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AGM BATTERIES what would you do?

J

JB73

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Aug 17, 2021
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Hello all, so I have acquired a total of 4 brand new AGM batteries. They are 12v 200ah/20hr batteries (qty of 4).
I have 4 panasonic solar panels as well but they are only 80w units (qty of 4). They are attached to a DOT trailer (see pics).
If you were given the items above, what usage ideas would you have for them, what would you add (obviously there is more equipment like inverter etc). I know there's not much capacity even though there are 4 batteries.
Just looking to see what some of you experienced diy solar guys would do if this stuff was sitting in front of you.
I enjoy prepping and thought maybe this could be beneficial in some way, if it were utilized correctly.
Thanks for any & all suggestions, feedback etc.
 

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The good news is you have the batteries to start the right way with a 48v system.

Then it's a question of how much do you want to overbuild your inverter/chargers for future battery upgrades, or just get off the ground with a well matched set.

Simplest way to get a lot of functionality might be an EG4 3k.

Would you imagine more like a mobile generator with 120v plugs, or working towards a 240v system you could plug into your house?
 
The bad news is you have a lot of AGM batteries and way way too little solar panels.

I can’t tell from the photos if you’ve got the batteries wired in series or parallel or a combo. Is it 12, 24 or 48V?

In any case, my experience is that AGM are difficult to charge on solar UNLESS you have a lot of panels.

AGM in particular requires a high current charge ideally everyday but at least every 7-10 days.

For example, a single 12v 200Ah would need a minimum of 0.3C rate, which equals 200x0.3=60A. And this charger needs to happen over a long period of time, like 3-6 hours. And consistently, too, shading or clouds will interrupt this important high amp charge.

So you need a minimum of a 60A charge controller for that one battery. If you intend to put those four in parallel, then you need a total of 60Ax4=240A. Three 80A charge controllers would do it but you’d need excellent sun conditions (and not too hot) and enough panels to power that.

Now if you go 48v, you only need 60A to charge, because it’s operating at 48v. Much easier, but you’ll still need to keep the four batteries balanced with each other, which is another topic but not too difficult with the right equipment.

60Ax48v=2880W
240Ax12v=2880W

I don’t want to burst your bubble but I also don’t want you to destroy your brand new AGMs like I did. I had 200Ah of 12v AGMs that never saw more than 9A on solar and 20A on shore charging, when they really needed 30A daily and 60A weekly. I replaced them after 2 years because they no longer held a charge and the replacements lasted even less as long. Also, don’t buy Renogy AGMs, they’re not very robust but sure are cheap. But I still wasted $800 to learn this lesson.
 
Thanks for all this input. I'm not to up to speed on solar & battery technology and is definitely one reason I'm turning to those who can point me in the right direction.
I would like to keep this simple, a few 120v outlets, use this equipment as a basic system for times of offgrid-emergencies...(what is an emergency? Lol maybe operate a cb radio, charge phones/computer, possibly run a small 120v mini fridge....very basic necessities) also have it mobile as well, seeing as it is already on a solar panel trailer.
I was told by a gentleman, that using what I have, as is, is a waste of time and I will never truly have a use for it, as I can't generate enough power over a long enough period to make it worthwhile....
We live in FL so lots of sun most days. I don't want to sink a ton into it if I can't get much use out of it.
The batteries are Renogy 12v 200ah/20hr AGMs. Batteries are brand new and are connected to the charge controller & panels, keeping them topped off. The trailer orig had same size batteries in it and was used for AM radio broadcasting of traffic conditions on roadways.
I have a solar charger which I will attach a pic of as well.... also acquired with this equipment.
 

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A lot of lead-acid batteries like charging at 1/8th of C, and AGMs in particular may want charging at as much as 1/5th of C. Let's look at the math to see what is proper for your hookup. Let's assume you wire them for 48V, and that you don't expect to get better than 85% output from normal panels. The low end and high ends are.....

[(200Ah/8) X 50Vcharging]/85% = 1470W of panels. Call that six 250W panels.

[(200Ah/5) X 50Vcharging]/85% = 2353W of panels. Call that eight to ten 250W panels.

So, what you really need is thousands of watts of panels, not hundreds. Look up the battery manufacturer's recommendations for the final determination.

Really, the 320W of panels that came with it are a joke. Depending on your final panel number, and how many you wire in series, you'll most likely want a high-amperage, high-voltage controller in the 50-60A, 150-200V range. Take a look at Epever's Tracer 5415, 5420, 6415, and 6420.
 
Nice, but whatever you end up doing, be sure you start out on the right foot, whether the batteries are used or new.

You'll need a 20-30 Amp charger, that has the typical AGM CV spec (usually 14.4 - 14.7 - see your batt manual), and charge each battery *individually* before placing them into service.

Solar won't properly balance these at least initially. In use, if you measure more than 0.1V difference between the batts, you are out of balance. And when that happens, one tends to blow the pressure-caps off the agm.

So crucial to buy a decent charger (not a wimp maintainer! for that size batt!), and charge each individually. You may want to charge off AC individually any time you see more than 0.1V difference between them.
 
1400 cycles (to 30% DoD) would be 4 years nightly.
These batteries are better for backup when the grid is down, rare enough that 50% or 70% DoD is reasonable to do.
AGM costs $0.50/kWh by the time they're worn out, so use as a luxury to keep the lights on, not as an alternative to grid power trying to save money.

As said above, you should have enough PV to charge at 0.2C rate (While also powering your daytime loads.
10 kWh of battery, so 2kW charging for several hours, in addition to loads.

Ideally you would have a charger and a battery shunt that work together to maintain 0.2C charge rate, but harvests more from PV when loads call for it. A couple expensive brands Midnight and Victron offer that. I would think any all-in-one could be programmed for it.
 
AGM batteries are not for the novice. I destroyed 1600 pounds of lead by undercharging them over the course of a few years. But wait!…. overcharging AGM batteries will cause them to vent and they’re toast.

It can be done, and many do it successfully every day, but finding that Goldilocks zone where you charge them hard right up to the point where you stop charging them hard is elusive and unforgiving.

They’re probably already sulfated but if you don’t have much invested in them, a good training course.
 
Hedges said:
1400 cycles (to 30% DoD) would be 4 years nightly.
These batteries are better for backup when the grid is down, rare enough that 50% or 70% DoD is reasonable to do.
AGM costs $0.50/kWh by the time they're worn out, so use as a luxury to keep the lights on, not as an alternative to grid power trying to save money.
Click to expand...

With them being second hand , if he got them really cheap it might well be worth while using as apposed to grid power
 
As the OP said, These are new batteries. AGM batteries are comfortable with up to 20% of their amp hour rating in current, so 40 amps is a good target. Do not charge below 10%. 60 amps is max and not necessary. As to charging voltage, it's no mystery. Charging specs are easy enough to find from the mfg. According to Renogy 14.4 to 14.8 volts is recommended for cycle use.
These are rated as Deep Cycle batteries. Use them as such and expect 5 -6 years out of them.

 
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The term Deep cycle is relative. I wouldn't recommend cycling down to 50 % with any batteries, obviously it affects their lifespan. This chart shows 300 cycles at 100% dod. It also shows over 1400 cycles at 30%dod. That is my typical daily dod and that will give you over 4 years before you are down to 80% of original capacity. I stand by my statement.
 
littleharbor2 said:
It also shows over 1400 cycles at 30%dod. That is my typical daily dod and that will give you over 4 years before you are down to 80% of original capacity. I stand by my statement.
Click to expand...

Yea I think you're right there, I bet it will give 4 years like that. But say what you like, 30%dod is NOT deep cycle
 
SamG340 said:
Yea I think you're right there, I bet it will give 4 years like that. But say what you like, 30%dod is NOT deep cycle
Click to expand...
Do you know anybody who drains their batteries 100%? Most the off grid people I live around don't go much below 30 % dod. That is because a well designed system will have 3 - 4 days of capacity. This is proper design. Also I don't know anybody who uses standard automotive batteries. That IS asking for a 300 cycle battery bank. Just because a car can go 120 miles per hour doesn't mean you have to drive it that fast, Deep cycle batteries CAN be cycled down low but if you are cycling that low on a regular basis you have a poorly designed system.
 
littleharbor2 said:
Do you know anybody who drains their batteries 100%? Most the off grid people I live around don't go much below 30 % dod. That is because a well designed system will have 3 - 4 days of capacity. This is proper design. Also I don't know anybody who uses standard automotive batteries. That IS asking for a 300 cycle battery bank. Just because a car can go 120 miles per hour doesn't mean you have to drive it that fast, Deep cycle batteries CAN be cycled down low but if you are cycling that low on a regular basis you have a poorly designed system.
Click to expand...

I think we've spoken about it before, good quality AGMs can be safely taken down to 0%soc (10.5v) without significant reduction in overall lifetime capacity

14% less total lifespan comparing 100%dod to 50%dod cycles

24% when comparing 100%dod to 30%dod cycles.




Cheaper AGM on the other hand are a different kettle of fish . According to renogy's datasheet using these batteries you get over DOUBLE the total lifespan when you do 30%dod cycles instead of 100%dod..
 
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