24 v system question

[Famtraveler24]

Hi all. New to the group. Will indicates for inverters larger than 2000 you should go with a 24v setup. Originally, I was thinking of a 12v setup connecting 3 12v 300a battery banks in series. But, I intend to use a MultiPlus 3000. So...instead of running 900a total, do I get the same (or a bit more) amount of power if I the number of cells and go to 24v? E.g., a 24v 600a system? I'm a bit confused as to whether I should be thinking of run time in amp hours or in terms of watts generated. My intended use is for a 32' RV where I intend to run the 13.5BTU AC for 6-8 hours per day.

Additional questions:

1. Correct to assume I need a BMS for each 300a bank?

2. Fuse and wire size for relatively short wire runs to the inverter and the MPPT?

Thanks all!

 

[Steve_S]

Yes, after you cross the 2000W line your better off to go with 24V system. 1000W @ 12V = 120A but at 24V it's only 60A draw from the batteries.
- Batteries in Series increase Voltage. Batteries in Parallel increase Amp Hours (stored energy capacity).
- 24V/900AH = 21.6kWh gross (100% utilization) with LFP we suggest 80% to maximize life cycle. * Lead Acid & AGM can only be used to 50% max.
- 24V LFP system requires 8 Cells in series and an 8S BMS. 12V system requires 4 LFP cells in series and a 4S BMS. (Battery Management System)

Best bang per buck is the 280AH cells from XUBA / Lyuan which are shipped DPP (all inclusive with duties etc)
Have a look here: https://szluyuan.en.alibaba.com/productlist.html?spm=a2700.icbuShop.88.17.2b5c273edTUJMA

Remember that having a Big Battery bank is nice to have, especially if you are looking at standby storage for autonomy (no charging possible) BUT a lot of folks underestimate what it takes to charge up a large amount of Amp Hours in a big battery bank. Solar Panels, Inverter/Charger, Dedicated AC-DC Charger have to be planned for relative to the battery bank you want to have. Seen far too many folks get big banks only to realize they cannot recharge them in the low sun hour days of the season, some not even when the highest sun hours are available, at which time they go off on a rant that solar does not work....

 

[Famtraveler24]

This is great info. Thanks. A couple of follow ups.

1. Using a 24v system with fewer amp hours but more voltage - does that decrease my run time, say of a 13.5 AC because of fewer amp hours?

2. I intend to use 1500w of solar on the roof. Way to calculate change time.for a 24v 900a battery bank?

3. The 8S BMS you mentioned. That would would work in a single 24v400a battery bank, I assume?

Thanks again!

 

[Steve_S]

12V X 100AH = 1200Wh / 1.2kWh | 24V x 100AH = 2400Wh / 2.4kWh

The amount of Solar Panels limits how much an SCC can output. With 200AH LFP cells for example, they can discharge / charge at 1C rate witch is up to 200A. Assuming that the pack is flat @ 2.50V per cell (0% SOC) or 20.0V, to get that up to 3.65V per cell (100% SOC) 29.2V it will take quite a bit. It all depends on the amperage the charger is pushing out. @ 100A, it would restore 100AH (or roughly 50%) in one hour. @ 50A double it.
!!!- This is just rough simple math to show the example -!!!

BMS' denoted by 8S means for an 8 cell in series pack. 16S for 16 cells in series (48V).
There are many types of BMS' out there. Many Features, Functions and Capabilities, all at different price points of course, more goodies = more pesos. BMS' are chosen for the Amperage they can handle (deliver), with FET Based BMS' it is best to derate them, so if you want a 150A BMS, your better off getting one that is capable of 25% more than your expected loads, some recommend to support the Max Amps Draw that is possible. BUT there is a gotcha, if all of a sudden your inverter demands MAX Capacity (375A) and the BMS can't handle it, it should shutdown for safety reasons. A BMS will limit (safety) Charge & Discharge amperages and Voltage cutoffs for Low or Hi Charge states or if the cells or out of operating temperature range.

NOTE: BMS' up to 200A can be FET Based or use Relays. After 200A, it is virtually impractical to make a FET based BMS due to the cooling / heat dissipation requirements (and they'd be huge !). Usually, past 200A people switch to using BMS' with relays / contactors because relay can handle up to 1000A. Most in residential, energy storage type systems use 200A or 300A systems at the 24V level.

A 3Kw Inverter @ 24VDC will pull 125A BUT as a Pure Sine Low-Frequency inverter, it can pull triple that for surges which is up to 375A.

SCC (Solar Charge Controller) is important. There are many 30A, 40A units out there... but the bigger the battery bank the bigger the SCC has to be and subsequently, the more panels (watts & amps) it will accept. Many SCC's can be linked together (get's costly) for bigger systems. My STRONG SUGGESTION, is that if you are running Victron, stay with Victron, they are a great Tier-1 product and damned good at it. You may pay a bit extra but peace of mind has it's valued too and beyond bucks.

Word of advice. Do NOT buy anything till you have a good working plan, know what parts & bits are needed, and have a handle on it, lest you have extra money you do not need or want. There are many factors to consider and options that will need to be looked at. Even things like how/where you are installing your panels and getting the right angles etc... can't just stick em up willy nilly and expect pure goodness. They need to have right angle & direction to get the most wattage out of them. Here is a good simple easy sun / panels angle calculator, fill in the information and it will show you the optimal angles needed for YOUR LOCATION. Note that if you want a Fixed Mount installation and are in North America, Mar. & Sep. are the median months. http://www.solarelectricityhandbook.com/solar-angle-calculator.html

Hope it helps, Good Luck,
Steve

 

[Famtraveler24]

Thanks again. You have been a big help in understanding the details. I saw your link to the 24v400a battery, which, at $3k seems reasonable, given it is self contained with a built in BMS.

For that battery, is there a need for relays? I'm still a rookie and learning about relays.

Seems I could just get the one 24v400a battery and be as good as purchasing 800a of 12v. And it would take up less room.

 

[Steve_S]

The SHUNBIN is a DISASTER ! DO NOT GO THERE !!!
It is the perfect example of using USED Paralleled cells to hide bad defective ones.
They have FET Based BMS's and no way of interacting or monitoring them either.

A BMS will turn off Charging or Discharging and when Temps are out of operational range. Some BMS's use relays / contactors as they can handle a LOT of amperage. Other FET based BMS's do not use relays, they re solid state but are limited due to how they work and heat dissipation etc.

Let me be clear on a point here. The ShunBin cost me $ 3000 USD. The reality was they 175AH cells inside actually only gave at max 350AH but was sold as 400AH. Well they are not 350 either, capacity is lower.

I bought 16 280AH cells from Xuba delivered to me DPP for USD 2,433.32
That built 2x 24V/280 AH packs for a total of 24V/560AH / 13.4kWh

I am using Chargery BMS8T-300 BMS' for each of my packs. Now waiting to receive the all new DCC Contactors to eliminate my relays.
As the Chargery DCC is a brand spanking new item and not listed yet, I can't say what the pricing is for those. I think somewhere around $90 USD for the 300A version of it to go with the BMS.
XUBA sells the Chargery BMS' and they have the BMS8T-300A for $125 USD / $171.00 USD shipped DPP.

This product is no longer available.

This product is no longer available.

www.alibaba.com

 

[Famtraveler24]

Whew! Glad I asked. So, to be clear, from a run time perspective, would 560ah of 24v batteries be enough to run a 13.5btu AC for the 6-8 hrs I'm after, or do I need a bank with more amp hours despite the fact that the system is 24v?

And maybe do the DCC contactors instead of the relays.

Finally, and again, thanks so much for your help, I see you have a shunt on each battery. I had planned on a single Victron Smart Shunt. Would that work?

 

[Steve_S]

The DCC system replaces the Relays/Contactors. It's a Bi-directional Solid State Relay that has a cooling fan, built-in optocoupler so one DCC can be used in place of the two relays, and uses a fraction of the juice that conventional relays / contactors use. The shunts are used by the BMS to determine state of charge and to monitor the voltage / amperage. You can use the Victron Smatshunt I believe but the BMS would have to be calibrated to it. I don't know if there are peculiarities with the Victron shunt as it's not one I use.

As for your AC... You would have to get the actual specs off it, how many Volts, Amps, Watts the "sticker" says and then to crunch the numbers. Labels are so-so. The best way to know what the beast eats is to put a Kill-A-Watt meter on the plug and watch it. If it is an older AC unit, it may not be very efficient, in fact quite likely not. Conservation is roughly 3x cheaper than generation and 5x cheaper than storage. You may find a new energy efficient one a better deal when you roll it all up. (batteries are $$$). The most common thing folks are doing these days is replacing old AC units with Mini-Splits which can be amazingly energy efficient.

I can't offer much advice on AC.... I gave mine away after I moved into my home, it cools itself and I have no need to waste energy on AC. I cheated, I built a Net Positive, off-grid, Solar Powered home that requires very little to heat/cool it any time of the year,

 

[Famtraveler24]

Cant thank you enough. I'll check everything out and will begin the detailed planning. Might reach out to you for some feedback once that is finished.

 

[Famtraveler24]

Steve, can you point me to the thread where there is a discussion about how to order from a Chinese vendor? Before I go with your 280a suggestion, I want to ensure I understand all the ins and outs of the order process, wing to wing. Thanks. John