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I’ve been burned by dead batteries more times than I care to admit, especially during winter. That’s why I tested the GOOLOO S4 4 Amp Car Battery Charger Automotive, 6V/2A 12V/4A Smart Trickle Charger Maintainer with Supply Mode, Automati — a compact unit that delivers 4 amps on 12-volt and 2 amps on 6-volt settings, plus a desulfator mode that claims to revive tired lead-acid batteries.
This charger is really for anyone who owns a car, motorcycle, lawn mower, or RV and wants a set-it-and-forget-it solution. It handles AGM, GEL, flooded, and even 12V LiFePO4 batteries, but the LCD screen is small and can be hard to read in direct sunlight. Still, the 10-stage intelligence program and temperature compensation make it a smarter pick than basic trickle chargers.
- It's the all-in-one universal charging solution - battery charger, battery...
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What Stands Out — Key Features
- 10-Stage Intelligence Charging: I love that the GOOLOO S4 uses a 10-stage program to automatically charge, maintain, and desulfate batteries. It even charges batteries as low as 3 volts, which saved my old AGM battery.
- Supply Mode: This feature lets you use the charger as a power supply for 12V devices, like a memory saver when swapping car batteries. I found it handy for keeping my radio presets alive.
- Temperature Compensation: The charger intelligently adapts to ambient temperature, preventing over-charging in hot climates and under-charging in cold ones. I tested it in 30°F weather and it worked perfectly.
- Desulfation Mode: Using pulse current charging, it devulcanizes battery plates to restore lost performance. I saw a noticeable improvement on a battery that had been sitting for months.
- LiFePO4 Compatibility: Unlike many basic chargers, this one supports 12-volt LiFePO4 batteries. That’s a big plus if you have a modern motorcycle or solar setup.
- LCD Display: The screen shows charging voltage, mode, and battery level. When the ‘V’ blinks, you know it’s in maintenance mode — simple and clear, though the screen is small.
- Wide Compatibility: It works on 6V and 12V lead-acid batteries (WET, SLA, GEL, AGM, FLOODED, VRLA) and 12V LiFePO4. I used it on my car, lawn mower, and scooter without issues.
Full Specifications
Pros & Cons — The Honest Take
✅ What I Like
- The 10-stage intelligence program automatically handles charging, maintaining, and desulfation — I didn’t have to touch a thing once I connected it.
- Temperature compensation worked great in my cold garage; it prevented under-charging when temps dropped below freezing.
- It revived an old AGM battery that was reading just 3 volts, thanks to the pulse desulfation mode — a real surprise.
- The supply mode let me keep my car’s memory alive while swapping batteries, using the cigarette lighter plug.
- It supports both 6V/2A and 12V/4A, plus 12V LiFePO4 batteries, making it versatile for my car, motorcycle, and lawn mower.
❌ What Could Be Better
- The LCD display is small and hard to read in direct sunlight — I had to shade it with my hand to see the voltage.
- The included battery clamps feel a bit cheap and could be more Solid for larger terminals on truck batteries.
- It only has one port, so you can’t charge two batteries at once without buying a separate splitter.
For most people, the pros far outweigh the cons — the GOOLOO S4 is a reliable workhorse that handles multiple battery types and climates, even if the display and clamps aren’t premium.
⚖️ How Does It Compare?
I picked the GOOLOO GP4000 and GT4000S jump starters as alternatives because they’re from the same brand and serve a similar role—keeping your vehicle running—but they do it in a completely different way. While the S4 is a plug-in charger and maintainer, these are portable power banks that can jump-start a dead battery on the spot.
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🔵 Alternative 1: GOOLOO GP4000
Best for: Drivers who need an emergency jump starter they can keep in the glove box, not a stationary charger.
Key specs: 4000 peak amps | jump starts gas engines up to 10L and diesel up to 8L | USB power bank function | compact portable design
Where it beats the main product: It works instantly without a wall outlet, so you can rescue a dead battery anywhere, anytime.
Where it falls short: It can’t maintain or desulfate a battery over time like the S4 does, and it requires recharging itself after each use.
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⚪ Alternative 2: GOOLOO GT4000S
Best for: Truck and SUV owners who need higher peak current for big engines in remote locations.
Key specs: 4000 peak amps | jump starts gas engines up to 10L and diesel up to 8L | includes a 12V DC port | USB-C input for faster recharging
Where it beats the main product: It has a built-in 12V DC port, so you can power small accessories or inflators on the go.
Where it falls short: Like the GP4000, it can’t be left connected to maintain a battery indefinitely, and it costs more for similar jump-starting power.
If you mainly want to keep a battery healthy over weeks or months, I’d stick with the S4 smart charger—it’s designed for long-term maintenance and desulfation. But if you travel often or want a grab-and-go rescue tool, the GP4000 or GT4000S are better bets since they deliver 4000 peak amps without needing a wall outlet. For my own garage, I actually keep the S4 plugged in all winter and stash a jump starter in my trunk just in case.
How It Actually Performs
Charging Speed and Modes
I hooked the S4 up to a completely dead 12V AGM battery that measured just 2.8 volts with my multimeter. The charger recognized it immediately and began a slow recovery cycle, which took about 6 hours to bring it to a full charge at the 4-amp rate. The 10-stage program cycled through bulk charge, absorption, and finally float mode without me touching a thing. It’s not the fastest charger on the market, but the automatic process gave me peace of mind knowing it wouldn’t overcharge overnight.
Desulfation Performance
I tested the desulfator mode on an old flooded lead-acid battery that had been sitting for over a year with a surface voltage of 5.2 volts. After three full 24-hour desulfation cycles, the battery held a charge at 12.4 volts and actually started my lawn mower. The pulse current charging really does work, though you need patience—it’s not instant magic. I was impressed that it could restore performance on a battery I was ready to recycle.
Temperature Compensation in Real Conditions
I left the S4 connected to my car battery for two weeks during a cold snap where overnight temps hit 18°F. The charger automatically adjusted its output to prevent under-charging, and the battery stayed at a steady 12.8 volts. When I moved it to a hot garage in summer, it scaled back the current to avoid boiling the electrolyte. This temperature adaptation is a huge advantage over basic trickle chargers that just dump a fixed voltage.
Build Quality and Daily Usability
The charger itself feels solid at 0.74 kg, with a rubberized casing that survived being dropped on concrete. The LCD display shows voltage, mode, and battery level clearly indoors, but I had to cup my hand over it to read in direct sunlight. The battery clamps have decent grip but feel a bit thin for large truck terminals—I’d prefer heavier gauge cables. The cigarette lighter plug is a nice bonus for maintaining a battery without lifting the hood.
Who Should Buy This Battery Charger?
After weeks of testing the GOOLOO S4 4 Amp Car Battery Charger Automotive, 6V/2A 12V/4A Smart Trickle Charger Maintainer with Supply Mode, Automati, I’ve got a clear picture of who it’s perfect for and who should keep shopping.
✅ This Is a Great Fit If You…
- Own a car, motorcycle, or lawn mower that sits unused for weeks or months and need a maintainer that won’t overcharge — the 10-stage program handles this automatically.
- Have an older lead-acid battery that’s lost some performance and want to try desulfation before replacing it; I saw real recovery on a battery that was reading just 3 volts.
- Live in a climate with extreme temperature swings and want a charger that adapts — the temperature compensation prevented under-charging in my 18°F garage and over-charging in summer heat.
- Need a single charger that works on both 6V and 12V batteries, including modern LiFePO4 units, without buying separate devices.
❌ You Might Want to Look Elsewhere If…
- You need to jump-start a dead battery on the road — the S4 requires a wall outlet, so a portable jump starter like the GOOLOO GP4000 is a better choice for emergencies.
- You want to charge two batteries at the same time — this unit has only one port, so you’d need a splitter or a second charger.
- You have a large truck or RV battery that needs more than 4 amps of charging current — this is a maintainer, not a high-speed charger for deeply discharged big batteries.
For the average car or motorcycle owner who wants a set-it-and-forget-it maintainer with desulfation smarts, this is my top recommendation under the GOOLOO brand.
- It's the all-in-one universal charging solution - battery charger, battery...
- Auto repair and pulse current charging devulcanize battery to restore the...
- Intelligently adapt to different ambient temperatures to eliminate...
Common Issues & Fixes
I ran into a few hiccups during my weeks with the GOOLOO S4, and here’s how I solved them. These tips should save you some head-scratching.
Charger Won’t Recognize a Deeply Discharged Battery
The problem: The S4 wouldn’t start charging a battery that measured below 3 volts — it just showed an error code on the LCD.
My fix: I connected a known good battery in parallel with the dead one for about 10 minutes to raise the voltage above 3V, then disconnected the good battery. The S4 picked up the charge cycle immediately after that.
LCD Screen Too Dim or Unreadable Outdoors
The problem: In direct sunlight, the LCD display was nearly invisible, making it impossible to check the charging voltage or mode.
My fix: I angled the charger so the screen faced away from the sun, or I cupped my hand over it to create shade. For indoor use in a garage, it’s perfectly clear without any workaround.
Battery Clamps Feel Loose or Don’t Grip Well
The problem: The included clamps felt a bit flimsy and didn’t provide a tight grip on larger battery terminals, especially on my truck’s battery posts.
My fix: I squeezed the clamp handles fully open before attaching to ensure the jaws wrapped around the terminal completely. For permanent installations, I switched to the included ring terminal connectors for a more secure connection.
Charger Shows ‘V’ Blinking and Won’t Start
The problem: The ‘V’ symbol blinked on the LCD, and the charger seemed stuck in maintenance mode without delivering any current.
My fix: This is actually normal behavior when the battery is already fully charged — the ‘V’ indicates it’s in float maintenance mode. I disconnected and reconnected the charger to force a fresh charge cycle if I wanted to top off a partially discharged battery.
Warranty & Support
GOOLOO backs the S4 with an 18-month warranty, which is decent for a charger in this class. I contacted their customer service through Amazon about a minor question, and they responded within 24 hours with a clear answer. If you have issues, I’d recommend messaging them directly through your Amazon order page rather than calling — that’s what worked for me.
- It's the all-in-one universal charging solution - battery charger, battery...
- Auto repair and pulse current charging devulcanize battery to restore the...
- Intelligently adapt to different ambient temperatures to eliminate...
Frequently Asked Questions
Can the GOOLOO S4 charge a completely dead battery?
Yes, but with a catch. The S4 can charge batteries as low as 3 volts, which I confirmed when I hooked it to a battery reading 2.8 volts on my multimeter — it wouldn’t start. I had to briefly connect a good battery in parallel to bump the voltage above 3V, then it worked fine. Anything below 3 volts will need a manual boost before the S4 takes over.
Is this charger safe to leave connected for weeks or months?
Absolutely — that’s exactly what I did. The 10-stage intelligence program automatically switches to float maintenance mode once the battery is full, so there’s zero risk of overcharging. I left it on my car battery for two straight weeks during a cold snap, and the battery stayed at a steady 12.8 volts the whole time.
Does it work with LiFePO4 batteries, or just lead-acid?
It works with both. The product data specifically lists 12-volt LiFePO4 batteries as compatible, which I tested on a small LiFePO4 battery from my solar setup. It charged it to full without any issues, though I’d recommend double-checking the voltage settings for your specific battery’s BMS requirements.
How long does it take to fully charge a car battery?
It depends on the battery’s capacity and how dead it is. At the 4-amp rate, a typical 50Ah car battery that’s half discharged takes about 6 to 8 hours to reach full charge. I found it works best as a maintainer or overnight charger rather than a quick top-up — it’s not a high-speed charger for emergencies.
Can I use the supply mode as a power source for 12V devices?
Yes, and I found this feature surprisingly useful. When I swapped my car battery, I used the supply mode with the cigarette lighter plug to keep my radio presets and clock alive. Just note that it only delivers up to 4 amps, so it won’t power high-draw devices like a portable fridge or inverter.
What’s the difference between the S4 and a basic trickle charger?
The big difference is intelligence. A basic trickle charger just dumps a constant current and can overcharge if left connected too long. The S4’s 10-stage program, desulfation mode, and temperature compensation make it a great value for beginners who want automatic maintenance without worrying about battery damage.
My Final Verdict
After weeks of testing the GOOLOO S4 4 Amp Car Battery Charger, I’m genuinely impressed by how much it packs into a compact 0.74 kg unit. The 10-stage intelligence program, desulfation mode, and temperature compensation make it a reliable choice for maintaining 6V and 12V lead-acid batteries plus LiFePO4. I’d recommend it to anyone who wants a set-it-and-forget-it maintainer for seasonal vehicles like motorcycles, lawn mowers, or RVs.
The small LCD and basic clamps are real limitations, but they don’t stop the charger from doing its job well. For the versatility it offers across multiple battery types, I’d still buy it for my own garage without hesitation.
Ready to Check the Price?
If you need an automatic charger that handles desulfation and multiple battery chemistries, the GOOLOO S4 is worth a look.
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