Best Solar Battery Tender for your Car, Boat, or RV
What does a solar battery tender do, and which ones are worth buying?
If you’ve got an RV, sports car, ATV, boat, or anything else that sits in your garage for most of the year, a solar battery tender might a worthwhile purchase. Read on to learn more about what they do, and how they work. And if you’re already looking at solar battery tenders and wondering which one to buy, we’ve got you covered there too.
All batteries naturally drain power when they sit idly. If it sits for a month or two, that’s no big deal. But if your batteries are sitting for 6 months or more, you might walk out – ready for your day of fun – only to find the batteries are dead in your boat.
And worse than that, batteries that are frequently left dead become unusable bricks of plastic and metal – unable to hold a charge and good for pretty much nothing beyond bringing it to Walmart for recycling.
Battery tenders – no matter where the juice comes from – prevent your battery from slowly draining away. They continuously trickle a small bit of electricity to the battery, keeping it well charged. It’s a cheap investment that can save you frustration and cash down the road.
What Is a Solar Battery Tender?
Conventional battery tenders plug into a normal AC electrical outlet (like in your garage). If you’ve got a snowmobile battery you want to keep charged up for the summer, just bring the battery to the garage and hook up the battery to the tender.
However, what if you can’t (or don’t want to) bring the battery in the garage or shed? Maybe you’ve got a boat or car outside that you’d rather just leave outside. With a solar battery tender, you can keep your battery charged up without moving anything!
Solar battery tenders create electricity via a small solar panel you can attach to your roof, siding, or anywhere else that receives sufficient light. The solar panel provides a minute amount of electricity (typically 5 – 10 watts) that flows through a charge controller to prevent battery overcharging, then directly to the battery.
They’re inexpensive, easy-to-use, and run off clean, no-cost sunlight. Pretty awesome!
Why Do I Need a Battery Tender?
First, let’s set one thing straight: a battery tender (or maintainer) is NOT a battery charger. A battery charger is powerful enough to completely recharge a battery after full use. A battery tender, on the other hand, is simply a device designed to keep your battery topped up when not in use. If you’re looking for something to completely charge your dead battery, a battery tender just isn’t going to cut it for you.
Why would you need a battery tender, if you’re not using your battery? Good question!
When batteries sit unused, they slowly lose their stored electricity. This phenomenon, known as self-discharge, is a characteristic of all batteries. Self-discharge rates vary by battery type and condition. Poorly manufactured batteries and abused batteries see higher rates of self-discharge than higher quality options, but even well-made batteries aren’t immune.
Respected battery manufacturer Trojan, for example, notes a self-discharge rate of 5% to 15% per month depending on temperature, for their popular flooded lead-acid T-105 golf cart battery. With the T-105 holding 225 amp-hours of electricity, by Month 12 you’d be down to 138Ah of electricity – 60% of its total capacity – at 5% self-discharge each month.
Of course, we’re not picking on Trojan. As we said earlier, all batteries suffer from self-discharge. Crown Battery claims a very small self-discharge rate of just 2% to 3% per month (pg. 2) for their 12-volt AGM batteries.
Lithium-ion batteries are a bit better than lead-acid when it comes to self-discharge. Speaking very generally, lead-acid batteries generally self-discharge around 5% per month, while lithium-ion batteries enjoy a much slower rate – around 1% – 2% per month. So you don’t have to worry about self-discharge quite as much with lithium batteries, but it’s certainly still present.
All this to say, battery tenders keep your batteries in tip-top shape by ensuring they’re always charged – even when left alone for months on end.
What to Look for in a Solar Battery Tender
Before buying a solar battery tender, consider the following factors to get the right model for you:
Battery maintainers are pretty cheap tools. A typical solar battery tender costs between $20 to $40 depending on the manufacturer, and you’ll spend another $20 or so on a charge controller if you decide to buy one (more on charge controllers below).
Wattage, aka the size of the solar panel, determines how much power you’ll have available to charge your battery. Sizes range from a tiny 1.5-watt panel to 20 watts or more, though most solar battery maintainers sit around 5 to 10 watts.
The size you need depends on how frequently you use your battery. The less you use the battery, the smaller your maintainer can be. Users report just a 2.5-watt inverter keeping their SUVs charged!
For battery tenders, you’ll be looking at crystalline silicon solar panels or amorphous silicon panels. Crystalline silicon panels are rigid and heavier than amorphous panels but are more efficient, so you’ll get more power from a crystalline panel with the same dimensions as an amorphous panel.
However, amorphous panels (a type of thin-film solar panel) are typically lighter and can even be flexible, allowing you to install the panels in more varied locations. They also perform better in low-light conditions, so it might be a better option if you live in rainy Seattle or somewhere similar.
Charge Controller and Diode
If you purchase a solar panel larger than 5 watts, experts typically recommend also purchasing a charge controller, which is a device that controls the amount of electricity going from the solar panel to your battery so you don’t accidentally damage it through overcharging.
Some solar battery tenders include an integrated charge controller, but most don’t. Purchasing a battery tender with an integrated charge controller simplifies the system and cuts out one more piece of equipment that could get lost.
Many users simply plug the battery tender directly to the battery without a charge controller. This works fine as long as everything’s working properly, but if you have a voltage spike or leave the maintainer connected to the battery indefinitely, your battery could become permanently damaged. Best to spend the $20 and buy a charge controller if you’re concerned.
Even if you decide against a controller, you’ll definitely want a model with an integrated blocking diode, which you’ll find in the better solar battery tenders. Blocking diodes allow electricity to flow from the solar panel to the battery while the sun is out, but blocks that connection when the solar panel isn’t producing electricity.
Without a blocking diode, your solar panel can actually leach electricity from your battery at night, leading to a dead battery in the morning. If your solar battery tender doesn’t have a diode, you’ll need to disconnect the battery each night.
Best Solar Battery Tenders
Now that you know the basics of what to look for, let’s take a look at the a few of the best solar battery tenders currently on the market. For our list, we focused on cost, quality, and customer reviews. There aren’t too many on the market, but we took the time to find our top four picks. At 2.5 – 15 watts, any of the options below can easily keep your car, boat, or ATV battery topped off and you’d do well with any of them.
Where possible, we’ve also linked to YouTube videos of the products, so you can see them in action. Pricing information is accurate as of August 2018.
Easiest Set-Up: Battery Tender (yes, that’s the brand name)
Customer Reviews: Great
Specs: 5-watt, 10-watt, and 15-watt options, integrated charge controller, integrated diode
Buying electric gadgets on Amazon can be a gamble – there’s a lot of cheap junk on there that breaks right when you open up the package (and yes, I’ve tried many of them). Thankfully, there are some well-known and reputable options out there as well!
While their name is about as generic as you get, Battery Tender is a well-known manufacturer of – you guessed it! – battery tenders, and you can find their products in brick-and-mortar stores across the country. (The featured image at the top of this article is actually a Battery Tender solar tender in action, keeping a boat’s battery topped off.)
Their bread-and-butter is plug-in battery tenders, but their solar-powered model is equally respected. They manufacture 3 different sizes: 5-watt, 10-watt, and 15-watt options. Their solar battery tenders are the only ones on our list that come with a built-in charge controller to prevent overcharging your battery, and an integrated diode to prevent the solar panel from sucking electricity out of the battery when the sun goes down.
At about one foot in diameter, Battery Tender’s panel is about the same size as others on our list, but it’s one of the more powerful options here. And with all equipment included, all you need to do is connect it to your battery and set it in a sunny place. Done.
It’s certainly the most expensive option on our list, but with Battery Tender’s 10-year warranty, you can rest assured you’re covered for many years to come. Couple that with the integrated charge controller, and you should be sold.
Most Portable: ALLPOWERS Portable Battery Charger
Customer Reviews: Great
Specs: 5-watt, 7.5-watt, and 10-watt options, integrated diode, no charge controller
ALLPOWERS’s solar battery charger is a cool little gadget. Coming in 3 different sizes, the efficient mono-crystalline silicon panel packs a good little punch. The panel is actually housed in a cloth covering with suction cups on the back. Simply stick the panel to the inside of your windshield or on your visor (with attached straps), plug it into the cigarette lighter (or directly to a battery with the included clamps), and let the sun charge up your car’s battery.
Of course, with a cloth case, the ALLPOWERS solar battery charger isn’t designed to be left outside long-term, but its small size and easy-to-use features allow you to set it up and put it away quickly. Unfortunately though, these products don’t come with a charge controller. You’ll certainly want to add one to your shopping cart, especially considering their larger sizes – all of them are 5 watts or above.
ALLPOWERS claims their solar battery tender comes with an integrated diode, but several users still report dead batteries after leaving them connected to the maintainer overnight. That’s certainly a strike for ALLPOWERS, but the 18-month warranty alleviates some of that worry.
Lowest Price: Eco-Worthy 5-Watt Battery Charger Backup
Customer Reviews: Good
Specs: 5 watts, integrated diode, no charge controller
Eco-Worthy’s 5-watt solar battery maintainer is the cheapest battery tender on our list. But even at their low price, you’re getting a pretty decent product: a 5-watt panel, a fabric case with integrated suction cups (similar to the ALLPOWERS product above), and 2 connection options (1 for the cigarette lighter and one to connect directly to the batteries).
Like the others on our list, it also includes an integrated blocking diode to prevent electricity from flowing in the opposite direction at night.
Despite the low cost, most customers report that the solar panel produces a good amount of electricity and it’s even quite durable. However, with the lowest customer review score of any product on our list (though not by much), be sure to read over a few reviews beforehand so you’ll feel prepared for any issues that might arise.
Most Trusted Brand: Schumacher SP-200
Customer Reviews: Great
Specs: 2.4 watts, no charge controller, no diode
If you’re in the car world, you might already be familiar with the Schumacher name. German Michael Schumacher is a world-renown Formula 1 driver, but we’re actually talking about the other Schumachers – the Americans known for drag racing for the last 50 years.
The Schumacher brand has been manufacturing battery products for decades, so you can rest assured they know what they’re doing.
Their SP-200 solar battery maintainer is the smallest on our list, at just 2.4 watts. However with its amorphous solar panel, it’ll perform well on cloudy days. And with its hard shell rated for both indoor and outdoor use, it can take a heavier beating than other products.
At 11” X 20”, it is physically larger than the crystalline options above, so keep that in mind if you’re looking to store it in the trunk of your car or another cramped space. Like the others on our list, the Schumacher battery maintainer comes with connectors for both a cigarette lighter and battery.
Customers report good performance, but it’s certainly the most basic option on our list. At just 2.4 watts, you don’t really need a charge controller, but without a diode, you’ll need to disconnect the maintainer from the battery each night to prevent electricity from flowing back to the panel – somewhat annoying.
We hope you enjoyed our overview of solar battery tenders! If you have any questions at all, ask away in the comments. If you’ve got the solar bug and are interested in installing solar panels on your home, talk to a few installers and see what they have to say!