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Old 06-26-2022, 04:09 PM   #1
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Is it possible to have more than one battery bank

I've been crunching numbers on my electrical needs for my 37ft skoolie. My wife and I want to go completely off grid with it or at least only be dependent on shore power when we really need to be. So from my math on it is all our electrical items I have come to a conservative 24kw/day to a robust 40kw/day (I rounded up on both ends for sake of just in case). So when I got to crunching the numbers on battery bank, a thought occured to me and now I am curious. Is it possible to have more than one battery bank to split the load of the system. Like if I were to put my 24 hour appliances like my fridge and air conditioner on one battery bank and then put all my smaller items on another? And if so would this be more efficient than putting it all one one system?

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Old 06-26-2022, 08:57 PM   #2
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It wouldn't typically be better because if one runs out and the other doesn't, it's just wasted capacity. That's said, if the idea is to conserve for critical loads, then two banks might make sense. For me, I have lights and heaters on the 12v battery bank. Air conditioners, TVs and and vanity items will go on the 48v bank.
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Old 06-26-2022, 10:34 PM   #3
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Yes U Can

Quote:
Originally Posted by Justinb.jackson903 View Post
I've been crunching numbers on my electrical needs for my 37ft skoolie. My wife and I want to go completely off grid with it or at least only be dependent on shore power when we really need to be. So from my math on it is all our electrical items I have come to a conservative 24kw/day to a robust 40kw/day (I rounded up on both ends for sake of just in case). So when I got to crunching the numbers on battery bank, a thought occured to me and now I am curious. Is it possible to have more than one battery bank to split the load of the system. Like if I were to put my 24 hour appliances like my fridge and air conditioner on one battery bank and then put all my smaller items on another? And if so would this be more efficient than putting it all one one system?

Yes, For example, we set up a separate, smaller 12V system that we charge with a dc buck converter from our larger 24V system. It just runs the lights and BiPap machines. Also on an isolator from the vehicle charging system so if we need to we can start the bus and charge the 12V system.


This way if we have several cloudy days in a row that are also hot, which, means we are running the minisplit, and we run our storage down to minimums we don't have to worry that our bipap machines will quit while we are sleeping.


We live full time in the bus and have only had to turn the generator on a total of 14 hours in the last 400 days. And, let me be clear, we didn't have to run the generator, we just wanted to run the AC and charge the batteries on the 9th straight day of heavy dark clouds. That almost never happens in SW New Mexico.

Our ideal system, the one we are saving up for, adds a separate inverter and battery bank for the minisplit.


We went to skooliepalooza this year and we were surprised that we were the only ones with a whole roof full of solar. I don't know why. We have about 3900W on the roof and are always thinking about how to squeeze a little more up there.



We are actually building a 1500W shade shelter where we park during the winter that will plug right into our main storage bank when we are parked.



We like being able to have a full sized fridge, deep freeze, electric cooking appliances, 50" TV,and most of all air conditioning.
When we fist started out we were pinching our power budget.



One of our tricks was to put the fridge and freezer on timers so that they only ran when the sun was shining.
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Old 06-26-2022, 11:08 PM   #4
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Yeah that was the kind of idea I was thinking like all the stuff that isn't too necessary on one and all critical items on the other. Also after looking at the prices of batteries if I plan it out right it might be just a hair cheaper to split it in the sense of two smaller banks
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Old 06-26-2022, 11:14 PM   #5
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Quote:
Originally Posted by Rock-N-Ruth View Post
Yes, For example, we set up a separate, smaller 12V system that we charge with a dc buck converter from our larger 24V system. It just runs the lights and BiPap machines. Also on an isolator from the vehicle charging system so if we need to we can start the bus and charge the 12V system.


This way if we have several cloudy days in a row that are also hot, which, means we are running the minisplit, and we run our storage down to minimums we don't have to worry that our bipap machines will quit while we are sleeping.


We live full time in the bus and have only had to turn the generator on a total of 14 hours in the last 400 days. And, let me be clear, we didn't have to run the generator, we just wanted to run the AC and charge the batteries on the 9th straight day of heavy dark clouds. That almost never happens in SW New Mexico.

Our ideal system, the one we are saving up for, adds a separate inverter and battery bank for the minisplit.


We went to skooliepalooza this year and we were surprised that we were the only ones with a whole roof full of solar. I don't know why. We have about 3900W on the roof and are always thinking about how to squeeze a little more up there.



We are actually building a 1500W shade shelter where we park during the winter that will plug right into our main storage bank when we are parked.



We like being able to have a full sized fridge, deep freeze, electric cooking appliances, 50" TV,and most of all air conditioning.
When we fist started out we were pinching our power budget.



One of our tricks was to put the fridge and freezer on timers so that they only ran when the sun was shining.
Yeah I've driven through New Mexico and Arizona in n the summer in my Ford escape a/c is a necessary life support item lol.....that is interesting about skooliepalooza I've never been but hey maybe when it's going and we're both there you won't be the only there with a roof full of solar....yeah I'm only in the planning phase and already thinking about what all can I put up there and get away with before hitting max height
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Old 06-27-2022, 03:46 PM   #6
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Interesting design question. Doable, but gets complicated very quickly.

What sources of charging would you have? Solar, alternator, shore power? Maybe all three?

And assuming you need 120VAC receptacles, you would need a dedicated inverter from that second battery bank...plus some form of battery management system.

Will you have dedicated receptacles and light fixtures, or try to power them from both battery banks (only one at a time, of course)?

The easiest way would be to connect the batteries together but put some of them on an isolator, but that route does not look promising. I did a quick check of isolators and I found none with programmable voltages-meaning that the isolators are only for lead acid batteries with cut in and cut out voltages favoring those style of batteries, voltages that don't really help you (13.5 cut in for charging, 12.5 cut out for discharging, meaning they won't work quite right with a LiFePo charge curve and/or will be almost totally depleted by the time the isolator cuts out).

If ALL your batteries were lead acid, you could put an isolator on the second bank, get about 60% out of the secondary bank before the isolator cuts power from it, then as long as the secondary bank's remaining ~10% of charge before it hits the 50% of charge is enough to carry your loads through the night....but that's just wrong on too many levels, so probably not recomended...

If you plan on dedicating receptacles (and lights) to the second, separate bank, one way to do this might be to power a separate battery charger connected only to the second battery bank, and only connect it to the dedicated circuits whether 12v or 120v through an inverter. You power the charger with a DCDC charger from the alternator (or for that matter from the load lines of an MPPT controller). While it might be a little inefficient, it would put control over the charge cycle on the second battery bank.

It starts to look complicated but if you truly want to ensure the second bank is not drained, I don't see any way you can connect to the first battery bank and ensure the second bank isn't discharged, including inadvertently if you put for instance a manual switch between the two (not really a smart idea for other reasons).
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Old 06-27-2022, 04:48 PM   #7
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Quote:
Originally Posted by Rucker View Post
Interesting design question. Doable, but gets complicated very quickly.

What sources of charging would you have? Solar, alternator, shore power? Maybe all three?

And assuming you need 120VAC receptacles, you would need a dedicated inverter from that second battery bank...plus some form of battery management system.

Will you have dedicated receptacles and light fixtures, or try to power them from both battery banks (only one at a time, of course)?

The easiest way would be to connect the batteries together but put some of them on an isolator, but that route does not look promising. I did a quick check of isolators and I found none with programmable voltages-meaning that the isolators are only for lead acid batteries with cut in and cut out voltages favoring those style of batteries, voltages that don't really help you (13.5 cut in for charging, 12.5 cut out for discharging, meaning they won't work quite right with a LiFePo charge curve and/or will be almost totally depleted by the time the isolator cuts out).

If ALL your batteries were lead acid, you could put an isolator on the second bank, get about 60% out of the secondary bank before the isolator cuts power from it, then as long as the secondary bank's remaining ~10% of charge before it hits the 50% of charge is enough to carry your loads through the night....but that's just wrong on too many levels, so probably not recomended...

If you plan on dedicating receptacles (and lights) to the second, separate bank, one way to do this might be to power a separate battery charger connected only to the second battery bank, and only connect it to the dedicated circuits whether 12v or 120v through an inverter. You power the charger with a DCDC charger from the alternator (or for that matter from the load lines of an MPPT controller). While it might be a little inefficient, it would put control over the charge cycle on the second battery bank.

It starts to look complicated but if you truly want to ensure the second bank is not drained, I don't see any way you can connect to the first battery bank and ensure the second bank isn't discharged, including inadvertently if you put for instance a manual switch between the two (not really a smart idea for other reasons).
You have given me something to really look at with this....I was wanting to go with lifepo4 batteries because of their performance advantages and physical specs of being a bit lighter than lead acid....if I were to say make both banks the same voltage and if possible charged from the same panels but routed the wires from each bankt to the same inverter for just 120 but go to two separate breaker boxes would that work....probably definitely would need a separate dedicated battery monitor for the second bank...it's highly recommended not to do alternator charging with lifepo4s so generator/ shore power and solar was what i was thinking for charging....I've also considered including a smart battery charger ive heard about....but yeah still in the planning number crunching side of it indont know if inverters come with 2 output ports but if i do have to get two separate inverters that is something to consider....to answer your question about dedicated vs a little on both itbwould be dedicated on both like one strictly for the larger energy items a fridge, a/c, and an air fryer with the other handling lights and the other ports
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Old 06-27-2022, 06:59 PM   #8
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Good to hear you are pretty set on LiFePo. There really isn't any reason in 2022 to do Lead Acid.

The biggest challenge I see is really just making the system automagic. That is to say, a system that requires no intervention to operate as planned.

You COULD have two separate battery banks that power the same inverter with a breaker/switch, but then you'd need to manually operate the switch, which just invites problems.

Better might be to put a smaller inverter on the second battery bank that feeds into an autotransfer switch ($100) on the dedicated branch circuit downstream from the primary inverter. The secondary battery system is a 'safety system'. If/when the main battery bank voltage drops too far the primary inverter will shut off and the autotransfer switch will jump to the secondary inverter power. Of course, your primary inverter will be squealing or beeping and wake you up, but even that can be remedied.

Alternatively you could just put a smaller inverter on a properly sized secondary battery bank (properly sized) that only feeds the dedicated receptacle.

You are correct that you can't just pump alternator juice straight into a LiFePo battery, you'd need a DCDC charger.

There's still a lot of detail and room for complexity, but there are some solution paths. I'm sort of surprised there's not an isolator out there that can have specific voltage setpoints for cutting in and out.
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Old 06-27-2022, 07:20 PM   #9
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Quote:
Originally Posted by Rucker View Post
Good to hear you are pretty set on LiFePo. There really isn't any reason in 2022 to do Lead Acid.

The biggest challenge I see is really just making the system automagic. That is to say, a system that requires no intervention to operate as planned.

You COULD have two separate battery banks that power the same inverter with a breaker/switch, but then you'd need to manually operate the switch, which just invites problems.

Better might be to put a smaller inverter on the second battery bank that feeds into an autotransfer switch ($100) on the dedicated branch circuit downstream from the primary inverter. The secondary battery system is a 'safety system'. If/when the main battery bank voltage drops too far the primary inverter will shut off and the autotransfer switch will jump to the secondary inverter power. Of course, your primary inverter will be squealing or beeping and wake you up, but even that can be remedied.

Alternatively you could just put a smaller inverter on a properly sized secondary battery bank (properly sized) that only feeds the dedicated receptacle.

You are correct that you can't just pump alternator juice straight into a LiFePo battery, you'd need a DCDC charger.

There's still a lot of detail and room for complexity, but there are some solution paths. I'm sort of surprised there's not an isolator out there that can have specific voltage setpoints for cutting in and out.
Yeah there's definitely still a lot of room for complexity and multiple ways to get results that work...the real question really is what's the most efficient and also effective way to do so monetarily and performance biggest bang per dollar kind of thing
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Old 07-02-2022, 03:17 PM   #10
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One of the 4 wheel drive van companies makes a system like you want.

Also, semi trucks use an APU (auxiliary power unit). It is a small diesel unit that powers your a/c and various other engine driven accessories. I have seen them listed on eBay.
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Old 07-02-2022, 03:41 PM   #11
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My solar system is essentially two separate systems running in parallel: each array of PV panels is producing slightly different power than the other because they are at different angles to the sun (they are all hinged to a central walkway). Each array powers its own Morningstar TS-MPPT-60 charge controller that charges its own bank of four golfcart batteries, and each battery bank runs through a 250A Schottky diode to prevent one bank from back-feeding into the other; while both banks feed into a common DC load bus, each bank is electrically separate from the other. An added benefit of this system is that I will never have less than 50% of my power if one battery bank or one charge controller or one PV array goes tits up and carks. Redundancy is good!

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Old 07-02-2022, 03:58 PM   #12
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There are aux load panels/outlets that are programmable to only be on when the batteries are above X volts/percent charged. I plan to use one or two to set up “nice to have” loads and “dump” loads to burn power when I have an excess.
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Old 07-02-2022, 07:59 PM   #13
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semi trucks use an APU (auxiliary power unit). It is a small diesel unit that powers your a/c and various other engine driven accessories. I have seen them listed on eBay.
Yes, there's a couple of different APUs out there which are essentially just generators small enough to hang off the truck frame. But if OP is looking for separate battery banks and already has the power source figured out, some semi trucks use a dual-pack system. My company's trucks use Freightliner's ParkSmart technology. There's a battery bank for the engine starting and a second bank for the driver comforts in the sleeper berth. Of course most trucks spend 10-12 hours a day driving so recharging isn't a big concern. But some trucks also have solar panels which significantly extend the battery life during an extended break. If the batteries begin waning, the monitoring system begins shutting down non-essential systems - usually interior lights, then outlets, then fridge, and finally heat/AC. The other feature is Optimized Idle which will start the engine and maintain the batteries as well as ensure interior comfort. The bad part is idling burns fuel and static idling has repercussions with regards to longevity and exhaust treatment system if equipped. For this reason an APU or generator is going to be more fuel efficient and hopefully quieter as well and preserve your engine. I have plans to adapt a similar scheme in my coach when I get to that point. One battery bay for native 12v applications and a second battery bay for inverter loads requiring 110v. Solar and generator are supplemental but I also plan to be moving almost daily so the engine alternator can replenish batteries.
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Old 07-03-2022, 12:07 PM   #14
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Question: I see no time factor in your numbers. Do you mean kilowatt HOURS?
You might want to consider paring down your usage. We seem to do just fine with 1080 nominal watts on the roof and 5040 watt hours of battery at 12vdc. (FLA, so 2520 at 50%) The battery bank has never gone below 75%, and is fully replenished by 1 pm every day, and we nearly always park in shade. However, we have no electric cooking appliances, nor A/C, and rely little on electronic entertainment. Cooking, water heating and space heating all with propane; 12vdc fridge, lights, radio. No TV. 2000 watt inverter is really for the small microwave, used a couple of minutes at a time. In winter, wife likes her 12vdc 55watt electric blanket for an hour or so to warm the bed up. I realized early in the solar electric design phase that using gas for any/every kind of significant heating was the way to go.
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Old 07-04-2022, 02:14 PM   #15
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Quote:
Originally Posted by Justinb.jackson903 View Post
My wife and I want to go completely off grid with it or at least only be dependent on shore power when we really need to be. So from my math on it is all our electrical items I have come to a conservative 24kw/day to a robust 40kw/day (I rounded up on both ends for sake of just in case).

SOMETHING IS WRONG WITH YOUR MATH or you have built a Tardis style bus.
Our current home burns only an average of 16 kWh/day and we are "all electric".


In your calculations did you account for:

"duty cycle"?
the fact that you won't run A/C and heat at same time?
Other items that won't run concurrently


Or perhaps your calculations have the inefficiencies and drains too high?



Remember, while a specific 9000 BTU mini split may pull 800 watts, it only does so when the compressor is running. The rest of the time it is pulling considerably less power (just running a fan and the miniscule wattage to run the solid state controls). So if yo figured two A/C's running 800 watts for 24 hours each, you've significantly over estimated your power requirements unless you plan to camp on the surface of the sun......or anywhere below 1500 feet in Arizona in the summer.


We bought a "kill a watt" meter and have checked ACTUAL power draws on those things we're bringing into the bus as well as a lot of things that we can use to estimate actual bus gear.


Our inefficient, big, old, refrigerator is about 900 watts/day over a one week test.
Our electric mattress pad is less than 600 watts for the entire night on MAX temp. We will definitely be letting the bus cool off at night and using the mattress pad.
Our 1800 watt inductive "hot plate" will pull 1800 watts but as soon as the pot or pan is up to temp it goes WAY down and cycles based on the temp range selected. But an hour using an 1800 cook top doesn't take 1800 watts is the point.
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