AGM’s on float

[a64pilot]

I’ve had my AGM bank now for I think going on six years, last year before we left, we had significant loss of capacity, pretty big loss, I had had one other significant loss of capacity.
I’ve had no loss of capacity when the bank was in use.
Both times the loss of capacity occurred after months of being in float, sitting plugged into shorepower. Neither time did an equalization charge recover any significant capacity.
Leads me to believe that float over time isn’t good for an AGM?

I have purchased three L16 Firefly batteries at a very significant expense and I’ve heard a couple of times In past threads about not floating Firelies, and the float voltage has changed etc.

So I’m wondering if it’s not a better idea whenever at a Marina for extended period to simply disconnect the bank, or if your charger won’t work as a power supply, to have a small second cheap bank that you can float?

Will AGM’s last longer if kept cool, fully charged and disconnected than if they are left floating?

On edit, I’m saying fully charge them and disconnect them, monitor voltage and maybe charge every three months or whenever a significant drop in voltage occurs?

[rslifkin]

I've personally never seen a lifespan issue with AGMs in long term float in boats or other applications. A too-high float voltage could be an issue though. What float voltage were you using for your AGMs?

Keeping them cool will definitely help lifespan. If you have solar and the batteries are floating along on shore power or will just have no loads to draw them down and they'll be sitting for an extended period of time, I'd personally drop the absorption voltage and/or time way down. Doing a daily absorption cycle on already-full batteries could be a bit rough on them.

[a64pilot]

I have Solar, I was last year setting absorption voltage to float levels, which for my batteries are 13.3V, so it never saw higher than float voltage.
I bought some extra parts so now Solar drops to float level when acceptance rate reaches .05% of the banks rated capacity, so no real time at absorption voltage.
Bank is 80F now and probably never varies much at all.

[Pitchondesign]

I have some AGM's on my boat that are going on 10 years with minimal loss of capacity. I these are my start and windlass batteries, which get very little use, mostly on float. In the off season (6 months) in my case, I leave them on float with solar, no shore power. I don't have enough experience with my house bank to make an observation.

I killed my previous house bank by disconnecting them in the off season, then reconnecting and charging. They lost very little charge in the 6 month period. The problem came with charging after the 6 months. I have 3 8D's all on the same bank. I began charging them and they charged unevenly. I wasn't paying attention. Charging appeared normal, but it wasn't. One of the three batteries charged faster than the others. The charger saw them all as one battery, they weren't. One battery overcharged, overheated, bulged its case, and was ruined. Replacing one of the three made no sense as they would be seriously unbalanced. So, I replaced them all. I learned I needed to charge each battery individually after a prolonged period of non use.

Now, I leave all of them on float with solar for an extended period, but when I go to my shore power chargers, I still bring up the batteries one at a time. Once balanced, they seem to stay balanced for the season.

There is lots of talk about balancing lithium batteries. My experience is the same applies to AGM batteries.

Just one data point.

[a64pilot]

Batteries in parallel can get out of balance.
Batteries in series can’t. All lead acid batteries above 2V are multiple cells in series, you can buy 2V batteries, but they are actually individual cells, not a battery.
Balance issues are one reason I am going with 3 large 4V batteries. They are each two 2V cells in series, they will be connected in series to make a 12v battery.

Issue with a battery is of course if you lose a cell, you lose the whole battery. Individual batteries in parallel, you can remove the bad battery

[gamayun]

Originally Posted by a64pilot

Batteries in parallel can get out of balance.
Batteries in series can’t.

This is an interesting comment I'd not heard before. I have 4 six-volt AGMs in series and in parallel that make up my 12-volt house bank. They are mainly on a 220-v charger with solar as a back up. I'm not off-grid (yet) and have had them for 5 years now. I've recently been noticing some funny things in which the charge drops quickly after a few days when I'm off AC charge. Would having batteries in parallel be a concern in such a set up for the long-term, off grid?

[Richv]

Originally Posted by a64pilot

I’ve had my AGM bank now for I think going on six years, last year before we left, we had significant loss of capacity, pretty big loss, I had had one other significant loss of capacity.
I’ve had no loss of capacity when the bank was in use.
Both times the loss of capacity occurred after months of being in float, sitting plugged into shorepower. Neither time did an equalization charge recover any significant capacity.
Leads me to believe that float over time isn’t good for an AGM?

I have purchased three L16 Firefly batteries at a very significant expense and I’ve heard a couple of times In past threads about not floating Firelies, and the float voltage has changed etc.

So I’m wondering if it’s not a better idea whenever at a Marina for extended period to simply disconnect the bank, or if your charger won’t work as a power supply, to have a small second cheap bank that you can float?

Will AGM’s last longer if kept cool, fully charged and disconnected than if they are left floating?

On edit, I’m saying fully charge them and disconnect them, monitor voltage and maybe charge every three months or whenever a significant drop in voltage occurs?

It depends entirely on your charger. Xantrex has a line of chargers called TrueCharge2 which are extremely smart. The version I'm using has provisions for up to 3 batteries, with separate monitoring. If used with a 2-battery isolator switch, it would be best to switch the batteries off and allow the charger to individually charge the batteries. If your house battery is made up of two paralleled batteries, I would use a separate battery isolator for the house batteries and use separate taps on a multi-battery charger.

[a64pilot]

Originally Posted by gamayun

This is an interesting comment I'd not heard before. I have 4 six-volt AGMs in series and in parallel that make up my 12-volt house bank. They are mainly on a 220-v charger with solar as a back up. I'm not off-grid (yet) and have had them for 5 years now. I've recently been noticing some funny things in which the charge drops quickly after a few days when I'm off AC charge. Would having batteries in parallel be a concern in such a set up for the long-term, off grid?

In theory I believe an equalization charge should bring them all up to 100% SOC, in fact I believe that’s what it’s called an equalization change and not a desulphation charge etc.

So far as charging each battery independently, there is no need if they are wired in series, it’s parallel connections that can drift in SOC. I think to charge a bank that is connected together, that you may need to disconnect them to charge independently. With them all wired together, how do you charge only one?

[Richv]

Originally Posted by Pitchondesign

I have some AGM's on my boat that are going on 10 years with minimal loss of capacity. I these are my start and windlass batteries, which get very little use, mostly on float. In the off season (6 months) in my case, I leave them on float with solar, no shore power. I don't have enough experience with my house bank to make an observation.

I killed my previous house bank by disconnecting them in the off season, then reconnecting and charging. They lost very little charge in the 6 month period. The problem came with charging after the 6 months. I have 3 8D's all on the same bank. I began charging them and they charged unevenly. I wasn't paying attention. Charging appeared normal, but it wasn't. One of the three batteries charged faster than the others. The charger saw them all as one battery, they weren't. One battery overcharged, overheated, bulged its case, and was ruined. Replacing one of the three made no sense as they would be seriously unbalanced. So, I replaced them all. I learned I needed to charge each battery individually after a prolonged period of non use.

Now, I leave all of them on float with solar for an extended period, but when I go to my shore power chargers, I still bring up the batteries one at a time. Once balanced, they seem to stay balanced for the season.

There is lots of talk about balancing lithium batteries. My experience is the same applies to AGM batteries.

Just one data point.

Most modern lithium batteries have their own BMS (battery management system) which automatically equalizes the cells. My son and I just installed two Battleborn Batteries 100 AH batteries at our house , which is on an Island in the San Juans (WA State), and they explicitly advise against equalization.

[Pitchondesign]

Correct. Lithium batteries don't need equalization, but they do need to be balanced. AGM batteries are the same. AGM batteries, in general, should not be equalized because of the risk of boiling off the electrolyte. AGM batteries do require balancing, just like Lithiums.