This is another mad scientist project, where you can cause a serious mishap if you don't do things right, so be very careful and don't fry your face off with exploding battery acid, okay? The process below only works on flooded batteries. If you've got gel cell, AGM, or any other sealed battery, you can't perform this kind of gross equalization.
I went to start my engine after a little overnight cruise, and no go. In a previous post I talked about various ways to keep a battery in reserve for such times, so I was able to start the engine from my house bank.
Checking the voltage on the battery, it was very low:
This pointed to either a deeply discharged battery, or a bad cell. Next I checked the cells with a hydrometer. Hydrometers are becoming less common these days, but they're still widely available, and cheap, less than $10 at an auto parts store. With the hydrometer I checked the specific gravity of each cell by drawing electrolyte into the bulb and seeing where the float rested. At the top of this page you'll see a relatively good cell--the specific gravity is in the green. As I went on to check the others, I found our bad cell trying to hide in the corner. You can see its specific gravity is way down in the red, while all the others were good:
So we've confirmed it's a bad cell. Each cell has a nominal voltage of 2 volts. Connect six in series and you've got a 12-volt battery. One bad cell gives you a 10-volt battery, which isn't going to do much for you.
A cell can go bad for a few reasons, none of which can be seen because they're inside the battery. Sulfates build up on the battery plates' surfaces, increasing resistance until the cell won't accept a charge. Its specific gravity will be very low compared to the other cells. The process of bringing all the cells back up the same specific gravity is called equalizing.
To do this we do a controlled overcharge, AKA abusing and boiling a battery. We need a charging source that is high voltage but low amperage. Five percent of battery capacity is a good rule of thumb for equalizing amperage. My battery is an 8D, so about 200 amp hours. Five percent of 200 amp hours is ten amps. But then we want to get the voltage up to 15-16 volts. It is this high voltage that overcomes the resistance on the cruddy plates, breaks through the crud, and hopefully resurrects the battery.
If we were to try this with some high amperage/high voltage charging source, like an alternator or big AC charger, the battery could heat up, boil, melt down, or maybe even explode. To keep the amperage down around five percent of battery capacity, we need a special charging source. There are sophisticated chargers that can do this...I haven't messed with one. An unregulated solar array or wind generator can be just the key. I have solar panels that put out about 13 amps in direct sun. Unregulated, the 'open circuit voltage' on my panels is about 18-volts. By connecting these, unregulated, directly to the battery I can get this high voltage, but it won't exceed 13 amps, which is close enough to my five percent.
Make sure the battery is completely disconnected from everything else aboard, because this high voltage can fry sensitive electronics.
I connect the unregulated solar panels, and up goes the voltage. It normally takes a few hours, sometimes all day. During this time we've got to monitor the battery closely, making sure it doesn't overheat or boil too much. But boil it will...lot's of bubbles will percolate up through the cells, with a pronounced rotten egg smell. You want to see the voltage come up well into the 15s, even as high as 16 volts.
If the battery gets too hot we can get into a thermal runaway situation, where the increased heat lowers resistance, the battery boils uncontrollably, and disaster strikes. Keep feeling the battery with your hand. It can get warm, but not hot. Some hydrometers have thermometers in them. We want to keep it below 120F.
Keep checking the cells with the hydrometer. If the bad cell's specific gravity comes up the same level as the other cells, eureka!, we have equalized. We can stop this abusive overcharging and our battery will have many more months of service. BUT, we now know we've got a problem cell or a problem battery, and prophylactic equalization might be in order, say every month.
In my case I had no such luck. I even went through two days of boiling, but that bad cell's specific gravity stayed dead low. Also, the bad cell never bubbled, when all the rest bubbled like a bottle of soda.
This tells me that my bad cell is beyond hope. Either it is too gummed up for the high voltage to overcome the resistance, or sloughed-off material has settled in the bottom of the battery, shorting out the cell completely. Also, the highest I could ever get the voltage was 14.9 volts:
In the past I've been able to get the voltage up to 16 volts by this method, so my bad cell probably wasn't accepting any charge at all, and held the overall voltage down. I will need a new starting battery, BUT, every time I've done this in the past it's worked. I've revived many batteries from the dead in this manner, sometimes getting another year out of them.
A friend in the automotive industry says it's also worth trying a deep discharge, like leaving a small load on the battery until it's down to nothing, then charge it back up from there.
If you've noticed what's happened to the price of batteries over the last few years, it's worth a little messing around before replacement.