From 12v to 24v

[wpns]

There are many 24 volt systems that are built up from 12 volt batteries.

I know, it seems like we see at least one a week here that's failed to deliver to expectations, and needs days or weeks of independent charging and careful tuning of voltages to get balanced without tripping cell over voltage protection.

Wire them up in 24 volt without taking them apart and use them as is.
Every 90 - 180 days, check the voltage of each one and see if action is actually needed.

I recommend inter-battery balancers, especially the ones with voltage displays, then you'll be able to tell at a glance how they are working, and won't forget to check and be surprised at anomalies.

 

[Vigo]

Problem with all in one at 12v is I can't seem to find one with any decent amount of power output. I'm currently running a 5,000w that runs all my house 120v loads. A 3,500 that runs my freezers, washer and one 120v circuit in my shop. I haven't found anything AIO to get close to either of my current inverters. My batteries are rated at 50a max continuous CC so I might be better off just throwing another controller at it.

I share this frustration even though i know of several good reasons why it is the way it is. The 12v AIOs don't go past ~2kw. Luckily they are cheap 'enough' that imo it is not ludicrous to buy them as JUST a 450v / 80a mppt at ~$250/pc. There are no >400v mppts of any sort that are cheaper than that, that i know of.

In my RV i ran a 5000w standalone inverter along with a ~1500w 12v AIO being used purely as an mppt/gridcharger, but im planning to convert it to a single 3000w inverter/60-100a charger combo unit sometime this spring and see if i dont regret it.
The high voltage mppt aspect is really only useful to me when the RV is parked at home anyway.

 

[Ronwattson]

Over Current Protection

There's no 'best' option other than than maybe buying new batteries at the intrinsic voltage you want with BMS with some kind of comms, plus new inverter and <throw everything away and start over>, so it looks like you've identified the three least-worst options:

1) Reconfigure your existing batteries as individual 24V strings of 12V batteries, each with a balancer. Expensive, but a known solution.

2) Tear apart your existing batteries and build new ones using an external BMS. Unknown complexity, cost, results.

3) Add more SCC, wiring, etc. Probably the most straightforward but feels wasteful/inefficient.

Honestly, 1) is my current favorite by a slight margin, at least until you cut one of them open and discover how much work it's really going to be to build 24V batteries.

I agree on that. This is probably a ridiculous option and certainly not the most efficient but I thought about just building a 24v system and charge the 12v off of it with a 100a Powermax RV converter and just run my scc's and PV on the 24v then I can run all the PV I have on hand.
My main goal is to be able to run my air-conditioner in summer and an oil heater in winter. So just a separate 24v or 48v for that purpose only and just keep trucking. If I just build another system I think I will just go 48v and just throw the 100/50's and 100/30's at the 12v and just lick my wounds. Thoughts?

 

[Ronwattson]

I share this frustration even though i know of several good reasons why it is the way it is. The 12v AIOs don't go past ~2kw. Luckily they are cheap 'enough' that imo it is not ludicrous to buy them as JUST a 450v / 80a mppt at ~$250/pc. There are no >400v mppts of any sort that are cheaper than that, that i know of.

In my RV i ran a 5000w standalone inverter along with a ~1500w 12v AIO being used purely as an mppt/gridcharger, but im planning to convert it to a single 3000w inverter/60-100a charger combo unit sometime this spring and see if i dont regret it.
The high voltage mppt aspect is really only useful to me when the RV is parked at home anyway.

You know I hadn't thought of that! Question. Would I be able to use say two of those on the same bank or even one and also connect a victron to the same bank?

 

[Ronwattson]

But 5KW at 12V is over 400A...

I don't allow it but 300 amps. I over size inverters so it doesn't have to work it's butt off lol it runs every light in my house, microwave, wife's hair dryer and refrigerator at the same time with no issues. I tested it for ten minutes and nothing got hot and every cable was room temperature. It's just me and the wife in a 24x24 house. To be honest the test scenario I just stated never happens. The most load it sees at once is a few led lights, 1.2a refrigerator and a 1500w microwave. My coffee maker, dvd and vcr run on a separate inverter so none of that draws from the 5,000w fuses to 300a but the 4/0 can take more in that short of distance but it would never see it anyways.

 

[sunsurfer]

If you don't want to run them in series with a balancer, you would be better just selling the batteries or just keeping your 12v system and build a second 24v system. Rebuilding the batteries and dealing with off the shelf BMS will be a risky move and its hard to find a good BMS.

I have a 12v system with 8 12v/200ah batts. I also have 8 100/30 Victron controllers and 2 inverters. I built a second 24v system thats basically the same. They both have TONS of redundancy and having two separate systems is redundant too! My batteries have good size bus bars.

Don't buy and AIO. The MPPTs and inverters they use are crap. Stick with Victron gear.

 

[sunsurfer]

I don't allow it but 300 amps. I over size inverters so it doesn't have to work it's butt off lol it runs every light in my house, microwave, wife's hair dryer and refrigerator at the same time with no issues. I tested it for ten minutes and nothing got hot and every cable was room temperature. It's just me and the wife in a 24x24 house. To be honest the test scenario I just stated never happens. The most load it sees at once is a few led lights, 1.2a refrigerator and a 1500w microwave. My coffee maker, dvd and vcr run on a separate inverter so none of that draws from the 5,000w fuses to 300a but the 4/0 can take more in that short of distance but it would never see it anyways.

You have the perfect life style for a 12v system to work just fine. I would just build a second 24v system.

 

[Ronwattson]

You have the perfect life style for a 12v system to work just fine. I would just build a second 24v system.

Thanks! I have done daily power audits for a year. I had the system going with much less with the intent to just run my freezers but it grew lol.
Our use is like clockwork seasonally. Right now my freezers only draw 800wh per day but that will turn into around 2,500 come summer since they are not climate controlled, just in a shed. Our TV time is non-existent in spring, summer and early fall because we rather be outside. Cold weather sees the most TV time. Our winter use right now is only 2.7kw and our summer peaks at 4.45kw for everything we run on 120v with the exception of heating or cooling. I currently just have the two 100/50 Victrons fed by 800w series and I haven't had to touch my transfer switch but one time since I got it all running. I probably didn't have too but the voltage was down to 13.1 and it was going to be heavy clouds for a few days. I only had it in time out for half of one day though. We simply just don't use much power really. 2 days ago I set the oil heater on low and let it run from about 8 am until about 5 pm. To my surprise the my yield for the two combined was 5,900wh! But they never hit absorption but we're at 13.5 at rest when I turned the inverter off to check it. We did get some heat time off of it in it's current configuration so I'm thinking I may can run heat/air at least during the day if I add the other scc's and PV. Over night on a cold 24 degree night set to low on eco mode set to 62 degrees they (2) oil heaters consume 7.5kw so that's not too bad I guess. Small open floor plan very well insulated house. Build it myself with the wife and my two employees help(contractor is my trade).
My bank is too small to me to run heating and air overnight and I'm reluctant to buy anymore 12v batteries but that may be doable. All of our use minus the daytime draw of the refrigerator and two freezers all of our other draw is at night or late afternoon (sunset) and before daylight. A disadvantage. So heater or air-conditioning included would put a big dent in my 12.8kw bank. 5 more 12v added to my 10 that I have or build another dedicated 24 or 48 for heating and air is my stuck point now. We do supplement heat with a propane heater but not at night. My wife thinks it will kill us. She's afraid of gas appliances big time. Our water heater, well pump, clothes dryer and heating at the moment is all we run on grid. We turn the water heater on when we get up for coffee and after we both have showered and are ready to leave we switch it off via double pole 30a switch so no standby used for it. She typically drys clothes 6 times a month for one hour tops so not a heavy use there. Water use isn't much in winter. We do water our garden in summer though so it's more for that in growing season. The batteries are a about 13 months old so I don't think adding 5 will hurt anything since the first have been very gently used. I may can manage with just two more and not worry about having 3 to 5 days of backup since we can just throw the transfer switch if need be. I need to do some math on it. I could heat up a buffer tank but our hot water use is already very low. I'd rather take the climate controls out off the grid.
In the future I'd like to knock the clothes dryer out so maybe a second system at 48v to take care of the dryers minimal use and heating and cooling is the way to go. Thank you for your time as well as everyone that has offered suggestions. You guys rock and y'all taught me a lot over the last 18 months since I joined the forum. I thank you all!

 

[740GLE]

If you main goal is to get more PV out of those SCC if it were me I’d buy a slew of victron battery balancers (one per 2s string) and a couple Lynx power in bus bars for OCP a 24v5k multiplus and be done.

Battery Balancer | Victron Energy

Equalize the state of charge of two series connected 12V batteries using the Battery Balancer. Find a Victron Energy dealer near you.

www.victronenergy.com

Sure it’s not the cheapest but is a nice robust system.

 

[David J]

I know, it seems like we see at least one a week here that's failed to deliver to expectations, and needs days or weeks of independent charging and careful tuning of voltages to get balanced without tripping cell over voltage protection.

I recommend inter-battery balancers, especially the ones with voltage displays, then you'll be able to tell at a glance how they are working, and won't forget to check and be surprised at anomalies.

I have a 2s3p 24 volt battery bank made up of 6 LiTime 12v 200AH plus batteries. They are connected as per the LiTime battery manual with 1/0 cables. Each string is fused at 200 amps, switched and then on to the buss bar. I use a LiTime balancer. A victron smart shunt is used to monitor the bank current etc. and the mid point voltage. This bank is connected to a Xantrex 24v 3KW inverter charger SW. I can use the Charger in the inverter to grid charge at the cheap electrical rate. I set the battery profile for a lead acid battery to give an output to max at 28.4 volts, seems to work great. Solar charging is 5KW 2s4p to an Outback FMX 80. The FMX80 has a relay output control to disconnect a contactor that normally feeds grid power into the inverter. When the battery bank voltage is up and solar is coming in and the utility power rates are highest the contactor drops out and the inverter transfers. Lots of relay logic added. This then lets the internal transfer switch in the inverter switch all the loads (2 15amp. circuits) to the inverter battery source. Thank you all. This is my first post.

 

[HarryN]

Professional Series Chargers

PRO Charging Systems

www.dualpro.com

You can always get one of these dual pro chargers and use that to keep things balanced if needed.

 

[Vigo]

To add an anecdote, I had 480ah of 12v in my small rv and i was able to run a 5k btu window unit overnight from that. I’ll be up to 620ah in a few weeks here.