12v or 24v on long camper with solar

[Nicks RV]

@astrocamper, I like your setup and might try to imitate it. I just reread your last post and am confused on the battery balancer. Could you explain a little more on why that would be needed?

thanks

[hclarkx]

Originally Posted by Nicks RV

So after watching youtube the last 2 nights and reading all the replys, I feel i have a relatively good handle on what I am about to do. I am seriously considering a 24v setup for various reasons, and thinking of running upwards of 1400W of solar.

The hang-up I have now is batteries and their mounting location. I would love Li, but living in Wisconsin I worry about ruining them in the winter and of course price is a huge factor. If I go FLA or AGM, then I have huge weights to deal with and space constraints. I did an energy audit of my camper and lifestyle, and found if I can get close to 400-500AH at 24V, I would be good.

I'm confused about your energy audit results. You are saying you can use 400-500 Ah at 24V between end of one solar day and the beginning of the next? That's huge. 450 Ah at 24V is 10,800 kWh. Did you mean 400-500 Ah of battery capacity, 50% usable? That's still huge. Are you going to run a lot of A/C?

You can't go wrong with 24V or 12V. We designed my son's 2000W solar array and 840 Ah battery system both ways and went with 12V. It has no long 12V runs (inverter is close to the batteries). My own system started small at 12V and will stay that way through 2200W of solar and 700Ah of battery. Again, not long DC runs. We both run mini split A/C and boondock exclusively.

I stuck with 12V and my son went 12V largely for simplicity and reliability. I.e., redundant solar controllers, no DC-DC converter, easily sourced replacement parts, etc.

My son went with 280 Ah LiFePO4 cells and wanted about 800 Ah and that also suggested 12V for his system since he would have needed 16 smaller cells to get 800 Ah at 24V.

You mentioned AGM. Staying with lead-acid or going LiFePO4 is a much larger decision than 12 vs 24V IMHO. With lead acid you have battery losses and other issues that greatly increase the amount of solar you need for a given daily energy use compared to LiFePO4.

[Fffrank]

Lots of good info has been posted in here but some terrible info mixed in as well.

I just did ~1200w of solar. 12v battery bank. Did the research on the steps to switch to 24v and the advantages just weren't there (it would have cost a lot more.) It's just not necessary unless you're over 2000+ watts.

Something is off on your battery calculations. I can run in the winter (in northern WI) and with the propane heat cranked and not being stingy on usage at all I was using a max of about 180ah/day. This is with an absorption fridge and not a residential fridge (which should use considerably less.) We almost exclusively boondock.

At 38' and two rooftop ACs + vent fants + shower skylight I don't think I have room for more than 1500w and that would likely make it very difficult to get around on there for maintenance.

If you don't have an existing battery bank I would 100% invest in LiPo4. You'll need to locate them in an interior/heated compartment (or waste the heat in an external compartment) but it's the way I would go if I was starting from scratch. They won't be damaged if you store them below freezing but they will not work (or charge) if ambient temp is below 0F. (And be honest -- are you really camping below 0F? Haven't seen many on this forum that are.)

I had 500ah of new-ish flooded batteries already. (And weight/storage space is completely different in a Class A. ) But I have enough capacity for what I need to run, enough solar to top it off every day (unless the weather is really nasty) and I didn't have $4000 burning a LiPo4 sized hole in my pocket. When my current batteries start to fade, I expect LiPo4 will only be 20-30% of a premium and LiPo4 will be even more on a no-brainer.

[Nicks RV]

Quick disclaimer.....I am not 100% sure on any math I have done and as of right now everything is only on paper and subject to change as I learn more.

To answer hclarkx, I meant to say 4-500 12v, but 24v would be awesome if I can fit the batteries.

I recently hooked up my BMV-712 and with only a single FLA battery to power, It was reading a draw of 165W with all lights, 12v fridge, co2 detector, and stove fan on ( for some reason I didn't turn the 2 vent fans on). Quick math on batterystuff.com says i'd need approx 190AH to run for 12hrs. Call it 200AH to be safe and knowing I still need to factor in small ac electronics and bad weather days I figure my starting battery bank should be at least 400ah 12V.

I would love to run the A/C while boondocking, but right now I don't boondock in the middle of summer. I am hoping to eventually(as funds allow) build a bank that would handle it.

As for winter. I agree that the furnace shouldn't pull much and most don't camp in sub 0 temps, but my issues is with the lithium batteries. and how to store them if stuff is still running off of them....now that I think of it, I guess I could just turn everything off for winter storage.. Sorry, sometimes things just need to be put on paper to see the error of my thoughts..

@Fffrank, I am starting this build mostly from scratch except for a few hard components like the BMV-712. I am still not 100% sure on the 24v system but have been considering it. My main reasons for 24v is supposedly can charge the battery bank faster, inverter size and more stable 12v.

I'm told less heat generated is best and as inverter size increases, so does heat on the same voltage. Stepping up voltage the inverter will run cooler and I've heard people having issues with their inverters shutting down due to poor ventilation. I'm not too worried about wire size as everything should be in a small proximity to one another. Also I just read that by using a DC to DC converter, I would get a more stable output to the 12v system. I guess this is why I am taking my time and doing it on paper first.

[Fffrank]

Nothing wrong with a 24v system except for added complexity and expense. It's better suited for a house where you may have multiple hundred feet of wire runs.

More things to keep in mind: if you run 24v you likely won't have a great way to charge from your tow vehicles alternator. Or if you do, it's another dc-dc-converter/charger.

Current inverter will need to be replaced with a 24v model.

You'll also need a 24-12v dc-dc converter to feed your 12v systems.

[axleman27]

I went against the grain on batteries and put in 4 150AH silicate salt batteries. They offer many of the benefits of lithium without the complexity and toxicity. Unfortunately, they are heavy FLA batteries but I was willing to deal with that to avoid potential BMS issues.

[WillT]

Originally Posted by axleman27

So this means that I can connect about 1000w of solar panels to the charge controller on a 12v system. Any more than that and the excess solar capacity will not get used.

Good point. When I put my 900W of panels on my roof I was told that any excess power put out by the panels over the capacity of my Victron controller would just be clipped as you describe. The reality is that even in the summer, because I am pretty much only in Oregon, Nevada, and Utah and do not tilt up my panels, I do not come close to exceeding the controller capacity. In fact, my 900W of panels average 450 to 500W max. on an average summer day. I may have seen as much as 600W once, maybe. I am considering adding another panel to take it to 1080W. That way I will have a little more production on average. And in the winter, that is needed if I never want to use the generator. Unless I go to Mexico in the summer, (and tilt the panels), I will still never have any clipped power.

The panels are the easy, and least expensive, part. I plan to add that additional panel, double my battery capacity and add an inverter so I have to go back to school on all of this!

[hclarkx]

Originally Posted by WillT

Good point. When I put my 900W of panels on my roof I was told that any excess power put out by the panels over the capacity of my Victron controller would just be clipped as you describe. The reality is that even in the summer, because I am pretty much only in Oregon, Nevada, and Utah and do not tilt up my panels, I do not come close to exceeding the controller capacity. In fact, my 900W of panels average 450 to 500W max. on an average summer day. I may have seen as much as 600W once, maybe. I am considering adding another panel to take it to 1080W. That way I will have a little more production on average. And in the winter, that is needed if I never want to use the generator. Unless I go to Mexico in the summer, (and tilt the panels), I will still never have any clipped power.

The panels are the easy, and least expensive, part. I plan to add that additional panel, double my battery capacity and add an inverter so I have to go back to school on all of this!

Agree. And the clipping that occurs is pretty minimal unless you "overpanel" by more than about 140% AND spend a lot of time out in June or July. One can use the National Renewable Energy Lab (NREL) PVWatts program to study how much clipping will occur. It's surprisingly low and not huge even at 140%. What one finds is they lose maybe 10% of the available energy on only a dozen or two days, usually in June or July. In much of the West, 140% results in 4 or 5% lost energy production from clipping IIRC. One can use PVWatts to optimize panel to controller ratings for the location where they spend June and July. It's easy to adjust panel to controller ratio and see changes in monthly production.

As for AGM vs LiFePO4, this bears a bit more study. AGMs need to be kept at high charge to prevent freezing damage and their capacity drops at lower temperatures. LiFePO4 needs to be not used below about 32F to prevent damage. It's worth looking into the specifications of individual batteries to do a proper comparison. Additionally, battery heaters are not a complex affair and are even built into some LiFePO4. If one has solar one can set the solar controller to not over-charge a battery but supply enough energy to keep the battery warm via a heater. With LiFePO4 the low weight can make it feasible to take the batteries indoors in winter. Not so much with lead-acid. And you need almost half as much battery capacity with LiFePO4 (LiFePO4 is about 80% usable while AGM is about 50% usable). This is a quick and not complete overview. And does not touch on the many advantages of LiFePO4. The recent drops in LiFePO4 prices (some decent 100 Ah ones under $500) make them hard to resist.

If you are a bit adept at spreadsheets, there are ones that can help sort out the energy usage to expect. I have one I can send you. It goes much further in RV solar/battery planning if you want to do more than just estimate your day and night Ah needs. For instance you can model A/C energy use based on weather at a given location (using weather data from NREL's PVWatts). You can adjust solar and battery capacities to limit generator use to a level you choose.

[Freqz]

You can use Li batteries under 32° but need to be really careful about charging them that cool— max charging current drops off a cliff. If they’re in a heated basement compartment keeping them warm enough to be usable shouldn’t be a problem. With a split 24/12V system you can, if you set it up that way, always turn off the 24V side leaving the batteries at half charge for storage and maintaining trickle charge on the 12V side if needed. Mine has that capability but it never gets winterized.

My current rig has batteries under the bed in my TT…420Amp hours @ 24V. I find I can run the 15kBTU AC for about 5 hours before I’m looking for shore power if I’m not getting any solar input. (I’m currently camping at parks with shades for the summer in far West Texas) When I didn’t have shade, I could run a second portable AC on 30A plus my 1000W of fixed panels no problems. (Which was the goal for my system)

I have the original 12V batteries in place and just charge them with the stock converter— you lose a couple of points of efficiency doing that but it’s cheap and easy to manage. Flip the inverter on for a couple of hours a day and they get topped off— at least through bulk. Most of the time, they just stay topped off like you’re plugged into shore power. I definitely leave the inverter off when I’m boondocking and not otherwise using AC power— it’ll be your biggest draw over 24 hours if not running the air conditioner (fridge and water heater on gas).

[astrocamper]

Originally Posted by Nicks RV

@astrocamper, I like your setup and might try to imitate it. I just reread your last post and am confused on the battery balancer. Could you explain a little more on why that would be needed?

thanks

When two 12 volt batteries (or banks of parallel batteries) are in series it possible for one battery to be over charged while the other is under charged. The battery balance will monitor the voltage between the two batteries and allow a one amp current flow from the battery bank with the high state of charge to the one with the lower state of charge.

I avoided the issue by remaining with one bank. My batteries are inside behind my sit and sleep sofa that folds down to make up the bed platform that it half over the front pass through storage. I was able to fit the seven batteries side by side in a space otherwise unused. The batteries stay closed to inside ambient temperature and have never been below freezing when my trailer is parked at my home in Reno. I have 3 temperature monitors on my Cerbo GX that graph the temperature data. I have about 13 months of data kept on the VRM portal. I have a temp probe in the center of the batteries, above the Multiplus inverter/charger in my pass through storage area and underneath my trailer to show the outside temperature.

My Battle Born batteries have a BMS that balanced the four banks of batteries internally that make up the 12 volt battery. It requires that the batteries periodically get charged to 100% in order for the balancing to happen.

Battle Born sells 24 volt batteries if you wish to avoid having multiple banks of 12 volt batteries for a 24 volt system.

[Nicks RV]

@hclarkx, Yes please, I would be interested in the spreadsheet. Any information I can get to help me make more informed decisions is always welcome.

So when my friend first started talking about going 24v on his next solar setup, I was really excited and wanted to do the same as it sounded like the best way to go. After talking to y'all, I come to realize that in larger applications it makes sense to do it, but for my approx 1K watt system I cant seem to find enough of a benefit. It almost seems like I am making a mountain out of a mole hill and should just stick with and expand my current 12v solar setup. I honestly don't ever see me getting over about 2kW of solar or over 800AH of battery.

[Nicks RV]

Random chance I came across an opportunity to buy 8 of these batteries for 25$ a piece. Worth it or continue to save for lithium?

[hclarkx]

Originally Posted by Nicks RV

Random chance I came across an opportunity to buy 8 of these batteries for 25$ a piece. Worth it or continue to save for lithium?

Interesting opportunity. For me they would be a non-starter. LiFePO4 prices are dropping and even today are good enough that they are worth the premium.

Sealed, clean
Lighter
Smaller
Zero maintenance
Require much less solar
Can be discharged to 20% SOC without measurable loss-of-life.
Fewer batteries so less cabling
BMS protection so much safer than lead-acid
Hugely longer life, especially relative to flooded LA
Efficient**
Faster charging (all done in bulk, no absorb stage)

**Keep in mind that not only are you limited to 50% discharge (for multi-year life) but the losses in a lead acid eat up a good part of that 50% that you can use (15-35% depending on charge and load current .. higher means higher losses).

I'll never go back.

[Charlesland]

As I read ur description, wire lengths for q2v or 24 are short. Wire between inverter and breaker panel is long. That wire is 120 volt so length is as much of an issue and is the same regardless of whether dc voltage is 12 or 24 volt. With short distances for dc wiring, savings on wire isn't great. Victron controllers are current limited, so you get twice the power with 24 volt system. But you will need a 24 to 12 volt buck converter and upgrade your converter to charge a 24 volt battery bank when charging batteries from Shore power.
Run the numbers. I bet the 12 volt option will win.