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diy solar

12 vs. 24v systems on motorhome

On further reflection since I am only using 2 cells in parallel, how I was originally planning to do things is good. If I wanted to parallel more cells then I would need to change how I wire them.
 
On further reflection since I am only using 2 cells in parallel, how I was originally planning to do things is good. If I wanted to parallel more cells then I would need to change how I wire them.
Yes that's right, for 2P it is simple. As long as you connect positive to one of the two cells and negative to the other cell, it will be balanced.

Plus I'm guessing you'll have equal-length bus bars joining the 2P cells. The bus bars will be much lower resistance than say 8' long cables running from one side of an RV to the other, so any imbalance will be a much smaller effect.
 
Yes that's right, for 2P it is simple. As long as you connect positive to one of the two cells and negative to the other cell, it will be balanced.

Plus I'm guessing you'll have equal-length bus bars joining the 2P cells. The bus bars will be much lower resistance than say 8' long cables running from one side of an RV to the other, so any imbalance will be a much smaller effect.
Correct on both points.
 
Some components are not available in 24V, for example the Maxxfan that almost everyone uses is available in 12V only. Not a big deal, the converters are very efficient, for example the Victron Orion 24/12 converters range from 92 to 96% efficiency.
The god damned maxxfan / fan-tastic fan ? this is the only commonly used component for a van or vehicle based build I have run into that can be found in 12v not 24v. Basically everything else I've identified at least one 24v alternative.
 
The god damned maxxfan / fan-tastic fan ? this is the only commonly used component for a van or vehicle based build I have run into that can be found in 12v not 24v. Basically everything else I've identified at least one 24v alternative.
I bought a MaxxAire vent hood and added my own 24V fan to it.
 
If I ever build a van from scratch I’m building a 24v system and will add a second 24v alternator dedicated to charging only the house battery.

Lithium batteries are hard on alternators and I don’t want to get stranded because the alternator burned up or the clutch went out.
It does require a 24v-12v DC converter since there aren’t many 24v RV appliances.
 
That is an interesting article.

Has implication for DIY batteries with parallel and series cells. For example I am building a 2P7S NMC battery pack consisting of 16 3.7V, 60AH cells, resulting in a a 25.6V, 120AH battery. I was planning on using his second wiring scheme which would end up resulting in uneven usage of the cell. I am going to rethink how I wire these cells to ensure the most even utilization of the cells possible.

I know that article is good for Lead Acid. For LiPso (lithium phosphate) it would be too, but at some point the Battery Management system could make up for this. I admit I know very little about BMS and I assume that a BMS that could make up for resistance in parallel strings, but I'm sure each battery would need to be monitored individually to compensate. I've seen posts where Battleborn with their each battery having a BMS compensates for resistance, but I don't see how that's possible without a wire going between each Battleborn Battery.

I will eventually upgrade to Lithiums, probably 24 volt at 400 - 600 AH's worth, but I will take my time learning about the BMS.

I think diagram #3 is the best way to wire the batteries, and if I did this with six batteries, I would get a 7 post bus bar for the positive, and seven post busbar for the negative, and hook the six terminals to their own post and run the paralleled power to the system from the seventh post.

Diagram #4, I don't see a good way to turn it into actually being wired. To me it comes back to having at least two cables on one terminal, and its good in theory but I am using 2/0 and 4/0 wiring for my batteries, and with wire that thick, just becomes too difficult. I have enough trouble making cables with wire that thick to fit one cable on, never mind two or three.

Also wish there was math with the resistance for diagram #3 and diagram #4 to see what the batteries are losing.
 
I know that article is good for Lead Acid. For LiPso (lithium phosphate) it would be too, but at some point the Battery Management system could make up for this. I admit I know very little about BMS and I assume that a BMS that could make up for resistance in parallel strings, but I'm sure each battery would need to be monitored individually to compensate. I've seen posts where Battleborn with their each battery having a BMS compensates for resistance, but I don't see how that's possible without a wire going between each Battleborn Battery.

I will eventually upgrade to Lithiums, probably 24 volt at 400 - 600 AH's worth, but I will take my time learning about the BMS.

I think diagram #3 is the best way to wire the batteries, and if I did this with six batteries, I would get a 7 post bus bar for the positive, and seven post busbar for the negative, and hook the six terminals to their own post and run the paralleled power to the system from the seventh post.

Diagram #4, I don't see a good way to turn it into actually being wired. To me it comes back to having at least two cables on one terminal, and its good in theory but I am using 2/0 and 4/0 wiring for my batteries, and with wire that thick, just becomes too difficult. I have enough trouble making cables with wire that thick to fit one cable on, never mind two or three.

Also wish there was math with the resistance for diagram #3 and diagram #4 to see what the batteries are losing.
Just another reason to switch to 24V or even 48V. Easier and cheaper wiring. Plus your solar charge controller can be a lot smaller and cheaper also.
 
I am using the Maxxair Mini. It fits under my luggage rack.

Hah, that is ironic. I just (as in literally 2.5 minutes ago) recommended this precise product for this precise purpose on another forum. What are the odds ?

Never used it myself but I was helping someone search for a low-pro roof vent van for under a roof rack.
 
what 24v fan did you use?
I got this surplus 24V Comair fan.

Really moves the air. It is a noisy than I would like though.

 
I got this surplus 24V Comair fan.

Really moves the air. It is a noisy than I would like though.

Tanks! so you felt that it moved the same volume of air? Did you just clip the wires on maxxaire fan and reconnect 24v fan? Is it louder than original Maxx fan?
 
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Tanks! so you felt that it moved the same volume of air? Did you just clip the wires on maxxaire fan and reconnect 24v fan? Is it louder than original Maxx fan?
I bought the no fan version of the Maxxair Mini and mounted the fan to a piece of plywood.
 
My motorhome already has a 12V AGM battery system that charges from shore power or the alternator. I have added a shunt based battery monitor so that I can keep track of the state of charge with reasonable accuracy. All it needs to be self sufficient is to do the 12v loads analysis and add the appropriate panels and charge controller to ensure that the batteries are able to be fully maintained by those panels. In my case this just takes about 200W of panels and an inexpensive 20a charge controller

Now if I want to be able to run my 120V system from solar/batteries as well, I need to add an inverter. When adding an inverter, most people seem to feel the need to integrate their 120V (inverter) batteries with their existing 12V system (regardless of what battery voltage they target)
I am choosing to design the 2 systems independently, which leaves me free to choose inverter battery voltage independently from the 12V system. My Lead Acid batteries are located in an area that is not a good location for LiFePo4 batteries, so I'm adding my Lithium batteries in an area which is more protected from heat and cold. I have no reason to remove the AGM batteries, so they'll just stay in place to handle 12V loads. My current plan is to run my 2.6kWh of Lion Energy UT1300's in series tied to a 24V inverter. These will be charged with 400-600w of panels using a separate 35a solar controller. Since the inverter is used to power mostly "luxury" items and uses a LiFePO4 battery which doesn't need to be fully charged every day to be happy, I'm not really worried about charging the inverter battery from the alternator while driving. In a pinch, I could re-wire the panel connections to shift generated power to the system that needs it most and if the 12V starts to get low, I can always switch on the converter for a bit to move energy from the 24v battery to the 12v side (through the inverter in a somewhat inefficient manner.)

Down the road I'm building a much larger LiFePo4 battery bank, more panels and may step up to a larger inverter at some point. Whether I stay at 24V or step up to 48v may depend on whether I decide to add a mini-split to the motorhome and whether the unit I choose runs on 120V AC or 48V DC. I may end up at 48V either way to allow me to double the solar panel power input without upgrading the charge controller, or wiring.
 
This is a good article that explains the drawbacks of paralleling batteries, the math to back it up, and different ways to overcome this resistance.


I did not want my batteries being sloppy, so made my own cables. For my battery charger, it is 26 amps and I'm not concerned about lug size on that, but when I am running a microwave and drawing 130 amps from the batteries, I want a nice, secure connection.
Excellent article at Smartgauge. Thanks.
 
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I got this surplus 24V Comair fan.

Really moves the air. It is a noisy than I would like though.

 
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