12 vs. 24v systems on motorhome

[neoflyer]

Just watched Will Prowse’ comparison of 12 vs. 24v systems. He had me convinced to go 24v except for a couple of things. He is opposed to charging from the engine alternator. He said there were other options but didn’t say what they are. I think using the alternator guarantees fully charged batteries at the end of a day’s drive. I plan a lithium battery system so will need a DC DC charger if I connect the alternator. Does anyone know what he is thinking for other options than just counting on the solar panels to keep the batteries charged while driving?

He said connecting more than three batteries in parallel leads to problems. if one battery starts to underperform it could lead to reduced battery life. Can anyone enlighten me on this? I originally planned four 105aH Lion Safari UT1300 LiFePo batteries in parallel but now wonder if I should go series and parallel, 2s2p, for 24 volts/210a.

As an aside, the new batteries have 1/4” lugs whereas the cables for the batteries I am removing have 5/16” lug holes. should I replace the lugs Or use them even though they are a little “sloppy”? Also, do all of the cables between the posts, be they series or parallel connected, need to be the same length for all of the baTerries?

2005 Holiday Rambler 40 PLQ Diesel Pusher

 

[Rocketman]

For charging a 24v battery from a 12v alternator, you need a DC-DC Charger. I know of 2 vendors (There are probably more). Victron and Sterling. Both have setting to only run the charger while the motor is running. They also limit the power from the alternator (probably a good thing), and have lithium settings.

Each paralleled battery need to have the exact same length of wire. A longer wire creates more resistance, so that battery will not charge as fast (and get out of balance and work at a different rate).

 

[Dzl]

Just watched Will Prowse’ comparison of 12 vs. 24v systems. He had me convinced to go 24v except for a couple of things. He is opposed to charging from the engine alternator. He said there were other options but didn’t say what they are. I think using the alternator guarantees fully charged batteries at the end of a day’s drive. I plan a lithium battery system so will need a DC DC charger if I connect the alternator. Does anyone know what he is thinking for other options than just counting on the solar panels to keep the batteries charged while driving?

Can you link to the video please.

He said connecting more than three batteries in parallel leads to problems. if one battery starts to underperform it could lead to reduced battery life. Can anyone enlighten me on this? I originally planned four 105aH Lion Safari UT1300 LiFePo batteries in parallel but now wonder if I should go series and parallel, 2s2p, for 24 volts/210a.

While the above may (or may not) be good advice, manufacture recommendations (if it is a reputable manufacturer) would in my eyes supersede generalized advice (particularly if there is a decent manufacturer warranty), so you may want to double check what your battery documentation has to say about series and parallel connections. It probably gives some guidance.

 

[ianganderton]

I think Wills video is probably relating to
lead acid with no BMS

Lifepo4 with a BMS makes the battery story different

I’m going to be using a 2000W inverter with 200AH Lifepo4 battery, 480W solar and I think this is right on the cusp of the 12V vs 24v

I’m going 12V. My fridge is 12V, the diesel heater is 12V, all my lights and water pump etc are lots easier at 12v. Saves me running a 24 to 12 converter

The feed to the inverter will need to be able to deal with +166A but the cable run will be short so it will be cheap and effective enough. A bigger inverter would get challenging and that’s where 24V starts to become inevitable.

 

[HaldorEE]

I think Wills video is probably relating to
lead acid with no BMS

Lifepo4 with a BMS makes the battery story different

I’m going to be using a 2000W inverter with 200AH Lifepo4 battery, 480W solar and I think this is right on the cusp of the 12V vs 24v

I’m going 12V. My fridge is 12V, the diesel heater is 12V, all my lights and water pump etc are lots easier at 12v. Saves me running a 24 to 12 converter

The feed to the inverter will need to be able to deal with +166A but the cable run will be short so it will be cheap and effective enough. A bigger inverter would get challenging and that’s where 24V starts to become inevitable.

Big difference is also in solar charging. A 24V system will make use of double the solar power for a given amp rating. That can save you quite a bit right there if you have more than 300W of solar panels.

 

[ianganderton]

Big difference is also in solar charging. A 24V system will make use of double the solar power for a given amp rating. That can save you quite a bit right there if you have more than 300W of solar panels.

Hmmm I’m not sure there is much difference there. Watts are watts and decent MPPT SCCs can deal with a wide range of input voltages that as long as they are at least a bit higher than the battery voltage and high enough to be efficient it’s all good

 

[Dzl]

Hmmm I’m not sure there is much difference there. Watts are watts and decent MPPT SCCs can deal with a wide range of input voltages that as long as they are at least a bit higher than the battery voltage and high enough to be efficient it’s all good

The point he is making is that you get double the usable output (Watts) for your money or for a given amp rating with a 24v system (relative to 12v)

A 40A charger controller =
40a x 12.8v = 512W
40a x 25.6v = 1024W
40a x 51.2v = 2048W

 

[HaldorEE]

Hmmm I’m not sure there is much difference there. Watts are watts and decent MPPT SCCs can deal with a wide range of input voltages that as long as they are at least a bit higher than the battery voltage and high enough to be efficient it’s all good

An MPPT SCC will deliver the rated current into the battery voltage.

A 10A SCC into a 12V battery (charge voltage ~14.4V) = 145W
Same 10A SCC into a 24 battery (charge voltage ~29V) = 290W.

The data sheet for the Victron SCC states this clearly.

https://www.victronenergy.com/upload/documents/Datasheet-SmartSolar-charge-controller-MPPT-75-10,-75-15,-100-15,-100-20,-100-20_48V-EN.pdf

 

[neoflyer]

Can you link to the video please.

 

[neoflyer]

I think Wills video is probably relating to
lead acid with no BMS

Lifepo4 with a BMS makes the battery story different

I’m going to be using a 2000W inverter with 200AH Lifepo4 battery, 480W solar and I think this is right on the cusp of the 12V vs 24v

I’m going 12V. My fridge is 12V, the diesel heater is 12V, all my lights and water pump etc are lots easier at 12v. Saves me running a 24 to 12 converter

The feed to the inverter will need to be able to deal with +166A but the cable run will be short so it will be cheap and effective enough. A bigger inverter would get challenging and that’s where 24V starts to become inevitable.

That sounds good for your size system. Now mine, first thoughts. 2000w inverter, 4 105a lithium batteries (might go to 5 or 6 batteries), 840 watts of solar monochrystilline panels, (could go 1260 or 1680 watts in future but I think 840 will probably do). Does this change the 12 vs. 24v equation especially considering adding DC DC charger or possibly more complicated (is it?) charger/converter for 24v?

 

[ianganderton]

That sounds good for your size system. Now mine, first thoughts. 2000w inverter, 4 105a lithium batteries (might go to 5 or 6 batteries), 840 watts of solar monochrystilline panels, (could go 1260 or 1680 watts in future but I think 840 will probably do). Does this change the 12 vs. 24v equation especially considering adding DC DC charger or possibly more complicated (is it?) charger/converter for 24v?

Probably

As pointed out above I misunderstood a good point made by @HaldorEE

Once you get above 500W of solar there are advantages in the solar charge controller by upping the battery voltage

 

[neoflyer]

Big difference is also in solar charging. A 24V system will make use of double the solar power for a given amp rating. That can save you quite a bit right there if you have more than 300W of solar panels.

Is that amp output rating of the solar charge controller, not the amp output of the panels?

 

[ianganderton]

Is that amp output rating of the solar charge controller, not the amp output of the panels?

Correct though in effect they are the same wattage.

The amp input can be a variable amount due to the way solar arrays can be configured very differently resulting in many different voltage and amp age combinations

The output is in specific voltages to charge the battery so ampage is based on that and wattage

 

[HaldorEE]

Is that amp output rating of the solar charge controller, not the amp output of the panels?

Exactly.

An MPPT controller takes watts in at the panel voltage then steps it down to the battery voltage. While doing this the current is increased (watts in minus losses = watts out).

The limiting factor is the current rating of the SCC output. 40A at 24V is double the power of 40A at 12V.

 

[Dzl]

Now mine, first thoughts. 2000w inverter, 4 105a lithium batteries (might go to 5 or 6 batteries), 840 watts of solar monochrystilline panels, (could go 1260 or 1680 watts in future but I think 840 will probably do). Does this change the 12 vs. 24v equation especially considering adding DC DC charger or possibly more complicated (is it?) charger/converter for 24v?

1. 2000W Inverter is right around the point where I think 24v starts becoming a more serious consideration, but could go either. 2000W (w/ inefficiency) @ 12v = 200A, @ 24v = 100A. If you factor in surge, which you should with a heavy transformer based Low frequency inverter, those numbers would double, if you have a high frequency inverter you probably can just stick to the continuous rating.
2. 840W @ 12.8V = a 66A, @ 25.6V = 33A, A 30 or 40 amp controller is much generally going to be much cheaper than a 60 or 70 amp controller. Price out your different options to see how much of a difference it would make.
3. A DC-DC charger will not be any more or less complicated if its 12v to 24v than 12v to 12v. With Lifepo4 you need one either way if you will charge via the alternator, so this doesn't add any complexity.
4. A DC-DC converter (to run 12v loads from your 24v system) will add a bit of additional complexity and cost to your system since it is a component you wouldn't need with 12v system. The more high power 12v loads you have the more of a factor this is. In the realm of things its not a big deal (in terms of cost or complexity), particularly if you select 12/24v loads or 24v native loads as much as possible. There is a small efficiency penalty here as well, which partially offsets the efficiency gains of moving to a higher voltage.

 

[HaldorEE]

1. 2000W Inverter is right around the point where I think 24v starts becoming a more serious consideration, but could go either. 2000W (w/ inefficiency) @ 12v = 200A, @ 24v = 100A. If you factor in surge, which you should with a heavy transformer based Low frequency inverter, those numbers would double, if you have a high frequency inverter you probably can just stick to the continuous rating.
2. 840W @ 12.8V = a 66A, @ 25.6V = 33A, A 30 or 40 amp controller is much generally going to be much cheaper than a 60 or 70 amp controller. Price out your different options to see how much of a difference it would make.
3. A DC-DC charger will not be any more or less complicated if its 12v to 24v than 12v to 12v. With Lifepo4 you need one either way if you will charge via the alternator, so this doesn't add any complexity.
4. A DC-DC converter (to run 12v loads from your 24v system) will add a bit of additional complexity and cost to your system since it is a component you wouldn't need with 12v system. The more high power 12v loads you have the more of a factor this is. In the realm of things its not a big deal (in terms of cost or complexity), particularly if you select 12/24v loads or 24v native loads as much as possible. There is a small efficiency penalty here as well, which partially offsets the efficiency gains of moving to a higher voltage.

Very well said. One thing I like is the smaller (20 A) or less Victron SCC have a load output with a programmable low voltage disconnect. I am powering all of my 24V loads from that on my system.

A StartSolar 100/20 can handle 580W of solar power with a 24V battery, but only 290W at 12V. That is the largest SCC that Victron makes with a load output. I could have just squeeked in with that at 12V since I have a 300W panel, but with my current system I can add a second panel without changing anything. It may turn out I value having more solar input over having a luggage rack on my roof. If so I can just add a second panel in series.

 

[chrisski]

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.

SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank

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.

 

[HaldorEE]

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.

SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank

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.

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.

Thanks.

 

[Airtime]

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.

Thanks.

A few months ago I was looking at this and came across the same article. I was looking at undermounting four Battleborn batteries, two on each side of the van. Using the cable lengths and sizes I would need, I saw variation of up to 6A charging between batteries on a 100A overall charge (i.e. 25A per cell if perfectly balanced), depending on the topology. That would have quickly and seriously imbalanced the four batteries.

 

[Airtime]

A couple other points on the 24V topic. I started out designing a 12V system and ended up with 24V. Main reasons:

1) It lets me use the full surge capability of a Victron 3000VA Multiplus inverter. For 24V, Victron specifies a 300A fuse, for 12V a 400A. Ideally there would be a 2X difference due to the 2X difference in current between 12V and 24V. But how many 600A fuses have you seen? And what wire size would you use for it, 2x 4/0 AWG? As @Dzl said earlier, above about 2000W inverter load it really favors 24V.

2) Wire sizing can be 4x smaller (6 AWG sizes) for same power, assuming the wiring selection is voltage drop-limited rather than ampacity limited. About 2x smaller if ampacity limited. Copper is bulky and expensive and large guage wire adds up.

On the flip side, if you go 24V you will need to add a 24V to 12V DC-DC converter for 12V loads. 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.