Battery Bank Solar Panel Expansion

Solar Generator – Battery Bank & Solar Panel Expansion

This is an update to our DIY How To series on How to Build a Large Solar Generator.   In this follow up post, I will show you how we can use the quick connects designed into our solar generator to expand both the solar charging capacity, as well as the battery bank for increased run times.

A lot of the feedback was asking how to expand the solar generator to an even bigger capacity system. When I designed the base unit, I wanted it to be very easily expandable using quick connects. There are two areas where we can easily do that with our system:


Increasing Solar Panel Array

First is solar charge capacity, which will increase the rate at which the unit can recharge our batteries.  We used a 30 amp charge controller in our build, as well as a single 100 watt solar panel.

When this panel is in peak conditions, we can expect slightly over 5 amps. The panel is rated with a 5.25 amp Optimum Operating Current, which is the current we would get out of the panel in 100% perfect conditions with the panel operating at peak efficiency.

There is a second rating, the Short Circuit Current, which for this panel is 5.75 amps. Unless there is a problem with the wiring from the panel, we should never see this full current, but its good to understand what it means and take it into account when sizing wire. This value is the maximum possible current the solar panel could create when it is in full direct sunlight, and the output wires are shorted together.

Knowing these specs, along with the manufactures recommendations for our charge controller not to exceed 80% of its max current rating, we can add up to four of these 100 watt solar panels in parallel.

By connecting these panels in parallel, the current from each panel will add together, giving a max short circuit current of 23 amps.  But the output voltage will balance between them, not add together. It is possible to connect panels in series as well, but it doesn’t make as much sense for our usage scenario to wire them that way.  I will explain the differences in a later post, so make sure you subscribe if your interested in that.

We can easily  wire our four panels in parallel using some 4 to 1 MC4 cable adapters.  We will use on adapter to connect all 4 positive (+) wires from the panels into the positive wire leading to the solar generator.  Likewise, we will use the other 4 to 1 adapter to do the same with the negative (-) connections.

Expanding The Battery Bank Capacity

We included a high current connection to our large solar generator, so that we could easily expand the battery bank. Adding 1 or more additional batteries will give your inverter an increased run time at higher loads, or when solar conditions are poor.

As a bonus, the battery bank expansion unit we build here can also be used independently with our quick connects, which give you an easy way to get a large 12v power source for jump starting vehicles,  powering a 12v winch on a trailer for example, running a 12v tire inflator, and with an inexpensive option, you can even have an extremely large capacity USB power bank.

We use two high current quick connectors on the battery expansion unit, that way we can daisy chain as many additional batteries as we want.  It also allows us to still use our other high current accessories with the solar generator, such as jumper cables, or a 12v winch without needing to disconnect the additional batteries.

The batteries are wired so that when they are connected together, they form a parallel battery bank. This means the total voltage will stay the same, but the battery capacity will increase. Our main solar generator unit has a Optima Blue Top battery, which has a 55 amp hour rating. We will use a VMAX SLR-125, which is rated at 125 amp hours in the battery expansion unit.  So the total capacity will increase to 180 amp hours when they are connected together.

The battery is quite heavy though, weighing in at 75 lbs.  It is also a Group 31 battery, so it is fairly big.  I purchased this battery case, which is compatible with group 31 batteries, as well as several of the other larger battery sizes. We will be using it as an inexpensive housing for our battery expansion unit.

Components Used in this Tutorial

To see the components used in the main solar generator unit, see the post for Part 1 in this series.

Solar Panel Expansion

Battery Bank Expander Unit

60 thoughts on “Solar Generator – Battery Bank & Solar Panel Expansion”

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  8. Mark,
    The parts have started arriving from Amazon, a shower of boxes.
    I am excited and can’t wait the get this built. I plan on using it for household emergencies, camping and other outside events and also for emergency operations for or local ARES ( Amateur Radio Emergency Service) who I volunteer with. Thank you again for this wonderful design.

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  12. Hi Mark

    I’ve been kicking this idea around for years so thanks for the inspiration. I like the fact that I could do this for far less $$.

    I built the unit with just a few small changes. I will be expanding the unit as well. I live in snow/fire prone area so this basic unit will keep my wife safe if I’m gone.

    This basic design is great for upgrading a small pull camper and that is what I’ll be doing


  13. Hi Mark, I have thoroughly enjoyed your videos and will be making one of these systems for myself. I am an amateur radio operator and I deploy for emergency communications.

    I want this system to be able to power my radio. It runs 13.8V DC, 100 watts at 21A for transmit, though could step power down. RX is 0.9A on standby, 1.25A maximum audio. Would the configuration with the blue top battery work, or would I need more?

    1. Hi David,

      It sounds like that should be no problem at all. I would power the radio directly off the battery. Wired this way you would not need to turn on the inverter unless you need AC power for something.
      Sounds like the standby / receive power usage is pretty minimal. So the only real power drain will be when transmitting, which I imagine is not usually continuous. Your battery run time would be dependent on how much transmitting you do.

  14. Mark – Understand and agree. My battery expansion unit is complete and works great. Have it strapped to the Milwaukee Tool tote carrier and can wheel it around wherever I need it. Total weight is about 70 pounds. It was a much simpler project to start with and not near as overwhelming since it is has a lot less wiring and physical parts to worry about. I did not put its own charger in it. But can easily hook it up to one via the extra terminals mounted on the front when needed. Now I will start collecting parts for the main unit. Thanks for your comments and feedback.

    1. You could, but if you are talking about an electric golf cart, you will need to be very careful that the inverter voltage matches what it was designed for. Electric golf carts are usually 36v or more. If you are using a 12v inverter, you will only be able to connect to 1 of the 12v batteries (or 2 6v in series). It can be very easy to accidentally connect to the wrong combination of batteries in these situations. Also, you will only be draining the batteries you are connected to, leading to an imbalance, so it is not real good for the life expectancy for the entire battery bank if you did this often.

  15. Sorry if this is a stupid question but when building the extra battery bank like Mark has done……how does that batter stay charged? Do you connect it via the quick disconnect and then since that wires the batteries in parallel then that means either the solar charger or trickle charger will maintain both batteries at the same time? Just trying to fully understand that part…..I am working on building the expansion battery bank now. Also, my main unit has a 400amp fuse inline connected between the battery and the inverter…..when connecting the expansion battery I should have enough fuse protection right? Or do you think an extra 200amp fuse in the expansion bank is still needed?

    1. Actually, it is a good question and I was going to put the same battery charger in the main unit in this so you could just plug into the wall the same way. Problem is, the unit is already pretty heavy and there is very little extra room inside, So I just slapped and extra set of red/black terminals ( on the left front top and ran some heavy duty cables to the battery. This way you can easily attach a battery charger or you can clip your regular battery cables on it to charge your car battery in a pinch. I put the whole thing and a Milwaukee Tool carry cart I found at Home Depot and have a vey handy 12 V battery cart that literally goes anywhere and doubles as the expansion pack for the solar generator. I actually built this first, to see how easy building this would be and in case I neve went any further, I at least I had something useful and it most definitely is.

    2. Not a stupid question at all! Like D. Peddie said as well you can recharge it a few different ways. As a stand alone unit you can connect an external charger, but when it is connected to the main unit it will stay charged in parallel with the main unit battery. The solar panels (or the AC battery tender/ charger) will trickle feed the main battery when it is alone, or trickle charge the entire battery bank when they connected together.

      The fuse you already have between the AC inverter and the main battery will continue to protect the AC inverter and its wire against direct shorts whether the additional batteries are connected or not. It won’t protect the wires between the batteries from a short however. Wires between the batteries are often left un fused because they are very short (in length) and adding fuses increases the internal resistance. However, if there is reasonable risk of an electrical short in the wires (say as the wire length goes up) then it would be wise to add a fuse between them as well.

    3. D. Peddie – That sounds like a good plan. I have found myself using the expansion battery by itself much more often than I realized I would as well. From jump starting vehicles, to powering a 10k lb 12V utility winch I have that mounts into a 2″ trailer hitch receiver so I can easily move it between trucks / or my flatbed trailer.

  16. Mark, thanks for the great articles, you have done an amazing job. I am following your articles and building a generator nearly identical to yours. What are thoughts on adding a quick connect to a relay then to the battery/alternator of my truck? could the truck be used as an alternative charging source through the high voltage quick connect?

    1. Thanks John! Yes you could create a setup as you describe. A couple of thoughts. Some trucks (my ’02 Ford F250 for example) already have a relay feeding 12V to the trailer hitch, that disconnects when the truck ignition shuts off. This was designed to provide 12V trickle charge to trailer brake batteries, or camper batteries, but could be used in the same way with the solar generator.

      If you prefer a more manual approach, you can get battery disconnects designed for adding a second automotive battery fairly inexpensive as well. For example:

  17. I am very interested in building my own using your list of components. Thank you so much for all of your postings and information on this subject. I am still skeptical I will be able to fully pull this off. I would like to build something that will have enough power to keep a refrigerator and freezer operational as well as being able to plug other normal daily used items and to be able to keep electronics charged, modems and a tv operational. Is all that even possible? I am not worried about weight or portibility. Also is there a digital read out of the total watts being used, this way my wife and I know when we are at the max so not to break anything. Thank you again for all your input and time

  18. Mark,
    I built your solar generator of Christmas and it truly has been an awesome experience. My wife calls it the 1.21 Gigawatts Flux Capacitor,….
    I made some minor mods regarding the cable wiring. I used some cheap wiremold cable routing [ ] to orderly route the cables to the fuse box and ground bus. It is also came handy to hide / organize the cables going up the LED light on the box cover.
    I had a question. You fused all the major circuits ( battery tender, solar charger, LED lights, outlets) but the battery extension to the quick disconnect is not fused and a I was thinking to get a another 200A blade fuse to protect this circuit. I believe that 200A should be sufficient since the disconnect is rated at 175 . Please let me know your thoughts in this matter.

    1. Andreas,

      Those all sound like great improvements, and I like the wife’s name for it too! Yes, adding a high current fuse to the high current wires is definitely an improvement. I hadn’t included one because I was worried about space and the cost. Those big 200A fuse holders can get pretty bulky and expensive. From a safety standpoint I had figured it was similar to using jumper cables which are also typically not fused, but there is certainly an improvement when adding one. I also think 200A should be adequate for the fuse value.

  19. Hey Mark!
    Thanks for the great tutorial on this build. I basically just built very similar to yours but added 2 35AH batteries in a slightly larger scuba case and changed the placement of a few things to meet my needs. I am very happy with the final product and plan to actually build a larger battery bank for the system to run as a back up power system for my home for emergency use, basically lights and my furnace.
    What will definately make it all worth it would be to adorn my case with a sticker if by any chance you have any more.

    Thanks Again!!

    1. Thanks for the great feedback Doug! I do still have a few stickers left. Please send me your mailing address to mark @ modernsurvivalists . com (delete spaces!) and I will get one sent out.

  20. Hi,

    This guide has been so helpful. Like other people have mentioned, I’m building this in stages and just did my basic set up this weekend. I live in Puerto Rico and having been hit hard by Hurricane Maria, I’ve had no power for over 2 months. It was great to have a fan on in my apartment for the first time in what feels like forever. I really don’t know anything about electricity and the easy to follow guide has been a god send and an awesome learning experience.

    1. Thanks so much Ricardo! I am so glad it has been helpful to you guys! : )
      You will find the electrical stuff is not as hard as you thought it would be!

  21. I am collecting parts to do a similar set up, the only thing significant I would like to change is using LiPO4 batteries. This should help with weight as well as the better draw-down capabilities and battery life if nothing else. There is conflicting information from what I have seen preliminarily as to whether these batteries are “drop in” or not. What say you??

    1. Hi Rick,

      I am interested in that as well. I have not done much research on this yet, but I think it will depend on the battery you go with, and the solar charger you are using. The inverter and other load side equipment should all be fine, but the way the LiPO batteries charge is different. Some of these LiPo batteries made to drop in have their own controller electronics integrated. This is important because these batteries must be protected from over current, over discharge, and over charging. If the battery has these functions built in, it may work. But the charge controller may not charge in a mode that is optimized for that battery type.

      If you try one, please report back what you use, and how it works out!

  22. Hi, just found this and am curious if there is any issues with internal heat? Or if there is a need to manage this? I ask because it gets stupid hot down here in the south of Texas

  23. I am going to attempt this project but I’m going to do it in stages. I plan to set up my panel, and everything else up to the inverter. I was going to take the unit out to the new shed and use the DC side to power some LED lights, maybe even some Off-Road lights. My question is, if I’m going to use those bigger LED set-ups, will I need to use relay’s like one would do in a car? I’m gathering my materials and I’m about ready to start testing, just need to know if I should add some relays to my list.
    Thanks again

    1. Hi Robert, Generally LED lamps are low enough current to run directly off the switch. You can double check be determine the total current draw for the lamps, (add them together if more than one) and compare that to the max current rating for the switch.

      If the lamps do not list current, but wattage instead, you can calculate it by dividing wattage by voltage (12) and that will give you the current in amps.

  24. Wow, I appreciate the help and the suggestions. I really won’t need AC power for a few months yet so that would give me time to get everything else set up and learn more about what calculations I need to learn. Electricity was never my ‘friend’ so the most I messed with it was adding lights to my motorcycles, This is a lot more challenging.
    You all are very helpful and non-judgmental unlike some of the sites I have been on, thanks again.

  25. To Robert M:
    Based on the excellent site , it says that a 15watt solar panel can charge 1 Amp-hour in one hour. So a 100 watt panel can recharge (100 / 15) = 6.66 amp-hours in one hour of direct sunlight.

    I don’t know the amp-hours of that battery you have, so I’ll use my 100 amp-hour battery as an example. If I discharge it to 50%, I need 50 amp-hours to charge it back to 100%. A 100-watt panel would take (50ah / 6.66ah) = 7.5 hours of direct sunlight to fully recharge my battery from 50% to 100%. As I understand it, temperature and battery age would affect how much actual power can be stuffed into the battery. Older batteries won’t hold the same amount of charge that they could when new.

    1. Ron, Thank you, I will read the article you included. The more I find out about this process the more I’m having second thoughts. I’m just wanting to power a shed but the more I read the more stuff I need to add to my shopping list……..
      Thanks again

    2. Thanks for looking up the calcs Ron! The Blue Top Optima is listed as 55 amp hours, so if ran down 50%, a 100 watt panel should recharge it in about 4 hours of direct sunlight.

      Try not to feel too intimidated with this stuff Robert. It seems much more complex than it really is at first. You don’t need to buy everything all at once either. What is your primary goal for the shed power? Lights? If so, you can skip the inverter at first and use some 12 volt lighting directly off the battery. Lots of accessories can run directly off a 12 volt battery. Then you could always add an inverter later if you decided you want 110v ac power as well.

  26. Great videos, thank you. I have one question though. If I’m using a 30 Amp controller, a 100W Monocrystalline Panel, an Optima Blue top battery, how long would it take to charge the battery if it was drained (being aware that it would never be “drained”) Do I need to plan on starting with 2 panels to make the recharge times reasonable? Thanks again for sharing your information.

  27. Hi Mark, thanks for this excellent series of posts! I am thinking of folowing your guidance to build a solar subtitute for a gas generator, to be used in light construction on areas of our farm that are not reachable from the grid. It would need to power a circular saw and charge battery packs for a full day of intermittent use. From my own calculations, it looks like I’d get about 15 minutes of continuous use from the 55AH battery using a 1500 watt saw (2400 watts start-up). Of course, the system will be charging continously so it should provide much more than that over the course of a full day, as long as the sun is shining. I looked throughout these posts, but didn’t find anything about the suitability of this system in that application. Should I consider keeping a second 55AH battery on charge at the house, to swap out? Could you please advise? Thanks very much!

  28. Mark, how about this for a waterproof switch?

    It looks like the Amazon search to use is “latching switch waterproof -momentary” That finds a lot of waterproof switches, most of them push-button kind (latching).

    Here’s a six pack of little ones like you used (rocker-type, not push-button), but with waterproof caps:
    It seems a shame to have all the other ‘ports’ on the Pelican case be waterproof, but not the switches… I’ve already received my 10 pack of the ones you already recommended, but I might return them and use some from this Amazon search. Some of them even have different color options.

    1. Thanks for the links Ron! I wasn’t searching with the best phrase, much better suited options with “latching switch waterproof -momentary”! I will add these to the list of updates to add.

      Another update I want to try is using an epoxy such as JB Weld to attach the high current quick connect. After repeated use, the hot melt glue is becoming weak on that part due the additional strain that part gets.

  29. The toggle switches you use, with the blue power light — they don’t appear to be weather-proof, like almost all the other external connectors on the Pelican case. I’m curious about that.

    1. You are correct. These are only considered “splash proof” which means they can handle light / occasional water. I wasn’t able to find a low profile fully waterproof switch. The water proof ones tend to be big bulky toggle switches with rubber boot covers, or extremely large rockers. I didn’t want switches so big that they stick out and get snagged, knocked off or accidentally turned on in transit. If you know of a good switch for this application, please let me know!

  30. Mark, are you sure the Quick Connect you list in part 6 Components Used is correct? It says 6-10 gauge, and the wire you use is 4-gauge. Also, the Quick Connect plugs you used earlier were 2-4 gauge.

  31. Awesome post…I’ve been eagerly awaiting it. One quick question…I thought that I had heard that it is best to have the same capacity for all of the batteries in a battery bank system. Because of that, I was going to go with a higher-capacity primary battery from the start, rather than the 55 amp hour Optima battery that you showed. But I like the size and weight of the Optima for the case, so if it doesn’t hurt anything to have them be different, then adding much larger external batteries would be my preference. Any thoughts on this?
    Thanks for the time and effort you put into these videos and posts!

    1. That’s a good question Scott. You are correct, ideally you would want to have matched batteries in a battery bank – same capacity, same construction, same age. The reasoning behind this is so that each battery will discharge and recharge at the same rate, and equally share the load, and consequently hopefully all wear out at the same time.

      In reality all batteries, even of the same model, will vary from each other some. But this is still good advise to follow when practical, and if this setup were a permanent installation, I would recommend matching all the batteries. Because this is a portable system, I had to make compromises to make the main unit of a manageable size and weight, like you mentioned. Since the expansion batteries may not always be connected at times, the number of cycles and service life for them will likely vary from the main unit anyways.

      The larger expansion unit batteries have more capacity than the 55 ah agm battery in the main unit. This will not really hurt anything. Since the batteries are all connected together in parallel, they will remain consistent with each other in voltage. As load is applied to the system, the batteries voltage will all drop in unison, and each battery will contribute current towards that load. Since the bigger batteries have the higher AH capacity, they will contribute more current than the smaller AH batteries. When the solar charger is charging the batteries, the same thing happens in reverse. We probably can’t expect them to all have the same service life, but if the expansion units are not always connected, we wouldn’t anyways.

      If you connect an expansion unit with a discharged battery to the main unit with a full battery, the full battery will begin charging the low battery until the voltages balance out. The same thing happens when you use jumper cables to connect a vehicle with a charged battery to a vehicle with a dead battery. Running the vehicle with the good battery allows the alternator to raise the voltage even further and speed up the charging to the dead battery.

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