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Sizing battery banks


noteven
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Is there a point at which a battery bank can be too large in relation to the solar array used to charge it? 

In my research I think I understand you should have panels matched as close as possible to your average daily use (considering sunlight, season etc) then have some additional battery bank capacity to carry you over on less efficient charging days? 

My tow vehicle has lots of unused payload capacity. If I built a battery bank on the truck connected to the trailer bank by a Big set of cables, are there precautions I need to take for situations where there may be a state of charge difference between the truck bank and the trailer? For example the trailer is left parked on a good charging day and is at 100% while the truck is used for a day trip and returns to base with a not yet fully charged deep cycle bank... does the flow of current that will occur when you connect the two banks (using proper method) harm deep cycle batteries in any way? 

Thanks 

Edited by noteven
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Could you set up the truck bank to charge while driving?  Use a solenoid to connect them to the alternator once the truck is running.

Regardless, how much battery do you need?  Would the extra in the truck be overkill?  I can't see a need for more than 600-800 Amp Hours and 6-8 Trojan T105s will get get you that.  Do you really need more?

Lenp

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20 hours ago, noteven said:

If I built a battery bank on the truck connected to the trailer bank by a Big set of cables, are there precautions I need to take for situations where there may be a state of charge difference between the truck bank and the trailer?

Good questions even, here are my thoughts:

As long as the cables have adequate ampacity and there's short circuit and overcurrent protection I don't see you need other precautions. Sure if one bank is at a higher voltage then the other some degree (depends on voltages and cable resistance) of charging current can flow until such time voltages equalize but proper overcurrent protection and cable ampacity  should handle it. A sufficient amperage in line diode can assure  current flows in only one direction, but it creates a voltage drop and resistance.

20 hours ago, noteven said:

does the flow of current that will occur when you connect the two banks (using proper method) harm deep cycle batteries in any way? 

 As long as the amount of current doesn't exceed charging including time and current and voltage limits into the deep cycle batteries, I don't see they are harmed. 

BOTTOM LINE If you consider a battery charger may operate at say 13 up to 14 + volts (in order to charge a 12.6 volt battery) while a full charged battery at rest and stabilized is only around 12.6 volts UNLESS one of the two banks is severely depleted and based on cable resistance,  I don't envision any HUGE current flows one way or the other you need to worry about. You could size the overcurrent protection devices (perhaps auto resetting breakers) at each bank conservatively which will prevent excess current concerns.

REGARDLESS if one bank is full charged at rest and is at 12.6 volts and the other is severely depleted maybe even 12 volts DUE TO CABLE RESISTANCE AND SUCH A SMALL POTENTIAL DIFFERENCE I DONT ENVISION ANY EXCESS CURRENT PROBLEMS (compared to what a 50 or more amp 14 volt charger or alternator might deliver) BUT IF SO  PROTECTIVE DEVICES WILL HANDLE IT.........  

The use of relays and diodes and/or switches can allow manual controls over charging problems from one bank to another if you're still concerned, but they need to have sufficient current rating and 100% duty cycle rating.

One other concern is for charging balance where its best if ALL batteries and banks etc are the same design and type and size and even age but if you try to charge (via alternator or solar or charger)  multiple connected batteries, say deep cycles versus starting batteries,  they may NOT receive equal charge.   

 

John T

 

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22 hours ago, noteven said:

Is there a point at which a battery bank can be too large in relation to the solar array used to charge it?

I don't think too large a bank is so much of a problem HOWEVER if there's inadequate charging capacity, be it solar or otherwise, such that the bank remains in a discharged state for long periods, that can be harmful to the batteries.

John T 

Edited by oldjohnt
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Quote

Is there a point at which a battery bank can be too large in relation to the solar array used to charge it? 

It is very important to get all lead acid batteries charged to 100% every few days to a week.  If you don't the plates sulfate and you begin to loose battery capacity.  An equalization cycle will help with the sulfation, but it is best to keep the batteries at 100% as much as possible.

So you want a solar array large enough to get the batteries to 100% or supplement with a generator for an hour or two to supply a good starting bulk charge and then allow the solar to complete the charge. 

Keep in mind that the last 5% to 7% (i.e. going from 93% or 95% to 100% charged) of the charging of lead acid takes several hours of low amperage charging.  Think of 1-5 amps of constant charging.

Edited by Al F
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12 hours ago, lenp said:

Could you set up the truck bank to charge while driving?  Use a solenoid to connect them to the alternator once the truck is running.

Regardless, how much battery do you need?  Would the extra in the truck be overkill?  I can't see a need for more than 600-800 Amp Hours and 6-8 Trojan T105s will get get you that.  Do you really need more?

Lenp

Hi Lenp- yes I would configure the truck to charge via large gauge cables and an isolator. My won ton and truck camper is set up like that. 

My trailer is a toy haul 5th wheel so battery box area is small. I could do some re arrangements in the generator compartment up front and set up a battery compartment in there.

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John T - thank you I hoped you would share your knowledge. 

Al F - yes that is my understanding of how wet deep cycle batteries charge - It seems it is better to have fewer batteries and a matched solar array rather than run a large bank at 85-90% state of charge all the time. 

My electric needs are generally low. 

 

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Lithium batteries look promising. 

I guess I should research discharge controls and how they like cold weather (as in western Canada -30F).  I understand they charge quickly, aren't bothered by partial charging, and have 80% amp hour rating available. Maybe I should invest in a lithium setup and an ultra-silenced generator box for the generator I already own? Then add some panels in future?  

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16 hours ago, noteven said:

- It seems it is better to have fewer batteries and a matched solar array rather than run a large bank at 85-90% state of charge all the time. 

Not at all.

Your primary focus should be on sizing your battery bank based on an in-depth energy audit without consideration to how you're going to refill it, just yet. That's secondary. You have multiple recharging options that give you a great deal of flexibility. You have few options if your battery bank won't provide what you require on a daily basis at a level that allows you to maintain your lifestyle, optimal battery health and longevity.

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12 hours ago, noteven said:

Lithium batteries look promising. 

I guess I should research discharge controls and how they like cold weather (as in western Canada -30F).  I understand they charge quickly, aren't bothered by partial charging, and have 80% amp hour rating available. Maybe I should invest in a lithium setup and an ultra-silenced generator box for the generator I already own? Then add some panels in future?  

Many people have installed their lithium batteries inside the living area, such as under the bed, or if you have a dinette in the box under the seat.  Now if you are going to store the RV in an unheated location at -30F, I am not sure if lithium will tolerate that temp even if totally disconnected from any load.  Heck getting to -30F is pushing it on a fully charged lead acid battery. 

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8 hours ago, Al F said:

Many people have installed their lithium batteries inside the living area, such as under the bed

Al, as many here know I support a persons free choice to do as they please in their own RV, but Lithium batteries under my bed would NOT be my choice. I'm not saying good or bad, I'm ONLY saying that's not where I would install them.

 

John T

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14 hours ago, oldjohnt said:

Al, as many here know I support a persons free choice to do as they please in their own RV, but Lithium batteries under my bed would NOT be my choice. I'm not saying good or bad, I'm ONLY saying that's not where I would install them.

 

John T

OK.  Not good. Not bad. To have your lithium battery bank under your bed.  Under the dinette seat, in the bottom of the closet, perhaps?  

Just a thought, it is kind of like saying I don't eat broccoli.  Not good or bad I just don't eat broccoli.  Others do, but I don't. 

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On ‎1‎/‎19‎/‎2018 at 7:43 AM, Al F said:

Not good or bad I just don't eat broccoli

Mornin Al,  for the record I HATE BROCOLI  lol  but support those who do like it SUCH AS MY FIRST WIFE

 Keep warm its been a bit nippy here in central Florida brrrrrrrrrrrrr

 PS   I'm  Not a fan of spinach either lol and my own "personal choice" (while respecting other choices)  is that batteries be located in places OTHER THEN living space air/environment,  but feel free to put them any darn place is fine by me...........Heck just because I wouldn't "choose" to put house batteries (any type) under my bed don't mean that may not be the absolute safest place (which I seriously doubt) possible ?????????

 Thanks Al, nice chatting with you

John T   NOT a broccoli or spinach or cauliflower or brussel sprouts or tofu or humus eater either lol NOT saying its good or bad however 

 

Edited by oldjohnt
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I can't remember if it's broccoli or kale that should always be cooked with olive oil or butter so it slides into the garbage easier...

I subject power tool lithium batteries to cold and it hasn't wrecked them so far but I'm a combination of a cheapskate and a chicken when it comes to subjecting $100's a dollar rv batts to cold during an emergency winterize and storage event. I guess a guy could take em on the plane back for the family emergency, but it would not be handy...

I just got looking at my USA Battery "T 105's" they went in service in 2009.  

About 60 days / Year use till full time in 2016...

during recreation use years stored in heated garage, put on charge every 60 days or so with a "smart" battery charger...

Moved from truck camper to trailer as needed, run to 10volts on an electric fencer once, pretty much abused most of the 60 days a year of use... half assed charged with gens, then plugged in  for days to shore power, kids left the lights on run them nearly flat...

i guess I should shop for something more long lasting? :lol:

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On 1/17/2018 at 7:51 PM, GlennWest said:

Can use them below 32 f but not charge at that temp. basement should stay above that or you in trouble.

Hi Glenn - My use is such that Lithium batteries would work if located inside the heated envelope. They would need to be removed to heated storage during a "unit shutdown event." 

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On 1/20/2018 at 1:11 PM, noteven said:

I can't remember if it's broccoli or kale that should always be cooked with olive oil or butter so it slides into the garbage easier...

I subject power tool lithium batteries to cold and it hasn't wrecked them so far but I'm a combination of a cheapskate and a chicken when it comes to subjecting $100's a dollar rv batts to cold during an emergency winterize and storage event. I guess a guy could take em on the plane back for the family emergency, but it would not be handy...

I just got looking at my USA Battery "T 105's" they went in service in 2009.  

About 60 days / Year use till full time in 2016...

during recreation use years stored in heated garage, put on charge every 60 days or so with a "smart" battery charger...

Moved from truck camper to trailer as needed, run to 10volts on an electric fencer once, pretty much abused most of the 60 days a year of use... half assed charged with gens, then plugged in  for days to shore power, kids left the lights on run them nearly flat...

i guess I should shop for something more long lasting? :lol:

What I am understanding from your description of your batteries, is that your 9 year old batteries are working fine for what you want to use them for. 

To have a good idea of how many amp hours (AH) are left in your battery pack would require a test.  It would be interesting to find out about how many AH's are left in the batteries. 

I believe that when Trojan rates their battery end-of-life, it is when the battery is down to having 80% of the AH's that it started with.  I would certainly be quite happy with 9 year old batteries that still gave me 80% of the capacity they had when I bought them.

The way I test the AH capacity of my battery pack is to insure they are at 100% charged.  Then using my Trimetric to monitor the AH usage, I hook up a 120V 1500 watt elect heater and run it until my batteries are down to 75% full.  Then after letting them rest for about an hour in a no load condition, I check the battery voltage and compare that to a battery voltage to percent full chart. 

I can't tell you just how accurate this test is.   However if anyone does this test and the ending battery voltage says 50% full and your AH's used show 75% full, you know you have a problem.

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