As of 5/13/2021, I have everything up and running! My DIY solar system with battery backup has produced 5.9 kWh over the last few days. It has been very wet and rainy this spring here in the Front Range of the Rockies and I don’t think we’ve had a full day of sunshine in a couple weeks. In terms of dollars, that 5.9 kWh is worth $0.649. Not much, but knowing I can power the whole house for 6 hours (based on average consumption) is a pretty good feeling. Check out my last post for some detail on how I got the battery bank set up – DIY Solar System with battery backup update.
The battery bank has performed great so far. I am in the process of doing a full discharge test. These cells did each come with a discharge test sheet from Battery Hookup, all showing more than 260Ah (most around 262Ah). Total for the batteries was $1082.
Under discharge in the flat part of the curve, the cells are extremely close in voltage. Below is a screenshot of my BMS app (XiaoxiangBMS, which is a super handy app. the fastest I’ve ever spent $6 on the pro version of an app) showing minor discharge current and the cells are within 6 millivolts of each other. It really doesn’t get much better than that.
The BMS is a JBD 8s 100A model I got from eBay that comes with bluetooth monitoring ability. It has worked flawlessly so far. I love it when technology just works. Every setting can be configured. It keeps track of every alarm event. And best of all, it was only $68.
The all-in-one inverter (inverter, battery charger, MPPT solar controller) is a MPP Solar LV2424 hybrid model. Everything works as expected. I’ve tested all the features successfully. I even got the monitoring software going. The main downside so far to this inverter (and I think this is common) is a relatively high idle power consumption. The BMS reports ~56-60W draw with the inverter on and nothing plugged in. The “idle” power draw decreases as a proportion of the total load on the inverter but it never drops to 0. For example, at 0 load, the inverter draws 60W. At 120W load (based on a questionable Kill-A-Watt), the inverter is drawing 160W, meaning the “idle” power dropped to 40W. This inverter was $665.
My $100 each solar panels from Craigslist have been hooked up on and off over the last few days. I haven’t seen more than 480W from the 2 of them combined but they are laying completely flat (they should be tilted towards the sun for maximum power). Overall, happy for the price. I need to get them mounted on the roof.
My DIY solar system with battery backup is commissioned! Things are functional. Things aren’t located optimally. I need to get the solar panels mounted on the roof and do some tidying around the batteries/inverter. I plan on mounting some drywall above the battery cells to protect from whatever and covering up all the battery terminals. The rear of the cells are covered. The mains are covered. Some of the intermediate terminals are not.
I’m already looking on Craigslist for more solar panels, but I need to 100% finish this project before adding on to it according to my lovely wife (I am sure some of you know exactly what I’m talking about!). With that, I’ll be signing off for the night. The baby is having a very hard time falling asleep tonight. Until next time!
As of 5/1/2021, all of the main components of my DIY solar system with battery backup have arrived. I posted about the requirements, component select, and some fun with shipping from China in my initial solar post – Planning my 600W DIY solar system with 6 kWh battery backup. If you want some background of how we got here, head to that link then come on back.
To recap, my DIY solar system with battery backup consists of a few main components:
8x 260 amp hour prismatic LiFePO4 battery cells. They will be placed in series for a 24V nominal system with around 6.0 kilowatt hours of usable storage.
MPP LV2424 hybrid all-in-one inverter. This device handles converting direct current (DC, like a car battery) to alternating current (AC, like household outlets) and charging the batteries
2x 310W Canadian solar panels. These will be wired in series for 72V maximum power point voltage.
8S JBD 100A battery management system – to protect the batteries from a number of undesirable conditions
The other miscellaneous things that I need are: battery bus bars, wiring, ring terminals, and general connection things.
Materials arriving and resting battery voltages
We were on vacation when the batteries (and inverter) arrived so they sat for a few days before I got a chance to unbox them. The batteries were very well packaged and I can’t thank Battery Hookup enough for how fast they shipped after what I’ve been dealing with from China.
Upon unboxing, I made sure to record the resting voltage of each cell. Below were the resting voltages:
Resting voltages of the 260 amp hour BYD LiFePO4 cells
Cell #2 had the lowest voltage and cell #3 the highest. This presented an easy chance to test out my bus bars for voltage equalization. I did not measure the cell internal resistance so I wasn’t really sure how much current would flow from cell 3 to 2 when shorted together so I did a quick estimate based on Ohm’s law (V=IR -> I=V/R). With an estimated internal resistance (IR) of 20 milliohms (I’ve had LiPo in this range after some degradation), and a voltage difference of 0.096V, that would mean a current of 0.096/0.020=4.8A. That wasn’t a huge number so I was comforable just connecting the cells with the bus bars. But first I wanted to actually measure with a multimeter.
I was expecting about 5A based on the calculation above. I’m not sure how I was 10x off on the estimate but after hooking up my multimeter, the equalization current was 0.6A. That was plenty low so I set my mind to balancing. But before that, I needed bus bars to connect all the cells in parallel.
Constructing copper bus bars
Bus bars are used to conduct high amounts of current in electrical applications. In essence, they are oversized, flat wires. I ordered 2x copper bars from McMaster Carr that are 1″x1/8″x3′. They arrived in two days with free shipping… Amazon is gaining competition. I got to work drilling holes.
Heat shrinking the cells
With the bus bars ready, it was time to heat shrink the battery cells. Battery Hookup said they were uncovered and needed to be heat shrunk for every cell. Turns out 7 of 8 had a covering on them already. I heat shrunk them anyways for two reasons: 1) to further protect the cells from damage and short circuits and 2) so they look better. The cell on the left will be re-done with more heat shrink. I guestimated how much I needed and was short a few inches.
Balancing in 2 sets of 4 cells each
When making the bus bars, I put together a mental picture of what I needed to make and how many I needed. This worked fine for the final battery but I would need double for balancing. I had the full extra 3′ copper bar but it took forever to cut so I just decided to balance the cells in 2 groups of 4. Balancing is charging all cells in parallel as one low voltage, giant capacity battery to their rated voltage (3.65V for LiFePO4). Doing 4 cells at a time meant it was a 3.2V battery with a capacity of 1040 amp hours. The resting voltages were in the upper end of the voltage curve chart so it wouldn’t take super long. If the battery was fully depleted, it would take 104 hours at the 10 amps my bench power supply puts out.
So I got the bus bars hooked up and started balancing by setting my power supply to 3.65V and current to max.
Measurement discrepancies and voltage drop
The first thing I did when I started charging was take voltage measurements to make sure things were right. I noticed some irregularities.
The first irregularity was the fact that the bench power supply was over-reporting the voltage by 0.034V or so when comparing the display to the terminals. This isn’t a huge deal and is actually a pretty decent level of accuracy for a $40 bench power supply from Amazon.
The next thing I noticed is that for a 10A power supply, it is putting out a lot less than 10A. So I measured the voltage at the bus bars.
By leaving the voltage set to 3.65V at the DC power supply, the voltage drop means I wouldn’t get anywhere the rated 10A. The actual drop would decrease proportionally as the battery voltage neared the terminal voltage.
Regardless, it only took a couple hours until the cells were topped off for each set of 4. They are currently resting. I will check their voltages again tomorrow morning. The amount the cells drop in voltage from where they left off indicates the strength of the cell, with larger drops meaning weaker cells.
This is where we will leave off for the day. The 8 cells have been balanced in 2 groups of 4 and are currently resting. After 24 hours I will recheck the voltages to see which cells are strong and which are weak. I need to buy more electrical tape to cover up the bus bar ends (I did start assembling the full battery but stopped because the potential for short-circuit was higher than I was comfortable with).
2021-04-06 – fleshing out the background and requirements
2021-04-29 – updated with parts ordered, reasoning for choices, and some more background for my DIY solar system with battery backup
I’ve always been interested in solar power. Being able to generate heat and electricity from the sun is just so cool on a fundamental level. When I was little, playing with magnifying glasses (read: setting things like plastics and mulch on fire) was always a good time. My mom got me a science book at one point that had a full letter sized (8.5×11″) fresnel lens.
That fresnel lens upped my lighting things on fire game dramatically. Even since then I’ve wanted to harness large amounts of solar power. I’ve had 50-100W solar panels for a good portion of my adult life running fans and charging small deep cycle 12v batteries, and it is now time to move up to the big leagues. Read on to join my thought process for planning a large-ish system.
The requirements for my DIY solar system with battery backup aren’t too strict. I’m looking for the following:
Run my homelab for 5-10 minute until it can be powered off
Provide a couple hours (1-2) of space heating/cooling for comfort with plenty of battery left over
Run the refrigerators for 6-12 hours
Run the cable modem/router/WiFi for ~6-12 hours
Run the furnace as needed
Ability for a generator (to be purchased) to charge the batteries
Ability for grid power to charge the batteries
Ability for solar panels to charge the batteries
Less than $2000 total to get started with a system that can grow
Use my 2x300W solar panels I picked up off Craigslist for $100 each
Nice to haves
USB/RS-232/RS-485/Ethernet Interface to read status via Raspberry Pi or similar
Decent warranty (I don’t usually worry about warranties but this will be a decent chunk of change)
Not waiting another two months to ship from China (I may have already ordered the batteries. Ordered Feb 26 2021. Still waiting for even a tracking number as of April 29.)
If we add up all the electricity requirements, we end up with a couple to a few kWh (I am being intentionally vague here. I’ll post details with my next update.). This DIY solar system with battery backup is intended to grow with me – I’m not building a data center-sized system to start. As such, I have a tentative list of the basics:
2x300W solar panels. They are Canadian Solar CSUN-something 36V nominal. Already have these.
8x272Ah LiFePO4 batteries in series for 24V nominal. These will total out to 6.9 kWh of storage assuming full capacity. For $101 per cell shipped, this deal is hard to beat even if it is taking the slow boat from China. 6.9 kWh divided by $101 per cell is $116/kWh.
A 2.5ish kW inverter. Current choices are MPP Solar LV2424 (2.4 kW 24V with most of my requirements for ~$700) or the Growatt SPF 3000TL LVM (3.0 kW 24V with basically the same features as the MPP for ~$700. but there will be at least a month shipping delay).
A quality 8S BMS (expect to spend around $150 for this)
I get an urge to troll craigslist for solar panels (and NAS’s) every couple weeks and came across a post that had 300W solar panels in Loveland, CO. They were in great shape and they were $100 each. $0.33/W is a pretty good price for solar panels so I jumped on them. I didn’t really have a use but knew I would in the future. There is a slight “prepper” tendency I always have in the back of my mind so part of me was thinking I’d be able to use them to charge stuff in the event of an extended power outage. Since I bought them, we have had 3 power outage – one for 2 hours, one for 1 hour, and another for 15 minutes.
[insert pic of solar panels]
For batteries, there are a lot of good options. Some better than others. There are a few big decisions:
Lead acid – the traditional “car battery” type but deep cycle. Old tech, heavy, usable capacity is relatively little compared to the full rated capacity (generally recommended to not discharge deeper than 50%). Pretty good price in terms of watt-hours per dollar. Almost all inverters/chargers are designed around 12V/24V/48V as defined by the lead acid cell voltages.
Lithium-ion – new tech. Used in many electric car batteries – primarily Tesla. Lots of used cells available (often in bulk). Each cell is about 10Wh. This means many wire connections (500-1000) and soldering. Does not handle overcharging/discharging well. Can cause fires/explosions if handled improperly. No good solutions for 12V standard stuff. 7S (7 cells in series) can work for 24V. 13S works for 48V
Lithium polymer – very power dense. Not very energy density. Quite hazardous. That by itself is enough to write these off.
Lithium iron phosphate (LiFePO4) – new tech, decent tradeoff for all other aspects mentioned above. Used in electric buses in China (which is a source for cells). Very large capacity per cell (>200Ah), which means minimal wiring. Cell voltage is 3.2V, which matches up perfectly with traditional lead acid voltages (4S is 12V, 8S is 24V, 16S is 48V). Good cycle count/capacity curve (it takes many cycles to reduce capacity). I will be using LiFePO4 batteries in my system.
Battery bank voltage – requirements are for a 2.5kW inverter.
12V – 200+ amps for a 2.5kW inverter. This would need large wires. Generally the amount of current at 12V throughout the system would be high. Ability to “start small” with only 4 LiFePO4 cells.
24V – 100 amps for 2.5kW inverter. Much more reasonable. I will be using 24V for my system.
48V – 50 amps for 2.5kW inverter. Even more reasonable but this requires greater up front investment to get enough batteries (16 cells for LiFePO4, meaning $2000+). Borders on what is considered “high voltage” for low voltage DC work (generally the cutoff is 50V).
Below is a table I created in Excel to help me make my decision. When I came across the group buy for the DIYSolar Michael Caro 272Ah cell group buy from China, I took 2 days to decide and ordered 8 cells. That was Feb 26, 2021. I still don’t even have a tracking number. I’ll probably cancel the order. Mid-April, 260Ah cells became available at batteryhookup.com. They weren’t the cheapest in terms of watt-hours per dollar, but they were in Pennsylvania and would arrive to me in a predictable amount of time. With my yearly bonus and tax refund firmly in my bank account, I figured I could have two orders opened at once. I placed the order with BatteryHookup. It took 6 days for 8 cells to arrive. I still don’t have a tracking number for the group buy from China. I can afford to wait. Or I could cancel the China order and get 8 more cells on my door step a few days from now… decisions, decisions.
For the inverter, it really came down to two options:
MPP Solar LV2424 – 24V 2.4kW 120V (able to be stacked for split phase and/or more current) – this is what I picked
Growatt SPF 3000TL LVM – 24V 3.0kW 120V (able to be stacked for split phase and/or more current)
I posted a poll on DIYSolar asking for the popular opinion. Most said go with the Growatt (5 votes to 2 as of 4/29/2021). Will Prowse (solar genius) said they’re basically the same. Both batteries allow charging by utility, have solar MPPT chargers, and monitoring via serial.
I ordered the battery and knew it wouldn’t take long to arrive. The option for Growatt involved waiting 3-4 weeks for a container to arrive at the Port of Long Beach from China. The MPP option shipped from Utah (I am in Colorado – one state to the east). I picked MPP mostly based on shipping time. Also because 8S 100A BMSs are pretty common (which works well for 2.4kW because 100A * 24V = 2.4kW) which usually have a trip limit of around 110A. The next step up is usually 200A which is a correspondingly large increase in cost.
Battery Management System (BMS)
The BMS is there to protect the battery. It protects from a number of conditions – overcharge, overdischarge, overcurrent, cold temperatures, short circuit, and others. The main criteria here is 100A nominal (with overcurrent kicking in around 110A), 8S for 24V, with some sort of monitoring capability (serial, bluetooth, WiFi, etc). An active balancer would be good but that appears to be in the next higher price range. I ended up going with the JBD 8S 100A BMS for $80. One of the things that really caught my eye was this thread about monitoring – it appears these are really capable of putting out data.
With all the main materials/parts ordered, it is time to focus on how to construct the system. When it is all hooked up and ready to go, I will have a small DIY solar system with battery backup to power a few select loads in the house. The main components are:
8x 260Ah prismatic LiFePO4 cells for a 24V nominal system with 6.6 kWh of storage
MPP LV2424 inverter for 2.4kW of 120V power with ability to charge from grid, solar, or generator as well as expand with more units in parallel
2x300W solar panels to charge in case of long term outage