Battery selection advice requested

[SolarSteve]

Thanks! I'll look at the pros and cons of the 35A charger vs. a programmable one. Price being just one of the considerations.

Steve

[Mike90250]

Several of the higher end charge controllers can be programmed for a Max Battery Amps Limit

[sensij]

If there's a regular problem with high amp load into the battery, could I not knock down the max amperage via the controller? Wouldn't that be better than losing a panel because I get more oomph during the earlier and later hours, not to mention the often present clouds?

Yes, removing a panel is only a last resort... Even remounting them to a less optimal angle is probably a better choice if you can. Really, sizing the charge controller so it can't overcharge the battery is the right way to do it, which is why I thought the 35 A victron would be a really good fit.

[SolarSteve]

How little I know about AGM! OK, will definitely keep the runaway problem in mind and opt for a temperature sensor. The box is outside and very well ventilated, which means the temp is rarely above 85 and usually around 70 during the day. Even in the middle of summer it can scrape into the 40s. So quite a range overall, though not too much during charging times.

If there's a regular problem with high amp load into the battery, could I not knock down the max amperage via the controller? Wouldn't that be better than losing a panel because I get more oomph during the earlier and later hours, not to mention the often present clouds?

Discharging at night will be steady, but unknown at this point exactly how much. Rough estimate based on limited observation is I might discharge as much as 6-8Ah after the sun goes down. Provided I'm not running low going into the evening, that seems pretty modest.

OK, I think I have the system planned out now. I learned a lot in the process, that's for sure! You all have helped enormously to keep my beer cold

Steve

[Sunking]

Thanks! That all makes sense to me.

You are welcome.

OK, looks like I can charge at a max of 52.5A into a 105Ah AGM battery. That right? If so, then it sounds like I should stick with the 3 panels and the 40A controller.

Steve you can get away with charging your AGM with C/2 or 50 amps, does not mean you should normally. Recharge is not going to be an issue for you because it is sized to fully recharge a completely discharged battery in a normal day. Charging at that rate and using AGM, I would be using Temperature Compensation. AGM are prone to Thermal Runaway, and one of the causes is Fast Charging. On Solar Thermal Runaway is far less likely to happen because they just not have the longevity of a AC charger. As the sun gets lower, power goes down along with the heating until sun sets and you cool off over night and begin the next day.

Anyway that is the risk you run charging that fast The real issue is how much you use at night and how deeply you discharge. As long as you do not over discharge you should be OK other than running Risk if Thermal Runaway. Easy to fix if you want or if it becomes a problem, loose a panel.

[BackwoodsEE]

Thanks! That all makes sense to me. Since this system is going to be used periodically in the sunnier time of the year, I am fortunate enough to not worry about the darker winter (unlike with my home system). This gives me some flexibility that generally doesn't exist for an all year use system.

Yes, it certainly does. In my neck of the woods (100 miles from the Canadian border), there is about a 4:1 difference in solar power I can collect in summer vs. in Winter. That's huge.

OK, looks like I can charge at a max of 52.5A into a 105Ah AGM battery. That right? If so, then it sounds like I should stick with the 3 panels and the 40A controller. That allows me to rapidly recharge the bank between uses, decreasing the chances that I'll start off a weekend with poor reserves. It also increases the ability to use the system longer, provided there is mediocre sun for some of the time. Also increases adding new draws on the batteries over time.

Makes sense to me. (By the way, don't fall into the trap of specifying and thinking in terms of three significant digits given Sunking's general figure of "C/2". You can pump about 50 A into that battery, a little more perhaps or a lot less if you choose.) If you have any shading issues at all, three panels with an MPPT charge controller (the only kind you should consider) will give the bypass diodes in those panels more chances to allow full current, even with some substrings shaded. And dropping your maximum power point voltage from around 85 V to 55 V is significant. Given all the other costs involved, panels are cheap.

Also, if it's cloudy the whole week while you anticipate a break in the weather and look forward to a visit to your little retreat, you will be glad you've overpaneled a bit. PV output under cloudy skies is 10-25% of what it is when sunny, the top end being for a light haze where the sun almost still casts a shadow on the ground.

I think I'm getting close to having something on paper that fits our needs!

I think so, too.

[AzRoute66]

I think it depends on "What's next?" With the 40A controller, your panels are the limiting factor, and with anything less than 'ideal' conditions you would be charging at less than C/3.17 anyway. If your next upgrade is to the batteries, for more capacity, then consider staying with the 40A charge controller and let the panels be your throttle. If your next upgrade is more panels, to get more power from 'less than ideal' conditions, then the 30A charger (to provide the limit) will be crucial for limiting current to your current battery bank. (Or just set the current limit in your 40A charge controller to 30A if it is capable of that, but that is another rant for another thread...)

[SolarSteve]

I agree so my answer to your original question would be yes to save $$$ by getting the matching 105 Ah AGM to wire in series.

Key is 3 x 265W = 795W max. 795W / 24V = 33.125A max. 105 Ah / 33.125A = C/3.17 which is a great rate for AGM.

If you want to save some money by going to a 30A charge controller go for it. That would make 105Ah / 30A = C/3.5 which is also great, maybe better.

Interesting thought. In theory that extra 3A charging sounds nice, but in reality... how often and long willl I get a rate that high? My guess is not all that often. Spending the extra on the 40A charger might not be worth it. Then again, at this point a cost variance of +/- $100 isn't by itself all that tempting to go after.

Steve

[SolarSteve]

Thanks! That all makes sense to me. Since this system is going to be used periodically in the sunnier time of the year, I am fortunate enough to not worry about the darker winter (unlike with my home system). This gives me some flexibility that generally doesn't exist for an all year use system.

OK, looks like I can charge at a max of 52.5A into a 105Ah AGM battery. That right? If so, then it sounds like I should stick with the 3 panels and the 40A controller. That allows me to rapidly recharge the bank between uses, decreasing the chances that I'll start off a weekend with poor reserves. It also increases the ability to use the system longer, provided there is mediocre sun for some of the time. Also increases adding new draws on the batteries over time.

I think I'm getting close to having something on paper that fits our needs!

Steve

[AzRoute66]

I agree so my answer to your original question would be yes to save $$$ by getting the matching 105 Ah AGM to wire in series.

Key is 3 x 265W = 795W max. 795W / 24V = 33.125A max. 105 Ah / 33.125A = C/3.17 which is a great rate for AGM.

If you want to save some money by going to a 30A charge controller go for it. That would make 105Ah / 30A = C/3.5 which is also great, maybe better.

[Sunking]

Steve you brought up the question or said something to the effect of lowering the charge rate on your batteries? While it is true you can charge a battery to fast, but not in your case. Flooded Lead Acid batteries you want to limit charge current to around C/6 to C/8 where C = Battery AH Capacity. Example on a 480 AH battery C/6 = 80 amps, and C/8 = 60 Amps.

That rule does not apply to AGM batteries because their internal Resistance is much lower, so they can be charged much faster. AGM can be charged as fast as C/2, or 240 amps in our 480 AH example.

With a fixed panel wattage can only generate up to X amount of watt hours everyday. Example say we have a 800 watt panel system and in winter you only have 3 Sun Hours. That only gives you 1600 watt hours of usable power in a day. Lower th epanel wattage and you lower your daily production form 1600 watt hours to something less.

[SolarSteve]

No wrong direction. You can charge AGM's as fast as you want. All slowing down charge is takes longer and less energy in a day.

Kind of like saying you are cruising down the highway at 60 mph with 60 miles to go and say I will slow down so I can get there in less than an hour.

I guess I'm misunderstanding the concerns raised in the first two responses. Maybe you can break it down for me so that I can understand better? Although I've lived off grid for more than 10 years, I didn't design my system so this is the first stab I've made at understanding more than the basics. I'm learning a lot

Steve

[SolarSteve]

Is this a Duracell MARINE "Deep Cycle" AGM Group 31M Battery?
Absolutely, you can use two 12 V AGM batteries in series = 24 Volts.

And if you have good SUN and your battery bank is 100% SOC then you can use way more than 30AH per day ...

Yup, this is why I don't want to up the total Ah of storage. As long as my uses and patterns don't change dramatically, it's overkill to go with more storage. I already have an overkill system at home (4080Ah) and therefore understand the costs that come with it.

If you have no sun then you would to need to reduce consumption dramatically that day.
Just two days of no sun and your batteries are down to 40% SOC, which is quite low.
We try not to go below 50% on a regular basis, but your batteries only cost $170 each.
Let's see if you get 3 years, or more?

The key thing is for the most part the system will only be in use for 2 days out of the week, therefore if I go into that two day period with a 100% charge I can just about have a blanket over the panels and come out OK on the other side. If the weather is that crap I might as well stay home

BTW, the cost of the Duracell AGM is actually +$200 for the places I checked out. The one I bought was $220.

Specifically, what are your LOADS ?

You can see details in this thread:

https://www.solarpaneltalk.com/forum...system-concept

The primary drain comes from a refrigerator. The 30Ah draw is based on warm temps and 4+ people (two of which are teens). That's the worst case scenario. Normally it's 2 people who don't believe a fridge should be left open for 5 minutes every half hour

Steve

[Sunking]

Here's a thought. Instead of upping the Ah of the batteries, how about lowering the charge going into the batteries? If I reduce my collecting capacity from ~93V @ 9A down to ~62V @ 9A I lose more in voltage drop as a percentage (roughly 3% vs. 2%), but I'm still at ~60V. As I understand it, that gives me sufficient voltage to maintain charging early/late in the day and suboptimal cloud conditions. It also puts the max charge at roughly 22A down from the previous 34A, which seems to be more inline with what a 105Ah system wants.

Do I have that right? Is the lower amperage sufficient to keep two 105Ah AGMs happy over the 5 day charge period?

No wrong direction. You can charge AGM's as fast as you want. All slowing down charge is takes longer and less energy in a day.

Kind of like saying you are cruising down the highway at 60 mph with 60 miles to go and say I will slow down so I can get there in less than an hour.

[NEOH]

Is this a Duracell MARINE "Deep Cycle" AGM Group 31M Battery?
Absolutely, you can use two 12 V AGM batteries in series = 24 Volts.

And if you have good SUN and your battery bank is 100% SOC then you can use way more than 30AH per day ...

Energy generated by the PV Panels
============================
795 Watts = 3 panels x 265 Watts
4,000 Watt-Hours = 795 Watts x 5 Solar Hours
166 AH per day = 4,000 Watt-Hours / 24 Volts

or even 200 AH total consumption = 166 AH (PV) + 30 AH (Battery)

If you have no sun then you would to need to reduce consumption dramatically that day.
Just two days of no sun and your batteries are down to 40% SOC, which is quite low.
We try not to go below 50% on a regular basis, but your batteries only cost $170 each.
Let's see if you get 3 years, or more?

Make sure you set your Charge Controller to AGM Type batteries.

Specifically, what are your LOADS ?