Charging an AGM battery

**SunEagle** · 02-02-2023, 11:54 PM

My fear is how those 8 batteries are wired. I presume that there is unequal charging currents so one or more of those batteries will never see a full charge. One or more may also be damaged so it can't every get back to full charge again.

I would try to charge each 12V separately using a single charger and then see if they all come back to being full.

**-robw-** · 02-03-2023, 09:01 AM

Originally posted by SunEagle

My fear is how those 8 batteries are wired. I presume that there is unequal charging currents so one or more of those batteries will never see a full charge. One or more may also be damaged so it can't every get back to full charge again.

I would try to charge each 12V separately using a single charger and then see if they all come back to being full.

HI. Two strings @ 48vdc (2p/4s). All batteries were fully charged using a NOCO before activation, and yesterday I took a voltmeter to each one: One battery was .2 volts off before I started EQ, the rest were closer. All pretty legit especially since my ammeter indicated the bank was only 75% charged.

Am I right to assume a higher absorb voltage would help?

**SunEagle** · 02-03-2023, 10:33 AM

Originally posted by -robw-

HI. Two strings @ 48vdc (2p/4s). All batteries were fully charged using a NOCO before activation, and yesterday I took a voltmeter to each one: One battery was .2 volts off before I started EQ, the rest were closer. All pretty legit especially since my ammeter indicated the bank was only 75% charged.

Am I right to assume a higher absorb voltage would help?

It really comes down to the resistance in each of the strings. The one that has a higher resistance will fight the current to charge it. Believe me when I say it does not take much of a resistance to slow down the charging of that string. It only has to be more then the other string to cause an issue.

**-robw-** · 02-03-2023, 11:16 AM

Originally posted by SunEagle

It really comes down to the resistance in each of the strings. The one that has a higher resistance will fight the current to charge it. Believe me when I say it does not take much of a resistance to slow down the charging of that string. It only has to be more then the other string to cause an issue.

A resistant battery/string would support a higher current need though? I mean, if a battery or string is not charging properly it would require more charging, not less. The issue I'm facing seems to be either 1) a battery string which is not 200AH, or 2) too low an absorb voltage to charge properly.

**SunEagle** · 02-03-2023, 07:39 PM

Originally posted by -robw-

A resistant battery/string would support a higher current need though? I mean, if a battery or string is not charging properly it would require more charging, not less. The issue I'm facing seems to be either 1) a battery string which is not 200AH, or 2) too low an absorb voltage to charge properly.

The charge will follow the lowest resistant string. Anytime you wire batteries in parallel you run the risk of one string getting more of the charge. I feel that is why some of your batteries are not getting full.

**-robw-** · 02-03-2023, 08:19 PM

Originally posted by SunEagle

The charge will follow the lowest resistant string. Anytime you wire batteries in parallel you run the risk of one string getting more of the charge. I feel that is why some of your batteries are not getting full.

Ok, gotcha on all that, but that is not the issue here. Coincidentally, I just got in from taking my voltmeter to every battery. They're as good as month old batteries should be: all voltages were right around 13.55 after 5 hours of EQ and a rest. And both strings had exact same voltage.

I've owned floodeds, lifep04's and, now, AGM's. With flooded we can jam as much current as we want passed them (CC outputting max amps), they don't care. With lithiums we cannot because they have imbedded balancers that kill the current flow once they're full. Are Renogy AGM's like that, with some kind of balancer? cause that's how they're acting. The tech at Renogy kinda inferred that when she said the battery would shut down if its voltage is too low., but I didn't believe her. She didn't come across as being particularly bright.

**-robw-** · 02-04-2023, 12:11 PM

Hey, I didn't say I believed it! Just saying that's how the bank is acting.

**SunEagle** · 02-04-2023, 01:20 PM

Originally posted by -robw-

Hey, I didn't say I believed it! Just saying that's how the bank is acting.

I understand your dilemma. I hope you can find the issue and save your batteries.

**Bala** · 02-04-2023, 07:59 PM

Originally posted by -robw-

I'm thinking Renogy voltage specs are wrong.

I'm having the damndest time charging 8 Renogy 12v/200AH batteries running for a few weeks now. I noticed something odd before, in that once they get to about 70% charge they won't accept enough current to finish charge by EOD. One of my FM-80's will drop off ("charged") and the other supplies only about 20 amps when far more is available via my array. But now I'm really lost. Us Texans were just hit with a bad storm and, for the first time since living here, we actually had 4 straight days of very dark skies. These batteries got as low as ~25% (47 and change), but when charging them today I had to put my CC's in EQ mode at around 60% SOC to finish charging, and even then my they wouldn't fully accept what my CC could supply.

That seems to me to be the result of too low an absorb voltage, but I've read that AGM's are susceptible to high voltages and don't want to kill them. Renogy specs call for 57.6 vdc absorb, which is 1/2 volt lower than my prior floodeds. I'd like to experiment with higher voltage but before I do I'm wondering if anyone else has had such an experience with Renogy/AGMs and found a solution. Or if this just how AGM's behave.

I've communicated this problem with Renogy but their team is utterly useless.

Thank you.

Follow the manufactures recommendations for charge voltage. You will possibly find their Max v is below gassing. Unless otherwise stated you do not want to equalise sealed batteries.

How are you determining soc?

**-robw-** · 02-05-2023, 10:06 AM

Originally posted by Bala

Follow the manufactures recommendations for charge voltage. You will possibly find their Max v is below gassing. Unless otherwise stated you do not want to equalise sealed batteries.

How are you determining soc?

Hi. With an ammeter. It's pretty accurate, now showing 74% SOC @ 49.33vdc under 7 amp draw.

I'm not sure I've been clear here. What I'm curious about is the following example:

Forecast calls for heavy clouds the next few days. The CC is in absorb mode but I only have 2 hours of daylight left and I want to top off bank by EOD. So I start EQ

Same wiring and interconnects:

Before with floodeds: max amps is delivered (50-80ah) - bank gets fully charged
Now with AGM's: EQ no better than absorb, with restrained ~30ah supply - bank gets to ~90 SOC and I run out of sunlight

Why? Is that just a characteristic of sealed batteries?

- recommended EQ for these batteries is .2v higher than absorb
- set at 4 hour absorb
- set at 10 ending amps
- don't pick on my example. same thing happens everyday, with CC going into float @ 90 SOC while delivering 30 ah

**Bala** · 02-05-2023, 03:47 PM

A couple of things to make communication easier.

An ammeter measures current draw, it will tell you the current in/out at any given time. A battery monitor like Victron BMV will use that info and calculate SOC.

Amps is what the panels are putting in or what is being drawn from the batteries.

Ah is the overall amount of power that has been used or is still available over time.

The victron BMV battery monitors use power that is not recorded by them. So the battery needs a full charge to reset the 100% factor. Yours may be the same, if you have not had 100%Soc for some time it may not be giving you an accurate reading.

The agms may well have different charging characteristics to your previous lead acids.

Have a look at this info. Max Smoke Method. Basicially just set Bulk, Absorb and Float to same setting.

Are You Killing Your Batteries? - Solar Panels - Solar Panels Forum

https://www.solarpaneltalk.com/forum/off-grid-solar/batteries-energy-storage/19316-are-you-killing-your-batteries

You have a battery or energy storage question, post your comment here. Talk about the various batteries, from lead acid, to lithium ion, to Ni.

Are You Killing Your Batteries Part 2. - Solar Panels - Solar Panels Forum

https://www.solarpaneltalk.com/forum/off-grid-solar/batteries-energy-storage/351396-are-you-killing-your-batteries-part-2

Catchy title but if you use Lead Acid Batteries and use Solar to charge them, you are part of the 90% population killing your batteries. Not really your fault

**-robw-** · 02-06-2023, 12:40 PM

Originally posted by Bala

A couple of things to make communication easier.
Have a look at this info. Max Smoke Method. Basicially just set Bulk, Absorb and Float to same setting.

Are You Killing Your Batteries? - Solar Panels - Solar Panels Forum

https://www.solarpaneltalk.com/forum/off-grid-solar/batteries-energy-storage/19316-are-you-killing-your-batteries

You have a battery or energy storage question, post your comment here. Talk about the various batteries, from lead acid, to lithium ion, to Ni.

Are You Killing Your Batteries Part 2. - Solar Panels - Solar Panels Forum

https://www.solarpaneltalk.com/forum/off-grid-solar/batteries-energy-storage/351396-are-you-killing-your-batteries-part-2

Catchy title but if you use Lead Acid Batteries and use Solar to charge them, you are part of the 90% population killing your batteries. Not really your fault

I'm aware of that smoke method, and I applied it to my last floodeds bank which was toasted during Arctic vortex of '21. I would not apply it to a new bank though, especially a voltage sensitive AGM bank. I can also reset SOC on my ammeter, which I do occasionally because most ammeters - like mine - don't have config adjustments to account for 85% efficiency of LA batteries.

Since no one has addressed my original question, I guess juicing my absorb voltage +.2vdc (allowed per Renogy but I'm skeptical - would rather charge at lowest voltages) won't help me here.

**Bala** · 02-06-2023, 04:02 PM

My thinking is you have either a problem with your controller wiring/setup or with your battery wiring/setup or batteries.

I have only used series batteries for my house. Firstly FLA and now sealed.

If I had your problem I would turn off all loads and use my external A/C charger and charge the bank taking note of the charge current. They should taper off to 0 amps current or almost. If all is well this will give you a true 100% Soc but faulty batteries can also taper off to 0 amps current.

I would do that coming up to dark so that you then start the night with a known 100% soc and reset meter. Turn the loads back on.

Assuming a nice sunny day next day monitor what happens.

It is just on daybreak here, my meter is telling me I am -70Ah, so I know I what I need to get back today.

**-robw-** · 02-06-2023, 04:31 PM

Ok, you have AGM's? So what result do you get if you charge your bank to 100% and then start EQ?

Charging an AGM battery

Charging an AGM battery

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