12v or 24v battery

[CampLife]

Hi,

I'm looking to build a system this week, but I'm torn between a 400ah 12v or 200ah 24v battery.

Because of the holiday, a few companies have a sale ending tonight, so I feel like I need to purchase today.

I did a small 12v lead acid build a few years ago (batteries dead) so I should have most of the wires I'll need (at least I think). A fair amount of 2ga wire and some 10 and 14awg. I also have a fuse block and kill switch from that build I'd like to use. I have an older 12v, 750 inverter but it's not pure sine wave so I'll upgrade that this week anyways (a 2000w invertor should be plenty for me at time).

I have a 100w Renogy foldable solar panel and plan on adding about 400w of additional panels (those will likely be foldable as well). Because all the panels are foldable, they won't be permanently attached to the outside of the van, instead I'll take them out when parked and I'm able to.

In addition to the 100-500w of solar I'd like to use a dc-dc charger to get power from the car while driving.

The best dc-dc charger I found for a 12v system was the Renogy 60a. For 24v the best I saw was a 15a with Bluetooth. I like that the 12v, 60a will charge twice as fast as a 24v, 15a and I don't really want Bluetooth, so because of all that I like the 12v option.

If I can get on average of 6 hours of good sun with 500w of solar that should give me enough to power all of my needs. On days when I can't get solar, the 12v, 60a dc-dc charger driving for 6 hours would be enough. The 24v, 15a dc-dc charger would be ok but about half my needs.

Here is my anticipated power usage:
Working on my computer for up to 10 hours a day (an 85watt laptop, I believe) - 850 watts
A small Dometic 12v fridge (CoolFreeze cf-25, 35 watts) - 840 watts
A 120v slow cooker for 7 hours every three or four days (says 250 watts on plug, it's a 6-quart Hamilton Beach) - 1750 watts
Enough lights for about 75 square feet, powering smaller portable batteries/phone, other misc. - 1,000 watts (a guess)
At some point I would like to add a large chest freezer from sunstar (says when room temp is 90F (32.2C) it uses 495 WHR but I'm not sure what whr means)
If possible, it'd be awesome to run a 120v electric, 530watt appliance for 6 hours a day (trying to be realistic so this may not work) - 3,180 watts
Another if possible, it would be awesome to be able to use a 120v air fryer two to three times a week for 45 minutes at a time using 1200-1500w.
Maybe at some point in the future some sort of smaller AC unit but no plans for that at the moment.

I keep hearing that 24v is better. However, in my situation since it's a relatively smaller system and I'd like to charge via dc-dc, I'm wondering if the benefits of a 24v system still make it worth wile? For example, I hear 24v is more efficient, does that mean the 500w of solar I have will charge the bank faster? Does it also mean that I'll use less power overall on my daily loads on a 24v system? I also hear with a 24v system you save money on wires, since I already have large 2ga wire, should I just go with the 12v?

Any help choosing a 400ah 12v system or 200ah 24v system would be appreciated!

 

[time2roll]

For a mobile RV type vehicle I recommend staying at 12v unless the inverter is 3000+ watts or the solar is 1000+ watts.

 

[Tomthumb62]

2 AWG isn't going to be big enough for a 2000W inverter on a 12v system, but it would be fine for a 24v system. You'll want 2/0 or larger if going for 12v. Or downsize to a 1000-1200W inverter.

Also, without doing all the math, I think you may be grossly overestimating how many loads you can run off a 12 or 24v battery. Freezer, airfryer, computer and 530W appliance for 6 hours a day = likely 800Ah or more of battery. Double that to 1600Ah if you want to run an A/C unit for 4-6 hours a day. At 60A, it would take 13 hours of driving to fully replenish a 800Ah battery bank, 26 hours for a 1600Ah bank At 500W of solar (assuming a 50A charge controller), it would take about 16 hours of perfect sun conditions to fully charge a 800Ah bank, which doesn't exist on the planet (you would need more like 1500-2000W of solar).

Do a full energy audit (instructions on this site and elsewhere): basically figure out how many watts your device uses over the period of time you intend to use it, and do this for every item, all DC loads and AC loads, then add up the numbers in watt-hours. That number will tell you how big of a battery bank you will need Our half-size fridge/freezer uses about 1500Wh (watt-hours) per day, measured using a kill-a-watt device over a 24-hour period, in the heat of summer in a mostly uninsulated wood+metal camp trailer (hot! and no A/C). The inverter alone uses about 500Wh per day, just to be powered on (look for those with low idle-draw, like the Victron models but $$$).

The main pros/cons of 12v/24v is the 24v stuff can be hard to find in retail stores, so if something breaks, you have to order it and wait for it to arrive, which may not be easy to do if you're traveling in a van. But 24v requires cabling only half the gauge of 12v, so you can save some money on cabling. And stuff thicker than 2 AWG starts to get hard to bend, so you'll want the super flexible welding cable stuff, $$...WindyNation and BatteryCablesUSA are the two places I like to use.

We have a single 200W foldable panel suitcase (in addition to 400W on the roof) and the issue with the foldable ones is you can't leave them there or they get stolen. If you're camped out in the boonies, not a problem, but if you're anywhere else, you're at the mercy of sticky fingers.

 

[Tomthumb62]

Here is my anticipated power usage:
Working on my computer for up to 10 hours a day (an 85watt laptop, I believe) - 850 watts
A small Dometic 12v fridge (CoolFreeze cf-25, 35 watts) - 840 watts
A 120v slow cooker for 7 hours every three or four days (says 250 watts on plug, it's a 6-quart Hamilton Beach) - 1750 watts
Enough lights for about 75 square feet, powering smaller portable batteries/phone, other misc. - 1,000 watts (a guess)

So for example, your computer uses 850Wh (watt-hours) per day. It's probably less than that and I suggest you get a kill-a-watt device and measure your actual use. I use this one. Same goes for the other AC devices...such as your slow cooker probably doesn't use 250W the entire time it is on. And if you use LED lighting, then 1000W would blind you. Even 500W of LED lighting is blinding in a small van and charging a phone is small potatoes.

But to use your numbers here to give you an idea of a 12v battery bank:

850+840+1750+1000 (Wh) = 4,440Wh of battery needed. If you go with something like the budget 300Ah CHINS, that gives you 3840Wh for $939. Two of those in parallel would give you 7680Wh for a bit less than $2000 and this would give you some extra for days you don't drive and it's cloudy (but you still need a massive charging situation to charge those puppies up).

At some point I would like to add a large chest freezer from sunstar (says when room temp is 90F (32.2C) it uses 495 WHR but I'm not sure what whr means)
If possible, it'd be awesome to run a 120v electric, 530watt appliance for 6 hours a day (trying to be realistic so this may not work) - 3,180 watts
Another if possible, it would be awesome to be able to use a 120v air fryer two to three times a week for 45 minutes at a time using 1200-1500w.

If you add in these devices to the above math:

Chest freezer - 495Wh (I find that number unbelievable, likely more, but let's go with it)
530W device for 6 hours a day - 3180Wh
Air fryer - 1500Wh

Total = 5175Wh

Added to the previous total, 5175+4440 = 9615Wh.

Three 12v 300Ah batteries in parallel would give you 11,520Wh, which would cover you and provide a very small buffer for cloudy days that you don't drive. On cloudy days, you would be required to drive in order to not run out of power. If you wanted to sit at camp for three cloudy days, then you need to double or triple the battery bank.

An air-conditioning unit (A/C) run for 6 hours a day could easily add another required 5000Wh.

Note that these numbers are accounting for running off the battery bank and not offset by your solar panels. Let's say you have 500W of panels and you're realistically getting 350W out of them (ideal conditions, angled towards the sun, 5-6 hours of direct sun), then in 5 hours you would produce 1750Wh. So you could subtract 1750 from the battery bank size numbers above. If instead of powering via solar panels, you drive for 5 hours, you could subtract 3364Wh from the battery numbers (60A*12.8V*0.85=3364). Also, you will need HUGE cables, probably 4/0 for the 60A DC-DC charger.

If all that sounds daunting and expensive, then do a real energy audit (like with the kill-a-watt) to find out your ACTUAL, real-world use. Consider the temperatures inside your van, in summer it could be well over 120F or higher, depending upon how well insulated your van is and where you plan to camp (big difference between Arizona and Alaska in summer). Then decide which devices you really do want or need and if they're worth the cost of a bigger system to support them. Some people go without air conditioning and simply find other ways to cool off, such as traveling further north in summer and/or getting out of the van and into a river or lake during the hottest parts of the day.

In our camp trailer, the only things being powered by battery/solar are our fridge, phones and occasionally a computer, plus all the regular DC stuff like lights, water pump and exhaust fan. We get by just fine with a 12v 200Ah lithium. Cooking and heating is all done via propane, doing so via electric would just be cost prohibitive for us at this point, plus the space required for a bigger system isn't something we have.

 

[Rocketman]

Put solar panels on the roof. That way there are always there charging- side benefit the direct sun is on the panels with an air gap underneath- instead of on the roof heating the place up too hot. Wire the roof panels to its own dedicated mppt. Then have another mppt for the ground panels.

The formula of watts /volts = amps.

Watts are the energy packet - so a 200w panel will charge a 12v system at 200w/13v = 15 amps. The same panel will charge a 24v system at 200w/26v = 7.5 amps. If your batteries are 200ah @12v or 100ah @24v (the same two batteries just hooked up in parallel vs series) and they are both half full (same energy used), it will take for the 12v system 100ah/15a= 6.6 hours to charge vs 50ah/7.5a= 6.6hours to charge - exactly the same.

The advantage of higher volts is less amps (and voltage drop) and smaller cables.

In your description of items used above you need to finish the tally - then convert into battery capacity.

For example - computer 85w for 10 hrs = 850wh/ 13v = 65amp- hours (which is just over half of a 100ah battery.

Don’t forget the inverter’s power usage - that is a super critical spec for an inverter is on a lot - especially if it is on 24/7.

Be sure and do both sides the usage side plus the recharge side. Flat roof panels give about 50% the rated power (for 5 to 6 hours) - tilted ground mount (as long as you move them every 2 hours) give about 75% the rated power for the 6 hours.

Also, Dc-Dc chargers (because of voltage drop), I find give about 75% rated power.

When you plan your system be sure and add a good shunt based battery monitor- like a Victron Smartshunt or BMV712. You need to know how much power is in the batteries to make good decisions while using the system.

Good Luck!

 

[HarryN]

The specific batteries that you purchase do not matter for 12 vs 24 volt as they can be wired together as needed.

As far as 12 vs 24 volt systems, if the question was 6 volt vs 12 - would you pick 6 volt ? of course not.

I can't think of any downside to a 24 volt system over a 12 volt system.

If your reasons are that you want to buy from renogy those are not very good reasons.

 

[CampLife]

Thanks for breaking down the numbers and explaining things Tomthumb62 and Rocketman! It has really helped to reduce my anxiety about the purchase.

Since I'll only be using the slow cooker and air fryer (if I use the air fryer at all) every couple days, I'm hoping that during the off days I'll be able to get my battery bank up to where I need it to be. I probably won't use the 530watt appliance or only use it on days when I already have a full battery and there is peak sun out. I'm also trying to overestimate my needs a bit so I have some wiggle room.

It seems that a 12v system is going to be my best bet. HarryN, I'm basing that off of my need to charge the battery while driving using a dc-dc charger. I havn't been able to find any 24v dc-dc options other than a 15a charger that uses bluetooth. If you know of any 12v to 24v dc-dc chargers over 15a that can charge lithium, let me know. I like the Renogy because at 12v, 60a it would charge twice as fast and should suit my needs. I'm a bit hesitant having to use such large wires 4/0 like Tomthumb62 suggests, but it should be doable. I may end up going with their 40a version but I'll not sure yet.

 

[mikefitz]

Don't know where the recommendation for 4/0 cable came from. Renogy recomend the following for the 12 12 60 battery to battery charger,
Cable from starter battery, fuse at battery 90A , cable 4AWG.
Cable unit to house battery, fuse at battery 75A, cable 6AWG
Over 10 ft use 4AWG for both.

Comments,
Can your alternator supply 70 to 80 amps needed by the Renogy charger?
Renogy charger needs D+ , alternator charging , as a control signal.

Alternative is the Renogy DCC50S, a 50 amp charger that has a built in MPPT solar controller for panels up to 22 volts.

Consider fixed panels on the van roof, as much as you can fit , you will soon lose the enthusiasm for deploying the external panels, but they will most likely disappear anyhow.

Mike

 

[Tomthumb62]

I'm a bit hesitant having to use such large wires 4/0 like Tomthumb62 suggests, but it should be doable.

With 12v especially, you need to account for voltage drop. There are calculators out there. Blue Seas makes a nice free app called Circuit Wizard to help figure this out. You’re aiming for 3% or less voltage drop. The longer the run, the greater the voltage drop, hence the need for bigger cables.

So I ran an example calculation and good news! I was wrong. Assuming 30 feet round trip (15 feet from engine battery to lithium battery (red wire), and another 15 feet back (black wire)), you only need 2 AWG for the 60A dc to dc charger for 6 hours.

 

[CampLife]

That's good to know, I should have enough 2ga wire to get the job done.

I'm hoping to install the dc-dc charger on a 2004 Honda minivan. Not sure the amps of the alternator or how I would determine that. Do you think any version of the Renogy dc-dc chager would work on my van? It comes in 3 versions, 20a/40a/60a.

Edit: If the 20a/40a/60a dc-dc charger won't work, would the Renogy DCC50S with its additional features be suitable for my van?

 

[Supervstech]

If I was planning on any serious ac loads like a freezer or air fryer, no way would I go 12V...
24V minimum.
Wire is just too expensive, and a 12V setup has MASSIVE voltage drop when a 1500W or larger load is applied.

 

[time2roll]

Odyssey? Go with the 20 amp max for the DC-DC.
2/0 is only needed if 2000w inverter is attached. And then only from battery to inverter. The rest is smaller wire.

https://www.customcableusa.com/coll...custom-battery-cable-marine-grade-by-the-foot

 

[mikefitz]

You are going to need a solar controller so having the DC to DC charger and MPPT solar controller in one unit will save space and make wiring slightly easier. The DCC50 gives 50 amps from alternator or solar ( up to around 800 watts of panels). If both are active the DCC50 limits at 25amps max each. It's easier to interface to the van as it auto detect alternator charging. The slight disadvantage is that panel volts are limited to 22 or 23 volts.
The 2004 Honda minivan is not a large vehicle, how do you expect to fit all the stuff you discussed Into it?
Alternator capability is not known, perhaps the 50 amp unit is just OK, else the 40 amp DC to DC.
It seems the stock alternator is 130 amps max, perhaps around 80 amps at low engine speed. Given that the van needs power , 40 to 50 amps for the DC charger is just possible.
For fool proof operation uprated alternators are available, 320 amp units.

My overall impression is that you should take a step back and re evaluate the complete concept, especially the use of such a small vehicle that has a limited internal space and a low load capacity.

Mike

 

[Supervstech]

You are going to need a solar controller so having the DC to DC charger and MPPT solar controller in one unit will save space and make wiring slightly easier. The DCC50 gives 50 amps from alternator or solar ( up to around 800 watts of panels). If both are active the DCC50 limits at 25amps max each. It's easier to interface to the van as it auto detect alternator charging. The slight disadvantage is that panel volts are limited to 22 or 23 volts.
The 2004 Honda minivan is not a large vehicle, how do you expect to fit all the stuff you discussed Into it?
Alternator capability is not known, perhaps the 50 amp unit is just OK, else the 40 amp DC to DC.

167A is way above 2/0 ampacity...

 

[HarryN]

Here is a sterling unit

Sterling Power battery to battery charging system - DC to DC smart battery charger, marine grade DC powered charger 12v to 24v

DC to DC converter. Charge your auxiliary battery bank with the correct multistage, temperature compensated charging curve by using the input power of any vehicles alternator thru the start battery system automatically while the vehicle is running.

www.sterling-power-usa.com

I think that Victron also makes some.

I use some very heavy duty extended temperature range ones but they are out of the price range of most DIYers.

 

[HarryN]

Honestly, I would try very hard to put 400 - 500 watts of panels on top of the van if you can.

I get it that you will try to park in the shade.

 

[CampLife]

Sorry to confuse everyone, lol. I plan on the first phase of my trip to be in the 2004 Odyssey. In 6 month's or so, I may be in a larger vehicle or possibly on a piece of land. The Chest Freezer from Sunstar and potential ac unit would be for the future if I have the space to allow for it. I'm trying to plan this system to work for me now as well as in the future.

HarryN posted about a 12-24v dc-dc charger that charges up to 35amps. I also saw time2roll suggest not to use over 20a for a 2004 Honda. Now I'm not sure what to do, lol. If I get a 24v, 200ah battery and use the dc-dc charger HarryN linked for the 2004 Honda that could be the ticket. Does anyone know if the Sterling power dc-dc charger will for sure work on my set up?

Now that I'm thinking about it, the air fryer has really changed my diet over the past two years, and I should have it at the top of my priority list. A problem though, I saw that the air fryer is rated at 1800watts not 1500watts (Ninja DT251). I don't need to use it every day, but the ability to use it a few times a week for about 45 min at a time is what I'm hoping to achieve.

If I go 12v, Supervstec mentioned that I would have a massive voltage drop when using high loads like 1800watts.... does that essentially mean I won't be able to use a 120v appliance using up to 1800watts for an hour? Or does it mean that it'll use a lot more power than the 1800watts? I just assume more watts means it'll drain the battery faster, but does the voltage drop compound how quick it drains or is it basically the same thing?

I may end up just putting some permeant panels on the roof. I just came across a roof rack and I am going to try and install it. Hopefully it'll work.

I like the idea of having a mppt charge controller and dc-dc in one unit like mikefitz suggests, however I do have a 30A Adventurer charge controller already attached to the 100w folding solar panel I have. I'm looking at other folding panels and many already come with charge controllers included. Was thinking I can use multiple charge controllers if necessary (since I'll already have them).

 

[timm]

If you are leaning toward a 24v system, you should consider a Kisae DMT2430 dc to dc charger with MPPT. You can configure how much current it will take from the 12v alternator. It will also take up to 50 vdc input from solar and charge up to 30a in a 24v system.

 

[HarryN]

Sorry to confuse everyone, lol. I plan on the first phase of my trip to be in the 2004 Odyssey. In 6 month's or so, I may be in a larger vehicle or possibly on a piece of land. The Chest Freezer from Sunstar and potential ac unit would be for the future if I have the space to allow for it. I'm trying to plan this system to work for me now as well as in the future.

HarryN posted about a 12-24v dc-dc charger that charges up to 35amps. I also saw time2roll suggest not to use over 20a for a 2004 Honda. Now I'm not sure what to do, lol. If I get a 24v, 200ah battery and use the dc-dc charger HarryN linked for the 2004 Honda that could be the ticket. Does anyone know if the Sterling power dc-dc charger will for sure work on my set up?

Now that I'm thinking about it, the air fryer has really changed my diet over the past two years, and I should have it at the top of my priority list. A problem though, I saw that the air fryer is rated at 1800watts not 1500watts (Ninja DT251). I don't need to use it every day, but the ability to use it a few times a week for about 45 min at a time is what I'm hoping to achieve.

If I go 12v, Supervstec mentioned that I would have a massive voltage drop when using high loads like 1800watts.... does that essentially mean I won't be able to use a 120v appliance using up to 1800watts for an hour? Or does it mean that it'll use a lot more power than the 1800watts? I just assume more watts means it'll drain the battery faster, but does the voltage drop compound how quick it drains or is it basically the same thing?

I may end up just putting some permeant panels on the roof. I just came across a roof rack and I am going to try and install it. Hopefully it'll work.

I like the idea of having a mppt charge controller and dc-dc in one unit like mikefitz suggests, however I do have a 30A Adventurer charge controller already attached to the 100w folding solar panel I have. I'm looking at other folding panels and many already come with charge controllers included. Was thinking I can use multiple charge controllers if necessary (since I'll already have them).

He is right - those older alternators were often only 50 - 80 amps total and targeting 10 - 20 amp draw rates are correct for it. Doesn't really have any effect on the 12 vs 24 volt decision though. It just means that you will be even more heavily dependent on solar and 120 vac charging. (which is what I also do )

I think of your honda odessey as being pretty similar to the 2000 dodge minivan that I used to have. Will look for some photos of it with solar panels on it.

 

[HRTKD]

Since this appears to be a from-scratch build and given the loads, I would go with a 24 volt system. In an RV where there are 12 volt loads it makes sense to use a 12 volt system.