12v or 24v

Hi Everyone, Thanks a bunch to William Prowse, I'll buy as much thru your Amz links as possible to support u. I need an off grid system for my TT. I will use 10-100W rigid panels, a 3000W inv, 1-80A or 2-40A CC and AGM Batts. As far as Batts, I can do as far as space, weight and $:
4-6V 220AH in ser/par=440AH 12V or 2-6V 400AH ser=400AH 12V or
2-12V 210AH par=420AH 12V
Now if I did a 24V:
4-12V 105AH ser/par=210AH 24V or
2-12V 210AH see=210AH 24V.
I understand all the benefits of a 24V system and would like to use it, but isn't it cutting my amp hours in half compared to a 12V system? Or since it's 24V and it goes thru the inverter more efficiently to 120V and it has to step down also to 12V, does it change the AH? Or is amp hours amp hours no matter the voltage? Please help unconfused me and please don't try to convince me to use lithium. For right now I can afford the AGM. Thanks.

24v cuts the amps to produce a given wattage in half, it doesn't reduce your power.

3kW at 12 volts = 250 amps, 3kW at 24 volts = 125 amps. You would use a 24v battery, a charger capable of charging a 24v battery and inverter capable of accepting 24v. Out the end comes '3000 watts' of AC.

Forget amp hours for the purpose of this, it is making things more complex. What you are after is watt hours. You get watt hours by multiplying the notional voltage by the notional amp hour rating. 2 x 12v 120ah batteries in parallel for 12v 240 ah gives you 2880 watt hours*. 2 x 12v 120 ah batteries in series for 24v at 120ah gives you 2880 watt hours.

3kW at 12 volts means very heavy cabling, more than you'd assume in order to work with the very slim voltage drop margins you have to play with.

If you are going to put 2 AGM batteries in series, IMO, you'd do yourself a favour by including a basic battery balancer. Lead acid batteries just love to drift out of step WRT state of charge.

*not really but keeping it simple

Hi,
Little more confused than before, where are you getting 3kw from? If I have 10-100W panels, isn't that 1kw of dc coming in? And when you say to use a 24V battery, you mean a 12V in series, correct? Because I don't see any AGM or Battle Born sold in 24V.
If we concentrate in watt hours, I am not loosing any night time storage usage capacity going with 24V, because the watt hours are the same in your example as the 12v system, correct? So to simplify everything, you are recommending I go with a 24V system, correct? I just taught of something. If Battle born batteries can be discharged 100%, could I get away with 2-12V 100AH in series for the system I want to build? I was figuring on a 50% discharge for AGM so that's why I needed more AH. It would raise my budget a little, but I could do the 2 Battle Born if you think that would be enough. Thanks.

You mentioned a 3kw inverter but since this is just addressing your concerns regarding power its only used as an example. Substitute what ever watt load you desire, results are in the same line. 24v = half the current do to the job.

Personally I'd use 24v for lighter wiring, and ease to move to a higher power system. You have a 3kW inverter so presumably you are looking at using that capacity at some stage.

If battery life is a consideration lithium wins. Lead acid down to 20% charge is usually around 400 cycles to reach 80% of original capacity. lifepo4 down to 20% charge is typically at least 2000 cycles to reach 80% of original capacity. If you have doubled your AGM capacity to extend life you could go with 2 x 100 ah lifepo4 batteries and still be in front in regards to battery life. Check with battleborn regarding series charging, the batteries will do it, but they may advise you to put a battery balancer across them, or maybe not

12v 420AH of AGM at 50% discharge, 2.5kWh of usable capacity, approx 800 cycle life
12v 200AH of lifepo4 at 100% discharge, 2.4kWh of usable capacity, approx 2000 cycle life

kWh is approximate, battery voltage changes as the battery discharges etc. Rearranging for series operation does not significantly change the available kWh.

joegambler0 said:

Hi,
I just thought of something. If Battle born batteries can be discharged 100%, could I get away with 2-12V 100AH in series for the system I want to build? I was figuring on a 50% discharge for AGM so that's why I needed more AH. It would raise my budget a little, but I could do the 2 Battle Born if you think that would be enough. Thanks.

Click to expand...

I see numbers all over the map for what the safe discharge is for AGM. Gnubie just implied 20% is safe.... but I have always used 50% as the deepest discharge you should ever do. Here is the interesting thing: From 50% to 80% lead acid chemistries will soak up all the current you can give 'em. From 80% to 100% they slow way down on charging..... but for battery life it is important to get it up to 100% charge.

LiFePO4 on the other hand, is rated differently. When it says 100 amp hours, it means there are 100 *usable* amp hours. Most of them are rated at 1C charge rate. That means you can charge a 100 amp hour battery at 100 amps....and it will take it almost all the way up to 100% SOC. That is why everybody talks about how 'fast' they charge as compared to Lead.

So.... that is the long answer to your question. The short answer is "Yes 2 battleborns would be about equivalent to your planed AGM batteries.

I recommend this video:

It was an example and if you check the specs for pretty much any AGM they rate to down 20% charge. If we are to compare the cycle count we need to take the battery down to the point where the rated lifetime of the battery is obtained. Taking an AGM to rated discharge will result in the battery getting approximately the rated number of cycles, low it may be, but that's what will happen. The world won't implode.

2 12v batteries in 24v is same capacity as 2 12v batteries @ 12v. Your TT is already ran for 12v, no good reason to go 24v.

AGM requires 2x the amount of solar to get filled by sundown and 1.5x the AH over lifepo4. Not really cheaper when you design a system from the ground up with full knowledge of what different chemistries require. Most people way underestimate how much solar is required for lead batteries.

IOW you can go with 500w of solar and 200ah of lifepo4 - ~$1500

Vs

1000w with 300AH AGM - ~$1700

Same amount of usable energy except youre paying less for a system that will last at min 2-3x longer and will be much lighter, easier to install, and not fretting everyday if youre getting fully charged.

P.S. Less panels - better. Look at higher wattage panels.

Thanks everyone for the help, seems like the more I learn, to more it's going to cost me. Even though it's only a 20' TT, me my wife and 2 dogs will be living in it full time for about a year completely offgrid while we build. So I need a robust system. The issue is not a smaller solar array or 12V vs 24V what I discovered after some online calculators and speaking to Battle Born is for the higher wattage kitchen appliances we have and power tools I need to run, is that I absolutely need, not want a 3000 watt inverter so that means 300 AH of Battle Born at 80% discharge to make them last gives me 2880 watts to feed the inverter, I was really hoping to get away with only 2 batts. So it's either $1500 of FullRiver 500AH 12V AGM at 340lbs lasting 3.5 years at 50% DOD or $2800+ of BB 300 AH 12V at 90lbs lasting 13.5 years at 80% DOD. When you put it like that, it seems like a no brainier, but almost 3K just for batts for a small TT is hard to swallow. Thanks.

Lifepo4 is easily capable of 1c/1350w. 200ah 2700w. Actually they can often do .5c but might be restricted by BMS. Theres nothing in your TT that needs 3kw except the AC startup current. A microwave is usually 800-1000w same with coffee pot and toaster.

Why BB? shun bin 200Ah is $800. 280ah of valence - $900 which would give you 4,050w continuous/8,100w peak for 30 seconds.

Yup... you ran into one of the downsides of a lot of the LiFePO batteries: They are specked at 1C discharge. The BB BMS enforces it too.

Make sure you read the threads about the Shun Bin.... some folks on the forum were not very happy with what they got from Shun Bin.

joegambler0 said:

the higher wattage kitchen appliances we have and power tools I need to run, is that I absolutely need, not want a 3000 watt inverter

Click to expand...

I'd go 48v then...but 24 will be ok too. I would not run a 3k inverter off 12v.

The shunbin cells were stamped 12.5% lower than advertised. Not sure if they actually tested low. Still a good deal.

We all have different needs. My wife's Ninja Foodie is 1460 watts, taking that away from her would be equal to her taking away my power tools, no deal. Instant hot water kettle to warm up my dogs raw food from the fridge 1500 watts, Vitamix blender which is the corner stone of our health, 1380 watts, you couldn't force me at gun point to eat from a micro, toaster oven 1300 watts. Use 2 of these at the same time and I have to use a power tool or my compressed kicks on, 3K watts doesn't seem to far off, does it? Just wanted to go with BB because I know they will do what they say they will. Didn't mean to stir up a war here, just bitching about how much I now know I will have to spend to get a system that will last and do what I need it to do for our lifestyle.

Yeah this aint like other forums. We are strictly fact driven not ego... or really try to be. The reason anyone tells you different is in case you dont know. Spittballing. Dont worry about offending or being offended.

I'm am OK with a 24V system, but that would mean 4 BB instead if 3 to get 24V, am I correct? BB was totally OK with 3 Batts at 12V and a 3K inverter.

If you keep inverter close to batteries and use large wire i cant see any advantage.

I wasn't offended, didn't mean to come out as such and also didn't mean to offend. I am here to learn and very happy I found Williams videos. I was just explaining why unfortunately I need more power than average.