The Ultimate Guide to Powering Your Telescope: Behind the Scenes with Celestron PowerTank and PowerTank Lithium Family of Products

October 4, 2019

For owners of computerized telescopes, there are now more choices than ever when it comes to portable power supplies. Our Product Manager for Astronomy, Lance Lucero, who’s responsible for developing our PowerTank Lithium LT, PowerTank Lithium, PowerTank Lithium Pro batteries will walk you through the details.

There are several ways to power your Celestron telescope. Of course, you can choose to run your telescope on AC power with an AC adapter. Some telescopes include an adapter, and others accept one that can be purchased separately. This is the most convenient and cost-effective choice for permanently installed telescopes or if your observing site has access to AC power. However, be careful when setting up your telescope that you leave plenty of slack in the cord, so you do not trip on it in the dark.

Many of our telescopes accept standard batteries, either in a separate battery pouch or a built-in battery compartment. These work well, but if you observe with any frequency, you’ll quickly get tired of buying new batteries. Unfortunately, rechargeable AA batteries do not put out enough voltage to reliably power a telescope (1.2 volts instead of 1.5 volts). This is where a dedicated astronomy power supply comes in. Celestron offers five portable batteries at a range of chemistries, capacities and price points: PowerTank, PowerTank 17, PowerTank Lithium LT, PowerTank Lithium, and the PowerTank Lithium Pro.

Let’s look at the differences between these batteries so you can determine which one best suits your needs.

Sealed Lead Acid Batteries (SLA)

Most people are familiar with sealed lead acid as the battery technology you’ll find in an internal combustion car. It works well for this application because the battery is constantly recharged as you drive. Lead acid batteries do not perform well when left idle for long periods. (That’s why you need to top off your car battery after not driving for a couple of weeks.) If you choose a sealed lead acid battery for astronomy, you’ll need to charge and discharge it with some regularity. Every month or so, it’s good to go through a full discharge/recharge cycle to keep the battery at optimum performance.

Sealed lead acid batteries are a bit of a relic, but they’re still popular for astronomy and work reliably when well maintained.

Also, like car batteries, sealed lead acid astronomy batteries do not last forever. Celestron sealed lead acid batteries have a lifespan of about 200-300 charge cycles. You can prolong the battery’s lifespan with proper maintenance, but after about two years, you’ll probably need to replace it.

Sealed lead acid is not a perfect solution (nor the most environmentally friendly), but astronomers have trusted these batteries for decades and they’re a cost-effective option for people willing to put in the effort to maintain them. The original PowerTank is a smaller capacity battery, best suited for telescopes like the NexStar SLT and below. The larger PowerTank 17 works well for larger telescopes and longer observing sessions.

Lithium Batteries

Lithium batteries come in variety of different chemistries, and not all chemistries are created equal. Here’s an overview of the most common lithium batteries:

Competitors’ Batteries: Lithium-Cobalt Oxide (LOC)

This is the most common type of lithium battery. Most people are familiar with them as the batteries in most laptops and cell phones. These are commonly referred to as “lithium-ion” batteries. They are a little more expensive than the traditional SLA battery, but offer a high capacity and can be charged and drained quickly. They do not require as much maintenance as the SLA.

While the LOC batteries have a longer 500-1000 charge cycle lifespan and can be drained flat without causing any permanent damage to the battery, they do suffer from major thermal instability issues. Charging or discharging the batteries at high current can cause the battery to heat up. If the temperature exceeds 150°C, the battery can go into thermal runaway and burst into flames. Most of the power packs sold on the market using a LOC battery have cooling fans that work to prevent the battery from overheating. Of course, the same battery that is overheating is now being used to power the cooling fans, which uses up more of your precious battery capacity. Puncturing the battery’s case and exposing the cells to air can also cause the battery to ignite.

Due to the natural voltage of the battery cell, these batteries cannot provide 12 volts. They will start discharging at 11.5 Volts and slowly decline from there. A Celestron telescope requires 12 Volts to operate nominally. When you get below 11.5volts, your motors may start to sound labored and your fast slew speeds will slow down. Below 11.0 Volts, you may notice glitches in the electronics. While LOC batteries offer a high capacity at a low cost, the amount of time you can use your telescope before it becomes underpowered is extremely short—usually one to two hours.

Whether it’s the TSA banning certain smartphones or the great “hoverboard” fiasco of 2015, it seems that unsafe LCO batteries were constantly in the news. When we decided to develop our own lithium battery, our primary concern was safety. Celestron does not use LCO chemistry in any of our 12 Volt power supplies.

PowerTank LT: Lithium-Nickel Manganese Cobalt Dioxide (NMC)

NMC batteries can commonly be found in power tools and e-bikes. They offer significant advantages over SLA and LCO batteries described above. They can be fully drained and recharged to full potential without suffering the same damage that an SLA battery would. While being a little more expensive than SLA or LCO batteries, NMC batteries offer twice the lifespan (1000-2000 charge cycles). But the most significant advantage is operational safety. The NMC chemistry offers a much higher thermal runaway temperature (210°C) making it significantly safer than the LCO battery.

Celestron uses the NMC battery chemistry in the PowerTank Lithium LT (#18763). To combat the lower 11.4 Volt output of this chemistry, we added a 12 Volt step-up voltage regulator to the circuit. This allows the PowerTank Lithium LT to deliver a constant 12 Volts until the battery is completely drained.

Comparing the Celestron PowerTank LT (#18763) to the original SLA PowerTank (#18774), you will pay a little more for the PowerTank LT, but you will have 3 to 4 times the lifespan, less maintenance, better performance, and similar capacity, all in a package that is less than half the weight and one-third the size.

PowerTank Lithium and PowerTank Lithium Pro: Lithium-Iron Phosphate (LFP)

LFP (or sometimes called LiFePO₄) batteries are commonly used in medical equipment and power tools. They can be fully drained and recharged to full potential and can hold their charge for years with minimal loss of power. They offer an incredible 2000-3000 charge cycle lifespan and the battery has a native voltage that is 12.8 Volts. This chemistry offers full 12 Volt power to your telescope for 95% of the battery’s capacity. Even though the LFP handles higher current loads (3 Amp for the PowerTank Lithium and 10 Amp for the PowerTank Lithium Pro) and charge faster than the other lithium batteries, the LFP has the safest thermal runaway temperature of all at 270°C.

Celestron uses this technology in our premium PowerTank Lithium (#18771) and large capacity PowerTank Lithium Pro (#18768).

The one disadvantage of our PowerTank Lithium and PowerTank Lithium Pro is their higher initial price points. However, with the long lifespan and safe technology, we think you’ll agree that it becomes quite cost effective compared to replacing cheaper batteries over the years. Plus, LiFePO4 requires no regular maintenance once fully charged, so it can work in your emergency kit as well as your astronomy kit!

The Connection Between Battery Chemistry and Telescope Performance

For comparison, Celestron tested the PowerTank Lithium LT (NMC), the PowerTank Lithium (LFP), and two competitors’ LOC batteries. The graph below shows the voltage as a function of time when using a steady load of 750mA. Notice how the Celestron batteries keep the voltage to 12 Volts over the full use of the batteries. The voltage of the LOC batteries drop below the 11 Volt line after only a couple of hours of use.

Since the earliest days of computerized telescopes, amateur astronomers have been improvising power solutions. In the 1980s, people ‘hacked’ old fashioned lead acid car batteries to power their telescopes and the rest is history. In that same vein, many competitors’ power solutions on the market today were not designed with astronomy in mind. The small 155 Watt-hour lithium batteries that have cropped up online are actually repurposed power supplies for portable CPAP breathing machines.

CPAP machines differ significantly from telescopes in the way they draw power. Telescopes are more demanding and need to consistently draw at or near 12V to operate optimally. When connected to a telescope, these batteries never deliver a full 12V and lose power quickly, tapering down to 11V in less than 2 hours.

Then there are the form factor differences. To power a telescope with a CPAP-style battery, you’ll probably need to use a small 6” cord that adapts the 12 Volt output to a cigarette lighter plug, then plug in a cigarette lighter adapter to the adapter. The cigarette lighter cord that many include is only 3 feet long, so it will not reach the power port on most German equatorial mounts if placed on the ground.

Our team at Celestron designed PowerTank Lithium LT, PowerTank Lithium and PowerTank Lithium Pro specifically for astronomy. The high-quality battery keeps a consistent voltage until the last 30 minutes of its life, and the form factor is perfect for telescopes, including straps to attach it to tripod legs. By placing the battery on the tripod leg, you can keep your cords short and out of the way, eliminating trip hazards and cord wrap issues.

Some aftermarket lithium batteries come with an AC adapter, so why doesn’t the PowerTank Lithium Pro come with one?

We thought about including an AC outlet on PowerTank Lithium Pro, since other batteries on the market offer this feature. However, after testing those batteries, we determined that it does not make sense for a battery of this size to have an AC circuit.

DC to AC converters are notoriously inefficient. What’s more, these 155 Wh batteries also require the use of an internal fan to keep the batteries cool when drawing from the AC port. Powering the fan eats up electricity that should be going to your telescope. All these factors combine to render the battery much less useful than its technical specifications might suggest.

To demonstrate this, we hooked up a competitor’s 155 Wh battery to a single 60-watt incandescent lightbulb via the AC port. It drained in 2 hours.

Of course, a lightbulb is the last thing you’d want to power when dark sky observing or imaging. So, we tested it with a laptop. The battery’s manual claimed the maximum output was 100 watts. Most laptop chargers need between 70 and 125 watts. But running autoguiding software and image capturing software at the same time requires more power from the laptop. Unsurprisingly, the draw exceeded 100 watts and the battery shut down after 2 minutes of use.

Rather than add an essentially useless AC outlet to PowerTank Lithium Pro, we decided to include an integrated “cigarette lighter” port, which is compatible with many telescopes and astronomy accessories.

PowerTank Compatibility List

Will power any computerized mount

Compatible
	PowerTank Lithium LT	PowerTank Lithium	PowerTank Lithium Pro	PowerTank 12V	PowerTank 17
CGX-L			X		X
CGX			X		X
CGEM II			X		X
ADVANCED VX	X	X	X	X	X
CPC GPS XLT			X		X
CPC DX			X		X
NEXSTAR EVOLUTION	X	X	X	X	X
NEXSTAR SE	X	X	X	X	X
SKYPRODIGY	X	X	X	X	X
NEXSTAR SLT	X	X	X	X	X
LCM	X	X	X	X	X
ASTRO FI	X	X	X	X	X
NEXSTAR GT	X	X	X	X	X
CG-5			X		X
CGE			X		X
CGE PRO			X		X
CGEM			X		X
CGEM DX			X		X

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