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Risky buisness
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# Posted: 30 Jun 2021 07:07pm
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Hey..... been looking around how to figure total amp hours for a battery bank.
8 12v lead acid 90 amp hour each wired series parallel to get 24v.
I know that a parallel connection doubles the amp hours for a single ' pair ', but does that carry through for the entire system, or 4 pairs..... ?
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ICC
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# Posted: 30 Jun 2021 07:31pm
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Maybe this thought process will help.
Eight 12 volt 90 aH batteries connected to make a 24 volt battery bank.
Connect two of those batteries in series and what have you got? 24 volts with 90 AH capacity.
Do that a total of four times. Then stop thinking about the series connections.
You have 4 "battery strings" that you now connect in parallel.
Connect two of those strings and you have 24 volts and 90 + 90 = 180 AH.
Add a third string and you have 90 + 90 + 90 = 270 AH
Add a fourth string and what is the end result? You do the math.
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gcrank1
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# Posted: 30 Jun 2021 07:50pm
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Then remember that with Lead Acid bats you are not to exceed a 50% discharge or it is very hard on the bats.
At 30% Depth of Discharge (dod) is even better on a regular basis with the 50% occasionally ok.
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ICC
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# Posted: 30 Jun 2021 07:58pm
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Yeah, there is a lot more to this than connecting the batteries. Eight such batteries, configured as described, are not going to have a long life.
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Risky buisness
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# Posted: 30 Jun 2021 08:53pm
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Thanks for the reply folks.
I understand that their is more to this than hooking the batts up.
Not trying to sound argumentative....
Why would these bats have a shortened life span?
I'm discharging only 25- 30%.
Chose to series/ parallel to keep volts up and amps low.
Would you have another option?
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ICC
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# Posted: 30 Jun 2021 09:27pm
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This is directed to the use of flooded or AGM sealed type lead-acid batteries.
There are no true deep-cycle 12-volt batteries. None. Maybe ones called marine or RV deep cycle but those are only slightly better than a standard automotive starting battery.
Theoretically, there is no limit to how many batteries or strings can be connected in parallel. In practice, there are problems more often than not. In fact, anytime the number of cells is increased the chances of problems go up.
If you search online you will find many references that will say three parallel is the limit, but most of those will also say limiting to 2 parallel strings is better. My experience has been that parallel is to be avoided if at all possible. This is why batteries are made in different AH capacities. A system built with larger batteries in series only will outlast a system with similar quality but smaller capacity batteries and with parallel strings.
I am not just repeating what I have read or been told. I have proved this to myself with banks of three parallel strings. All can be fine for a while but when one starts to go bad it starts a cascade.
Lithium such as LFP are better, much better, but require a larger initial outlay of cash, or credit.
If you need to keep the initial cash outlay down I believe you will be much better served with 6 volt golf cart batteries.
6 volt golf cart batteries are group GC2. A typical GC2 is 6 volts at 200 to 225 AH. Let's call it 6-volts at 200 AH. Buy 8. Connect 4 in series for 24 volts. Do that for two strings of 4 batteries. Each string will be 200 AH. Put the two strings in parallel and you have 24 volts with more AH than the 12-volt batteries. Plus you have batteries that have thicker lead plates and will last longer than any 12-volt bank you assemble with 8 batteries. Depending on how deep they are discharged on average, whether they are used daily or more on just weekends and the ambient temperature where they are used, and how good you are at keeping the fluids up, and other factors, such a bank can last 9 years. I have a friend who got 9 years before things went badly enough to require replacement. His were in part-time use and he is a fanatic for keeping the cells full and not abusing them. Plus he is in cool mountains, not a hot climate. That makes a huge difference.
Hopefully, you are asking questions prior to purchase, not after.
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gcrank1
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# Posted: 30 Jun 2021 10:06pm
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When bats are series connected they become 'the battery' After all, that is what an individual commercial battery is, just a bunch of cells series connected.
So when you connect 2 bats (of any matched voltage) in series you get a bigger 'battery' of 2x the voltage. This can be repeated.
As long as you are series connecting you are just building a bigger battery.
This changes when you join them in parallel. The knowledgable consensus is that you dont go more than 4 in parallel, and that is stretching it. Lots of info online as to why.
However, it is still done. You must use heavy gauge wires of the same length to join them, BETTER yet, use buss-bars. They way they are arraigned is waayyy important to keep downstream batteries from receiving decreasing amounts of voltage. This creates an unbalanced discharge and recharge cycle with some never getting fully charged. This is death to FLA, sealed or not.
The position of the input charge wires and the output discharge wires is critical also.
Fwiw, my 'inherited' system was 4 parallel huge UPS 12v bats. they were just stacked in a row, charge controller wires on one end, inverter wires on the other. All hooked up with only 6ga. stranded wire with few properly crimped on eye- ends. And even if the wires would have been bigger and proper ends the set up was ALL Wrong, right from the start using ups / telecomm type batteries.
The batteries died one by one starting at the inverter output end where they got the big load/draw and never got a decent recharge.
But when the guy 'built it' some 10yrs ago he did not have the resources and info we have now. The hacked together system actually worked to a fashion for those 10+ years and Ive been refining it since.
If you do a proper s/p build it can work. Just know that you will likely be replacing the battery bank 1st, and perhaps sooner than you would like, will learn a lot and will do it better each time around.
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Steve_S
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# Posted: 1 Jul 2021 04:46am
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Victron does an excellent job explaining battery wiring configurations here: https://www.victronenergy.com/upload/documents/Wiring-Unlimited-EN.pdf
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Risky buisness
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# Posted: 1 Jul 2021 08:08am
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Thank all you folks for the interesting replays. The system I initially described is one that came with a property we just bought, I know just enough about solar power as to be prone to screw something up.
I was around big equipment starting systems enough to understand the advantages of a series system but not much else in that regard.
I think the 6v golf cart system is probably what I'm going to look into, and thanks for the link to the wiring configuration too.
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scott100
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# Posted: 1 Jul 2021 08:31am
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Is this a setup you are already using or one you are planning? You mentioned discharging it 25-30%. Seems like that might be a pretty large off grid system. As a point of comparison. We have a 24v system with 8 x 6v gc2 golf cart batts like ICC mentioned above. Running led lighting, occasional tv use and power tool use, a 10.1 cu ft fridge, water pumping, microwave, ceiling fans, and computer/phone/various other charging, we rarely drop the bank below 90% on a daily basis. When we do, its after a cloudy, rainy day. So, if this is a system you're planning, do a careful energy audit first.
Trojan, Deka, and Duracell (made by East Penn, the same company that makes Deka) all offer 6v, 8v, and 12v versions of their deep cycle golf cart batteries. The 6v ones will likely be the biggest bang for the buck.
As mentioned by others, how you wire the bank is critical to proper charging. Bus bars are best. I personally use heavy cables, but I have them made to my specs and lengths by a marine service provider who installs crimped ends far better than I can. If you are using a Victron Smart Shunt for battery monitoring, you can set it up for mid point monitoring so you might be able to see a problem caused by one of the batteries sooner than later.
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Nobadays
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# Posted: 1 Jul 2021 08:45am - Edited by: Nobadays
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Quoting: scott100 Bus bars are best.
Beat me to it! Rather than just linking seriesed batteries one to the other in parallel take your cables to busbars, from there to the inverter and SCC. The hypothesis is they will charge and discharge more evenly.
THESE work pretty well.
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Risky buisness
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# Posted: 3 Jul 2021 09:32am
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I have an updated system planned for my next project. I have found that ' solar system batteries ' are pretty scarce, I'm going to source the 6v batteries mentioned in an earlier post for a split system to power a load that the system I ' inherited' is trying to run now.
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gcrank1
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# Posted: 3 Jul 2021 11:12am
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It isnt that they are scarce just not in all the big box stores. If you go someplace battery specific (example: not to endorse but 'Batteries Plus' or maybe a cart parts place like NAPA which may be able to order from their battery supplier or maybe even you go to that bat supplier) you might find some, though not cheap.
As if often said, the big-box GC2's are the best bang for the buck in FLA. A lot of caveats there, but it sounds like you get it.
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Snowbound
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# Posted: 5 Jul 2021 08:55am
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When we bought our property it had 5 12v-100ah batteries in parallel. The first thing I did was to replaced the inverter, it was stupidly oversized that I calculated was using more than half my stored power just running.
After studying up on solar and consulting our local solar guy I dropped it to 3 in parallel. After that everything worked pretty well.
I'm now in the process of replacing the batteries and inverter, going with 4 Rolls Surrette 6V 445 ah batteries in series for a 24 volt system.
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