How do I Limit the charging Speed?

[sun walker]

7.5kw pv's , still researching mppt's.
To avoid details, I want to install a very small lithium batt's probably 48v.

Solar Battery Charge Time Calculator (12v, 24v, 48v) – Dot Watts®

Use our solar battery charge time calculator to find out how long it will take to recharge your battery using solar panels.

dotwatts.com

says I need 300Ah batt, taking 2hr's to charge for batt health.

but I want <100Ah, its mostly for a cool air con system during summer days.

Is there a device that can limit charging? to 2 hrs or more if little load is being used.

 

[JWLV]

100Ah is way undersized even if you intend to run a small window air conditioner unit.

But to answer your question, any decent MPPT charge controller will have a setting to limit the charging current. For a 48V 100Ah battery, you would set the charge current limit to 50A or less.

 

[sun walker]

100Ah is way undersized even if you intend to run a small window air conditioner unit.

But to answer your question, any decent MPPT charge controller will have a setting to limit the charging current. For a 48V 100Ah battery, you would set the charge current limit to 50A or less.

So am I wrong in thinking that during the sunlight period & even if the pv are producing 1/2 their watts, say 3-4 kw - That alone won't power a 1200w ac? with a small batt to cover the start up surge of the ac motor?

 

[JWLV]

Just as an example, right now (Oct 25) in Las Vegas, sunrise is 6:58am. Sunset is 5:53pm. You might think you'll get solar power between 7am to almost 6pm. That's 11 hours. Realistically, you might only get usable solar power for about 5 hours (10am to 3pm). But that's here in Las Vegas. In December, you might only get 3 or 4 hours of usable solar power. In the summer, you'll get more usable solar power, probably as much as 8 hours, even though you can visibly see daylight for 14 hours. Your location on earth matters. The farther away you are from the equator, the less usable solar hours you will get.

For a 1200W air conditioner, you'll probably need to be thinking 1500W due to loss, inefficiencies, and DC-AC conversion.

 

[RoadTurtle]

roughly speaking, each 100ah of battery will run a RV airconditioner for 45 minutes. Is your solar charging batteries or supporting heavy loads? You can't have it both ways.

 

[sun walker]

Just as an example, right now (Oct 25) in Las Vegas, sunrise is 6:58am. Sunset is 5:53pm. You might think you'll get solar power between 7am to almost 6pm. That's 11 hours. Realistically, you might only get usable solar power for about 5 hours (10am to 3pm). But that's here in Las Vegas. In December, you might only get 3 or 4 hours of usable solar power. In the summer, you'll get more usable solar power, probably as much as 8 hours, even though you can visibly see daylight for 14 hours. Your location on earth matters. The farther away you are from the equator, the less usable solar hours you will get.

For a 1200W air conditioner, you'll probably need to be thinking 1500W due to loss, inefficiencies, and DC-AC conversion.

Generally I'd only want to use the ac for a few hours midday - afternoon, & probably only for a few weeks over summer (when temp is >25c, 77F). Even in summer nights tend to be cooler (coastal climate). Plus my home is straw bale, with an 1" of cement plaster, so it stores & moderates the temp well, once you've heated or cooled it, it can maintain that temp for a while, R36 thermal rating.

What I'm not clear about is, can't you harvest the power from the panels kinda directly with the batt simply as a buffer eg when a cloud floats by & to pick up the surge with the motor starting?

 

[sun walker]

roughly speaking, each 100ah of battery will run a RV airconditioner for 45 minutes. Is your solar charging batteries or supporting heavy loads? You can't have it both ways.

During the summer I really only need the power in the afternoon, when I'm home (1 person), I don't have a fridge as such (not big on animal products, but useful for storing apples from my trees over winter & cooling beer), so a very small chest freezer may be on my list if I can put the noisy heat producing compressor outside (issues over refill refrigerant, not so diy) I figure I could put it in my insulated cool store/pantry, & it should cool the whole space to ~fridge temp. Never ending list of projects, sigh.
Other than that just the modem & lap top, be nice to upgrade to a laser projector but their wattage is small upwards of 250W.

 

[740GLE]

Generally I'd only want to use the ac for a few hours midday - afternoon, & probably only for a few weeks over summer (when temp is >25c, 77F). Even in summer nights tend to be cooler (coastal climate). Plus my home is straw bale, with an 1" of cement plaster, so it stores & moderates the temp well, once you've heated or cooled it, it can maintain that temp for a while, R36 thermal rating.

What I'm not clear about is, can't you harvest the power from the panels kinda directly with the batt simply as a buffer eg when a cloud floats by & to pick up the surge with the motor starting?

So to answer your question yes and load that is drawn from the system will come from the PV once the battery is charged.

Change the load, less W from the PV, again assuming your battery is full which it will be in short order.

The only short coming I see is if you would need this AC on muggy overcast days which are very common here.

What kind of AC is the unit? 1200w on a very efficient house (well done on R36) may actually be over kill. What’s the square footage?

One good thing to keep in mind is that if you start small, with say 100ah at 48v, and find you need more storage, plan the system around adding more storage down the road.

It’s very easy to add more so that your PV array is more “balanced” to the battery bank.

 

[JWLV]

Yep. 740GLE summed it up perfectly.

 

[sun walker]

So to answer your question yes and load that is drawn from the system will come from the PV once the battery is charged.

Change the load, less W from the PV, again assuming your battery is full which it will be in short order.

The only short coming I see is if you would need this AC on muggy overcast days which are very common here.

What kind of AC is the unit? 1200w on a very efficient house (well done on R36) may actually be over kill. What’s the square footage?

One good thing to keep in mind is that if you start small, with say 100ah at 48v, and find you need more storage, plan the system around adding more storage down the road.

It’s very easy to add more so that your PV array is more “balanced” to the battery bank.

Actually I got the size wrong. I'm looking at Midea Aurora Plus Smart Inverter Heat Pump 2.5kW, Chinese, one of the biggest appliance & ac manufactures, they sell under a lot of brand names, its all still new to me. Its as small as they come & dirt cheap, I guess < $500 us. Its unlikely I'd use the heater, as I cut my own fire wood & the burner heats water & has a cooking top, ideal for winter day temp ~ 10 - 15c, 50F - 59F , night -3c to 10c.
Sorry can't convert room size to ft, but I have a 10ft long hall with all the rooms connected ( so plan to install midway in the hall, which isn't normal), bedroom 40sqm, living room 50sqm. the Heat Pump 2.5kW is suitable for a room ~30sqm. But I was thinking along the same lines as you with the batt & ac, start small & can expand if needed.
I have read a lot of post of people losing their batts due to a bad mppt fault, & batts are a huge if not the biggest investment here, that said I'm thinking of adding extra overcharge protection if possible to the b. bank, still researching. So I will increase the Ah over time.
As the Heat Pump 2.5kW are cheap, if its undersize I can always add another directly to a room, but hopefully not, I should be able to direct the cooling by closing the doors on unused rooms etc (in theory). Still all guess work.
Thanks for your help.

 

[740GLE]

IMO any battery worth owning will have its own protection in case MPPT failure and possibly over charging a battery.

I can’t imagine any LiFePo that doesn’t have OVP or UVP as well as OCP all to safe the life of the battery.

Now I think lightening strikes are the one thing that could fry a BMS but that can be mitigated (to some degree).

 

[Hedges]

What you want is to have current into the battery limited, but provide however much current the A/C or other loads requires.
The battery will supply starting surge (assuming BMS allows it to) and ride through brief shadows.

That is something any hybrid/AIO should do. It has MPPT to take in PV, a charge controller, and an inverter.

To do it with component system, a battery shunt is required.
Midnight Classic with Whiz Bang Jr., or Victron charge controller with Cerbo + shunt.

Given BMS limitations, AGM battery might be easier to get working. But need to prevent deep discharge, so it lasts and doesn't wear out by discharging each evening.

My whole-house backup works this way. PV could supply 3x the charge current I want, but the inverter/charger is set for 0.2C.