Resting 12v Battery Voltage Reading Is 13.2v; What Am I Missing?

[john61ct]

The devices and standards used for cranking batteries by the automotive industry are completely different than evaluating SoH for a deep cycling bank.

And yes, very few SoC meters even costing hundreds are self-calibrating as Ah capacity declines with age.

A few that are better than most

Merlin SmartGauge
Victron BMV-712
Xantrex Link-Pro

Balmar SG200 looks good, but too new to know for sure.

The link I provided to http://marinehowto.com is a great learning resource, as I said, read all the DC electrical / battery resources there carefully.

[john61ct]

If any device claiming to measure SoC other than SmartGauge does not include a shunt wired at the bank counting Ah flows from all loads and charge sourced, it will not be at all accurate.

Even with such a shunt, if it does not have user-configured AH capacity, Peukert coefficient and Charge Efficiency Factor settings, and is not very regularly reset at 100% Soc by the user, it will not be very accurate.

[john61ct]

Trailing amps.

As you hold charging at the Absorb Voltage specified by the battery maker, you will see Amps lowering (tapering), as battery resistance climbs with SoC.

endAmps is the point at which SoC is by definition at 100% Full.

If that spec is not easily obtained from the bank mfg, use .005C by default

That is, half an amp, per 100Ah bank capacity.

Only after endAmps is reached, should your charge source stop charging, IOW allow voltage to drop, from Absorb setpoint, to Float.

Default settings will almost always be premature, you need to adjust Absorb Hold Time to match your (perhaps varying) circumstances.

Very few charge sources can integrate with a shunt-based BM to terminate charge based directly off endAmps.

They are of course more expensive.

[john61ct]

Battery University is a good 101 level general learning resource, but has too many errors and over-generalizations IMO.

So when you see a conflict between its statements and another resource, do **not** assume BU is the correct one.

[john61ct]

Originally Posted by cstan22

Hankook Power Control Sealed Maintenance Free Battery

That is not a quality battery for true deep cycling usage. ATLASBX is a Korean firm.

If you require AGM, then a maker like Lifeline or Odyssey would be a much better source.

FLA would be much better value though.

Where are you located?

[mikecambrai]

Only a smart charger, allowed to get to its float voltage will charge the batteries properly. let them rest for half an hour with no load and you should be reading around 12.6 to 12.9 volts.
Make sure that your voltmeter is accurate. What can you calibrate it against? Difficult to find a standard voltage against which to set it.
Capacity meters are only a guide.
Calcium batteries tend to be slower than lead acids to reach the at rest voltage.
As you have the batteries out, I would smart charge them individually and then measure them If they are within 0.1 volts I would think that they are all the same. If they are all around 13.1 volts then I would thank my lucky stars and just keep using them. They sound all right to me.

[Taipe]

Fully charged new 12 volt batteries normally have a resting voltage of about 13.1 to 13.2. They are just marketed as 12 volt batteries. But, the resting voltage says very little about the condition of a battery in so far as its ability to hold a charge and it's full storage capacity. Putting a sizeable load on each battery while keeping an eye on the volt meter will let you know the battery is capable of holding a charge if the volt reading does not drop
significantly. Delivering at least 12.8 volts would be acceptable while operating a microwave oven momentarily, setting the timer on the oven for about 5 seconds. Knowing when a battery is fully charged is another matter, and can be determined by reading the specific gravity of each and every cell, if you can obtain the mfg's data for that specification, as similar size batteries do not all have the same reading when new and fully charged with an equalizing charge. You can usually get a graph showing the percent of charge relative to a spec.grav. reading by calling the company's tech dept if you can't find it on their web site. Doing these two checks provides an independent reference to confirm that your charging systems and batteries are doing well. Many years ago a down to earth explanation of the basics of trouble shooting charging systems and keeping batteries charged and healthy when off the grid, was in a book titled "Living on 12 volts with Ample Power". You can pick up a used copy for cheap on the internet.

"Have fun, 'cause if your not, your wasting your time!"

[boatnerd]

I believe this chemistry is designed mainly for starting applications for high amperage draw. If you peel up the stickers on top there are usually screw off caps underneath. Even though these are considered maintenace free they will need rewatering towards the end of their lifecycle(agm have similar caps that are glued down -dont rewater). The low water level could affect the soc indicator. If you do rewater add just enough to cover plates. . At one time i bought and installed many batteries as a marine electrician. One of our battery suppliers showed me this when i went to replace 8 of this type that served as the starter bank for 2 MTU V12s.

[cstan22]

Thanks everyone! You're a bunch of wizards, and this has been immensely helpful. I've bookmarked Marine HowTo, John, by the way - great resource.

I'm currently moored at Paros, Greece. The chandlery here sells Solite batteries, and the chandlery in Leros, where we plan to have the boat hauled out later this summer, sells Exide.

In the meantime, although I don't think I'll have a 20hr window to do a proper test anytime soon, I think I thought of a sort of cheat - correct me if I'm wrong.

Since my battery monitor reads both the AH capacity, based on the original max capacity (I'm assuming it doesn't calibrate) AND voltage, once I receive the accurate voltage-to-SOC chart from Hankook, can't I simply drain the batteries down to roughly 12volts (assuming the chart confirms that's 50%), and then check that against the AH reading? Let's say, it's inaccurately saying 60% or 348 AH, but the voltage confirms that we are in fact at 50%. I could roughly assume that 232 AH used is my new 50%, and that my batteries are at 80% their original capacity. Thoughts?

[john61ct]

Nothing accurate about SoC can be determined from voltage other than zero (10.5V).

If you've been using a batt like that for actual deep cycling for a few years, and get any signs it's worn, just replace it.

Unless your situation makes "unexpected failure" NBD, in which case keep going, they're very unlikely to explode.

[Peeew]

I killed a bank of lead acid batteries by never fully charging them. As with you I had no way to do that while cruising. I am switching to firefly carbon foam AGMs in the hope that they really can tolerate PSOC time without damage. Time will tell I guess.

[john61ct]

They can, but do follow their "capacity restore" protocol as often as you can

[Capdave360]

You've gotten a lot of good advice here about the supply side and storage side of your electrical system.

My advice is about the demand side. I don't know the details of your inverter, but I'm going to guess that it's high capacity compared to your laptops - like 1kW+.

Running that inverter to run your (maybe max, likely less) 100W laptop is likely very inefficient. You pay the overhead load of the inverter, and likely so low on the capacity curve the efficiency is way below peak for the unit.

Install a couple of 12V cigarette lighter plugs, and buy a "high" power laptop car charger - really good ones @65W are like $50.

Way more efficient, will make a noticeable difference on your battery consumption if you're using your computers all day like we do.

My $0.02

[lewisa]

You can buy a reasonable quality battery load tester (a big resistor and a meter) on line or at NAPA for about 25-35 US dollars. Well worth having aboard and useful to monitor battery condition.

[exMaggieDrum]

Originally Posted by Mikado

You can pull them out and take to your local auto parts store and have them load tested for free. That will answer your concerns.

The load testing that they do at the parts stores tests only for cranking type loads that are short duration, like starting your engine. They don't, and cannot, use their testers for deep cycle usage for usable capacity.



Note: many of the anecdotal stories I hear from friends who use laptops all the time lead to dead batteries. If you are only going down to 70% of capacity (not sure how you determined that from what you have said), then the problem is never getting them completely recharged regularly to avoid steady loss of capacity over time.


That's the main reason I have installed lithium batteries on my boat. They charge quickly to as full as you need them (and can easily get overcharged if you don't manage that). But you need to rethink your charging system which can be intimidating and expensive. Along with the expense of the battery itself, this means that many will never do that.


Somehow you will need to find a way to routinely recharge your batteries (probably new ones) to as close to 100% as possible. A good SOC monitor will help you know what you have perhaps but it will not replace the need to get back in the Ah to a full battery.