To 12v batteries or 24v batteries, this is a question.

[johnxhc]

In the process of designing 1,200w system for my motorhome
I already decided to use 24v battery system.
Should I use 24v batteries in parallel connection or use 12v batteries series & parallel connections to construct the 24v system? does it make any difference at all either way?
All lithium batteries are identical and will be purchase same time.

Any pro and cons?
Please advise.

Thanks!

[pianotuna]

3000 watt are not often maxed out. Mine caught on fire, due to a rather nutty design "feature". Hence moving to a higher voltage is desirable.

Actually my microwave is the largest demand (1591 watts, measured). I wanted to be not having to load balance, so I can run the toaster and the microwave at the same time.

If I were starting over, I'd definitely go 48 volts.

[valhalla360]

pianotuna wrote:

valhalla360 wrote:
What are you trying to achieve going to 24v?

My reason to jump to 48 volts would be for large wattage inverter use. 3000 watts is right on the edge of a cliff for a 12 volt dc input.

24 volts or (even better) 48 volts makes that much easier.

There is an excellent 48 volt dc air conditioner.

24 volts would save on the cost of the charge controller as many of them can do 24 volts--so they have twice the capacity at 24 than 12 volts.

3000w inverters are commonly run off 12v. I know lots of people who do it...you just need to keep it close to the battery bank. And really if you are going DC on the air/con, not sure why you would need one that big. The next biggest item would typically be a microwave and even the high powered versions are only around 1500w.

Of course I was asking the OP why he was looking at a higher DC voltage. Not many try to run air/con off battery.

[dedmiston]

Hi Johnxhc - I'm curious. Why do you need 24v?

[pianotuna]

DrewE,

Do you have a link to the device?

[DrewE]

johnxhc wrote:
Interesting, so the battery balancer is better than DC-2-DC converter?

"Better" is hard to say for every situation. Internally a battery balancer is just a special arrangement of a DC-DC converter, with the output voltage set to always be one-half the input voltage (and so varying with the input voltage).

The advantages I know of for an RV with a 24V system over a straight 24V to 12V DC-DC converter are that the battery balancer only needs to supply half the 12V load on average, as the other half comes directly from that half of the battery bank. It also allows for transient loads greater than its rating since the instantaneous extra power can come from the battery, and then it gets rebalanced as time goes on. With some models, at least, you can also get some charging of the full 24V battery bank using a 12V charger.

The straight DC-DC converter has the advantages of a constant output voltage regardless of battery voltage, and in some cases galvanic isolation between the input and the output (though that's not typical). For a typical RV electrical system these are relatively unimportant, but in other applications they can be essential.

[pianotuna]

valhalla360 wrote:
What are you trying to achieve going to 24v?

My reason to jump to 48 volts would be for large wattage inverter use. 3000 watts is right on the edge of a cliff for a 12 volt dc input.

24 volts or (even better) 48 volts makes that much easier.

There is an excellent 48 volt dc air conditioner.

24 volts would save on the cost of the charge controller as many of them can do 24 volts--so they have twice the capacity at 24 than 12 volts.

[valhalla360]

What are you trying to achieve going to 24v?

It's relatively common on large boats as they often run anchor winches or bow thrusters that have cabling runs of 30-50ft from the battery bank to device and may draw 100-150amps...that makes voltage drop a big deal and requires massive cables. Double the voltage and the amperage drops in half allowing for much smaller cables.

What DC items are you running that require high amperage? Inverters are normally placed close to the battery bank to address the voltage drop concern with short cable runs. Then the 120v AC is fine with longer distances. Most other DC items simply don't draw a lot of amps, so voltage drop isn't a big deal.

[time2roll]

johnxhc wrote:
Should I use 24v batteries in parallel connection or use 12v batteries series & parallel connections to construct the 24v system? does it make any difference at all either way?
All lithium batteries are identical and will be purchase same time.

Lithium? Just get a 24volt battery already. This will allow the BMS to better keep all in balance. Stack in parallel to meet the peak amp requirements of your inverter.

Something like this: (shop around)

https://nexgenbattery.com/shop/batteries/24v-100ah/

[pianotuna]

MEXICOWANDERER wrote:
You found 12-cell batteries?

This appears to be 24 volts:

https://www.lithiumion-batteries.com/products/24-volt-lithium-batteries/24v-100ah-lithium-ion-batter...

The company below makes packs in many sizes.

https://lithiumionlifepo4battery.com/productinfo/503932.html

They make a 12, 24, 36, 48, 72, 80, and 120 volt.

They have a cold weather case which includes heaters allowing -35 C operation. Good enough for most folks--but....alas not for me.

I posted a "real life" link for SiO2 jars in a thread I started.

[pianotuna]

It would be just as easy to jump to 48 volts.

Consider SiO2 batteries.

[johnxhc]

No, need to get 24v to 12v converter...

Lwiddis wrote:
You have found 24 volt RV appliances? Costly?

[johnxhc]

No sure, I am looking at battle born...

MEXICOWANDERER wrote:
You found 12-cell batteries?

[johnxhc]

Interesting, so the battery balancer is better than DC-2-DC converter?

DrewE wrote:
Internally, a 24V battery is exactly the same thing as two 12V batteries in series.

If you're using a battery balancer to extract 12V from your 24V batteries, you need access to the 12V midpoint voltage of the two halves of the battery bank. If you aren't planning on using one of these, and haven't considered it, I would suggest taking a look; they're efficient and effective and get around some of the limitations of other ways of getting 12V power from the 24V supply. It is a moot point, of course, if you don't have any 12V stuff to power.

[KD4UPL]

The fewer sets of batteries in parallel the better. Do it which ever way let's you achieve your desired capacity with the fewest parallel connections.
It is good so see the lithium batter manufacturers making 24v and 48v batteries. It's a far better voltage for a large solar system.