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

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

  1. 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.

  2. 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.

  3. 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!

  4. 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.

  5. 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!

  6. 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.

  7. 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.

  8. 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.

  9. 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.

  10. 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

  11. 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!

  12. have you ever thought about making ihis projec as a hybrid with wind turbine to take advantage of the breeze for some night time charging

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