Harbor Freight 35AH Deep Cycle Battery Review

Sometime in July last year I had a 25% off coupon so clutching it in my sweaty palm I drove to Harbor Freight. I wanted to upgrade my onboard batteries from my 7AH gel batteries to a 35AH SLA battery. With that coupon the $70 Harbor Freight ThunderBolt Magnum Solar rechargeable deep cycle battery was just $55. What a bargain!

On the box it says “AGM” yet on the battery it doesn’t. That was pretty strange. Anyway,it was plugged into my solar system a month or two after purchase and not really used although it was kept on solar chargeUIKeyInputDownArrow using one or other of my charge controllers. That brought me to about September or October by which time I did not need to power my extraction fans. Thus the battery was used solely for powering lighting and tablet/phone charging which it did quite well.

Moving on to this summer and I needed the extraction fans. That was when I found running them for 5 minutes on the hour was all the batteries would take. Now not being bothered to do the calculations I hadn’t thought “gee - two 2.5A fans for 5 minutes is too much for a 35AH battery”. So when eventually I got my watt meter and measured current usage at 3.7W which shouldn’t flatten the battery. At the time I had 35W of solar power which by my calculation even if I got just 15W an hour should have provided enough for 15 minutes of fan power every hour.

In frustration at the battery continually being flat by mid afternoon (I had a timer that set the fans going for 5 minutes on the hour between 11AM and 6PM) I added two 30W solar panels. That didn’t help matters much either. 

Next I switched out the charge controller as that decided to blow up in April. It was allegedly a 10A controller and 5A proved too much for it. I replaced it with a different model and got no more out of the battery with that though that controller had a software crash every time the fans started. I replaced that with a much simpler controller then found after a week that would be undercharging the battery as it was designed for SLA (which has a maximum of 12.8V as opposed to the 13.8V of AGM). Thus I obtained a different charge controller. This one was much improved though the battery voltage never climbed above 13.4v and often stayed at 13.1v. That was bizarre as AGM is supposed to be 13.8v

Having changed everything in the system to make the battery perform, I took a careful look at how much power my system was using. I was using far less power than I’d imagined from the fans. The lights and so on used about what I expected and the USB chargers used very little power. So why was my battery going flat?

I disconnected everything and waited a few days with 95W of panels soaking up the sun’s rays. The two 30W panels were kept aimed perfectly at the sun. Now if those produced just 30W between them, my 35AH battery would receive 2.5AH per hour and 5 hours would give me 12.5AH in the battery. I kept everything off for 3 days and let my 95W of panels soak up the sun. The charge controller told me the batteries were full pretty quickly but even so I waited the full 3 days. That’s at least 35AH of sunlight available. Needless to say though within a few short hours of having the solar power going into the battery my watt meter was reading that the charge controller was only taking about 3W from the panels. That’s not because the light was poor. It was because the controller reckoned the battery was full and had gone onto float charge.

So, the inevitable conclusion is that as I have ample solar panels which produce ample power and a new charge controller, know how much power my panels produce and how much power I’m using, the battery must be as dead as a Dodo. To test that theory, after the 3 days I ran my two fans and kept everything else turned off. After 20 minutes my system shut down as the battery was declared empty by the charge controller.

I know the solar panels are good. I know they produce plenty power. I know the charge controller is good. I know how much power I am using. 20 minutes at 3.7A is 1.23AH. That strongly suggests that the battery from Harbor Freight is a piece of worthless junk and that I got robbed by Harbor Freight. It does not get cold here in South Carolina and the battery has not been misused nor mistreated. It was likely dead before I bought it or died very shortly thereafter. Three different charge controllers cannot be wrong. My two watt meters cannot be wrong. My two multimeters cannot be wrong. All agree that there’s something amiss and it all points toward the battery.

With my hand on my heart I cannot recommend the Harbor Freight Thunderbold Magnum “universal” deep cycle 35AH battery. It might be OK as a paperweight but I doubt it’ll be good for anything else. 

My recommendation having used the Harbor Freight battery is that instead of being a cheap arse and paying $55 for a battery type that normally retails for $100 - $120 you spend that $100-$120 now rather than $55 now and $100-$120 later, totaling $150-$170 and having the problem of a battery to dispose of. If you’re going to go cheap (like I am) then use a $20 lawnmower battery but don’t use it below 75% full. I’m only going to use a lawnmower battery (or try it) because I happen to have a spare lawnmower battery. The story behind that is the lawnmower wouldn’t start so a new battery was installed only to find the starter was the problem. So there’s a spare, used, lawnmower battery available. Thus instead of my buying a new battery for the bus and spending another $100, I’m going to slip the lawnmower battery into place and adjust the programmable charge controller settings so that it doesn’t discharge too far.

 Say NO to Harbor Freight’s junk 35AH deep cycle battery.