popular choice for digital cameras, portable CD players, radios, & flashlights.

Sealed Lead Acid (SLA)

Sealed Lead Acid batteries, similar in design to the battery that your car uses, are one of the oldest battery technologies going, and yet their unique attributes mean that for some applications there is no viable substitute. Although not disposable due to toxic lead & acid components, SLA batteries are easily recycled. They charge much more slowly than the other battey technologies (up to 5x slower), but are able to provide larger amounts of power to support larger appliances, and are very tolerant to overcharge & undercharge.

The biggest drawback with SLA batteries is their weight, having the lowest charge density of any of the rechargeable battery types. However, the availability of materials & ease of manufacture means that they are not only very well understood and safe to use & transport, but they also come with a much lower price tag.

So the big question becomes: which battery type is best suited for my application? The answer is simple, and is actually pre-determined for you by the very nature of each battery technology.

For Small, Low Power Electronics

Lithium batteries included with PDAs, iPods, cell phones, & blackberries, are put there for a reason. They provide fast charging, lightweight & compact forms, & no maintenance. You may find that you change your phone or blackberry model before your battery reaches the end of its useful life.

Vehicle adaptors are available for most of these electronics, and these adaptors enable you to plug them directly to any 12V solar panel (typically up to the 10W size). There are even DC vehicle adaptors, like the 100W Universal, that enables you to charge your laptop from the sun.

For Digital Cameras, Radios, Flashlights

As a replacement for standard alkaline 'AA' or 'AAA' batteries, NiMh cells are the right choice. They provide reasonable power drain characteristics for things like camera flashes, are available just about everywhere, and good quality home & vehicle chargers are also widely available.

Some NiMh charger models, like the Targus Rapid & PowerFilm chargers, offer DC vehicle adaptors that allow you to charge batteries from your car's cigarette outlet. These can also be plugged into any solar panel's socket outlet for solar charging.

For Solar Power Systems

When it comes to storing power from the sun, the SLA battery is still king. Home power systems uses banks of deep-cycle golf cart batteries because of their low cost, availability, and ability to hold a charge for months with very little self discharge. When you invest in solar panels, it is extremely important that you lose as little of the power generated as possible, and that performance over years of use will be stable and predictable.

Smaller solar power products that are designed for use with small, low-power electronics often integrate Lithium-ion batteries in order to keep designs small & lightweight. The popular Solio & Solar Mio products are examples. Other products like the Solar Uno & Solar ePower choose to charge removeable NiMh cells that can be easily charged & used in other devices or replaced if they fail.

Why don't we see Lithium or NiMh batteries in the larger solar products like the SolarPort 4.4 or any of the Outpost Power packs?
A couple of reasons, but primarily because of the issue of power discharge rates. External laptop batteries, like the Tekkeon MyPowerAll 3450 & 3300, will not provide output above 65 Watts. As a result, they are not able to drive AC inverters, or handle the loads of larger household systems without astronomical cost. It is also true that Lithium & NiMh chemistries become unstable in larger formats. This is why at the moment, external laptop batteries are about as large as they come. The circuitry to manage them becomes far too complex & costly, and one bad cell can adversely affect the safety & performance of all the others. It would be very nice to be able to give up some of the weight of an SLA battery, but unfortunately, at the moment, Lithium & NiMh packs are best suited to smaller DC applications.

NiMh batteries are hard to come by in larger formats, and our own experience at Modern Outpost was that a battery pack over 4AHr in capacity at 12V (equivalent to 10 'D' cells wired together) required special permission from Panasonic (the cell manufacturer). If charged incorrectly, a NiMh battery can vent Hydrogen gas. Not a real problem at the 'AA' battery level, but certainly one with 10 cells wired together. It should also be noted that when a NiMh battery eventually fails, it does so over a short amount of time. One minute it is working, the next it won't take a charge - not a good scenario for power needs miles from the nearest outlet.

Lithium batteries come with special safety circuits that prevent them from being charged too quickly or too high, and also restrict the discharge rate. Most lithium cells will not put out more than twice their rated amp-hour capacity. This means that the largest laptop batteries can only support approximately 100 Watts of power. Try plugging an AC inverter into a 12V Lithium battery, and it will not even acknowledge the presence of a battery. Both Tekkeon's 'myPowerAll' & the PowerBank Lithium power packs will not support the smallest of AC inverter while under load. Like NiMh batteries, Lithium cells also tend to fail abruptly when their time comes. Many people may be familiar with having a cell phone that suddenly will not take a charge. Again, not the sort of reliability you want when you are out in the wild.

So what remains is the lowly SLA standby battery like those used in the Outpost Power kits. Durable, easy to use, and predictable. These batteries act like reservoirs, storing your solar power until needed. They also act like a buffer for times when your solar panels are not providing the power your equipment needs. They can also be charged at the same time they are connected to appliances - as a so-called 'float battery'. A function that Lithiums do not handle well (see our note on the Quirks with Lithium). Even a 4 amp-hour SLA battery, like the fist-sized model used in the Outpost Power 4 pack, is capable of supporting laptops, battery chargers, solar panels, and more, and only weighs 2-3 lbs.

Take a few moments to read through our note on solar charging to learn more about how a solar power system is designed to provide reliable power far from home.

Following are a few other notes on the quirks of the various technologies...

Lithium Batteries

Can provide up to 500+ charge cycles
Longest life if only discharged to 80% full
Can charge in as little as 1-2 hours
Can operate in freezing temperatures, but do not like to be charged in freezing temperatures
Cannot be trickle charged
Cannot support high-drain equipment above 100W (for the largest Lithium batteries)
Does not require conditioning maintenance
Self-discharges at approx 10% per month
Store in a cool place at 40% of full charge
Larger battery packs are regulated for air travel, and will not be permitted
Low toxicity, but should be recycled rather than disposed

Nickel Metal Hydride (NiMh) Batteries

Can provide up to 300+ charge cycles
Can charge in as little as 2-4 hours
Can operate in freezing temperatures
Cannot be trickle charged - low tolerance to overcharging
Can support high-drain equipment below 200W (for the largest NiMh battery packs)
Requires periodic conditioning maintenance (every 3 months)
Self-discharges at approx 30% per month
Store in a cool place at 40% of full charge
Not regulated for air travel
Low toxicity, but should be recycled rather than disposed

Sealed Lead Acid (SLA) Batteries

Can provide up to 300+ charge cycles
Can charge in 8-16 hours
Can operate in freezing temperatures
Tolerant of trickle charging & overcharging
No maintenance required
Can support high-drain equipment over 200 watts
Prefers to be cycled above 50% full
Self-discharges at approx 5% per month
Store at room temperature and fully charge
Not regulated for air travel
Toxic materials, and should be recycled rather than disposed

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