Battery wiring question.

jrcent · July 2019

Not sure if this is in the right section? I have been looking for an answer for the battery bank wiring for a system but can't find anything specific. I keep seeing it stated that batteries don't like to be wired in parallel. So for example, wiring up a 24v system that has 8 x 100 ah 12v batteries would have to be 2 in series x 4 then the 4 24v batteries connected in parallel or 4 banks. What if a larger bank of 20? Supposed to ideally be 2 parallel banks, 3 max used? Is there any better way to get 24v from batteries (I realize electrically not possible). Smaller banks with separate controllers/inverters? One of those necessary evils that you have to live with having a larger system? Am I thinking wrong about battery banks? Thanks

mcgivor · July 2019

With 12V monoblock LA batteries there are limitations to their capacity due to the need for 6 cells within each, if larger cells were used they would become extremely large and heavy, Rolls Surrette has a 12V 357Ah (20 hour) 123 Kg (270 lbs).

To increase capacity using a single string topography, it's easier to use high capacity lower voltage higher capacity batteries like 6V, or for really large capacity single 2v cells, only minor downside is more interconnect links are needed.

jrcent · July 2019

So basically more than 3 banks is the normal way to increase battery capacity without buying higher capacity batteries.

Tecnodave · July 2019

The only way to get real decent life in a large capacity battery bank is to use 6 volt, 4 volt , or 2 volt batteries. Using paralleled batteries will result in battery balance issues and reduced battery life as a result of that imbalance.

I am using two battery sets, both 6 volt, 4 units in series, large format "L-16" deep cell batteries but they are not in parallel.....I have 100% redundancy with two very similar solar systems, each with their own controller, one system is using Rolls-Surette S-530's, now 14 years young and going strong, the other system is Interstate L-16's at 8 years old and going strong. Key to this long life is very conservative use, even though I have 10 day long extensive "marine layer" where I cannot locate the sun in the sky. I never deplete below 50 % state of charge.

Batteries in in parallel rarely get 6 years life

mike95490 · July 2019

jrcent said:

So basically more than 3 banks is the normal way to increase battery capacity without buying higher capacity batteries.

NO. parallel banks are problematic to keep charge and discharge evenly distributed. You start using lower voltage batteries.

12v 100ah

-------------Gives 12V 200ah , but the batteries are in parallel

6v 200ah - 6v 200ah ------ in series, gives 12v 200ah and no parallel problems.

Batteries also come in 4V and 2v versions, as the voltage goes down, the cell capacity increases

http://www.smartgauge.co.uk/batt_con.html

jrcent · July 2019

So the videos or posts with huge battery banks, are only lasting a few years?

if I have 2000 watt solar

8 - 12v 100ah batteries

and I am running a 24v inverter then what would be my best options

I would be better to have just 2 batteries in series? How would you increase the battery backup power to last 3 days without sun? Jump up to 2 x 24v system?

4 x 6v 400 ah batteries would be nice but expensive and can't afford it right now.

jrcent · July 2019

Tecnodave said:

The only way to get real decent life in a large capacity battery bank is to use 6 volt, 4 volt , or 2 volt batteries. Using paralleled batteries will result in battery balance issues and reduced battery life as a result of that imbalance.

I am using two battery sets, both 6 volt, 4 units in series, large format "L-16" deep cell batteries but they are not in parallel.....I have 100% redundancy with two very similar solar systems, each with their own controller, one system is using Rolls-Surette S-530's, now 14 years young and going strong, the other system is Interstate L-16's at 8 years old and going strong. Key to this long life is very conservative use, even though I have 10 day long extensive "marine layer" where I cannot locate the sun in the sky. I never deplete below 50 % state of charge.

Batteries in in parallel rarely get 6 years life

So having redundant systems how does that work for you? You have to manually switch between the two of them as needed?

Are both ac outputs tied into the same distribution panel?

I am planning on 2 separate systems with a large day and small night system.

I thought about having multiple size inverters with a battery bank that could conserve power as needed.

Two redundant systems that could automatically transfer the load back and forth while turning off the inverter would be nice for winter time.

BB. · July 2019

Off grid solar is not cheap... Trying to do it on the cheap can give you variable results.

If you have the batteries (and you have kept them charged)--You can use them and see how it works out.

2x series 12 volt batteries = 24 volts
4x parallel strings of above 100 AH = 400 AH

You can go with 3 days of storage and 50% (6x daily energy usage) maximum discharge if you want... I would suggest that 2 days and 50% discharge (4x daily usage) is fairly optimum for most people. If you are trying to avoid genset usage, then maybe 3x day is better for you...

Genset fuel and batteries tend to be expensive--Solar panels are relatively cheap these days (for most people) and last 20+ years. Adding panels is usually a better way to reduce genset runtime vs a bigger battery bank (which may need more panels and genset runtime because of its larger capacity).

Simple math (I think I went through the details with you before)... If you have 2 day battery bank, your suggested "no-sun" usage for 1 day would be:

24 volts * 400 AH * 0.85 inverter eff * 1/2 days storage * 0.50 max discharge = 2,040 Watt*Hours per day

To charge the battery bank, 5% is "OK" for weekend/sunny season usage. 10%+ for full time off grid:

400 AH * 29.0 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 753 Watt array minimum
400 AH * 29.0 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 1,506 Watt array nominal
400 AH * 29.0 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 1,958 Watt array "typical cost effective" maximum

You are in the far north of the US (near Canadian border). If you use a fixed array at 45 degree tilt, you get "hours of sun" by season like this:
https://pvwatts.nrel.gov/pvwatts.php

Month	Solar Radiation ( kWh / m2 / day )
January	2.03
February	3.22
March	4.26
April	5.54
May	5.80
June	5.66
July	6.78
August	6.23
September	5.69
October	3.65
November	2.43
December	1.97
Annual	4.44

Less than ~3 hours of sun per day--Not great for solar harvest... With your 2kWatt array, you would expect (assuming no trees, not on north side of mountain, etc):

2,000 Watts * 0.52 off grid system eff * 1.97 hours of sun (December long term average) = 2,246 Watt*Hours per day (Ave Dec Day)

And once the sun gets over 4 hours per day:

2,0000 Watts * 0.52 off grid system eff * 4.26 hours of sun per day = 4,403 kWH per day

Lots of power for a near normal/none genset/highly conservation minded home (using wood/propane/etc. for heating/cooking/hot water).

If you doubled the AH capacity of your battery bank--You would need a larger array (to keep bank charged at 10%+ rate of charge), you would need a larger genset (possibly) for winter charging. And you would have 8x parallel strings. And lots of connections to check, ideally fuse/breaker per string--more costs.

And if flooded cell batteries, you have (12 caps * 8 parallel strings = ) 96 batteries to check/fill every month... Not fun.

There are lots of reasons I suggest 1-3 parallel strings (more maintenance, more things that can go wrong, I like larger AH cells--I think they tend to be more "rugged", etc.).

For example, your present 4x parallel string with 12 volt @ 100 AH batteries--Using 6 volt @ 200 AH "golf cart" batteries (generally cheap and rugged) would take 4x 6 volt batteries in series (24 volts) and 2x parallel strings of 200 AH (2x 200 AH) for a 400 AH battery bank.

Only two sets of parallel connections/cells, and 1/2 the numbers of cells to check (24 Cells for 6 volt bank, vs 48 cells for 12 volt bank).

The answer is--You can do pretty much anything. And trying to make use of stuff you have on-hand, it can work out OK for the short term... Long term, trying to keep the system running can be frustrating.

What are your electrical needs? What are your expectations? Without understanding those issues, it is difficult to figure out what system will meet your needs or not...

Building out the system you have, and seeing how it functions (what your harvest is, how much you need to use the genset, etc.)... You can get good numbers and experience from your first try... And assuming you choose good hardware (panels, charge controllers, AC inverter), the only thing you should need to change is your battery bank down the road...

And if it does not meet your needs (too small, etc.)--You now have data and can redesign your system based on your experiences.

-Bill

jrcent · July 2019

Bill, following your recommendation with 24v 400ah batteries and 2100 watts solar. I was looking for a better way of connecting the batteries so not to be using parallel or at least not more than 2. Seems like higher capacity lower voltage batteries are the only better option.

My power consumption is mainly DC led lights (we still also use kerosene lamps and candles) , propane frig with DC controller, and internet service (35 watts with kill a watt). I use some other small appliances but only during daylight hours when enough sunlight. We've basically chosen a lifestyle off grid and switched to non electric power versions of things that we need whenever possible but my wife is still missing her electric coffee machine. Wife grew up completely off grid with no electric at all, and I was raised in a very conservative country life.

I have a 115vac deep well pump that runs 9 amp and only runs a couple times a month for 30 minutes to an hour to fill a water tank. Up till now we just run the generator for it and might continue. A small propane on demand water heater with a DC RV water pump.

Heat is a woodstove with plenty of free firewood which is also is used for cooking for the winter. Propane stove for cooking or small rocket stove.

My ultimate goal is to make enough solar power to be able to make it through the winter with very little generator if possible and keep batteries above 80%. We usually were running the generator every 2 or 3 day for a couple hours to recharge batteries which isn't a problem but would rather not have to.

So far I've been upgrading from any cheaper equipment as much as possible. Still have a problem with spending $1000 per battery though. I really want to upgrade to 48v also which I've been wanting for a few years.

I have a night / low power system with MS mppt 15 amp charge controller, MS 300 watt inverter, 4 x 100 watt panels, and 4 x 6v 110 ah golf cart batteries configured for 12v.

For a day / higher capacity system; midnight classic 150, 3500 watt pure sine and 300 watt pure sine inverters, 7 x 300 watt 24v canadian panels, 8 x 12v 100 ah agm batteries configured for 24v .

I've got the math down and initially figured out around 3000 watts for what we would like to use to include anything possible. I also ran the numbers backwards with the solar panels and batteries I have to get a more conservative 1500 watts of usage. I can upgrade the batteries to higher capacity, lower voltage cells and 48v as time goes or these batteries fail. I will use what I have for now.

Estragon · July 2019

IMHO, the problem with having multiple non-paralleled banks intended for use to extend time before running genny to charge is this:

In this example, with 4 strings in parallel, you have 400ah@24v. Staying above 50%SOC gives you 200ah before starting the genny. For simplicity, say you use 50ah/day,starting genny on day 4.

With a single bank, SOC looks like; Day1-350ah=88%, day2-300ah=75%, day3-250ah=62.5%, day4-200ah=50%. Start the genny to charge a single bank. Assuming C/5 (20%) charge rate and 3hr absorb, run time ~2.5hrs bulk + 3 absorb =5.5hrs. The whole bank spends ~ 2 days under 75%SOC.

With 2 separate banks; bank1/day1-150ah=75%, bank1/day2-100ah=50%, bank2/day3-200ah=100%, bank1/day4=50%. Start the genny to charge 2 banks. Same C/5 charge rate +absorb, but x2 to charge banks one after the other, so 11hrs. Bank 1 spends 3 days under 75%, 2 days at 50%. You also have to remember to switch banks when needed during charge/discharge cycles, or devise an automated way of doing so.

With 4 separate banks, even worse.

If you run large-ish surge loads on a small bank, voltage sag will be increased, possibly causing problems.

When the time comes for battery replacement, I may consider using lithium as a primary bank, sized to handle usual loads and daily cycling, with lead acid backup providing for extended crummy weather. Lithium handles low SOC for extended periods and higher charge rates vs lead acid, so this may make some sense. To me, doing it with multiple L.A banks doesn't.

FWIW, given you already have the batteries, I'd go ahead and do 4 (individually fused) strings, but watch carefully for imbalances and be prepared to charge individual batteries separately as needed to remediate.

BB. · July 2019

Your system is reasonably capable as is... It is almost large enough for running a full size refrigerator--Or get a smaller 12/24 VDC refrigerator and skip the AC inverter step. (DC refrigerators are not cheap--But do allow you to build out a much smaller solar power system--If that is your goal).

A 3,300 WH per day system--That sounds like it would run everything you want/need. Use 4x 6 volt @ ~220 AH in series for 12 vols * 3 parallel strings for ~660 AH... Or 12 GC batteries * ~$100 each, or $1,200 for the bank...

That bank would support a ~3,000 Watt AC inverter... But to conserve energy (if you run the inverter 24x7), I would be suggesting a full time inverter in the smaller size range (1,500-2,000 Watts to use less "tare" energy--Energy used just being turned on).

Also, sticking with a 24 volt battery bank, using 24 VDC appliances (and lighting), and especially the 12/24 VDC refrigerator, I don't see a reason (at this point) for going with a 48 VDC battery bank.

I am not sure what the "3,000 Watts" refers to. 3kW array? 3kW inverter? etc...

A little confused as to your energy needs... You sound like you are trying to go with a minimum amount of electricity (35 Watt load * 24 hours per day = 840 Watt*Hours per day). Not sure if 24 hours per day, what about laptop/tablet computer/LED lightning/etc...

Without a refrigerator, 1,000 WH per day can run a minimal cabin (LED lighting, laptop, cell phone/wireless modem) pretty well.

Energy usage/needs is a highly personal set of choices. And off grid solar power systems are not cheap. Generally you design the system for your known and planned loads... And not for an indefinite future set of loads.

Based off of your need to run a genset a few hours a couple times a week in winter--I would guess that your daily loads are >2,000-2,500 WH per day.

With your existing system, I would use the battery bank as is (check battery voltages with meter every week, keep terminals clean) and and up size the array to 3,000 Watts+ and see how the system performs for you.

You may shorten the life of your present battery bank--But it will give you "real data" for designing your future system upgrade.

Yes, your present 2s x 4p battery bank at 24 volts @ 400 AH could be rewired (with existing batteries) to 4s x 2p for a 48 volt @ 200 AH battery bank.... That would cost you a new AC inverter (and possibly addressing any 24 VDC loads you may presently be using).

At this point, I would not suggest rewiring/new AC inverter... Unless you are really looking to upsize your system (>4,000 WH per day, >3,600 Watts peak)--And then you are looking at a bunch more panels/solar charger upgrade/larger 48 volt inverter/larger 48 volt @ 400+ AH battery bank). That is a big step. And I am not sure that it is worth it to you at this point (vs a 3kW array and some genset runtime in winter during bad weather)...

-Bill

jrcent · July 2019

After the winter here and trying to keep things going with very little sun and lots of snow I was planning with a bigger is better mentality. Bigger maintenance and replacement costs go along with it. I was thinking about a lot larger system and getting 3000 watts of solar panels. 2100 watts should be fine. A lot of my problems were related to cheap crap and the associated issues with keeping what I had alive.

From just what I've learned in the last month or so from this site, have rethought my direction. I now have reliable (hopefully lol) name brand equipment. I also "backward engineered" with what I had already had size wise and incorporated more propane and non electric solutions. Backup gensets are always available.

Propane tankless water heaters are really nice and only require 3vdc for igniter. Smaller sizes are very efficient and when off grid any hot water is nice! A propane fridge works good but can be pricey to replace. A smaller electric fridge would be nice and ice boxes are plentiful here in the winter. Sweeping snow from solar panels 3 or 4 times a day is a pita,

I will wait and upgrade to better higher capacity batteries when these agm batteries give up. Hopefully battery technology will be better buy then. I will remain with 24v now also and replace with 48v inverter when it comes time for a new one.

jrcent · July 2019

Estragon said:

IMHO, the problem with having multiple non-paralleled banks intended for use to extend time before running genny to charge is this:

In this example, with 4 strings in parallel, you have 400ah@24v. Staying above 50%SOC gives you 200ah before starting the genny. For simplicity, say you use 50ah/day,starting genny on day 4.

With a single bank, SOC looks like; Day1-350ah=88%, day2-300ah=75%, day3-250ah=62.5%, day4-200ah=50%. Start the genny to charge a single bank. Assuming C/5 (20%) charge rate and 3hr absorb, run time ~2.5hrs bulk + 3 absorb =5.5hrs. The whole bank spends ~ 2 days under 75%SOC.

With 2 separate banks; bank1/day1-150ah=75%, bank1/day2-100ah=50%, bank2/day3-200ah=100%, bank1/day4=50%. Start the genny to charge 2 banks. Same C/5 charge rate +absorb, but x2 to charge banks one after the other, so 11hrs. Bank 1 spends 3 days under 75%, 2 days at 50%. You also have to remember to switch banks when needed during charge/discharge cycles, or devise an automated way of doing so.

With 4 separate banks, even worse.

If you run large-ish surge loads on a small bank, voltage sag will be increased, possibly causing problems.

When the time comes for battery replacement, I may consider using lithium as a primary bank, sized to handle usual loads and daily cycling, with lead acid backup providing for extended crummy weather. Lithium handles low SOC for extended periods and higher charge rates vs lead acid, so this may make some sense. To me, doing it with multiple L.A banks doesn't.

FWIW, given you already have the batteries, I'd go ahead and do 4 (individually fused) strings, but watch carefully for imbalances and be prepared to charge individual batteries separately as needed to remediate.

Actually that makes good sense. The more moving parts the more it can get very complicated . I'll end up spending all my time taking care of my system. I now realize less, higher capacity batteries in series is probably better. a couple separate low maintenance systems is good enough.

Battery wiring question.

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