diy solar

diy solar

Can too much battery capacity be a problem?

T

Tony_

New Member
Joined
Nov 17, 2022
Messages
20
I'm installing a 900W of solar on top of a van intended for "full-time" use. It will also have alternator-based charging, and maybe shorepower someday.

I want to buy lithium phosphate batteries, but need to decide how much. My goal is to operate "off-grid" as much as possible, and not have to worry if I decide I want power-hungry items in the future. I realize it might be overkill, but can someone explain what would be the disadvantage of buying 600Ah, or even 800Ah? Please avoid obvious things like cost and weight... I can figure that part out. Thanks ;)

I've been looking at these from the mobile-solarpower.com site:
A) 2 of the EG4-LL Lithium Battery (12V 400AH) Server Rack Batteries or
B) 3 or 4 of the 12V 206Ah SOK Batteries

If you have actual experience, I'd find your advice especially helpful! Thanks

By the way, the figure below shows my setup, involving 4x50W panels and 4x175W panels, and a table for how I figured the 900W. It also shows my electrical panel, which is small right now because I only have the 4x50W panels so far.
solar.jpg
 
Last edited:
Subject line question, answer:

Yes - for lead acid. They need to be charged at about 0.07-0.20C.

LFP are far more flexible. Just don't exceed their rated charge current. 0.2-0.4C is typically a sweet spot.

Your PV determines your daily energy usage.

Your Battery determines how long you can go without charging.

Have you conducted an energy audit and determined your needs? See link #1 in my signature.
 
One problem I can think of too much battery is if they become severely discharged, and you don't have the capacity to recharge them. Sitting a long time in a low charge state is as bad as sitting a long time in a fully charged state.
 
DIYrich said:
One problem I can think of too much battery is if they become severely discharged, and you don't have the capacity to recharge them. Sitting a long time in a low charge state is as bad as sitting a long time in a fully charged state.
Click to expand...
Thank you for directly addressing my question. The only thing I don't understand about what you've said is, "What is 'a long time'"? A day, month, year?

Let's assume I fully discharge a 800Ah battery system. Then according to my (maybe too simplistic) calculations this would mean about 10,000Whr needed to recharge. So with my 900W solar array, let's say roughly, operating at 500W, I would need about 20hr to fully recharge them. To me, that's doable if you're just trying to recover from a bad situation.

Of course, I don't want to get into situations like this. And in fact, the whole point of my question is to learn about the bad things that can happen so that I know what to avoid.

Thanks again.
 
One thought on battery selection is to keep in mind the BMS current limit. A 400Ah battery with a 100A discharge current won't do well with a 3000W inverter. Having multiple batteries of lower capacity makes sense when you consider adding the individual discharge currents.

I have three 280Ah batteries on my 2000W inverter and have plenty of overhead. I can take a battery out of service and not be handicapped.
 
TacomaJoe said:
One thought on battery selection is to keep in mind the BMS current limit. A 400Ah battery with a 100A discharge current won't do well with a 3000W inverter. Having multiple batteries of lower capacity makes sense when you consider adding the individual discharge currents.

I have three 280Ah batteries on my 2000W inverter and have plenty of overhead. I can take a battery out of service and not be handicapped.
Click to expand...
@TacomaJoe, there are several things you say here that are interesting to me -- but I'm just not educated enough yet to understand them. I appreciate that you are experienced with a system you feel is working well. Can you give me some more details? Here's what I don't understand:

1) What would you advise specifically regarding a BMS if I had an 800Ah system like I mentioned?
2) You seem satisfied with the 2000W inverter -- is that what you'd recommend, or something different -- for my proposed 800Ah system?
3) I think you are recommending "multiple batteries of lower capacity" -- if so, what exactly do you mean? Using many 100Ah? 200Ah? or is two 400Ah okay? Or is your three 280Ah the sweet spot?

Sorry for all the questions. I'd also appreciate references for where I can educate myself. I've read "Mobile_Solar_Power_-_Made_Easy" but it seems we are talking beyond the level of detail in that book.

Thanks again
 
  • Like
Reactions: Mia
Note - I'm a spreadsheet jockey and ECE engineer but still learning at solar.

I'm planning out my Sprinter 2500 170WB 900W build at the same time as you. I'm targeting 600aH in the battery bank with 9x 40"x20" 100W panels, all with the 40" length paralleling the length of the van, 3 abreast, on 60" roof rails, extending slightly over the edge of the roof, but staying within the 92" mirror to mirror width of the vehicle. This leaves me space for a fan and a roof mounted dometic 12V AC. Loads include the fan, AC (12h cycle), espresso machine, lights, and dometic fridge.

I'm adding a 30A DC2DC charger, with the intent of adding another if I find I really need it. I intend to use that more in the winter when my loads are just a heater, lights, espresso, and the fridge.

my daily power budget says 505Ah, but that includes about 80A of laptop usage for 2 people. At 450ah (10h x 5A x 9) I'm running at a deficit. Less work, more biking and skiing can solve that. Or drive to Starbucks in the morning instead...
 
sunshine_eggo said:
Subject line question, answer:

Yes - for lead acid. They need to be charged at about 0.07-0.20C.

LFP are far more flexible. Just don't exceed their rated charge current. 0.2-0.4C is typically a sweet spot.

Your PV determines your daily energy usage.

Your Battery determines how long you can go without charging.

Have you conducted an energy audit and determined your needs? See link #1 in my signature.
Click to expand...
@sunshine_eggo, thanks for your response but I just don't understand several things you've written here: things like, "0.07-0.20C" , "their rated charge current" "PV" -- but again, thanks for responding anyway.
 
I have 600W on my truck camper, 280Ah 24V DIY battery. I will install another 200W of panel once the roof air is off. I have the inverter setup on a remote switch, this allows turning the inverter off when we don't need inverter power.

If you want to go above 2500W on the inverter, I strongly suggest moving to 24V and using a buck converter for the 12V items.

What exactly are you planning to run and how many hours per day? That is what determines how large the battery will need to be. If you will be stationary for a few days and not charging off the alternator will make a difference also.

900W x 5 hours good sun in a day is 4500Wh. Anything over that is a bonus. In order to recharge totally off solar, your usage will need to be under 4500Wh. My 24V 280Ah battery is about 6000Wh of usable capacity. A 12V 560Ah battery would be similar.

We don't spend a lot of time in the camper using power. We do run a Dometic CFX95 for 24 hours a day off the system. Water heater is propane, we do use the fans occasionally and recharge phones both off the truck and solar. Microwave is a large consumer and I suggest a 3000W inverter with 24V system if you want to use a higher power microwave. Usually a 1500W minimum load. I will be removing the roof air and installing a mini split for air conditioning. I can run the roof air now, it draws 1200W no matter what. One might get 4 hours using the roof air off the battery with no solar. That is a losing proposition. We do charge the electric bikes if we have the bikes with us and I've even run a smoker all afternoon with no problems.

I've had rainy days where we ended up plugging into shore power in order to charge. Have a backup plan such as shore power or generator. I can charge off my truck using a 2600W inverter on the truck which feeds the AIO inverter and it's AC charger. Not as efficient as a DC to DC but I can push a pile of amps in a short time.
 
IMHO, a "long time" is a week. The problem is days of gloomy weather, and your generation is barely able to keep up with usage. You should plug into shore power to bring them up to 80%. The more batteries you have, the longer that takes.

FYI: it can sit forever between 20-80 (30-70 is better). So you only have to bring up into that range. But you will want to charge to 80 so you have power to use.

If you have enough battery so you can cycle between 80-20 most of the time, (99-10 when bad weather), that will extend the life of your battery.

Regarding "c": C is the capacity of your batteries. Charging at . 20c means 20% per hour, or no faster than 5 hours for a fully depleted battery.
 
Well - we're talking Lithium here. Lithium doesn't have cell memory etc and can sit at any point over the discharge curve for long periods and not be troubled.

I have an 'overly large' (121 kwh and growing) 18650 Lithium-ion powerwall on purpose so I can run 35% average DOD and hope for exteeeeemly long life. I'm only at 1,600 cycles on the oldest battery so the Jury is still out - but maybe I can get 3,000 or 5,000 cycles?!? and if I do I'll attribute it to oversized = low average DOD = much longer life.

For LifePo4 we read that limiting operations to 80% max DOD will increase the cycle life. Not sure if 40% DOD will increase it further of not - but it might? Larger batteries have lower stress (mah in/out) per cell which is usually a good thing.
 
Tony_ said:
@sunshine_eggo, thanks for your response but I just don't understand several things you've written here: things like, "0.07-0.20C" , "their rated charge current" "PV" -- but again, thanks for responding anyway
Click to expand...
 
Last edited:
Can always add more battery. I consider 800 AH at 12v to be medium at best with 900 watts solar. Leave room for another 800 if there will be an air conditioner etc in the future.

Actually this project seems on the cusp of going 24 or 48 volts. Although as a closed system higher voltage has advantages it does add complications to working with the van 12v system.
 
Tony_ said:
@sunshine_eggo, thanks for your response but I just don't understand several things you've written here: things like, "0.07-0.20C" , "their rated charge current" "PV" -- but again, thanks for responding anyway.
Click to expand...

C = C rate

If a battery has 100Ah, 1C = 100A

Therefore a 100Ah lead-acid typically needs to be charged between 8 and 20A, generally.

Rated charge current: LFP batteries have a BMS. Both the BMS and the cells of the battery have limits. Generally speaking, a 100Ah battery can be discharged at 1C (100A) and charged at 0.5C (50A).

PV = Photovoltaics = solar panels that produce electricity.
 
I appreciate the many posts. Thank you @Zwy especially, giving the numbers based on your experience means a lot to me. As I mentioned, the intention is "full time" van use, so week-long periods without battery use will be rare; for example, when parked while flying off for a visit somewhere. So, unlike your situation ("We don't spend a lot of time in the camper using power"), in my case power will be used the majority of the time, most of the time parked and moving from place to place every few days. Regarding a power audit, it will be dominated by things like whether I get air-conditioning or not, an AC vs. DC fridge, etc... and the whole point of my post is that I want go get "more than enough" for future contingencies like that == and to know what the pitfalls will be. According to @time2roll it looks like the 800Ah (max) I was considering would not be a problem -- and I could even get more. That's really interesting to me. Thanks again for all this help!
 
Last edited:
sunshine_eggo said:
C = C rate

If a battery has 100Ah, 1C = 100A

Therefore a 100Ah lead-acid typically needs to be charged between 8 and 20A, generally.

Rated charge current: LFP batteries have a BMS. Both the BMS and the cells of the battery have limits. Generally speaking, a 100Ah battery can be discharged at 1C (100A) and charged at 0.5C (50A).

PV = Photovoltaics = solar panels that produce electricity.
Click to expand...
@sunshine_eggo thanks for explaining. Unfortunately, your posts are still leaving me scratching my head. For example, in each post you talk about lead-acid batteries and yet my post doesn't mention those at all ("I want to buy lithium phosphate batteries, but need to decide how much" -- I even listed which lithium phosphate batteries I was looking at). Also, in the first post you write for the "charge current. 0.2-0.4C is typically a sweet spot." and yet in this last post you write that a lithium phosphate battery is "charged at 0.5C"

But still, I wanted to apply your numbers to my specific question, so I took the lowest charge current value you give (0.2C), and I think that would mean an 800Ah battery system should be charged at a minimum of 160A. I'm understanding that right? If so, I don't think I'll ever supply that at one time with my 900W solar array even if I added alternator charging with 2 Victron Orion 12|30's. So, once again, I'm bewildered by your posts.

As I said in my original post, "If you have actual experience, I'd find your advice especially helpful!" -- I said this because, as someone who has spent plenty of time in both theoretical and experimental science, I've come to value experimental results much more than theorizing. In my experience, theorizing is easy but there's usually something you miss. Actual experience/experiments are much more helpful for a simple man like me. Thanks again.
 
Last edited:
Tony_ said:
I appreciate the many posts. Thank you @Zwy especially, giving the numbers based on your experience means a lot to me. As I mentioned, the intention is "full time" van use, so week-long periods without battery use will be rare; for example, when parked while flying off for a visit somewhere. So, unlike your situation ("We don't spend a lot of time in the camper using power"), in my case power will be used the majority of the time, most of the time parked and moving from place to place every few days.
Click to expand...

You will find the limited real estate on the roof will be the limiting factor. You will need load management is the point I'm making, you can get by if you are conservative on usage. If you move every few days for extended time frames, you might have enough alternator charging to cover some shortfalls.
Tony_ said:
Regarding a power audit, it will be dominated by things like whether I get air-conditioning or not, an AC vs. DC fridge, etc... and the whole point of my post is that I want go get "more than enough" for future contingencies like that == and to know what the pitfalls will be. According to @time2roll it looks like the 800Ah (max) I was considering would not be a problem -- and I could even get more. That's really interesting to me. Thanks again for all this help!
Click to expand...
 
160 amp charging is not necessary for LFP. Even a 20 amp charger on utility power is fine. Just takes longer.
 
Rated charge current: LFP batteries have a BMS. Both the BMS and the cells of the battery have limits. Generally speaking, a 100Ah battery can be discharged at 1C (100A) and charged at 0.5C (50A).

Wouldn't .5C be 200 amps instead of 50 amps?

I thought it was C= capacity/ the following number?

100 amp hour, C/10 would be 10 amps, C/5 would be 20 amps?
 
You must log in or register to reply here.

Similar threads

B
Lithium battery user experience
Replies
8
Views
532
bobsh2000
B
A
Choosing Components for System
Replies
16
Views
530
anniewise05
A
J
Opinions on a pair of 12V 100Ah Smart Lithium Iron Phosphate Battery
Replies
18
Views
1K
Benjamin Yang
B
OurVentures
Too much Panel for the SCC?
Replies
13
Views
673
sunshine_eggo
sunshine_eggo
S
Can you guys recommend me a system before I do something stupid and spend too much?
Replies
11
Views
735
Quattrohead
Quattrohead
Back
Top