The Mystery of Schroedinger's Batteries

JeffBurright · #1 08-23-2010

Are they dead or not dead? Right now I have evidence of both.

Hi all, I'm hoping you can help me diagnose a situation with my house bank. A couple months ago I bought a pair of 200AH maintenance free heavy duty batteries (not quite true deep cycle but close enough) that had been pulled off a swanky yacht after a year of service. The owner was revamping his whole electrical system and the yard workers let me have the batteries for a super cut rate.

When I hooked the set up to my boat they appeared to take a full charge and work fine, but during a recent weeklong trip I discovered that after an hour or so, the voltage would steadily and quickly drop down to ~8V when put under any kind of load. Since the heavy duty batteries didn't have screw posts I had put tighten-down terminal posts on the smooth pos/neg nubbins. Fearing a bad connection there, I checked for looseness but didn't see any issues. I also tried hooking my system up to just one of the batteries rather than as a parallel bank, but the problem persisted.

It was so bad that my running lights wouldn't even shine at full brightness, so we ended up needing to buy a new deep cycle battery in the middle of the trip. The replacement battery worked fine, so I figured I'd been sold a couple of duds.

Now this week I took the batteries to an auto shop and when they tested them under load, the guy claimed they were fine. What am I missing here?

Thanks,
Jeff

mitiempo · #2 08-23-2010

Maybe get a second opinion? They sound like they have had it from your description.

Sixpoint · #3 08-23-2010

Car batteries are designed to deliver a large "jolt" of discharge over a short time - like for starting an engine. For running of peripherals when operating your car, the alternator takes care of all that, while the battery is quietly recharged. That would explain the batteries passing a test at an automotive shop.

Deep cycle batteries on the other hand, are designed to be more fully discharged over a longer period of time, such as is needed for your house bank without any type (or only minimal) charging happening at the time.

Generally speaking, any "normal" auto battery will do fine for a little while, and then drop off quickly, unless you have a genset or solar power or some other form of charging going on.

Hope this helps,

SP

mitiempo · #4 08-23-2010

That makes sense. In other words they will probably start a car if charged after the amp or 2 is removed for starting. But long term as you are using them no go.

bartender · #5 08-23-2010

I will assume you have good connections. Second I am going to assume they are connected exactly as the old house bank was wired. One thing worth mentioning here is with mutuple batteries being connected. And then connecting your charger to the batteries you should connect the positive charger lead to the positive side on battery #1 and connect the Negative to the negative swide of battery #2. if you only hook the charger to the pos and neg of the one battery you will never charge the entire bank conpletely. Since you have had no trouble with the new deep cell battery try charging one of your used batteries and replace the new battery with the used battery if the problem persists or functions ok. Do the same thing with the second used battery if the problem is created you have a bad battery, If it functions ok you have to look at the wireing again. Note though if one of the batteries is ill you will never get the voltage above the bad battery no matter how man batteries you put in the bank. The will always seek the voltage of the worst cell. Plus never mix wet and dry cells deep cel and standard batteries. It's better to buy them as a group.