Will Batteries Die If Not Used?

[Maine Sail]

I just had the opportunity to test this on an AGM battery. Lots of dock-talk suggests that a battery not used, such as in a reserve battery application, will die and lose capacity. This can be true if the self discharge rate is enough, the storage temps are also high and you let the battery self discharge to a dangerous level. Flooded batteries self discharge faster than AGM or GEL and warmer temperatures do accelerate this process on all lead acid batteries. Flooded batteries will also suffer from stratification if the electrolyte does not get to gassing voltages often enough, but on a sailboat that gets used the electrolyte is continually moving.

The Battery:

In March of 2015 I had one of a customers May 2014 date code Deka G-31 AGM batteries in my shop for capacity testing and to see if it could be have any capacity regained from an overly deep discharge event....

He brought it to me because his Rule bilge switch stuck in the on position (go figure never heard of that before) and the bank was discharged to an approx 10.2V rebound/resting voltage, once the load was removed. He caught it quickly but the capacity testing still put the approx 1 year old AGM battery at 75.91Ah of capacity out of a 105Ah rating. It had lost approx 28% of its rated capacity and only had about 15-18 actual cycles on it. The failed Rule switch murdered the bank (don't even get me going on these switches)...

After the testing had concluded I recharged the battery at a 30A rate and and held a 14.6V absorption voltage until 0.1A of current was flowing into the battery. I then set it aside for him to pick up.

Long and short he simply decided to purchase new batteries and the Deka sat in the corner. Two days ago I noticed it and decided to see how if faired after well over a year sitting uncharged through an entire summers worth of heat and a winters worth of cold.. The resting open circuit voltage, when I got it on the bench, was 12.48V, not full but not dead either. I recharged to 100% SOC using the identical protocol as above 30A to 14.6V then absorption was held until 0.1A was flowing into the battery..

I then ran another capacity test at the same battery temp as the previous one, 75F. This time the battery delivered 75.23Ah which is a mere a 0.68Ah difference. This difference is well within the range of error for any 20 hour capacity test, on a battery this size. Despite a rather low resting end point voltage, from sitting so long untouched and uncharged, it really had a minimal to zero net effect on the batteries actual Ah capacity.

For those using AGM batteries as reserve batteries, only rarely topped up, it seems that as long as you keep them charged, resting voltage over 12.7V, better than I did on this one, that you should have no problems for a good long time letting it sit there ready for emergency use.

Ah Testing vs. Impedance Testing:

How does this compare to an impedance tester? Both the Midtronics and Argus analyzers put the CCA at right around 879CCA. This is 79CCA better than the brand new rating of 800CCA. The actual usable capacity at the 20 hour rate however is coming in at just 71.6% of the Ah capacity rating. Once again this shows why impedance testers can be entirely misleading and incorrect guidance for Ah capacity. The impedance testers show a battery in better than new condition yet an actual 20 hour test, done accordance with BCI testing standards, shows it at 71.6% of new. The only reliable & accurate method of testing Ah capacity is a physical discharge capacity test. Once again the impedance testers fail to deliver reliable accuracy.

March 2015 = 75.91Ah > Sit for 14 months > June 2016 = 75.23Ah

This battery sat 100% untouched and uncharged for approx 14 months...

The next steps for this battery will be a battery desulfator as I have a good history and baseline to draw from and to track any improvements. This will just add to my growing desulfator data...

 

[hutch5151]

Thanks for the information. I always enjoy your articles and technical expertise.

 

[Benny17441]

Maine I have heard that it is not good to keep a battery bank on the charger 24/7 for an extended period of time (fact or myth?). Lets assume we are using a quality smart charger. How does this warning relate to the need to maintain the battery fully charged so it does not sulfate? My gut tells me any damage from keeping it on the charger is minor compared to having it discharge .

 

[Gunni]

Yep, my 7 year old AGM starting battery is still running strong. Never gets used, and sits in the boat year-round.

 

[frankies56]

I think temp. is more of a problem for batteries. I had two batteries in my small power boat. group 24 maintance free car batteries the boat sat for over a year. so I decided to replace the batteries they have been in the boat for over 5 years. well the batteries are still good and test well. i would not trust those batteries but its wierd they were never charge regularly. as for the amg,s I got 1 year old batteries from work 4 d,s for free. so I put the amg,s in garage well they are still good with no charging and they are over 7 years old. I am just keeping them till I start cruising they seem to be the best for the money. but what I do think is if you keep a constant charge it fights of sulfation which is a battery killer.just keep them on auto charge. so it just tops them off. and not boil them to death

 

[Maine Sail]

Maine I have heard that it is not good to keep a battery bank on the charger 24/7 for an extended period of time (fact or myth?). Lets assume we are using a quality smart charger. How does this warning relate to the need to maintain the battery fully charged so it does not sulfate? My gut tells me any damage from keeping it on the charger is minor compared to having it discharge .

This is always a tough one because boats at a dock are usually not just sitting there with no loads. Today they now have propane sniffers CO detectors, electronic bilge switches etc. etc. etc.. For most boat owners "floating" is necessary.

One of the best battery engineers I know, the inventor of the Carbon Foam technology, insists that floating is not desirable in VRLA (GEL or AGM) batteries yet accepts that it sometimes has to happen. For his batteries he wants the lowest possible float voltage and that is 13.2V.

The other side is that there are really more crappy chargers out there than there are good ones. Ripple, inability to maintain float when loads such as a fridge kick on and dip switch type chargers that don't allow for proper programming are but a few of the drawbacks.

For flooded batteries a charger that reverts to absorption every few days can be hugely beneficial to prevent stratification but this feature ideally needs to be used with healthy batteries..

I am not a fan of constant floating with an AC charger, it often causes more issues than it prevents, but I also recognize most owners need to due to parasitic loads.

I am also not at all a fan of leaving large 100A +/- inverter chargers running to satisfy a 0.1A load on a chock full battery bank. They don't and can't do this well so they PWM in an attempt to provide the extremely low current necessary to a full battery. Unfortunately pulsing on and off a massive 100A + transformer is less than elegant and the battery does not get a true low float current but rather massive pulses, then off, then pulse. In general the larger the charge source floating the batteries the shorter the battery life I generally see.

Another drawback of large inverter/chargers is when a battery cell goes bad or shorts, and this does happen. You now have three or four other batteries plus the full force of a 100A+ charger driving current into the shorted 10V battery. This spells bad news and can be rather dangrous...

For dock side charging I prefer solar with a good quality controller. Like battery chargers there is a plethora of junk out there in terms of controllers. Alternatively fit the smallest and smartest charger you can find that will maintain the dockside loads and maintain the battery at full. I also prefer a charger that can be permanently set for a float voltage while dockside and unattended.. This way it can not return to absorption when you are not there or every time a fridge kicks on.

Any charger on a boat MUST be a UL Marine charger an should have on-battery temp sensing.

In a perfect world all marine battery charge sources would have the ability to wire in as many temp sensors as you have batteries and any one of the sensors could then shut the charger down.

Sadly we don't live in a perfect world and the marine charge equipment market has failed us on the safety front. One could easily argue that the ABYC has let us down on this front too. Why is one sensor standard practice and "acceptable" on a boat with ten batteries..?? The simple answer is that it should not be acceptable.

The technology to do this is easy, industry gold standards such as the LM335 temp sensors are 96¢ each. Added a bit of wire and a terminal with some epoxy and charge $40.00 each and they are actually making money in a big way. It would at least be nice even if it was an add on option but as of yet not one single charger company I know of offers multiple battery temp sensors. The Balmar MC-614 can monitor two batteries, a step in the right direction, but that is still not enough on boats with multiple battery banks..

When leaving a vessel unattended it's a best practice to use the absolute smallest charger necessary to maintain the bank (solar or AC). One of the most unsafe practices I see at docks is using 100A - 150A + inverter chargers as your maintenance charger. Having a 150A potential in order to supply mA of charge current, when your not there is simply not my bag of tea from a safety stand point...

Why is this a poor practice?

With a 5A charger if you have a shorted battery, that may now be a 10V battery, that battery will begin sucking charge from the 12V batteries but the differential voltage is still around 1.5V or less. When you stack a small 5A charger on top if this, it does not create or add a real huge difference in voltage between the good and bad batteries.. Add a 150A charger on top of four other 12V batteries feeding the shorted one and you've got real issues. That 1 temp sensor the charger maker supplied is almost never going to be on the shorted battery. This is the failing of the charger industry I talk about.

Internal shorts most often start out small and present themselves in faster than usual voltage decay when not charging. When charging they present themselves in a manner that the charge current never drops as low as it once did. For example with no loads on, with a typical float voltage, your bank may have normally been accepting 0.1A - 0.5A of charge current and now it never drops below 9A.. This is a red flag.. If you pay close attention to your ammeter and battery voltage, using an accurate voltmeter that reads to the hundredths (12.72V) not just tenths (12.7V) then you can learn to spot any irregularities in the bank voltage and full charge amperage performance.

Large chargers left unattended are simply going to remain a dangerous practice until charger makers begin to provide us with the ability to sense the temp of every battery on board.

I won't even go into corrosion issues potentials created by plugging into shore power without an isolation transformer (solar).

As a human are still the best defense against battery failures but you have to be there! I rarely see shorted batteries fail rapidly, though AGM's can. More often than not I see them fail starting out as a small leak/short that slowly gets worse...

Course if you are on-board your Model 1 Schnoz should alert you to a shorted battery, if you;re not on-board........ Not much you can do other than using solar, not floating or using the smallest possible & smartest Marine UL charger you can....

This Shorted Trojan at 154F was discovered by the Model 1 Schnoz, the owner was on-board, what if he had not been......????

 

[hewebb]

I use a Battery Tender brand on the boat and understand it is not a constant charger but it cycles the batteries somewhat. My boat does not have anything that uses electricity when in the slip, unless the bilge pump comes on. I have been using them on vehicles that set for months at a time not used.