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DIY solar battery pack. Phase 1: Standardize Your Cordless Tool

DIY solar battery pack. Phase 1: Standardize Your Cordless Tool

    How to build battery bank for solar power system by using deep cycle batteries

    there are 4 major types of solar power system according to its application – on grid solar, off grid solar, and hybrid solar power system.

    1) On grid – solar farm

    It is solar power plant, also called “solar farm”. This is mega-watt huge solar power system works as a power plant to generate electricity to publics or private use. Solar panels connect to DC- AC inverter and send electricity to the AC grid. In most cases, it doesn’t need any battery as energy storage since all solar energy is sent to grid.

    2) On grid – net metering

    It is home or office size solar power system. On Grid – net metering system can save your electricity bills. AC loads are directly supplied by solar power instead of grid. When solar panel generate more electricity than demand of AC loads, those surplus solar energy will be sold back to grid.

    solar, battery, pack, phase

    3) Off grid

    This is independent stand-alone solar power system without connecting to AC grid. Our LWI high performance gel deep cycle solar battery can provide long service life for off-grid solar power solution. Off-Grid solar power system gives you completely self-sustainable solar energy anytime, anywhere.

    4) Hybrid

    Hybrid solar power system combines the advantages of “On Grid net metering ” and “Off grid” system. AC loads are supplied by solar power directly, the surplus solar energy can be used to charge batteries or sell back to grid. The batteries are charged by AC grid in cloudy day and night. Battery can be used to supply AC loads in day or night whenever grid electricity rate is expensive. AC loads also secured by LWI high quality gel deep cycle batteries when grid is failure (blackout).

    What is battery bank for solar power system ?

    After understanding 4 major types of solar power systems, let’s talking to battery bank. The battery bank means several batteries with parallel and series connection set up into an energy storage bank, which store solar energy from solar panel and provide electricity to loads via DC-AC inverter. Battery bank is core item in a solar power system as an energy storage.

    First, we should talking about the application of battery Generally speaking, there are two major application of industrial battery : standby use and cycle use.

      Standby use – emergency power backup for UPS, telecom base station, and security system. Battery is always fully charged and in standby condition as a power backup, the battery are used only when grid power failure, the battery power supply AC loads via DC-AC inverter during blackout.

    VRLA AGM and VRAL GEL battery are designed for standby use.

    The GEL battery is specially well knowns as its longer standby life in extreme weather environment.

    • deep Cycle use – power source for e-scooter, e-mobility, e-bike and renewable energy

    Battery is always being used every day as a power source. We call it “one cycle” when battery being fully charged and deeply discharged once.

    VRLA AGM deep cycle battery, VRAL GEL deep cycle battery and Tubular deep cycle battery are designed for deep cycle application.

    The GEL battery is specially well knowns as its longer standby life in extreme weather environment.

    The Tubular battery is named by using tubular positive plate for extra longer cycle life which is almost triple times than AGM and Gel deep cycle battery

    Why should we use deep cycle battery for solar power system ?

    Obviously, battery in a solar power system is being charged in daytime by sun and discharged in cloudy day or night. Battery is acting as a solar energy storage device, it reserve solar power in sunny time and provide power in raining time or night. So solar battery is always being fully charged and deeply discharged. We should choose deep cycle battery for battery bank of solar power system.

    it depends on how much AC loads, which are usually home electric appliance and how long they will be powered by batteries. For example, if you have 1500W AC loads and need to be powered by batteries for 3 hours. The calculation is :

    1500W x 3 hours = 4500Wh, which means you need 4500Wh battery capacity If we use 12V 150Ah gel deep cycle battery, ( Gel link ) The single battery capacity is 12V x 150Ah = 1800Wh 4500Wh / 1800Wh = 2.5, so we need at least 3 pcs battery to support AC loads.

    However, the solar inverter system for household use is usually 48V system, It means solar panel and battery bank should be 48V. Single gel deep cycle battery is 12V, so we need a multiple number of 4 which make it into 48V system by series connection. In above case, we need to use 4 pcs 12V 150Ah batteries for the battery bank

    Calculate Your Load

    The first step in designing your DIY battery bank is calculating how much electricity you typically use.known as your electricity load. There are two methods to calculate your load:

    • First, you can look at your previous electricity usage. If you are already connected to the grid, simply look at your total electricity use for the last 12 months and divide by 365 to get your daily average.
    • If you aren’t connected to the grid, you probably don’t have any data on your previous energy use. In this case, you’ll need to calculate how much electricity you need by adding up the wattage of all the electrical devices in the home and estimating how many hours you’ll use them each day.
    • 5 LED light bulbs8 watts (the wattage of each light bulb)3 hours per day = 120 watt-hours/day
    • 1500 watt blender.05 hours per day = 75 watt-hours/day (I can’t go without my morning smoothies!)
    • 50 watt laptop6 hours per day = 300 watt-hours/day

    As you can imagine, this process takes time and there are a lot of numbers to keep track of, so be sure not to rush this step! The size of your entire battery bank will be based on these calculations, so you need to make sure they are as accurate as possible!

    To help you keep track, use a spreadsheet like Microsoft Excel or Google Spreadsheets (which is free with Gmail!). There are also many online tools to help you in this process, including calculators from websites like Wholesale Solar and Affordable Solar.

    Example: Let’s say we have a small off-grid mountain cabin. The space heater, water heater, and stove are propane, so only a few key items need electricity. Along with the LEDs, blender, and laptop above, we’ll also have to power our cell phone, fans, TV, and clothes washer. Our usage probably looks something like this:

    solar, battery, pack, phase

    Our total load for each day is 1.22 kWh and about 36.6 kWh a month. Just to note, this is a VERY small cabin with only a few electrical appliances! For reference, the average kWh usage per month for grid-connected homes in the US is 900 kWh!

    Amount of Back-up Power and Depth of Discharge

    Batteries allow you to store the electricity your solar installation generates for later use, and after you find your daily electrical load, you need to decide how many days of backup power you want. Most homeowners choose between 1 and 4 days, though this depends on your needs and weather.

    Example: We’ll choose 3 days of back-up power, meaning our battery system needs to provide at least 3.66 kWh (1.22 kWh per day multiplied by 3 days) for those days when it’s rainy or cloudy.

    To make the process a little more confusing: battery capacity is measured in amp-hours – not watt-hours or kilowatt-hours like the electricity generated by your solar installation. Lucky for us, finding amp-hours is easy! Simply divide watt-hours by the voltage of the solar installation. Off-grid solar installations can be 12 volt, 24 volt, or 48 volt – the voltage you choose depends on your installation’s size, location and layout, and needs.

    Example: Our small installation will be 12 volts, meaning we need a battery with 305 amp-hours.

    (3660 watt-hours/12 volts = 305 amp-hours)

    Hold on though, there’s one more step. If you discharge the batteries down to their full capacity, you can hinder their ability to fully charge in the future. Because of this, battery manufacturers recommend only using a portion of the available battery, usually only 25% to 50% for lead-acid batteries (the most common type of battery for solar). Of course, only using a small fraction of your batteries’ power is annoying, but just consider all the batteries an investment. If you only discharge your batteries down to 25% or 50%, they’ll provide you with years of reliable service.

    We’ve decided that we’re only going to discharge about 40% of our batteries’ capacity, so we need to divide our battery size by.4 to account for this: 305 amp-hours.4 = 763 amp-hours.

    So, our batteries need to be 12 volts and have capacity of at least 763 amp-hours.

    Connecting Batteries in Parallel vs in Series

    Now that you know the voltage of your installation and the battery capacity you need, it’s almost time to start looking at batteries! In your battery system, there are two ways to connect multiple batteries together – in parallel or in series:

    • In Parallel: Connecting batteries in parallel simply means that each battery’s positive terminal is connected to the next battery’s positive terminal (and each negative terminal is connected to the next negative terminal). Batteries that are connected in parallel add up all their amp-hours together, allowing you to increase the total capacity of your battery bank.
    • In Series: Connecting batteries in series means connecting the positive terminal of the first battery to the negative terminal of the next, and so on. When connecting in series, amp-hours don’t increase, but voltage adds up amongst all the batteries. It’s also possible to create a system where batteries are connected both in parallel and in series to both increase voltage and amp-hours!

    We need 768 amp-hours for our 12 volt solar installation. If we connect in parallel, we could have two 12-volt 400 amp-hour batteries, giving us 800 amp-hours but keeping our 12 volt system. If we connect in series, we could have 2 6-volt 800 amp-hour, giving us a 12 volt battery system with 800 amp-hour capacity. Whether to connect in series or in parallel is a matter of what batteries are available and the structure of your solar and storage installation.

    All this can be confusing, but just remember: connecting in parallel adds amp-hours; connecting in series adds voltage! Knowing what options are available to you will help you build the most cost-effective installation that suits your needs.

    Planning my 600W DIY solar system with 6 kWh 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/Wi-Fi 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.)

    Initial plan

    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)

    Solar panels

    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 each33/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:

    • Battery chemistry
    solar, battery, pack, phase
    • 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 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.
    • 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 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.

    Phase 2: Purchase Vehicle Chargers, Not Inverters

    For large solar power projects, I am a firm believer in using high-quality DC to AC inverters which allow using standard 120-volt AC appliances and power tools. Inverters are becoming much more reliable and less expensive, which allows using your existing house wiring instead of having to rewire everything for DC. However, powering 120-volt AC power tools requires a 1,500 to 3,000-watt inverter and very heavy battery bank. Some small inverters costing less than 50 are now available to power your laptop computers and video devices while in your car or truck.

    Unfortunately, many of these lower cost inverters do not generate the same waveform as the utility grid, which can cause problems with the more sensitive electronic devices you want to power. It is also true that many battery chargers for recharging power tools will have very poor charging performance when connected to a low-cost modified-wave 120-volt AC inverter. Most of these low-cost inverters also have a low power conversion efficiency, and can quickly drain your car or truck battery if the engine is off while powering any 120-volt AC device.

    I was pleasantly surprised, however, to find that most manufacturers of battery-powered construction tools now offer a version of their power tool battery chargers in a 12-volt DC portable model, typically called a “vehicle charger.” Although harder to find and a little pricey at 65 to 95, these DC-to-DC chargers provide the ability to recharge your 12 to 24-volt battery-powered tools from a 12-volt battery without inverter or generator. Here are some examples from Bosch and DeWALT.

    There are many advantages to using portable 12-volt power without the need for an AC inverter. Not only will this make all wiring easier and safer than dealing with 120 volts AC, but powering 12-volt DC devices directly from a 12-volt battery is much more efficient.

    This can be a real advantage if your construction project or weekend retreat is located in an area where hauling generator fuel and equipment up a mountain trail is a major effort. Although this project was intended primarily for powering tools at a remote job site, you can also use this portable solar-power system during a power outage or when camping to recharge your cell phone or power a laptop computer, since most of these devices include charging adapters to fit a 12-volt DC vehicle auxiliary outlet.

    Phase 3: Build the System

    I designed this project to require a minimum number of parts and very few wiring connections. I selected a standard Group 31 RV/Marine battery which is designed for multiple deep charge/discharge cycles while still being reasonably priced. I also found an inexpensive plastic battery box, 10 amp in-line DC fuse, and female cigarette lighter receptacle (Here’s one with battery terminal attachments and fuse built in). I decided to use this type of power receptacle for this project since so many portable tools and electronic devices have charging adapters that fit this type of 12-volt DC receptacle. As shown in the photo, I mounted the cigarette lighter receptacle in the box cover and wired it through the fuse to the battery using #10 standard copper wire and crimp on ring terminals. The center post of the cigarette lighter receptacle is always connected the battery positive and the outer shell is always connected to the battery negative (-).

    The Solar-Tech 85-watt solar module I selected for this project includes a full-size conduit box mounted on the back. (Note, we had trouble finding a model with attached conduit box, so you may have to improvise when attaching the charge controller. One option is to mount it inside the battery box, and purchase a cable that ends with male and female MC4 connectors (typical of most solar panels). Wire the bare end of the cable directly to the charge controller, and you can use a short, 2-conductor cable with ring terminal ends for quick connect and disconnect to the battery terminals using wing nuts. This also allows for quick disconnect near the panel.—Editor)

    Also make sure the solar module is advertised for a nominal 12 volt charging voltage (17 volts peak), as manufacturers are increasing the physical size and wattage of their modules so fewer modules and wiring connections are needed for the same array total wattage. However, this increased module size also requires increasing the nominal voltage to 24 volts (35 volts peak) to keep current and wire size as small as possible, and this is too high for directly charging a 12-volt battery. While solar charge controllers are available to allow a mismatch between the solar array voltage and battery voltage so you could use a higher voltage solar module, these solar controllers tend to have a much higher cost and are too large to use in this very basic portable solar charging system.

    I purchased a Morningstar SunKeeper-12 charge controller, which is designed to mount into the standard ½-inch knockout opening in the solar module’s conduit box and is suitable for mounting out in the weather. You can locate the solar charge controller on the conduit box attached to the back of the solar module, if you can find one with a conduit, (or follow the MC4 instructions detailed above).

    Phase 4: Estimate Your Power Needs

    Each tool charging cycle consumes an average of 7 amp-hours of battery capacity (7 amp charge rate for 1 hour). The Group 31 RV/Marine battery used for this project has 100 to 115 amp-hours of charge capacity, depending on price and brand. To avoid discharging this battery below 50% (which will help increase battery life), we will have approximately 50 amp-hours of useful charge capacity. This equals seven battery tool recharges (50 amp-hour/7 amp-hour) before the RV/Marine battery will need to be recharged. Of course, the actual number of tool recharges will depend on ambient temperature, battery age, and depth-of-discharge of the tool battery.

    We estimated this Group 31 solar battery will require 50 amp-hours of solar charging to replace what the battery tool charging took away. Assuming we have an average of five hours of full sun per day, this will require a solar module capable of providing 5 amps of output to fully recharge this size battery in two days. (50 amp-hours/5 amps = 10 hours).

    A typical 85-watt solar module designed to charge 12-volt batteries will typically have a peak output of 5.1 amps, so I selected an 85-watt module. This smaller wattage module is also fairly easy for one person to carry, while still large enough to provide a reasonable amount of solar power. Your solar module can be larger or smaller than my 85-watt module selection, which will reduce or increase the number of days it takes to fully recharge the RV/Marine battery.

    I have also omitted solar and charging efficiency considerations to simplify our example calculation. I have also assumed a clear blue sky all day, no module shading, and proper module solar orientation. When these factors are taken into consideration, you will most likely only convert approximately 70% of any solar module’s nameplate output rating into useful battery charging. Do not be surprised if it actually takes a little longer to fully recharge the battery you select.

    Phase 5: Put it to Work

    It feels really rewarding to build something off-grid in a remote area with the convenience of labor-saving power tools without having to deal with a noisy generator. It’s also nice to have a portable solar-charging system instead of having to keep your truck running while using a DC to AC inverter to power your tools and tool chargers. When not needed to recharge power tools at a job site, this portable solar-charging system can be used for camping or during emergency power outages. This solar module with built-in solar charge controller can even be used to recharge your RV camper batteries when dry camping.

    While most major manufacturers of battery-powered hand tools offer an “in-vehicle” charger, these are not easy to find in your local retail store. If you cannot find them locally, there are several Internet sites that sell in-vehicle chargers. Order the charger that matches your brand of battery-powered tools, and be sure the charger matches the voltage and chemistry of your battery packs.

    DeWALT #DC9319 7.2-volt to 18-volt vehicle charger:

    Makita #DC18SE 18-volt/Lithium-ion vehicle charger:

    Bosch #BC006 7.2-volt to 24-volt vehicle charger:

    Milwaukee #M12 12-volt Lithium-ion wall and vehicle charger:Milwaukee #M18 18-volt Lithium-ion wall and vehicle charger: This is one of the few that is also an A/C charger, so it’s double-your-value.

    Ryobi One 18-volt dual chemistry in-vehicle charger:

    About the Author: Jeff Yago is a licensed professional engineer and certified energy manager with more than 30 years of experience in the energy conservation field. He has extensive solar and emergency preparedness experience, and has authored numerous articles and texts.

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