24v system: series vs. parallel battery options

[Jennifer-3456]

I'm a real beginner at this, so this is surely a kind of "stupid" question... I'm considering buying a 24v all in one inverter (probably Growatt, maybe MPP Solar) for my camper van. I want the equivalent of 400 ah 12v batteries (or 4800 watt hours). I could get A) two 200 ah 12v batteries and connect them in series, or B) one 200 ah 24v battery.

If I go with A, how do I expand the system? Any additional batteries in the series would increase the voltage past 24v. Could I make another set of two 200 ah 12v batteries connected in series and then connect the two sets in parallel? And is there any way to add another 100 ah at 12v (rather than 200 ah)?

If I go with B and want to expand later on, do I have to add another 200 ah 24v battery? Could I add only another 100 ah 24v battery and connect it in parallel (or does it have to be another 200 ah 24v battery)?

I don't want to get four 100 ah 12v batteries because I simply don't want to manage four batteries and take up that much space -- unless there's a really good reason to do it that way.

Thanks for any help you can offer!

 

[Al D]

Option A
You have to make sure you are able to series connect 12V batteries. If you are allowed by the manufacturer, then yes, you can place two 12V 200Ah batteries in series to make a nominal 24V 200Ah battery.

To expand that, you would need to add another 24V battery in parallel. It is advisable to match the capacity of the original battery (in your case, 200Ah). Connecting different capacity batteries in parallel can cause undesirable effects over the lifespan of your bank.

Once you have a 24V battery, do not connect a 12V battery in parallel to it.

Option B
This is preferable if you know you're wanting a 24V system. However, a 24V battery is ~2x heavier than a single 12V battery. You'll have to decide if moving it is feasible for your circumstance. With two 12V batteries, you can disconnect them from each other before moving them. Not so with a pre-assembled 24V battery.

To expand the 24V battery, you would add another 24V battery. Again, matching the capacity (200Ah) is advisable.

---
I have a 24V 280Ah Lifepo4 system in my converted campervan. I built it from cells which allows me to disassemble and move them much easier. I'm sure it cost less than the pre-assembled ones. And I get the experience. But that's me. You do you.

 

[Northernchateau]

Let me ask something before i comment.
What batteries were you thinking abut, FLA, AGM or LifePO4?

 

[FilterGuy]

If I go with A, how do I expand the system? Any additional batteries in the series would increase the voltage past 24v. Could I make another set of two 200 ah 12v batteries connected in series and then connect the two sets in parallel?

You could do parallel first or serial first

And is there any way to add another 100 ah at 12v (rather than 200 ah)?

I like to keep the capacities balanced, but people do mix and match LiFePO4 batteries. The 'problem' is that the higher capacity set will have lower internal resistance and will tend to wear slightly faster. However, if this is a solar setup, the calendar life is likely to get you long before the cycle life gets you.




Warning. This should not be done with lead-acid batteries.

If I go with B and want to expand later on, do I have to add another 200 ah 24v battery? Could I add only another 100 ah 24v battery and connect it in parallel (or does it have to be another 200 ah 24v battery)?

You could expand with a 100Ah LiFePO4 battery



As I mentioned before, mixing capacity is not ideal, but probably not a big deal.

You did not ask but: One thing a lot of people do not account for is the discharge capability of the pre-built batteries. Most of them have a continuous current limit of 100A. A 24 3000W inverter should have at least 150A of discharge capability from the batteries.

 

[Jennifer-3456]

Option A
You have to make sure you are able to series connect 12V batteries. If you are allowed by the manufacturer, then yes, you can place two 12V 200Ah batteries in series to make a nominal 24V 200Ah battery.

To expand that, you would need to add another 24V battery in parallel. It is advisable to match the capacity of the original battery (in your case, 200Ah). Connecting different capacity batteries in parallel can cause undesirable effects over the lifespan of your bank.

Once you have a 24V battery, do not connect a 12V battery in parallel to it.

Option B
This is preferable if you know you're wanting a 24V system. However, a 24V battery is ~2x heavier than a single 12V battery. You'll have to decide if moving it is feasible for your circumstance. With two 12V batteries, you can disconnect them from each other before moving them. Not so with a pre-assembled 24V battery.

To expand the 24V battery, you would add another 24V battery. Again, matching the capacity (200Ah) is advisable.

---
I have a 24V 280Ah Lifepo4 system in my converted campervan. I built it from cells which allows me to disassemble and move them much easier. I'm sure it cost less than the pre-assembled ones. And I get the experience. But that's me. You do you.

Hi AI D,

Thank you very much for explaining all of that. I have the general idea, but sometimes I get a little confused or think there's something I might be missing. Best wishes!...

 

[Jennifer-3456]

Let me ask something before i comment.
What batteries were you thinking abut, FLA, AGM or LifePO4?

Lithium iron phosphate. Sorry, and thanks for asking!

 

[FilterGuy]

A 24 3000W inverter should have at least 150A of discharge capability from the batteries.

In case you are wondering, Here is how I came up with 150A.

I assumed the inverter is 90% efficient. With this assumption, a 3000W inverter will draw 3000W/.9 = 3333.3W when running at it's max continuous power.

The current draw for the wattage will be the highest when the battery voltage is low (24V) This means the battery needs to provide 3333.3W/24V=138.9A when the inverter is running full out. I rounded that up to 150.

 

[Jennifer-3456]

You could do parallel first or serial first

View attachment 120808

I like to keep the capacities balanced, but people do mix and match LiFePO4 batteries. The 'problem' is that the higher capacity set will have lower internal resistance and will tend to wear slightly faster. However, if this is a solar setup, the calendar life is likely to get you long before the cycle life gets you.

View attachment 120809


Warning. This should not be done with lead-acid batteries.


You could expand with a 100Ah LiFePO4 battery

View attachment 120810

As I mentioned before, mixing capacity is not ideal, but probably not a big deal.

You did not ask but: One thing a lot of people do not account for is the discharge capability of the pre-built batteries. Most of them have a continuous current limit of 100A. A 24 3000W inverter should have at least 150A of discharge capability from the batteries.

Hi FilterGuy,

Thank you so much for your helpful explanations. This is wonderful. I am taking notes and studying it all. I will look carefully at the continuous current limit and discharge capability.

Have a good weekend!

 

[Jennifer-3456]

In case you are wondering, Here is how I came up with 150A.

I assumed the inverter is 90% efficient. With this assumption, a 3000W inverter will draw 3000W/.9 = 3333.3W when running at it's max continuous power.

The current draw for the wattage will be the highest when the battery voltage is low (24V) This means the battery needs to provide 3333.3W/24V=138.9A when the inverter is running full out. I rounded that up to 150.

Awesome! Thank you! I wasn't wondering yet.....but that was my next step. I'll get there soon.

 

[time2roll]

I suggest staying with a 12v battery and 2000 watt inverter as it makes alternator charging easier and will power 12v lighting and accessories directly.
Can't imagine in the simple life of a van that more than 2000 watts is needed.

 

[Jennifer-3456]

I suggest staying with a 12v battery and 2000 watt inverter as it makes alternator charging easier and will power 12v lighting and accessories directly.
Can't imagine in the simple life of a van that more than 2000 watts is needed.

time2roll: Thanks for the feedback.

 

[Rocketman]

One option with”B” is to get two 24v 100Ah batteries and put them in parallel. Then each battery will be half the weight of a 24v 200Ah battery. I believe they are also more common. And (assuming a 100a bms in the battery), you will have 200 amp available for the inverter.

After running a Victron Multiplus 12/3000 full- time for 2 yrs, I wish I could go up to 24v. But that would be difficult in my MotorHome.

There are lots of 24v stuff and a 12v to 24v charger from the alternator.

Good Luck on your project!

 

[Jennifer-3456]

One option with”B” is to get two 24v 100Ah batteries and put them in parallel. Then each battery will be half the weight of a 24v 200Ah battery. I believe they are also more common. And (assuming a 100a bms in the battery), you will have 200 amp available for the inverter.

After running a Victron Multiplus 12/3000 full- time for 2 yrs, I wish I could go up to 24v. But that would be difficult in my MotorHome.

There are lots of 24v stuff and a 12v to 24v charger from the alternator.

Good Luck on your project!

Hello Rocketman,

Thanks for your sharing your thoughts. Early on, AI D commented about the weight of the batteries. I will be traveling alone at times, so this is an important factor. I need to be able to manage the batteries on my own in general. Even 50 lbs. is a lot. I'm rethinking this---four 12v 100 ah batteries, with 2 each in series and then the 2 of those sets in parallel might be the way to go. The 12v 100 ah batteries are about 25 lbs each---do-able for me.

Yes, I've looked at the 12 to 24v dc-to-dc chargers, and I could also get a step-down transformer (24 to 12) if I want to use any 12v items.

Thanks for the well wishes!

 

[mikefitz]

A problem with two 12v lithium batteries in series is that there is a strong possibility the 'mid point' will get out of balance over time, there have been a number of issues reported on this forum. With a 24volt battery a single BMS is looking after all 8 cells in the series battery thus there is no problem . A further issue with the two battery series setup, is that the two BMS operate independently and are not synchronised, perhaps leading to more issues.
I suggest 24 volt batteries, a 100 Ah is manageable weight wise, multiple 100Ah 24 volt batteries in parallel will alow easy expansion and redundancy .

Unless the inverter power exceeds 2000 watts a 12v system is more practical .

Do you really need 4800 watt hours of battery, unless you have high power use, like Induction cooking, many modest coversions get by with 2400 watt hours. The fast recharge of lithium is a great advantage over LA.

Review the decision to use an 'all in one'. Whilst attractive there is a continious power consumption that needs to be taken Into account.

My recommendation is to consider Victron products if budget allows. Victron Multiplus and seperate MPPT solar charger.

Mike

 

[Al D]

Not sure about OP's situation, but consider heating the van. A 1500W electric heater will draw 125A at 12V from the battery (more really due to the inverter [in]efficiency). A 24V system would draw half that. The BMS on a 12V system would need to handle that 125A draw, too (and the C rating on the batteries needs to be thought about). Also, at 1500W, the 5kW batteries wouldn't really be enough to run the heat all night (unless, maybe, there is good insulation or the temperature setting is low).

Even if not heating the van, the math applies to any 1500W appliance (microwave, toaster, etc). So 12V systems are a bit under performing.

A 2kW inverter will typically handle a 4kW surge. Going by the general rule of thumb for in rush current made by inductive loads, you would only be able to run an inductive load using around 500W of power. That's probably fine for most devices in a van.

Personally, I chose to oversize my inverter at 3kW for robustness and longevity. My BMS is rated at 100A so I should never be able to max out the inverter at 24V.

 

[Steve_S]

Let me put it another way.
12V essentially is Maxed out @ 3000W - which is 250A draw uncorrected. Most CAP at 2000W max - 166A.
24V is maxed out at 5000W - 208A uvcorrected.
This excludes Surge pulls which are momentary.

Battery Packs in Parallel on a common DC Bus (connected to HD Busbars and then to Solar Control/Inverter divide both the Charge Incoming and the discharge. They also increase Fault Tolerance & Fall Back capabilities should one or more packs shutdown for any reason. Placing Batteries in series is NOT recommended as that increases the potential of failures and provides no fault tolerance.
Batteries in Parallel increases the amount of Amphours available but does not increase voltage.

It is BEST to stay NATIVE meaning 12V battery packs for 12V system, 24V battery packs for 24V and so on.

There are excellent step down converters to 12V for 24V & 48V battery backed systems BUT there are also some really crappy ones too, so one must do their research.

Using an Alternator to charge 12V still needs to go through an intermediary device otherwie you can destroy the batteries ! There are also Alternators that output 24V with Step Down to 12V taps so you can run your vehicles 12V systems without issue. Again, this still need an intermediary for LFP Battery Packs.
Have a Look at these Samlex Power systems components to get an idea. Samlex is a High Quality Tier-1 Product line and I will warn you NOT cheap, but you get what you pay for. https://samlexamerica.com/industries/mobile-fleet/

These find themselves in Ambulance, Fire & Even Military applications as well as Marine.

Hope it helps, Good Luck.

 

[Rednecktek]

Warning. This should not be done with lead-acid batteries.

OK, in layman's terms, why?

 

[Jennifer-3456]

Let me put it another way.
12V essentially is Maxed out @ 3000W - which is 250A draw uncorrected. Most CAP at 2000W max - 166A.
24V is maxed out at 5000W - 208A uvcorrected.
This excludes Surge pulls which are momentary.

Battery Packs in Parallel on a common DC Bus (connected to HD Busbars and then to Solar Control/Inverter divide both the Charge Incoming and the discharge. They also increase Fault Tolerance & Fall Back capabilities should one or more packs shutdown for any reason. Placing Batteries in series is NOT recommended as that increases the potential of failures and provides no fault tolerance.
Batteries in Parallel increases the amount of Amphours available but does not increase voltage.

It is BEST to stay NATIVE meaning 12V battery packs for 12V system, 24V battery packs for 24V and so on.

There are excellent step down converters to 12V for 24V & 48V battery backed systems BUT there are also some really crappy ones too, so one must do their research.

Using an Alternator to charge 12V still needs to go through an intermediary device otherwie you can destroy the batteries ! There are also Alternators that output 24V with Step Down to 12V taps so you can run your vehicles 12V systems without issue. Again, this still need an intermediary for LFP Battery Packs.
Have a Look at these Samlex Power systems components to get an idea. Samlex is a High Quality Tier-1 Product line and I will warn you NOT cheap, but you get what you pay for. https://samlexamerica.com/industries/mobile-fleet/

These find themselves in Ambulance, Fire & Even Military applications as well as Marine.

Hope it helps, Good Luck.

Thanks, Steve S. That is very helpful!

 

[Jennifer-3456]

Not sure about OP's situation, but consider heating the van. A 1500W electric heater will draw 125A at 12V from the battery (more really due to the inverter [in]efficiency). A 24V system would draw half that. The BMS on a 12V system would need to handle that 125A draw, too (and the C rating on the batteries needs to be thought about). Also, at 1500W, the 5kW batteries wouldn't really be enough to run the heat all night (unless, maybe, there is good insulation or the temperature setting is low).

Even if not heating the van, the math applies to any 1500W appliance (microwave, toaster, etc). So 12V systems are a bit under performing.

A 2kW inverter will typically handle a 4kW surge. Going by the general rule of thumb for in rush current made by inductive loads, you would only be able to run an inductive load using around 500W of power. That's probably fine for most devices in a van.

Personally, I chose to oversize my inverter at 3kW for robustness and longevity. My BMS is rated at 100A so I should never be able to max out the inverter at 24V.

Thanks for your feedback!

 

[pollenface]

OK, in layman's terms, why?

I'm going with misinformation

Like $15 MPPTs with usb charging ports.