Will my alternator overcharge AGMs?

[Sailing Paul]

My current house bank needs to be replaced. It consists of 2 85Ah SLA batteries which are basic wet flooded chemistry. I am considering replacing with AGMs for all the known advantages. However, my alternator outputs a regulated 14.5V and I am concerned this may overcharge my batteries and so shorten their life. My shore charger outputs 14.5V for 6 hours after powering up as well, then drops to 13.5V. Again a potential source for overcharging. The charging setup was designed with FLA in mind with any overcharging compensated for by topping up with water.

I keep the boat in the UK and the batteries stay cool much of the time, eg <20C, should that make any significant difference.

Whilst at anchor I intend to use a separate LFP bank, topped up with solar, so the AGMs should stay at a fairly high SOC most of the time, just used for instruments and auto pilot under sail, and when using the windlass.

Should I be concerned about my alternator overcharging, or just replace and not let it bother me?

[sailorboy1]

well yes if you use voltage higher that you suppose to it will be a problem

why don't you adjust the voltages to be correct?

[Kettlewell]

Check with the battery manufacturer for their optimum charging voltages. 14.5 is in the range that some recommend for an absorption charge. And yes, temperature does make a difference. This article covers things pretty well. https://workshoppist.com/agm-battery-charging-voltages/

And, here is an article on typical alternator charging problems with AGMs, if you don't have some sort of smart regulation. https://workshoppist.com/car-alterna...e-agm-battery/

[Wolfe10]

First question is NOW LONG are you running the engine.


14.5 VDC for an hour or so, particularly with discharged batteries is fine and very much like bulk mode on a smart charger or smart inverter/charger.


Motoring all day long (like long ICW trips) is a different matter.


There are some smart regulators that will have your alternator following the basic 3 stage charging that would be more battery-friendly.

[SailingHarmonie]

None of the answers you have gotten have asked you an important question: is your alternator internally regulated? If it is, then the output voltage into a full battery is going to be about 13.4 to 13.7. (Most certainly NOT 14.5V!) For routine motoring of a sailboat that’s going to be fine. My AGM’s live a long and happy life with an internally regulated engine alternator. I don’t need more sophisticated alternator control because other charging sources are normally used.

If on the other hand you have an externally regulated alternator, then just adjust the regulator to the battery manufacturer’s specifications for absorption and float voltages.

[Sailing Paul]

Originally Posted by SailingHarmonie

None of the answers you have gotten have asked you an important question: is your alternator internally regulated? If it is, then the output voltage into a full battery is going to be about 13.4 to 13.7. (Most certainly NOT 14.5V!) For routine motoring of a sailboat that’s going to be fine. My AGM’s live a long and happy life with an internally regulated engine alternator. I don’t need more sophisticated alternator control because other charging sources are normally used.

If on the other hand you have an externally regulated alternator, then just adjust the regulator to the battery manufacturer’s specifications for absorption and float voltages.

Interesting question! I have a Yanmar 3GM30F with a standard 55A Hitachi alternator. No external regulator and I believe internal regulation is essentially temperature regulated. So as it gets hotter the voltage is reduced. Is this what you mean by internally regulated?

This makes we wonder whether AGM batteries would be more at risk of under charging than overcharging.

[Sailing Paul]

Originally Posted by Kettlewell

Check with the battery manufacturer for their optimum charging voltages. 14.5 is in the range that some recommend for an absorption charge. And yes, temperature does make a difference. This article covers things pretty well. https://workshoppist.com/agm-battery-charging-voltages/

And, here is an article on typical alternator charging problems with AGMs, if you don't have some sort of smart regulation. https://workshoppist.com/car-alterna...e-agm-battery/

These are really helpful links and expand upon my concerns. A dumb alternator setup as described both under and over charging AGMs.

The articles say that AGMs are more sensitive to over/under charging than flooded, but I would not be interested in buying traditional flooded batteries that require topping up with water. The alternative to AGM would be still be sealed VRLA type batteries, just using flooded construction. So the question then is whether AGMs are more sensitive to over/under charging than sealed VRLA flooded batteries? If they are then I may not get improved longevity from AGMs with my existing charging kit. Could AGMs wear out even more quickly?

[Sailing Paul]

Originally Posted by Wolfe10

First question is NOW LONG are you running the engine.


14.5 VDC for an hour or so, particularly with discharged batteries is fine and very much like bulk mode on a smart charger or smart inverter/charger.


Motoring all day long (like long ICW trips) is a different matter.


There are some smart regulators that will have your alternator following the basic 3 stage charging that would be more battery-friendly.

We aim to use the engine as little as possible! There are though times when we will have to motor all day. I guess I could, maybe should, turn off the lead acid bank in such circumstances to prevent it being overcharged. The instruments, etc. could then run straight from the alternator.

[Sailing Paul]

Just had a thought about how smarter alternators and other chargers are supposed to work. When charging AGMs I believe you are supposed to lower to float voltage once the absorption phase finishes. But if the batteries are connected to a load, how can the "smart" charger detect the end of the absorption phase?

[SailingHarmonie]

Yes AGM batteries are more sensitive to undercharging. That would be a problem if your only, or principle, charge source was your engine alternator. That is not the system you described.

Yes, AGM are damaged by overcharging. However, a properly set up alternator does not go high enough in voltage to cause this. Think about this for a second. Lots of cars use AGM batteries with dumb alternators. They can spend many more hours running than your boats engine.

If you want to regularly charge your batteries with your engine alternator, you need to have a smart regulator. That’s true for any battery, not just AGM. If you just pump some extra energy into the batteries as a side effect of running the engine, a dumb alternator will work fine.

My Lifeline AGM bank lasted 7 years, cycling virtually every day, charging from a dumb alternator when the engine was running. We move a LOT, and our engine runs about 500 hours a year. Is that good enough for you?

[s/v Jedi]

If you motor for more than a couple of hours you simply -must- have an externally regulated alternator with a smart regulator. Every other option is technically flawed.

I can recommend a kit from Balmar, they have them for every engine.

[Wolfe10]

Do NOT turn off alternator B+ output to at least A/one battery with the engine running. Likely to blow the alternator diodes.

[Stu Jackson]

Overcharging with Combiners or ACRs The MYTH:
http://c34.org/bbs/index.php/topic,7052.0.html

[donradcliffe]

Originally Posted by s/v Jedi

If you motor for more than a couple of hours you simply -must- have an externally regulated alternator with a smart regulator. Every other option is technically flawed.

I can recommend a kit from Balmar, they have them for every engine.

Balmar regulators are good kit, unless you put them in the engine compartment--they don't like it hot. You can get a plug and play dumb regulator from NAPA for $25 as a backup if you fry the Balmar.

[sailorboy1]

Originally Posted by donradcliffe

Balmar regulators are good kit, unless you put them in the engine compartment--they don't like it hot.

Probably true, but my Balmar regular is in the small engine enclosure and about 1 foot away for the engine and it has been fine for 22 years. Just saying.