Basic battery types;
- Lead Acid or FLA or "Wet Cell"
- Sealed or Maintence Free (proof!)
- Gel Cell
- AGM
- and then there are exotics like Lithium Iron Phosphate (new) and I have even heard that someone used Nickle Cadmium(!)... but let's try to keep this simple.
The best bang for the buck at this time is in Lead Acid batteries.
Ummmm which "lead acid" batteries...?
Arghhhhh the semantics kill me some days....
ALL BATTERIES WHETHER GEL, AGM or FLOODED ARE
LEAD ACID....
The proper way to check the "health" of the battery with only a volt meter is; disconnect all loads from the battery, charge the battery, and then let it rest for 24hrs to see what the voltage is. (you could pick nits with this, but I am assuming that your test equipment consists only of a DVM)
There is no way to test the "health" of a battery with a volt meter. All you can test is SOC. Open circuit voltage readings do not represent the
health of the battery, nowhere even close.
Battery
health can only be determined by a physical capacity test (reserve minutes @25A or 20 hour test) or a capacitance test. Even expensive capacitance testers only test for short duration "impulse" or cranking capacity and are not accurately representative of the actual Ah capacity left in the battery.
Per Nigel Calder, at 70°F
- 100% - 12.6-12.7V (wet) 12.85-12.9V (gel) 12.8-12.9V (AGM)
- 75% - 12.4V (wet) 12.65V (gel) 12.6V (AGM)
- 50% - 12.2V (wet) 12.35V (gel) 12.3V (AGM)
While Nigels chart is one of the better ones, open circuit voltage for SOC ideally needs to be determined from each manufactures product or spec sheets. Better yet physically testing your own batteries.. Because of the way batteries are "formed" today the old on-line voltage points for batteries are no longer very accurate when applied across the board..
I am pretty sure Nigel would agree that the latest technology AGM batteries we are currently field testing do not fall into any of the voltage windows above. I have one of them on my bench today that has been at over 13.0V now for the last 7 days in a shop that has been above 70F for 7 days....
The group 31 prototype battery that is currently undergoing a full 20 hour test is at 12.716V with 7.614 Ah's removed and was loaded at the 20 hour load when that voltage reading was taken.... I paused the capacity test when writing this post and the voltage rebounded to over 12.97V (and was still rising) yet this battery has had a solid 7% of its capacity removed. A resting 12.97V OCV is not full on this battery....
7.614 Ah's removed over 1:23 minutes at 5.5A and the battery voltage still climbed past 12.974V when the test was paused, and voltage was not yet stable and still climbing. (NOTE: This is a calibrated lab grade tool I use for capacity testing and constant load applications so the accuracy shown here would be very difficult to reproduce on your own boat.)
Back under 20 hour capacity test:
Different batteries support different voltages and each owner really needs to determine what
full is for their bank in order to get an accurate reading..
Another huge factor is battery temp. With cool batteries, like the bilges of boats in the North East, batteries will take many days to attain a resting voltage and 24 hours simply may not be enough..
Your reading of 14+ volts is simply a reading of the charger's output...
Bingo, even LiFePO4 don't rest at 14+V and any of the lead acids will drop below 14V pretty quickly, though they may slow the voltage decay at 13.6V or so. Some AGM's in cooler bilge areas will sit at over 13V for a number of days but not 14........