2nd Alternator - does 24v charge faster? Winter Weather / Summer AC

[Elator]

Hi All - read some very useful threads on the technical sides of a secondary alternator, but struggling to find a conclusive answer on their benefits to certain use cases of a VS30 i'm designing to live full time in, so hoping someone can help.

In simplest form, does a 24v aux alternator recharge Lithium batteries faster than a 12v alternator? (assuming all other parts are designed to achieve that).

If useful, my two most-related use cases are:

1. Winter - Backcountry snowboard/ski - so offgrid, often snowing for days - but driving to a new spot most days. 2-3 meals per day inside on induction hob, plus more demand on general electrical use as in van more hours/shorter days.
2. Summer - Kitesurfing - mostly offgrid, typically weeks at each spot, driving to explore and pick up groceries. A/C (12 or 24v) used to cool van in the evening, then Eco mode through the night and mornings whilst working.

Obviously, summer will be helped a little by the solar on the roof, but I hope a second alternator can be used as a significant charging source. So with limited driving, i am really keen to understand if a 24v alternator will recharge the house batteries in a shorter drive than a 12v alternator?

If 24v had this benefit, then i was thinking to use Victron MPII 24/3000/? and focus on 24v parts throughout the build, and use a series of 200AH batteries - starting with 400AH with cab space to grow to 800AH if/when I realise its needed.

Lastly, am I right in my understanding that the batteries in a series (such as 4 x 200AH 24v) also help with this benefit by effectively allowing more AH usage to be restored from a more powerful alternator, than a 12v system (which would be parallel)?

Thanks for any feedback on this in advance!

Doug

 

[kcshoots]

An alternator's speed to recharge is solely dependent upon it's energy output--maximum average energy, or watts. Since alternators are "sized" by amperage, it's simply voltage times amperage to determine maximum power output, but power output will be less and vary with alternator speed which is driven by engine speed, which is what determines energy produced over the charging time. If one alternator is rated for 300 amps at 12 volts, and another at 150 amps at 24 volts, then they should charge the battery bank at the same rate. However, the higher voltage alternator and system will likely have lower losses to component and wire resistance, as well as most likely slightly higher efficiency of the power electronics such as a DC to DC charger. So, a higher voltage alternator should charge at a slightly higher rate, but will likely not be a material difference. The main benefit of a higher voltage alternator is smaller wire size--roughly half the size--and these are big wires from the alternator to your battery bank, so you will save on reduced weight and cost of the wires as well as well reduced wire resistance and easier running of the wires since they are smaller and bend easier than larger wires, as well as easier crimping, etc.

 

[3Play]

Hi All - read some very useful threads on the technical sides of a secondary alternator, but struggling to find a conclusive answer on their benefits to certain use cases of a VS30 i'm designing to live full time in, so hoping someone can help.

In simplest form, does a 24v aux alternator recharge Lithium batteries faster than a 12v alternator? (assuming all other parts are designed to achieve that).

If useful, my two most-related use cases are:

1. Winter - Backcountry snowboard/ski - so offgrid, often snowing for days - but driving to a new spot most days. 2-3 meals per day inside on induction hob, plus more demand on general electrical use as in van more hours/shorter days.
2. Summer - Kitesurfing - mostly offgrid, typically weeks at each spot, driving to explore and pick up groceries. A/C (12 or 24v) used to cool van in the evening, then Eco mode through the night and mornings whilst working.

Obviously, summer will be helped a little by the solar on the roof, but I hope a second alternator can be used as a significant charging source. So with limited driving, i am really keen to understand if a 24v alternator will recharge the house batteries in a shorter drive than a 12v alternator?

If 24v had this benefit, then i was thinking to use Victron MPII 24/3000/? and focus on 24v parts throughout the build, and use a series of 200AH batteries - starting with 400AH with cab space to grow to 800AH if/when I realise its needed.

Lastly, am I right in my understanding that the batteries in a series (such as 4 x 200AH 24v) also help with this benefit by effectively allowing more AH usage to be restored from a more powerful alternator, than a 12v system (which would be parallel)?

Thanks for any feedback on this in advance!

Doug

Charge speed is relative and not really the issue.
A higher voltage DC system is more efficient, uses smaller wire and has many good non-RV products available.
I went with 48v on the DC side. I might have gone 24v if I was using a second alt. because there is a good 24v alt available for under $150.
With 24v or 48v you can use rack mount lifepo which are much cheaper than rv and marine products.
I ended up using a growat 3000w 24v inverter charger.
I have to deal with upconverting the 12v side to 50v which I am still experimenting with.
I went heavy on the solar.

 

[blutow]

Higher voltage components (alternator, inverter, etc.) are typically a bit more efficient, but wire size and potential voltage drop (from undersized wiring) is the biggest think in my opinion. Wire size between components can be dramatically reduced with higher voltage and the alternator is often a long run with high current. The problem with 24v or 48v with RV's is that 12V is still king when it comes to fridges, heaters, fans, etc., so you significantly limit your choices if everything needs to be 24v (or even more limited at 48v). A common approach is to go higher voltage with the battery bank and big current items (inverter, alternator, etc.) and then use a converter to handle the 12v loads. That approach requires you to design your system for 3 voltages, so it's really a trade off between the advantages of higher voltage and the added complexity of managing 3 voltages. I really wanted to go 24v with my system, but I ended up going 12v and it's been really good (my wire runs are very short). I don't have a 2nd alternator though and I just charge from the stock MB unit through a couple DCDC chargers (only ~75a drawn and a little current drop doesn't matter since the DCDC chargers manage the charge voltage).

 

[Midwestdrifter]

As mentioned, alternator power output (watts) is mostly dependent on alternator physical size. The major limitation being cooling. Ohms law and all.

If installing a second alternator, choose one that matches the nominal voltage of the battery bank its charging.

Choosing a system voltage is an involved decision, and requires taking numerous needs and trade offs into account. You will likely encounter evangelists for varying voltages. While their choice works well for their usage/system, don't take blanket recommendations at face value. You need to assess your personal requirements.

A few pertinent questions.

What is your peak power consumption (watts)? For systems that will be supplying less than 3kw peak, 12V system can work fine, and for systems under 2kw, I suggest 12V unless there is a very good reason to go higher. Such as an existing 24V system, reusing hardware etc. For systems over 3kw peak, 24V or greater systems make sense from the wiring and component selection standpoint.

What is your expected battery capacity (kwhr)? For systems over about 8kwhr, 24V+ tends to be easier, as there are fewer parallel battery strings. For systems under 3kwhr, 24V or 12V systems are often easier to source components for.

How much solar are you planning on? For folks who need less than 400W of solar panels, it can be hard to find panels with high enough voltage in that range, for systems over 24V nominal.

What are your loads, and are they available in the voltage you are selecting? Some components can be found in 12 or 24V variants, but other than inverters, and some chargers, very little is available in 48V. In many cases a 12V converter is needed to supply lower voltage items. Some folks opt for 120VAC components across the board. This can work well, but is typically best suited for large systems. Residential 120V items such as lights and switches are not designed with RV/Vans in mind, and are often large, and/or difficult to install.


Blanket claims that one voltage is more efficient than another aren't accurate. There isn't anything unique about increasing voltage that inherently increases efficiency. In some cases a specific appliance/form factor is more efficient at a higher voltage design. For example 12V inverters have essentially the same efficiency as 48V units (with a percent or two usually). There are differences in the size of wiring required for various voltages, and higher voltage inverters tend to be a bit lighter. Some higher voltage alternators produce more output (~5-8%%) due to lower parasitic losses. This may be the case between 12 and 24V I believe. Going from 12V to 36V would be larger gains, but it depends on how the unit is built.

 

[Elator]

An alternator's speed to recharge is solely dependent upon it's energy output--maximum average energy, or watts. Since alternators are "sized" by amperage, it's simply voltage times amperage to determine maximum power output, but power output will be less and vary with alternator speed which is driven by engine speed, which is what determines energy produced over the charging time. If one alternator is rated for 300 amps at 12 volts, and another at 150 amps at 24 volts, then they should charge the battery bank at the same rate. However, the higher voltage alternator and system will likely have lower losses to component and wire resistance, as well as most likely slightly higher efficiency of the power electronics such as a DC to DC charger. So, a higher voltage alternator should charge at a slightly higher rate, but will likely not be a material difference. The main benefit of a higher voltage alternator is smaller wire size--roughly half the size--and these are big wires from the alternator to your battery bank, so you will save on reduced weight and cost of the wires as well as well reduced wire resistance and easier running of the wires since they are smaller and bend easier than larger wires, as well as easier crimping, etc.

Thanks for your explanation Kcshoots - I'll focus on the watts from now on.

Since my post I also found this explanation on 48v alternator output over 12/24v



which showed alternator output in watts - the Nations 12v and 24v being pretty similar, but a significant jump in watts output in 48v. New to how this can work, i was surprised the batteries were still 12v in their 48v system, but kind of understand how that works now.

 

[hertfordnc]

there is a good 24v alt available for under $150.

With 24v or 48v you can use rack mount lifepo which are much cheaper than rv and marine products.

I'm jumping into this thread. If a 24V alternator is out there for $150, and if rack batteries are cheaper, this sound like something I want to think about.

Would you just step down to 12V for the van's systems?

 

[Kajtek1]

...
Blanket claims that one voltage is more efficient than another aren't accurate. There isn't anything unique about increasing voltage that inherently increases efficiency. In some cases a specific appliance/form factor is more efficient at a higher voltage design. For example 12V inverters have essentially the same efficiency as 48V units (with a percent or two usually). There are differences in the size of wiring required for various voltages, and higher voltage inverters tend to be a bit lighter. Some higher voltage alternators produce more output (~5-8%%) due to lower parasitic losses. This may be the case between 12 and 24V I believe. Going from 12V to 36V would be larger gains, but it depends on how the unit is built.

x2
The only advantage of 24V system are thinner wires, what can save you very few dollars on wiring and a pound or 2 of weight, but with much higher cost of components and much higher repair bills. You are still using 2V battery cells and if I remember it right, solar panels operate on 3 v.
Semitruck historically run on 24V as they run heavy wires, however I was surprised that latest generation of semis run on 12V as well. I guess somebody in semi industry share our logics.
Start with checking how much 24V light bulb cost.

 

[Midwestdrifter]

x2
The only advantage of 24V system are thinner wires, what can save you very few dollars on wiring and a pound or 2 of weight, but with much higher cost of components and much higher repair bills. You are still using 2V battery cells and if I remember it right, solar panels operate on 3 v.
Semitruck historically run on 24V as they run heavy wires, however I was surprised that latest generation of semis run on 12V as well. I guess somebody in semi industry share our logics.

The cost difference may have been the case previously, but depending on your power needs, 24V and 48V base components (batteries, inverters, and chargers) are similarly priced to 12V, and in some cases cheaper for the same capacity. As far as accessories, such as fridges, lights, etc, many simple aren't available in 24V or higher voltages, so a DC-DC converter is needed.

For lower power users, a modest 12V system is simple to install and service, and can use the vehicles alternator for power/charging.

24-48V systems aren't inherently harder to service, and the safety factor is pretty similar. It can be harder to find replacement components in a pinch, at least for 48V, as many fuses and breakers top out at 36V, but thats a bit of a different discussion.

When folks start needing 3kw or greater power output, and we break into the 8kwhr or greater sized battery packs, higher voltage systems come into their own. These systems were not common outside of high end class As 10 years ago, but with LFP becoming so cheap, an increasing number of builds are installing large battery packs to allow nearly "shore power" like performance when boondocking. Its not cheap though!

 

[Elator]

Charge speed is relative and not really the issue.
A higher voltage DC system is more efficient, uses smaller wire and has many good non-RV products available.
I went with 48v on the DC side. I might have gone 24v if I was using a second alt. because there is a good 24v alt available for under $150.
With 24v or 48v you can use rack mount lifepo which are much cheaper than rv and marine products.
I ended up using a growat 3000w 24v inverter charger.
I have to deal with upconverting the 12v side to 50v which I am still experimenting with.
I went heavy on the solar.

thanks for the tip on rack mount - i'll take a look at that. It sounds like you are using a mix of 24v and 48v with 12v alternator - i have a lot more to learn as didn't realise that was possible.

 

[Elator]

Thanks for all the feedback guys. Although cost is a concern, i am more concerned by future proofing as the build will be a heavy investment in a new van and the fitout - so more concerned I make sure the cabling and base system parts have the ability to expand or a simple upgrade path.

After your comments and watching the video i linked above that refers to the 48v alternator delivering over 20% more watts than the 24v version, i realise i need to research much more.

The main heavy components in the build (in my head) will each be capable of 12 or 24v as far as i know:

12/24 Fridge Vitrifrigo drawer
12/24 Freezer Vitrifrigo Topload
12/24 A/C (Prob dometic as based in Europe, but would prefer cruise n comfort)
12/24 Single Induction Hob

I'll run a bit more research on what you have each mentioned. Thanks again for each of your help

 

[autostaretx]

@Graphite Dave takes a slightly different approach: he uses inverters to create 120 vac, and then runs that to everything.
If he needs lower DC voltages, he installs converters (AC-to-DC devices) sized to match the needs.
Yes, there are losses involved in the up/downs, but he accepts those for the convenience of his approach.

(added: as per his note below, he's a 12v DC bus ... i'd forgotten that bit)

--dick

 

[Graphite Dave]

@Graphite Dave takes a slightly different approach: he uses inverters to create 120 vac, and then runs that to everything.
If he needs lower DC voltages, he installs converters (AC-to-DC devices) sized to match the needs.
Yes, there are losses involved in the up/downs, but he accepts those for the convenience of his approach.

--dick

I have both 12-volt DC and 120-volt AC.

House battery is 12 volt. The 12 volts powers a Blue Sea DC fuse block and the 1000 watt house inverter/charger/transfer switch. The fused items running 12 volt DC are the refrigerator, lights, roof fan, shower water pump, water pump, lights and the 12v receptacles. The house inverter provides power to the single string of 120 volt duplex plug outlets. All 120 volt AC loads use cords to plug into the duplex outlets. AC loads are the shower water heater, ebike charger, air compressor, vacuum, "600" watt microwave (950 watts), trickle charger to charge vehicle battery and device chargers, tools etc. The only 120 volt AC load that is always plugged in is the microwave oven.

I also have a 1000 watt vehicle powered inverter to create 120 volt AC power with the engine running. The output from the vehicle powered inverter is used to power the house inverter's charger or the 750 watt electric air heater or the 750 watt shower water heater. Shower water heater can be powered either by the house inverter or the vehicle powered inverter. If house battery SOC is not low, then I use the house inverter and utilize the excess solar power. If house battery SOC is low or in bad weather then I would start engine and heat the water with power from the vehicle powered inverter.

Electrical diagram:

68b108_dc953400e3a74b568b7d32938ca1fc7e.pdf (ortontransit.info)

 

[HarryN]

A simple example - suppose that you have a 170 wb sprinter, battery pack in the very back on the drivers side. The aux alternator position is in the very front of the van - passenger side.

The round trip wire distance approaches ~ 40 ft.

Moving 3 kW @ 12 volts is ~ 300 amps, and no matter the wire size, there will be voltage drop so electrically it starts to be interesting to use 4/0 cable, which is not all that easy to route.

The same project at a higher voltage results in quite a bit thinner wire - not super thin, but wire that is much easier to route.

24 volt, mobile / vibration resistant systems are pretty easy for anyone to build.

48 volt, mobile rated / vibration resistant systems take more effort. I build them, but most people make the mistake of trying to use components rated or 24 volts like blue sea stuff. BS makes excellent items, but practically none of it has the required ratings.

Server rack oriented items are 48 volt rated, but not mobile rated or rated for the temperatures expected.

It is a significant task and often takes 2 minds working together to pull off.

 

[3Play]

I'm jumping into this thread. If a 24V alternator is out there for $150, and if rack batteries are cheaper, this sound like something I want to think about.

Would you just step down to 12V for the van's systems?

So to clarify, what I advocate is to build a large capacity 120v system so you can specifically stay away from expensive (and not so great) 12v. RV and Marine appliances.
The money saved on buying readily avaiable 120v. consumer appliances will more than pay for the entire power system.
With this being the goal, it is definitely more efficient (Not just electrically) to go with a higher voltage DC system.
Just the wiring alone is much smaller, easier and has more options.
I went with 48v. The 800w of solar is series wired for an average 76v-ish output. I only had to use 12 gauge wire.
I cut up high quality extension cords for this.
I do have the hassle of using voltage upconverters for the alternator charging side, but I wasn't looking for a high current
alternator charge. I used a DB Electric 220A alt. which was only about $130 and puts out over 120A at idle.
I will probably only get about 800w out of the 2 cheap upconverters. I'm working on that now.
The goal there is to set them at a voltage that takes over when the solar drops to a certain point off peak.
DB electric has a 24v 100A alt for a "Scania" big rig, it is about $130 and would be ideal for a second alt.
I was originally was going to use it as the main alt. and regulate for the 12v side, but high capacity DC to Dc regulators
are extremely expensive.
The rack mount batts come in 24v and 48v. Lifepo is a slight mismatch to 12v systems so the higher V. plays better with charging profiles.
The 3000w Growatt inverter charger was about $650.00 It has been good, but I may go back to a separate charge controller, there are some advantages
and they are not expensive.
To give you an idea of savings, you can get an 8kbtu 15 SEER inverter based window AC for $375.00 Many of the 12v AC systems are over $3k
I only spent about $3500 for my entire electrical system, panels, inverter 5100WH batt, second 12v. inverter, welding cable, solenoid switches,
wiring crimpers, enclosure under the van....

I have built out blue water sail boats, buses, 3 other vans and 2 of my own and have been living completely off grid for 12 years.
I wouldn't even consider 12v appliances of any kind, even for a small build out. My friend has been experimenting with small builds
using solar generators (Jackary, Bluetti, etc.) and has proven you can run a microwave, small griddles and other appliances from a
very small system that is alternator based with minimal house battery.
That is another good approach, especially if you are actually road tripping and not staying in one place
for long periods.
The options keep getting better, Bluetti just came out with a 5KW system with separate batteries...

 

[Kajtek1]

I wouldn't even consider 12v appliances of any kind, even for a small build out....

What exactly you mean by that?
Even in my house I do have 100's of 12V LED lights, who operate with power supplies, just like computers.
In RV all lights, fans, water pump, heater and refrigerator controls operate on 12V
Good luck replacing them all with 24V.

 

[3Play]

What exactly you mean by that?
Even in my house I do have 100's of 12V LED lights, who operate with power supplies, just like computers.
In RV all lights, fans, water pump, heater and refrigerator controls operate on 12V
Good luck replacing them all with 24V.

re-read the post.....

 

[Kajtek1]

re-read the post.....

I get it, You are saying to install 120V alternator coupled with 120V solar system

 

[3Play]

Sort of, if it were me, and I wanted a second alt. it would be 24 or 48v. 24v is much cheaper and has no issues with
batteries, IE lots of available alt power equipment.
The focus is on the 120v AC system for all the appliances...

 

[HarryN]

The 48 volt alternator project that I did in partnership with another member late last year was a lot more then $200 and also much more powerful.

There are marine alternators on the market for small boats that potentially could be adapted, but this takes some fairly clever work on the drive belt side.

We didn't want to do that part so we used a setup specifically designed for the NC3 and had a pro do the mechanical installation of the bracket and alternator. IIRC, the alternator, related parts, controller and mechanical installation alone were in the ~$5K range.

It was some real effort but it can run the roof top top air conditioner, cook with an induction cook top, and still charge at > 3 kW in high idle, so pretty successful IMHO.

The electronics layout was an extension / expansion of what I already was offering as an over the wheel well system product offering so it took some effort, but was not massive leap. We were able to use the same general components and concepts.

The total project ended up in the $25K range, including batteries, inverters, chargers, safety related items, 8020 work, high end 120 vac air conditioner, etc. The van was used this summer for some hot weather trips and it all seemed to work.

The 12 volt items are run from a good quality DC - DC converter.