Last fall, when I used my lawnmower for the last time, I put it in the shed, connected the Harbor Freight Solar system to the battery and closed the shed.
Today, I went to mow the lawn for the first time this season. Disconnected the solar system from the battery and hit the start switch.. Starter motor runs but doesn't engage the flywheel. Suspecting a stuck bendix, I pulled the shroud from the motor. The bendix had free movement by hand. Hit the start switch and the starter was turning BACKWARDS! Got my trusty digital volt meter and found the battery polarity was BACKWARDS! Apparently, when I connected the solar system last autumn, I hooked it up backwards. Over the winter, the reversed connection apparently drained and reverse charged the lead acid battery. Learn something every day but I'm still curious as to why the reverse connection didn't blow something in the Harbor Freight controller.

 

I'm still curious as to why the reverse connection didn't blow something in the Harbor Freight controller.

Perhaps because the controller was operating in constant current mode and configured so that battery polarity was not sensed.

 

After some searching with Google, I found a schematic of the controller. It has two diodes in parallel that are in series with the battery connection. Apparently, the battery went totally dead over time on its own and then the solar cells took over and charged it backwards. Also, during my internet search, I found that it is not too uncommon for wet cell batteries to be charged backwards, but their capacity is greatly reduced.

I also learned that the old designed lead/acid batteries polarity was determined by the polarity of their first charge. At that time, the plates (+/-) were made of the same type materials and design. The +/- plates in modern lead/acid batteries are made of different types of materials and different grid structures.

So far, I've just turned the battery around in the lawn mower and it started right up. I think I'll put a load on it, discharge it completely and recharge the proper polarity. I probably have a new battery in my near future.

 

It has two diodes in parallel that are in series with the battery connection. Apparently, the battery went totally dead over time on its own and then the solar cells took over and charged it backwards

If you connect the charger backward, the diodes are still forward biased, so the charger likely simply started charging the battery in reverse.
The charger is probably current limited so it didn't care what the voltage was.

 

Haha, I didn't even know this was possible. Learn something new every day.

Put the battery on ebay:
"One of a kind, reverse battery."
A friend got $14 for a chicken strip shaped like south America. I would think you could at least exceed the core charge with this unique specimen.

 

Back in the early 80's I worked for a lighting services company. Mostly emergency lighting systems. Some had Mercury batteries in them. When they went bad they were replaced by SLA batteries and the chargers were adjusted accordingly.

Wanting to mess with some of these Mercury cell batteries I took several scrapped batteries home. In retrospect it was a stupid idea, especially since I had children in the house. Nevertheless, I shorted the leads on one of the better batteries and discovered it was capable of quite high currents. Quickly disconnected that one. Others, with very low voltages, I connected to a charger and left them for a few days. Inadvertently I connected them backwards and didn't realize it. Those darn things TOOK a reverse charge, which blew my mind. I never thought such a thing was possible. And yet, here they were, revers charged and seemingly to be full of life. So I started discharging the remaining batteries and then reverse charged the lot of them. Hurricane Gloria came through Connecticut and knocked power out for days. Yet, I had plenty of light from just those several Mercury cell batteries.

Now, before anyone yells at me, I THINK they were Mercury batteries. For the life of me I can't imagine how mercury could possibly be used as an electrolyte. I could be wrong. Oh, and yes, as I said before, I realize how stupid and dangerous it was to even mess with those things. Back in my youth I did some explosive experiments with mercury. I refuse to go into details, but I did figure a way to get a rather large and energetic release of energy using some mercury. Don't ask for details. And what I did back then was LONG before terrorist concerns. Today I don't mess with explosives of any kind. PERIOD!

 

Thanks for the replies. I'm glad to see that I wasn't the only one that didn't know it was possible. Never too old to learn, they say.

 

Now, before anyone yells at me, I THINK they were Mercury batteries. For the life of me I can't imagine how mercury could possibly be used as an electrolyte. I could be wrong.

If they were mercury batteries (and I doubt they were since most mercury batteries were primary cells and, hence, nonrechargeable) then the mercury wasn't the electrolyte. The mercury (probably mercury oxide) was the cathode. The electrolyte was likely either sodium hydroxide or potassium hydroxide.

Mercury batteries were typically used where either very long shelf-life (such as in Civil Defense stockpiles) or very constant voltage over the bulk of the discharge curve (such as photographic light meters and early non-regulated electronics such as hearing aids and calculators) was important.

Once interesting special-purpose mercury battery was for smoke detectors. It was a 12 V battery but one of the cells was deliberately of lower capacity than the others so that the need to change the battery could be easily detected when the terminal voltage dropped but the remaining cells still had enough energy to keep the smoke detector functional for quite some time afterward.

 

Reverse charging happens a lot in long strings of series cells if you don't maintain even charges and discharges of each cell or have a weak cell. When one cell reaches discharge level ahead of the rest, the remaining cells will force the current through the discharged cell and generate an EMF on the internal resistance of the opposite polarity of the normal EMF.

A primary job of a BMS is to prevent this with balancing and equalization.
https://forum.digikey.com/t/cell-balancing-and-battery-replacement/873