Goal Zero “Extreme 350” Battery Replacement

Goal Zero certainly has a positive product lineup. They’ve been refining their offering for the past several years. The flagship mobile power pack they currently offer is the Goal Zero Yeti Lithium Portable Power Station. You can find out more about this batteries and current costs here: Goal Zero Yeti

Goal Zero Extreme 350
Extreme 350 disassembled

When Goal Zero launched it’s initial product lineup in 2010, the flagship powerpack was the Goal Zero Extreme 350. Essentially they provided a good looking and functional case around the RITAR RA12-33 gel battery. It’s a 33 amp hour battery. These batteries are generally used in applications like wheelchairs and uninterruptible power supplies (UPS). The Goal Zero enclosure is a stackable and sturdy design that gives the user a solid carry handle, Anderson Power Pole connectors, a charging port, slots to attach the 400 Watt Goal Zero power inverter, access to fuses, a simplified charging port and an intuitive power level meter that was easily understandable by pretty much anyone.

Here’s the problem. These batteries are notorious for going bad. I imagine that most of the units sold have simply sat and discharged and then sulphated over time. They are difficult to revive and simply can’t be recharged well with the provided AC wall charger.

My father and I each acquired two Extreme 350 units about the same time, 2011-2012 ish. He stuck his in the garage and never paid any further attention to them. I checked on mine and realize that they discharged pretty rapidly according to the on-board power meter. Even if left plugged in they seemed to not keep a charge very long. As I was somewhat concerned about that, I partially disassembled mine by disconnecting the internal batteries from the top part of the case which contains all the electronics and connections. From then on I stored them without the top on my workbench next to my other batteries. I’ve regularly used a Battery Minder charging unit to re-charge and maintain these batteries. five or six years later, my RITAR RA12-33s seem to hold a consistent 12.7 – 12.8 volts over time.

Meanwhile I recently discovered that my father’s Extreme 350 batteries were dead. They were deader than dead. The power meter was blank. I took them home and over several weeks I tried everything to revive the RITARs. I tried the Goal Zero charger. I used various other charging methods including my Battery Minder. I ultimately connected one of the bad RITARs with a good one in parallel to trick the charger into thinking they had voltage. This got the process going but ultimately “bulged” the battery. It was clear they were dead and dangerous.

I thought it would be a shame to waste these Extreme 350 cases. The enclosures are really solid. I like the idea even though it’s not the latest and greatest. I googled all over looking for a write-up of replacement options and “how-to.” Seeing none I made some measurements and took a small risk and ordered new batteries for my father’s enclosures.

It works!

The battery you want to order if you do this is the Mighty Max: ML35-12 – 12 VOLT 35 AH SLA BATTERY. Click the link for current pricing. I’m sure there are others out there that have the same dimensional specifications that you could use. I know for sure these work.

Mighty Max: ML35-12 - 12 VOLT 35 AH SLA BATTERY
Mighty Max: ML35-12 – 12 VOLT 35 AH SLA BATTERY

Here are the tools you’ll need:

  • 4mm Hex key
  • 3mm Hex key
  • #1 Phillips Screwdriver
  • 10mm inch box end or combination wrench

How to replace the battery – Goal Zero Extreme 350:

  1. Remove the handle using the 3mm hex driver
  2. Lift the top up carefully
  3. Slide the wrench in and loosen the terminal bolts. Pay attention to not touch the wrench to both terminals at the same time.
  4. Remove the side screws using the 4mm hex driver
  5. Carefully pop the plastic sides off
  6. Turn the battery on its side and remove the four Phillips head screws from the base
  7. Detach the battery from the base
  8. Remove the metal supports from each side and place them on the new battery
  9. Re-attach the base to the metal side supports. The Mighty Max fits perfectly.
  10. Replace the plastic sides and attach with the 4mm hex driver. Don’t force anything too hard.
  11. Connect the top to the new terminals.
  12. Tighten the terminal bolts. Pay attention to not touch the wrench to both terminals at the same time.
  13. Set the top in place. Pay attention to replace the heat sensor to the side of the battery. Make sure you are not crushing any wires or circuit boards etc.
  14. Re-attach the handle using the 3mm hex driver.
RA12-33 vs ML35-12 Terminals
RA12-33 vs ML35-12 Terminals
Extreme 350 Power Meter
Extreme 350 Power Meter
The Mighty Max Fits Perfectly
The Mighty Max Fits Perfectly
Extreme 350 Heat Sensor
Extreme 350 Heat Sensor

Why they discharge so quickly

I’ve done some minimal experimenting with the Extreme 350 by comparing the two new ML35-12s side by side. I believe that the design flaw is that the Goal Zero power meter is always on. It’s got a constant drain on the battery. After replacing the batteries I removed the fuses from one of them, thus disconnecting the power meter and essentially rendering the whole thing inoperable. I set the two Mighty Maxes next to each other and compared voltages over several days. The one that was disconnected remained constant while the other lost voltage. It’s my intention to check on them after a few months and see where they’re at.

The Solution?

Store your Goal Zero Extreme 350s with the fuses removed. You can set them loose inside the fuse compartment. Then check the voltage every few months. You can do this with a volt-meter or just plug the fuses back in and use the on-board meter.

Check Amazon for the latest pricing. Current prices may vary. After re-assembling them I gave them back to my dad as a Father’s Day gift. He seemed mildly impressed. 😉

Click the Amazon link to the left to purchase this replacement battery. FREE SHIPPING!

Update on Voltage September 2017

As of September 2017 the new Mighty Max batteries are holding at 12.8 volts. I would expect they won’t need to be recharged for another several months.

Here are some other Goal Zero products that are related which you might be interested in:

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Moses Roni

Moses Roni

Prepper, commenter, diy'er since the '90s. -- "Enviest thou for my sake? Would God that all the Lord’s people were prophets, and that the Lord would put his spirit upon them!"

2 thoughts on “Goal Zero “Extreme 350” Battery Replacement

  • May 1, 2019 at 10:22 am

    Thanks for this info! Very helpful!!

  • August 24, 2019 at 12:45 am

    Was gifted an Extreme 350 that had a battery that was deader than Hillary Clinton’s presidential chances. Ordered up the battery from Amazon, followed the instructions, and everything worked fine. Thanks for posting this…it really made a difference.


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