what to do with a 36volt battery bank????

[h2ocean]

hello all I acquired 6 6v crown 305 batteries from a lift. they do not have an amp hour rating it is amp hrs and time so after searching they are 20Hr 15.25A 305AH / 5hr 24A 270AH. they are currently in series for 36Volt but there is no 36 volt inverters.....so to use all 6 if i wire 3 series strings 12V and parallell the three i should get 12Volts 960AH.
after kinda figuring out the C-rate and i said kinda, if i want to run a 2000 watt inverter at 12V i will be pulling 190+ amps from the bank ( I assume 31 amps from each battery) but if i only use 4 in series i would have 1220 ah at 24V and only pull 80 or so amps total

not sure which scenario is better for the batteries and have not figures out a charge controller situation since i figure if i pull 190 amps out i need to put 190 amps back in and since panels are only like 9amps i would need a field of dreams to put them in.

any basic help on this to clarify would be great.....it seems everything i look up gives me an answer that raises 20 more questions.
I started this project to run 2 freezers and a fridge since i work 12 hour shifts and the power always goes out soon after i leave and although i have a generator im not there to run it and if i start it and leave it will run until its out of fuel wether the power comes back on or not.
all 3 units pull 4.76 WH a day or 198 watts, the reason for the larger inverter is i would like to run the furnace blower which is around 2280 watts but without 2 more batteries to make a 48 volt system i dont think i can run a 3000 watt inverter since the amp draw would be so high. tired of calculating and scratching my head and figured i reach out for some help.

thanks to all who may reply

 

[sunshine_eggo]

20hr is the standard rating - sometimes referred to as C20, so those 6V cells are 305Ah.

There are 36V inverters. They're a little less common and a little more expensive. Many 48V charge controllers will work as 36V.

2S3P for 12V would get you 915Ah of 12V.

Charging is different than discharging. Optimal charging is around 10% of C20 rating, so 92A would be about right. You can easily get away with as little as 5% charging or 46A. This would be about 700W.

Given the application, 2000W inverter sounds like overkill. With a max continuous of 200W, the optimal size inverter would be about 600W; however, that may not have sufficient surge capability to handle them. You'd need about 1000W to be confident you can handle the surge.

2280W for a furnace blower sounds absolutely absurd. Is the blower motor itself 2-3hp? That's moving a massive amount of air. My 5 ton heat pump blower is < 400W.

 

[Vigo]

I agree with looking further into that furnace requirement. I suspect if what you actually need is far less than 2000w continuous, then a 12v arrangement would be the easy button, especially if you can locate your inverter right next to your batteries to keep cabling length down.

I run a ‘5000 watt’ 12v inverter in my rv off some very undersize cabling, technically. Thing is, absolute peak momentary startup load is 300a for maybe a whole second IF i run the big rooftop ac (i usually run a smaller other ac) and the max continuous draw is maybe 150a, but typically not more than 70a or so. That’s my actual usage. I only really need to size wires for my actual usage.

Now, there is a caveat that if you want to fuse the system then the fuse must be rated higher than your continuous draw, but if that same number is enough to overheat your cabling then you do technically have an unsafe system. But if you can fuse your wiring safely without popping it during your normal use, you can put whatever wire is sufficient on the system even if its undersized for the ‘peak rating’ of the inverter. My .02…

 

[h2ocean]

I will have to look at the furnace motor the manual listed requirement is 19A @129V and it is on a 20A breaker so I am sure that is the high end or start load.

cabling is not an issue I do industrial maintenance and work with 480 3 phase and up and several 15-20 hp Dc motors so i have plenty of wire lying around.

also i looked at the spec shhet for the battery and it says it will charge in 3 hours @75 amp but im not sure how thet calculated when you start tying them together?

 

[Vigo]

Well that math is oversimplified in the first place.. It’s abbreviated info to get a daily user who has the batteries in a machine to do a ‘good enough’ job charging them that they wont come back during the warranty period. But thats sort of a long subject of how best to charge them. I think where you have to start from is how much power you have available to charge them. You said you have solar, but how much? You also have a grid connection, and are you ok charging from grid? Let’s start there, in my opinion.

 

[sunshine_eggo]

I will have to look at the furnace motor the manual listed requirement is 19A @129V and it is on a 20A breaker so I am sure that is the high end or start load.

cabling is not an issue I do industrial maintenance and work with 480 3 phase and up and several 15-20 hp Dc motors so i have plenty of wire lying around.

also i looked at the spec shhet for the battery and it says it will charge in 3 hours @75 amp but im not sure how thet calculated when you start tying them together?

charge time is:

Capacity needed / charge current * 1.2, e.g.

100Ah/10A * 1.2 = 12 hours.

The 1.2 is an approximation to account for charging inefficiencies.

 

[h2ocean]

well

Well that math is oversimplified in the first place.. It’s abbreviated info to get a daily user who has the batteries in a machine to do a ‘good enough’ job charging them that they wont come back during the warranty period. But thats sort of a long subject of how best to charge them. I think where you have to start from is how much power you have available to charge them. You said you have solar, but how much? You also have a grid connection, and are you ok charging from grid? Let’s start there, in my opinion.

1st I have not bought panels yet I was going to buy four 325 W panels Friday but am ready to pull the plug on that, I do not have a grid tie in just figured I could use the leftover batteries to run the freezers and let them go on their own... so far according to the kill-o-watt they use 4.89kw in a day Four 325 panels would make 6.4 kw per day which gives me 2,000 watts to charge the batteries and I do not think that is enough unless i am missing something. and for the life of me i cannot figure out how to calculate how long the batteries would run 4.9kw since it all has to be converted from amp hours through the inverter to the appliances. so I don't know if it will work or just end up with a pile of solar and dead batteries.

I really wonder what these people with a 100 watt panel and 12v 100 ah battery run? a laptop 3 hours a day?


by the way I put an ammeter on the furnace and it is 6.5A 120V so even if it only ran 15 min each our that's another 4.7kw per day so defiantly out of the question

 

[Vigo]

So i think what you’re getting at is that during the daylight hours you have to BOTH recharge from the previous night’s use AND support the ongoing load.

Remember that the ~4.89kwh per day for your equipment does not ALL come out of your batteries. During the daylight hours it will mostly come from your solar! If you assume you can make 200w (your averaged continuous load number) for say 6 hours a day from solar, then 6x200=1200 aka 1.2kwh which gets subtracted from the 4.89, leaving ~3.69kwh that actually comes out of your batteries and needs to be recharged the next day. If you divide 3.69kwh by the same 6 hours, you get ~600w of continuous charging needed to replenish the 3.69kwh of overnight consumption during 6 hours of solar power. Add the 200w from the continuous loads and you need to average 800w or better (out of a possible 1300w) of solar production while the sun is up. 800w / 12v = ~70a of charging. Not a number which requires HUGE cabling.

6v X 305ah = 1.8kwh/ea, x 6 batteries = ~10.8kwh of battery if they were in perfect condition and you drained them from 100% to 0%. In reality they are probably somewhat less than 100% health/capacity, but the 3.6kwh you need to take from them on average, is still probably less than 50% of their real capacity and thus a good sustainable long term arrangement that would still allow some wiggle room for bad solar days where you couldnt put all 3.6kwh back. You could run a little behind on charging for several days, or maybe go without any solar for one whole day. So, a pretty decent arrangement if you rarely have terrible solar conditions. I dont know what numbers you used to get the 6.4kwh/day theoretical production from the solar array, but if that holds true you could make only half of that and the batteries would still be able to cover the difference for several days.

So it kind of depends on how bad you expect solar conditions to get, and how long you would like to have backup power if solar doesnt cover it all, but in general i think 1300w of solar and 10kwh of battery to support a 5 kwh daily load is a tenable idea. If i were to improve one thing, it would be more solar panels to buy more wiggle room for poor solar conditions.

 

[Alkaline]

What do with a 36volt bank? Go buy a boat with a 36v troller motor of-course Then come on down to toledo bend for some bass fishing.?