SLA battery bank: 6v x2, 12v x3, or hybrid 6v x2+12v x1 for high current draw system for RV

[NOBO19.5]

I am moving the Solar system from our RPOD trailer to a NOBO and am expanding the system for basically double capacity.

The old system used 2 x 12v standard Marine batteries with ++500CCA each. This was connected to a 4x 100W renogy PV array with PWM 30A charger.

The (chinese) Pure Sine Wave inverter is rated at 3000W and I was able to run the AC, and microwave (not at the same time) with this setup for short periods. (both battery bank and solar limited). Short= minutes

The new system is 1060W of solar. I am upgrading to a 50A MPPT Victron charger (will retain the built-in 10A PWM charger). And I will use larger cables all around to limit voltage drop (10-12ga on each panel, 4ga at charge controller, and 1/0ga battery connectors).

Now my question, which is battery related.
I am considering buying 2x 6v 230AH batteries connected in series (true deep cycle https://www.batteriesplus.com/productdetails/sligc115) . These have no rated CCA, but are rated at 75A draw for 125 min, so I know they can at least handle that. However, being a 6V series, vs the 12v parallel system I had before, I worry about maximum current draw from the inverter.

From what I have read the PV panels themselves will be supplying large amounts of current, perhaps 40A on a sunny day (theoretical max current of panels is 48A to the 50A MPPT controller, and 9.3A to the PWM controller), and I will only run the AC on those sunny days. (the fridge will stay on propane).

So 75+40ish= 1300-1400 watts continuous. Since the AC will draw 1500w and spike higher than that, I worry about voltage drop if the 6V FLA cannot produce enough current, thus tripping the inverter, and leading to not running the AC.

I am limited by space and weight, or I would just go to 4x 6v series/parallel anyway. Lithium is way out of budget. I dont understand why people go lipo for RV when 230AH batteries can be had for $144 vs $900 for a 200AH LiFe, perhaps its the weight/space issue

I was originally going to go with 3x 12v in parallel to combat the current problem, but a group 24 12v 75AH battery costs the same as a 6v 230AH battery, of the same manufacturer (Duracell in this case). Hence my decision to go to 2x 6V in series.

Q1: would it make sense to scrap the 6v idea and just use 3x12v so that current demands are limited (1/3 current from each battery), and just cry a little at my limited storage capacity (75AH x3 aint that bad)
Q2: since the trailer is new, would using combination batteries make sense (i.e. 6v x2 in parallel with a 12v standard that came with the unit). That way the current draw would be cut in half at least.
Q3: scrap the whole idea of leaded batteries and go unleaded, and break the bank for a LiFePo4 system, and redesign everything?
Q4: find another place to put 2 more batteries and run a proper 4 x 6v system in series-parallel?

Any other thoughts?
Thank you in advance!



PS: Trial run in the Rpod trailer was successful with the 2x12v system, duty cycle should be 50% or less, so heat gen on the 3000W inverter is not that bad (I hope), and it will only be run for at most 2 hrs a day.

 

[MisterSandals]

++500CCA each.

This is not a meaningful number for this application, it refers to how many amps it can deliver starting an engine (cold cranking amps). Does it mention amp hours (Ah)?

considering buying 2x 6v 230AH batteries

2 x 6V x 230Ah = 2760Wh
Lead acid limited to 50% discharge so 1380Wh usable

Your proposed 1060W of solar:
1380Wh / 1060W = 1.3h to recharge

So 75+40ish= 1300-1400 watts continuous.

Huh?

Whatever is using(?) 1400W continuous...
1380Wh / 1400W = .98h runtime (1 hour if operating at 100% efficiency... which nothing ever does)

$900 for a 200AH LiFe,

12.8V x 200Ah = 2560Wh, nearly 100% usable.
2560Wh / 1400W = 1.8h runtime (80% more than your other battery setup)

If you use this basic math, comparing usable Wh, you can do a real cost benefit (and size/weight) comparison.

 

[acdoctor]

CHINS LiFe 100 AH Amazon $304.00. They can be paralleled. I don’t think they have freeze protection. I would not put that much money on the tongue anyway.
Two of these would give you. 200AH X 0.9=180AH X 12.8V= 2304Wh.

 

[NOBO19.5]

This is not a meaningful number for this application, it refers to how many amps it can deliver starting an engine (cold cranking amps). Does it mention amp hours (Ah)?


2 x 6V x 230Ah = 2760Wh
Lead acid limited to 50% discharge so 1380Wh usable

Your proposed 1060W of solar:
1380Wh / 1060W = 1.3h to recharge


Huh?

Whatever is using(?) 1400W continuous...
1380Wh / 1400W = .98h runtime (1 hour if operating at 100% efficiency... which nothing ever does)


12.8V x 200Ah = 2560Wh, nearly 100% usable.
2560Wh / 1400W = 1.8h runtime (80% more than your other battery setup)

If you use this basic math, comparing usable Wh, you can do a real cost benefit (and size/weight) comparison.

Thank you for the response, and useful info on lithium vs lead battery chemistry. But I am not interested in run time at all. I am concerned about high continuous current draw. if the 2x 6v in series will not work, its not worth comparing them at all.

500CCA is a perfectly meaningful number when you are pulling high current into a 3000W inverter to start an AC. Transients probably spike at over 2500W, which for a 12v system is a lot of current/amps, and will trip most low power generators and inverters 2000W or less.

As you know, current capability, both continuous and CCA, is very different between a Car battery, Marine, AGM, Lithium, and 6v golf cart battery.

My point was that the system worked with 2x12v using Marine batteries (which have high peak current capability (hence the 500+ CCA), but I have no idea how a 6v designed for continuous draw would do with the surge when the AC starts up. A parallel 12v system would see 1/4 the current of a 6v system, which exacerbates the problem.

I am not interested in AH, since even the largest battery bank will be drained fast with the AC running any significant length of time.

Hopefully, I explained it more clearly now.

 

[jberger]

Compared to (1) 12v Lead Acid, the 2x 6V should actually perform better, at least that is my experience with Golf Carts. The form factor usually allows for a thicker plates inside the case.

But you would still be better off looking at LiFe for charging, discharging and adding capacity later. You could buy 1, and then add other one in a couple of months, you can't do that with Lead Acid.

Those lead acids will have to be stored outside on the tongue, they cannot be stored indoors and charged at a high rate. Plus you are going to constantly have to keep the water levels right or you will risk killing them with high discharge. That means checking them at least once a month and keeping distilled water on hand to top them off. They will be permanently damaged if you skip a month. High discharge/charging will just wear out a Lead Acid in a shorter period of time, so think about the cycle life and calendar life when you consider the costs.

You cannot believe the marketing materials for Lead Acid, they are simply not accurate. And Temp is going to impact that discharge rate.
You might get that rated capacity for a floor buffer used in a climate controlled room, but you will not see it for long with a camping application. That floor buffer will get a mild recharge rate, you are planning for a rapid discharge and rapid charge so you are burning the battery on both sides.
If you really wanted to do lead acid right, you'd double the bank to (4) 6v, this would half the discharge rate and really extend the lifespan of the batteries. Or you could use AGM's instead, that would give you better performance, longer life and much less maintenance concerns.

BTW: I'm not sure where you get the $144 pricing, your link says those GC2 cells are 181 without a core, and I assume since this is a new application you have no cores. That puts you at 360 for a pair of 6v.

 

[NOBO19.5]

Compared to (1) 12v Lead Acid, the 2x 6V should actually perform better, at least that is my experience with Golf Carts. The form factor usually allows for a thicker plates inside the case.

But you would still be better off looking at LiFe for charging, discharging and adding capacity later. You could buy 1, and then add other one in a couple of months, you can't do that with Lead Acid.

Those lead acids will have to be stored outside on the tongue, they cannot be stored indoors and charged at a high rate. Plus you are going to constantly have to keep the water levels right or you will risk killing them with high discharge. That means checking them at least once a month and keeping distilled water on hand to top them off. They will be permanently damaged if you skip a month. High discharge/charging will just wear out a Lead Acid in a shorter period of time, so think about the cycle life and calendar life when you consider the costs.

You cannot believe the marketing materials for Lead Acid, they are simply not accurate. And Temp is going to impact that discharge rate.
You might get that rated capacity for a floor buffer used in a climate controlled room, but you will not see it for long with a camping application. That floor buffer will get a mild recharge rate, you are planning for a rapid discharge and rapid charge so you are burning the battery on both sides.
If you really wanted to do lead acid right, you'd double the bank to (4) 6v, this would half the discharge rate and really extend the lifespan of the batteries. Or you could use AGM's instead, that would give you better performance, longer life and much less maintenance concerns.

BTW: I'm not sure where you get the $144 pricing, your link says those GC2 cells are 181 without a core, and I assume since this is a new application you have no cores. That puts you at 360 for a pair of 6v.

I am thinking more and more about lithium.

Technically, I have 4 cores. 1 is a brand new 2022 battery that came with the new trailer. One is 2 years old, one 3, and one about 8 yrs (dead). I was going to sell the old trailer with the 2yo one, and exchange the 3yo and 8yo for new batteries if running a 4 batt system, or sell the trailer with the new battery, and use the 2 oldest ones for a 2 batt 6V system.

So, I just have these SLA 12v batteries sitting around.

Maybe, ill just let them die over the next year or two, then upgrade to lithium....

 

[corn18]

It looks like you are going to run an A/C off of this setup. How can you not be concerned with run time? An air conditioner running off an inverter will pull 150A. Not sure how long you want to run your A/C but two golf cart FLA won't last very long.

 

[NOBO19.5]

It looks like you are going to run an A/C off of this setup. How can you not be concerned with run time? An air conditioner running off an inverter will pull 150A. Not sure how long you want to run your A/C but two golf cart FLA won't last very long.

part of it is proof of concept. it worked on a trial run in Moab with the Rpod with two SLA car batteries. Basically, every forum poster says the same thing, it wont work. I don't think they are correct, and the only way to prove it is to do it. I am just asking for suggestions and guidance. The Nobo is winterized and with good insulation. With the roof covered in panels, its basically in the shade at all times. I am thinking the A/C duty cycle wont be more than 50% (could be very wrong here). AC would kick on at something like 80 degrees, which should only happen on full sun days, midday. My 1060W solar system should be around 800W minimum, producing around 40 A, taking into consideration losses. This will drastically reduce drain on the batteries. Mostly this will be as a lifesaving measure for the dog if my morning hike/ride goes too long and I am not back before midday due to mechanical on the bike or personal injury. I expect 3-4 hours of emergency run time, which should get past the heat of the day. In an ideal world safety measures kick in and the battery is protected, until it is able to provide juice again. But, I wouldn't care as long as the dog made it past the heat of the day.

Lithium 200AH batteries are capable of 200A continuous current.

So, using your 150A number. 50% duty cycle gives 75A on average, with a continuous 40A solar (more if I am lucky), so only 35A draw, on average to the battery. Obviously the fax for the Inverter will come on, and draw a few amps, and I am sure the panels will get hot, reducing their power output. But, I think it might work.

Thoughts?

 

[time2roll]

It will work. It will work until the inverter shuts down from low battery voltage. My 2000 watt sine wave inverter would run my Dometic 13.5k btu air just fine with 4x GC2 batteries. Drew about 125 amps. Never went the distance to see the run time.

You should attend the full cycle run a few times to verify what your set up will do.

 

[NOBO19.5]

Definately will do a trial run first. I dont want to spend $2000 on premium batteries, so will be using $600-800 200AH lithium most likely. Cheap chinese batteries will be at increased risk I presume. My experience is with Lipo, and that chemistry earned my respect. LiFEPO4 should be much safer. I am thinking about placing the batteries inside the cabin (storage under the bed), to protect from cold, but am concerned about fire. I will most likely construct a battery box on the tongue, which will hopefully be able to contain a fire should it be needed. (steel double wall with insulation).

 

[grizzzman]

Definately will do a trial run first. I dont want to spend $2000 on premium batteries, so will be using $600-800 200AH lithium most likely. Cheap chinese batteries will be at increased risk I presume. My experience is with Lipo, and that chemistry earned my respect. LiFEPO4 should be much safer. I am thinking about placing the batteries inside the cabin (storage under the bed), to protect from cold, but am concerned about fire. I will most likely construct a battery box on the tongue, which will hopefully be able to contain a fire should it be needed. (steel double wall with insulation).

Be mindful as you are aware Moab can get quite hot. Lifep04 on the tongue could cook them in short order. ( under the bed is a better idea)

 

[jberger]

If you are concerned with fire risk, just buy a metal cased battery like SOK instead. There isn't much fire risk at all with LiFEPO4 chemistry, so you should be fine to bring them inside. Using a prebuilt model in a metal case is going to further reduce your risk. They do need to be comfortable to operate effectively, so watch the temps.

There is no question that you CAN get the AC to operate from the batteries, the issue is repeated use will kill the lead acids much quicker than you'd expect. You should also think about adding an easy start (not just a soft start) to the AC.