Is My Battery Dead or Not?

[nickfox45]

Thanks to everyone for their thoughts, this has been really useful. Looks like I'll be opting for the new battery!

[senormechanico]

This says it all

https://www.youtube.com/watch?v=MH7KYmGnj40

[westwinds]

Not sure if this is desulfation: I had a battery that sat for months and discharged to 50% charge. I used a hydrometer for testing. It took a month with an intelligent charger on it full time, but I did eventually get the battery to 100% charge.

[btrayfors]

The following six identical deep cycle flooded batteries are at "100% charge", as measured by hydrometer readings, resting voltage, and amperage draw (less than .01C):

Battery# - Description - Remaining Capacity

#1 Almost new, broken in well - 100%

#2 Two years old, well treated - 85%

#3 Two years old, not well treated - 65%

#4 Five years old, well treated - 80%

#5 Five years old, not well treated - 50%

#6 Five years old, repeatedly abused - 25%

Lesson: batteries which are at or near "100% charge" may have wildly different residual capacity.

State-of-charge (SOC) means practically nothing in terms of actual residual capacity of a LA battery.

So, too, does "ability to hold a charge".

Bill

[skipmac]

Originally Posted by btrayfors

The following six identical deep cycle flooded batteries are at "100% charge", as measured by hydrometer readings, resting voltage, and amperage draw (less than .01C):

Battery# - Description - Remaining Capacity

#1 Almost new, broken in well - 100%

#2 Two years old, well treated - 85%

#3 Two years old, not well treated - 65%

#4 Five years old, well treated - 80%

#5 Five years old, not well treated - 50%

#6 Five years old, repeatedly abused - 25%

Lesson: batteries which are at or near "100% charge" may have wildly different residual capacity.

State-of-charge (SOC) means practically nothing in terms of actual residual capacity of a LA battery.

So, too, does "ability to hold a charge".

Bill

Hi Bill,

Thanks for posting this, but if I may one question.

When you test the various batteries to determine they are at 100% charge by hydrometer and resting voltage are you saying all the batteries had the same readings for these two factors, say resting voltage on all would be 12.6-12.7 V?

What about their ability to hold a charge? Would the worse batteries show lower resting voltage over a short or shorter period of time?

[btrayfors]

Originally Posted by skipmac

Hi Bill,

Thanks for posting this, but if I may one question.

When you test the various batteries to determine they are at 100% charge by hydrometer and resting voltage are you saying all the batteries had the same readings for these two factors, say resting voltage on all would be 12.6-12.7 V?

What about their ability to hold a charge? Would the worse batteries show lower resting voltage over a short or shorter period of time?

Good questions.

The answer to both is, "it depends". It depends on why the batteries have lost capacity. There are lots of reasons, including:

- sulfation of the plates (PbSO4 crystals on or embedded in the plates);
- other physical damage to the plates;
- stratification, due to different concentrations of electrolyte at different levels;
- physical damage to other internals in the battery;
- buildup of contaminants beneath the plates;
- age;
- contamination of the electrolyte;
- etc.

In the frequent case of sulfation, what is happening is that a portion of the plates is being effectively removed from the chemical process. If, say, 50% of the plate surfaces are covered and now "removed", then the capacity of that battery is greatly reduced. But, in this case, the battery is still very likely to "hold a charge" and to yield the same or nearly the same hydrometer readings as would a healthy battery.

As has been said here and elsewhere dozens of times, the ONLY way to know for sure the residual capacity of a battery is to do a 20-hour load test under controlled conditions. Short of that, there are numerous proxies, some of which are useful (like the inductance/capacitance tests using very pricey instruments) if you know what to look for, others of which tell you little about the general health of the battery.

Again, consider the lowly start battery in your car. It works great, every morning. Despite abuse over the past 5 years, where ambient temps ranged from well below freezing to perhaps 130F or more. Hey, it started your car yesterday, right?

Would you bet the house that it will start your car today? Tomorrow?

That poor battery may only have 30-40% of capacity left. Or even less. But it takes very little energy to start the car....less than 1AH....so you probably haven't noticed as your battery gradually loses capacity over the months and years of abuse, during which it still starts the car.

Bill

[Maine Sail]

Originally Posted by skipmac

When you test the various batteries to determine they are at 100% charge by hydrometer and resting voltage are you saying all the batteries had the same readings for these two factors, say resting voltage on all would be 12.6-12.7 V?

What about their ability to hold a charge? Would the worse batteries show lower resting voltage over a short or shorter period of time?

Bill's answer above is spot on. This may also help give some perspective to SG readings vs. actual Ah capacity.

How to Murder Batteries in Half a Year

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[hsi88]

One nice thing about a lead acid battery with removable caps is as mentioned, using a hydrometer to check individual cells. Usually tells me what I need to know. Being someone with too many vehicles of all types, I don't always pull the batteries and of course over time get some sulfation along the way. As long as its not too bad, I end up using a pulse charger to desulfate them, sometimes replacing the electrolyte. Feel like an EMT for batteries because I have revived a lot of batteries left for dead. Learned a lot about what a battery is capable of along the way. The most important lesson of all? Use the battery and keep it charged. They last for years!