Dual battery bank. Off grid in the car. Am I about to make a mistake?

**jflorey2** · 07-25-2016, 11:29 AM

Originally posted by Tamnakz

Here's where I can't get my mind around the math to make a decision.. ALL of my loads are 12v, I'll only really use A/C when I'm on shore power or need power tools without access to it. I know that if I get a 30v range panel, and a 24v capable charge controller, and build a 24v battery bank, all my charging runs that much more efficiently. If I went 24v, I'd get a 24-12 buck converter so I tap the batteries evenly. This would also let me run 24v and convert it to 12v right at the use source, which would minimize line loss.

As long as:
1) Your inverter remains 24 volts
2) You disconnect the buck when it is not in use and
3) The buck is adequately rated

that should work.

Battery bank size, and the panels that go with. Bigger? Smaller? Opinions?

Target C/10 charge rate. (As always the battery manufacturer specs are the final word.) Don't use more than 2 or 3 (max) batteries in parallel. So for your 20ah batteries that would be 6 batteries - 3s2p, 60ah pack. That means you'll want about 6 amps of charge current, which means about 180 watts of panels with an MPPT controller.

Using 24v for the charging efficiency, but losing ?5-15%? when I convert it down to 12v. Is that a crazy idea?

Not as long as you follow the caveats above.

**Tamnakz** · 07-25-2016, 02:58 PM

Originally posted by jflorey2

As long as: 1) Your inverter remains 24 volts 2) You disconnect the buck when it is not in use and 3) The buck is adequately rated that should work. Target C/10 charge rate. (As always the battery manufacturer specs are the final word.) Don't use more than 2 or 3 (max) batteries in parallel. So for your 20ah batteries that would be 6 batteries - 3s2p, 60ah pack. That means you'll want about 6 amps of charge current, which means about 180 watts of panels with an MPPT controller. Not as long as you follow the caveats above.

I would run any inverters or converters from the 24v bank. I'd never individually tap two batteries that function as a bank. To me that's as crazy as tapping into three cells on your cars battery to use a 6v light.
Also, I would ONLY use the 20ah 12v AGM batteries for the small bank in my car. To be clear, I'm looking at one 40ah bank and one 200-300ah or larger bank.

My recipe as I have it so far? Subject to change 100w panel + pwm CC + 40ah at 12v (two 12v 20ah) = small stuff. If I'm correct, the AGM batteries (Duralast Gold) will not suffer at the hands of the 100w panel. I thought I'd found a c/5 charge rate for them but can't seem to find the spec sheet again... I'd verify before I put them together, but I've also got a few 50w panels on hand should I need to downsize.

I am only considering the larger bank at 24v. I would do that with either four 6v batteries or two 12v. Though, I haven't done any reading into 24v batteries to see if there's bang/buck. It wasn't until this morning that it occurred to me I might gain a lot from going 24v even though my uses are all 5-16vdc.

...But your math still stands, and thank you, by the way. I'm still wrapping my head around how much changes going from 12-24v. I was confused until I realized you obviously meant 6amps at 24v.

**Tamnakz** · 07-25-2016, 02:58 PM

duplicate

**Mike90250** · 07-25-2016, 04:28 PM

A 30V panel is not going to be able to fully charge and EQ the batteries. Just skip the small 20Ah batteries, they are going to be more trouble than they are worth, spend that $ on another panel, or a battery isolator so you can charge from the car alternator (12V bank only)

A 24V battery bank needs at least a 35V PV array to be sure EQ happens on a hot day when the panel voltage drops.

But before you jump to 24V (which has advantages over 12V), how often will you have the engine running on the tow vehicle ? Will it be often enough to skip solar and keep the batteries charged ? or use a small inverter generator to charge the batteries, Solar panels require good strong direct sun, and I would want to park in the shade, which would disable SOLAR panels. You can put the panels on a long cord and stick them in the sun, but then be careful they don't wander off.
Lots of things to consider before you spend any money. PV panels flat on a roof, loose efficiency because they are not aimed well.

Collect ideas for a day or 3 before you spend too much $

**Tamnakz** · 07-25-2016, 05:06 PM

Originally posted by Mike90250

A 30V panel is not going to be able to fully charge and EQ the batteries. Just skip the small 20Ah batteries, they are going to be more trouble than they are worth, spend that $ on another panel, or a battery isolator so you can charge from the car alternator (12V bank only)

A 24V battery bank needs at least a 35V PV array to be sure EQ happens on a hot day when the panel voltage drops.

But before you jump to 24V (which has advantages over 12V), how often will you have the engine running on the tow vehicle ? Will it be often enough to skip solar and keep the batteries charged ? or use a small inverter generator to charge the batteries, Solar panels require good strong direct sun, and I would want to park in the shade, which would disable SOLAR panels. You can put the panels on a long cord and stick them in the sun, but then be careful they don't wander off.
Lots of things to consider before you spend any money. PV panels flat on a roof, loose efficiency because they are not aimed well.

Collect ideas for a day or 3 before you spend too much $

I've been collecting ideas for months, slowly but surely. I've got a lot of the facts in my head, but no experience with practical application.

The small batteries would only be used for a small bank. 40AH max. I have a 100w panel for those batteries, or 50w if it's necessary depending on spec'd charge rate(AGM batteries). I also account for the fact that I'm operating said panel on a flat roof and with a pwm CC, I know it'll never put out near 100w to the battery. This small bank may get hooked up with an isolator to charge off the alternator, but plan is not to be running my car much. Park somewhere for a few days to a week while I camp/hike/work etc.

I have a few flex panels (lightweight easy to store and move) and the cabling to Y into my charge controller to be able to move a panel away into the sun. Accounting for distance losses... I will also have a long extension cord and (though I haven't purchased it yet) a proper charger to be able to charge my larger bank on shore power.

The larger bank is what I'm considering running at 24v and using higher voltage panels on. I WOULD NOT USE THE 20AH batteries for this. I've looked at panels with open circuit voltages between 30-42 volts, but I can't choose a panel until I choose batteries. For the larger battery bank, the best I seem to have found so far are Trojan T105 6v batteries, which I have a line on for under 100 new. That was when I thought two T105's was enough for me though. I'm worried it may not be. I could easily run series/parallel with them and have a 24v bank, but that might just be blatant overkill for me...

But... I know I'm better off going too big than too small. I'd rather discharge to 75% than 25%. I'm a thrifty person by nature, but I also realize that with what I'm doing, making the right decision now will save me a lot of money in system revamping/changes later.

My thoughts? Given my widely varying use and light conditions, logic makes me think I should shoot for a 5-7day reserve instead of 2-4 like I see recommended. I don't want this so much for the extra reserve, but the extra charging capacity. If my bank can handle 300w of panel, then a daily 10-20ah useage wouldn't be hard to replenish, even in sub-par light.

Is the train of logic I'm following worth pondering? Or am I missing something that makes it a bad decision outright, and I just don't see it yet?

**jflorey2** · 07-25-2016, 05:13 PM

Originally posted by Tamnakz

But... I know I'm better off going too big than too small. I'd rather discharge to 75% than 25%. I'm a thrifty person by nature, but I also realize that with what I'm doing, making the right decision now will save me a lot of money in system revamping/changes later.

Normally yes. However you have to lug these batteries (and replace them periodically) so consider that cost as well as the additional gas and wear and tear on your rig.

**Tamnakz** · 07-25-2016, 05:16 PM

My thought process is this - most of what I've read on battery life is, your batteries last as well as you treat them. If I can ensure my batteries are useable for a full working life, and the next set, by making a better investment now in what I'm going to charge them with, I'll save myself down the line. Assuming I don't break a panel or fry a controller lol.

**Tamnakz** · 07-25-2016, 05:21 PM

at

Originally posted by jflorey2

Normally yes. However you have to lug these batteries (and replace them periodically) so consider that cost as well as the additional gas and wear and tear on your rig.

I'm aware, though please don't think I don't appreciate the input.

I'm trying to determine if the added weight and investment of the larger array will pay off in the long run. My biggest concern is getting to a place where I'm draining my batteries further than I'd like and then not seeing a full charge before I'm discharging again.

It seems like the larger system, even though it's more than I need for my regular use, would be far easier for me to maintain at a healthy level. Given the low discharge use compared to high charge.

The other consideration I have is using AGM batteries for my large bank as well. Purely for the fact that you can throw a much higher charge current at them, which lets me use a larger panel to make the best use possible of the light I have available.

**jflorey2** · 07-25-2016, 05:29 PM

Originally posted by Tamnakz

The other consideration I have is using AGM batteries for my large bank as well. Purely for the fact that you can throw a much higher charge current at them, which lets me use a larger panel to make the best use possible of the light I have available.

Given that you want to do that, is there a reason you are not considering lithium iron phosphate?

**Tamnakz** · 07-25-2016, 05:51 PM

Originally posted by jflorey2

Given that you want to do that, is there a reason you are not considering lithium iron phosphate?

I don't want to do AGM. It's simply a possibility I've considered, while I try to figure out what would work best for me. What I want? A battery bank that will meet my needs and that won't struggle to fully charge somewhat regularly given my cruddy light situation.

I know that my lack of regular light (as in, the same light consistently, I'll get plenty of sunlight) makes a typical system design less than favorable for me. The only reason I'm considering AGM or oversizing is to make it easier for me to keep my batteries as full as possible as often as possible. I'd rather extend their life than cut it short with system design. AGM batteries would allow me to charge fully more quickly with a larger panel. That doesn't necessarily mean it's worth the additional investment over just a flooded deep cycle. An AGM battery would simply be easier for me to keep from undercharging.

LiFePo4 hasn't ever seemed to be worthwhile to me. Initial investment is so much higher I could replace my batteries at least once, probably twice before reaching the same investment as lithium. I think if I make sure I e wind up with solid plate batteries I'll survive (from what I know, batteries are batteries, electrolyte is essentially the same between manufacturers depending on battery type, plates are what you pay money for, which seems dumb, 'cause lead is cheap)

**Mike90250** · 07-25-2016, 07:02 PM

For the simplest system to start with, forget the small batteries and just don't bother with them or their panels. Invest that $ into another panel for the 24V system. You never end up with too much solar (extremely rare), and forget the expense of AGM, Batteries belong on the trailer tongue, not indoors (unless you are -20F).
And you are going to need a battery charger and generator to run it (harbor freight has some small generators, with small warranties) 3 days no sun, your battery bank is flat and no way to charge it, you ruin your batteries,

**Sunking** · 07-25-2016, 07:19 PM

You do not need or have a solar application. You need a House Battery and Electronic Battery Isolator to use the vehicle alternator to charge the batteries. Your vehicle alternator can generate more power in 30 minutes than the panel can generate in a week.

Even if you use panels, still requires either a generator or the vehicle alternator. It will cost you less than $40 and do more work in 5 minutes that you panel can do in a day.

**Tamnakz** · 07-26-2016, 01:22 AM

Originally posted by Sunking

You do not need or have a solar application. You need a House Battery and Electronic Battery Isolator to use the vehicle alternator to charge the batteries. Your vehicle alternator can generate more power in 30 minutes than the panel can generate in a week.

Even if you use panels, still requires either a generator or the vehicle alternator. It will cost you less than $40 and do more work in 5 minutes that you panel can do in a day.

I've got very small consumption for a 'House Battery' compared to most folks I've seen with similarly designed setups. It helps that I'm a watt counting person even in my house. A large consideration is that I don't plan to use the 'large' bank except for down time. I'm partially disabled and will always deal with downtime to recover my aching bones. The largest constant consumption I could see myself with is about 150watts. That's for a TV, larger fan, both of my small devices charging, and is a liberal estimate.

My vehicle does have a 100amp alternator, but at idle puts out 20-30amps at best. I haven't ruled out an isolator as source altogether, but I will not be driving every day, and I'd rather have that only as a backup. I'd have no problem running my car once in awhile if I'm parked without shore power for too long without sun, but I'd prefer that to be the backup than the norm.

I am aware an isolator, a bumped idle, and patience would charge any batteries I want charged for a few dollars a day, but I'd rather spend 4-600 now if It's not a waste of money to do so to avoid that 345 days out of the year.

**Tamnakz** · 07-26-2016, 01:36 AM

Originally posted by Mike90250

For the simplest system to start with, forget the small batteries and just don't bother with them or their panels. Invest that $ into another panel for the 24V system. You never end up with too much solar (extremely rare), and forget the expense of AGM, Batteries belong on the trailer tongue, not indoors (unless you are -20F).
And you are going to need a battery charger and generator to run it (harbor freight has some small generators, with small warranties) 3 days no sun, your battery bank is flat and no way to charge it, you ruin your batteries,

The biggest reason I'm looking at the small batteries is because I can get them so cheap... $1 an amp hour for batteries less than a year old. I've already got panels and a CC to work with them. I'd never put more than a 10-20watt load on them, and it'd be easy for me to treat my larger battery right even in long term shade. Just a side thought...

AGM is of very little interest to me, unless it's charge characteristics (c/4-6) make it more likely for me to be able to charge a battery to full.

My trailer is an enclosed utility trailer for storage, and my car a station wagon for sleeping. A battery in the trailer is out of the elements, but not so much 'indoors.' I've said before I plan to make use of shore power whenever available. I'll not run a generator. No way no how.

Dual battery bank. Off grid in the car. Am I about to make a mistake?

Dual battery bank. Off grid in the car. Am I about to make a mistake?

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