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These notes apply to FLA (flooded lead acid) batteries only. Lithium battery diagnostics should be automatically handled by their BMS
This is, frankly, still a dark art to me. But I have had to figure it out because I am getting much less range from my system than I should be so I've got a "gremlin" somewhere.
Update - I found a battery with nearly normal voltage readings - but one cell is reading as "water" - low SG indicating the acid is gone. Not quite sure how you can have a decent voltage reading in those conditions.
Name your batteries
No not because they're cute. You need to start keeping records of exactly how each battery performs over time - and especially which one might need special attention because of overly low voltage readings after a run.
Damaged Batteries
Here's an example of what you see when batteries are either damaged or heavily sulphated. In my system, I am getting between 30-50% of normal capacity. However...it appears that it is always one battery in the string which is the weak link - ironically I can actually get more capacity by pulling a battery out of the series.
I did almost everything wrong when I first set up my system - using mis-matched batteries, not using enough batteries, running down too far, etc - so these have definitely been abused.
Note the nearly straight discharge curve for a brand new string loaded at 40amps vs the older batteries. They show a strong exponential decay.
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In this graph, Bank 3 is my "quarrantine" bank - every time I would go out I would measure which battery was weakest in the string and consign it to that bank - I have 12 to play with). So that's why such poor performance on that set. Most, but not all of the batteries in this bank ended up being my original Walmart batteries purchased in Aug 07 (so of course they are just out of warranty). BTW these amp ratings are not Peukert corrected - so for example 35AH on this graph is probably more like 50 total. I stopped all the testing at 40 volts - one odd finding is that almost immediately all the batteries pop up to over 12 volts - but a load tester quickly reveals 1 or 2 weak ones in the string.
I have not done extensive specific gravity measurements as I just have a cheap meter and don't trust it's accuracy. But I did check the new batteries after 35AH discharge and they showed 50% charge by SG. So getting exactly what's expected there.
Battery Life/Health
I believe the definition of battery life, in cycles, is the number of charge/discharge events it can handle before capacity falls below 80% of original. I have replaced 1 bank of batteries, but it's clear that by conventional definition my other 8 appear "shot". But...it's also clear that only 1 or 2 batteries are the weak ones, so I am playing around with different strategies to try to improve the health of those batteries before replacing entire banks (and I may just go Lithium at that point).
One of the interesting conclusions from this graph is that perhaps just tracking your voltage under load can give you some sense of the health of your batteries. If you see exponential decay (it just requires 3-4 readings, 5-AH apart to see) you know you have 1 or more weak batteries in the string. A load tester can pinpoint which - but this only works after they've run down a bit -- when freshly charged all of my batteries appear healthy under a 140Amp load)
Sulfation & De-sulfators
Based on reading, the main cause of battery failure is sulfation [sometimes spelled sulphation]. There seems to be a combination of good products and snake-oil salesman when it comes to de-sulfators - devices which pulse a high-frequency alternating current through the batteries in an attempt to break down the large and insoluble sulphate crystals which form in the battery on each charge/discharge cycle.
The best description/site I've seen - which seems to not have an overt profit motive, and doesn't seem to over-hype is http://leadacidbatterydesulfation.yuku.com/topic/379/t/Battery-and-Pulse-Conditioning-Primer.html
I am currently experimenting with an inexpensive de-sulfator I picked up on eBay (I can tell it's doing _something_ because it drives my digital voltmeter crazy). I will post results when they come, but it takes weeks for the effect (improvement) to show up, esp on large AH batteries. If it doesn't work I will shell out for the BatteryMinder which has a plethora of positive reviews and acts as a charger as well.
Symptom:
Amp/Volt meter shows a sudden voltage drop. Boat is noticeable slower.
Possible cause:
Probable 1 or more bad batteries in the pack. After running the system for a while, get a load tester (100amp or higher) and see if any batteries in the pack are significantly weaker than the others.
On a recent trip I had this happen and found one of the batteries was reading only 11.3 volts while the others are still at 12.4. So it had stopped providing any useful voltage to the string under load - sending 48V down to ~ 36V. I've got it running on a de-sulfator I recently purchased to see if it will recover - if not - back to Walmart.
Here's a page with some seemingly sound battery diagnostics
And another:
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