down sides to over-sized battery bank

[panderson03]

thanks for the words of warning. while we've purchased the components of the system, we have not yet hooked anything up. because of that the batteries are not yet used so we feel adding new panels to the system now would not in any way impair the health of the ones originally purchased. would you agree with that interpretation? how many new batteries do you suggest we purchase to round out our system? thank you

[Mike90250]

Battery size really depends on the loads your system has, you want to only cycle the batteries about 20% daily

Solar size is obtained via your location, weather and panel site.

[SunEagle]

thanks for the words of warning. while we've purchased the components of the system, we have not yet hooked anything up. because of that the batteries are not yet used so we feel adding new panels to the system now would not in any way impair the health of the ones originally purchased. would you agree with that interpretation? how many new batteries do you suggest we purchase to round out our system? thank you

As Mike states you first need to know what your daily watt hour usage is going to be and size your battery system from that value. Those existing 207Ah 12V batteries really should only be charged with 18 to 25 amps which is way less then what those panels can put out.

If you do not know how many watt hours you plan on using then I would suggest reducing the panel wattage so you only get charging amps in the 18 to 25 range. That way you can safely charge your existing battery bank and use it before you find out what you really need.

[panderson03]

Battery size really depends on the loads your system has, you want to only cycle the batteries about 20% daily

Solar size is obtained via your location, weather and panel site.

good morning. average monthly usage is 300 KWH with a spike in July to 400 KWH. daily usage can sometimes spike to 17KWH.
sometimes we're away from the cabin for a week or three. the intent behind the oversized battery bank is to capture the suns energy while we're away for our use when we are there living in the structure. thanks very much for your help

[panderson03]

As Mike states you first need to know what your daily watt hour usage is going to be and size your battery system from that value. Those existing 207Ah 12V batteries really should only be charged with 18 to 25 amps which is way less then what those panels can put out.

If you do not know how many watt hours you plan on using then I would suggest reducing the panel wattage so you only get charging amps in the 18 to 25 range. That way you can safely charge your existing battery bank and use it before you find out what you really need.

thank you. does the charge controller control how fast the panels charge the batteries? thanks again

[SunEagle]

thank you. does the charge controller control how fast the panels charge the batteries? thanks again

The charge controller will put all of the amps it gets from the panels into the batteries until the batteries are "full". The problem is that putting a lot of amps into the batteries too fast can hurt the batteries. I don't believe the charge controller can adjust the amount of amps going in or the speed of those charging amps when the batteries are first being charged. That is why you limit the amount of charging amps going into the battery by limiting the amount of amps going into the CC.

[SunEagle]

good morning. average monthly usage is 300 KWH with a spike in July to 400 KWH. daily usage can sometimes spike to 17KWH.
sometimes we're away from the cabin for a week or three. the intent behind the oversized battery bank is to capture the suns energy while we're away for our use when we are there living in the structure. thanks very much for your help

The range of 10 to 17kWh daily requires a variable solar/battery system. A static solar/battery system may be too little one day or too big another. IMO I would keep the existing battery system of 48V 207Ah for your normal usage and find another way for the months (like July) of high usage.

Since you have a lot of panel wattage maybe reducing the amount of panels and charging amps to about 20 to 25 amps to keep your existing 48V battery system happy and find a way to use the rest of the panels to charge a second battery system when you need it for the higher usage months. There are a number of ways to get power for your needs.

[Bala]

we've purchased 9 370 watt panels and 4 12V SOK LiFePO4 batteries. seriously considering purchasing another 4 SOKs

If these are Sok lithium 200ah battery then you get to use a lot more capacity and and probably have a higher charge rate than lead acid.

I have a single 12v 200ah lithium. Charge at 40amps and have about 70% usable capacity. I could use a little more capacity but it drops off really fast at the end so monitoring and cut outs are fitted.

[panderson03]

If these are Sok lithium 200ah battery then you get to use a lot more capacity and and probably have a higher charge rate than lead acid.

I have a single 12v 200ah lithium. Charge at 40amps and have about 70% usable capacity. I could use a little more capacity but it drops off really fast at the end so monitoring and cut outs are fitted.

thank you, Bala. would very much like to hear more about your experiences!

[Bala]

thank you, Bala. would very much like to hear more about your experiences!

Can you post up the specifications sheet for the battery you have.

[panderson03]

spec sheet for our batteries https://drive.google.com/file/d/1_Q5...ew?usp=sharing

[SunEagle]

spec sheet for our batteries https://drive.google.com/file/d/1_Q5...ew?usp=sharing

Based on that battery info you can just about forget what I stated in my previous posts. I was thinking FLA type not LiFe type battery. The LiFe battery can handle a much faster charge rate then the FLA type. Yet you can provide almost 70 amps for charging which is higher then the Max charge current for those batteries. You still may have to reduce the panel wattage so you don't charge those batteries too fast.

[panderson03]

Thanks SunEagle

[Bala]

thank you, Bala. would very much like to hear more about your experiences!

I have the single 200ah lithium in my caravan, so not used full time. When at home I turn the system off. Lithium do not need to be kept at full charge and are better kept at Partial State of Charge. PSC

You do not really want the internal battery system getting to the low voltage disconnect point so best to fit an adjustable external unit in combination with battery monitoring. Lots of options there.

I have my LVD set at 40% State of Charge SOC. If it does cut out I can turn it straight back on and have a bit of time to get it charging again before it cuts out on the internal cut out.

So I have about 60% of my capacity to use, you could go to about 80% but the internal cut out may come into play. So you can decide on what capacity % you do your energy budget on.

You do need to be aware that some solar charge controllers are damaged if the battery is not connected so that is a factor if the battery cuts out on LVD.

Have a look online at lithium discharge curves, you will see that the voltage drops very quickly at the end.

[Ampster]

......we are using SOK LiFEPO4 12V batteries. we have an Outback Flexmax 80 charge controller.
the idea is that our battery bank would be oversized for what our panels can output.
that way, when we're away from the cabin for a week or 2, the batteries can install an abundance of energy
....

In that case the only downside is the extra cost but the upsides are many. By using less of your pack everyday the life will be extended. You will have reserve power when the solar cannot power your loads and fully recharge your batteries.LFP batteries are more efficient so more of your solar energy wil be stored in your batteries.