Question on how batteries work in this ACR+Solar situation

[jeegro]

In short, my plan is to run dual batteries, with a Blue Sea ML-ACR, and a 2x50w 12v panels wired in series to a 15A MPPT controller connected to the AUX battery.

Q1. With the car off, and the solar running, the ACR should open and connect both batteries, correct? What happens when the discharge is greater than the incoming amps from the controller?
Q2. Assuming yes to above, the wise choice would be for both batteries to be deep cycle / high AH specced, so that the discharge is spread across 2 batteries, for greater longevity. I realize that if the discharge is higher than the charge from the panel, at some point the starter battery will get disconnected, correct? Or will it? because the solar controller would be pumping out 14+V volts, is the ACR too dumb in that the overall voltage is likely 12.6+, so everything stays connected? It wouldn't be until the starter battery is nearly totally drained does the overall voltage get pulled low enough so that the ACR disconnects?

I'm making a lot of (likely wrong) statements above, so please correct me if so.

On to how this affects my product choices:
- My tray (google "Columbia overland Discovery 2 dual battery tray") will support 2x Group 34 batteries, but I've been unable to get accurate dimensions on it yet. The guy who makes it says 16"x 11.5"(+/-) footprint. I'm thinking, depending on how wrong I am above, that I might be better off getting a smaller CCA starter battery, and then a larger (AH speced, >100) battery for the AUX to fill as much of the space as possible (because the starter is literally only used for starting). Or, if my assumptions above are somewhat correct, then I get matching high aH deep cycle batteries to distribute the discharge (because the elevated voltage of the controller will keep the batts connected?). In the best case scenario, I think 2x golf cart batteries wired in series would give the best AUX setup, but there isn't space for it.

In other words, how the heck does the ACR work when a solar setup is added to the AUX battery? When the car is running and when it is not.

Goal: maximize battery aH capacity in my given footprint, and maintain a dedicated starter battery so the car will always start no matter what. Let's leave out my load demands for now, I just want to understand how this whole thing will work, and accordingly which batteries to choose given my available space in the tray.

 

[DiploStrat]

Quick Answer

The long answer is to read the documents on my website.

Basically:

-- When the voltage of EITHER battery rises above a threshold (about 13v) and remains there for a determined period (to prevent false cycling) a voltage sensing/automatic/intelligent relay closes.

-- When the voltage of EITHER battery drops below a threshold (about 12.7v - nominal "full charge" for a 12v battery) the relay opens to isolate the batteries.

-- The batteries remain isolated until the voltage of either battery rises above the specified threshold voltage, that is until you repeat the first step.

In most cases (like mine) the starter battery is connected to the vehicle's charging system and the camper/auxiliary/house/domestic battery is connected to solar and shore chargers. Thus a charge applied to either battery will cause them to combine and a drain that is enough to lower the voltage of either battery will cause them to isolate. The result is that running the microwave on a dark night cannot drain your starter battery.

With this set up you can (as I do) use a smaller starter battery for the engine and a larger deep cycle batter for the camper, as long as they have grossly the same charging profile. Obviously, your vehicle's starter battery should match the manufacturer's specs and your shore and solar chargers should be set for your camper battery.

This system works very well.

N.B. Some intelligent relays have variable settings, for example, the Australians sometimes use a "winch" setting which holds the batteries combined until the voltage drops to something like 12.5v. I prefer to simply use a manual override, in my case, a switch on the dash. (Of course, I don't have a winch, but that is another discussion. )

Hope this is helpful.

 

[jeegro]

DiploStrat, thanks this is helpful. I will read your website.

I agree with your setup.. it's the same way I've mocked mine up so far. Outback Joe's article on "Design guide for 12v systems", the very last diagram is how I envision my setup to look.

So there is no benefit in the starter battery being a higher aH slash deep cycle battery?

I understand that if the vehicle is running, the alternator is pumping 100 or whatever amps into the starter battery, raises the volts to 13+, and thus the relay closes.. charging the AUX battery as well.

The part I am fuzzy on is when the vehicle is off, and the solar is pumping amps into the AUX battery at 13V+. Let's say the batteries are 75% charged, so around 12.4V. What voltage does the ACR see from the batteries? If its 13V+, but the panels are pumping in fewer amps than are being discharged from loads, is the relay open or closed? Based on your feedback, it sounds like the ACR can more or less detect the State of Charge of the battery, rather than it's elevated voltage due to charging, and open the relay accordingly, isolating the batteries.

I'm concerned that the ~65 aH AUX battery I could fit space-wise won't be sufficient. That's not a lot of juice. So then I'd have to look into adding another bank (or adding to the AUX bank) to the vehicle cabin, complicating the setup quite a bit.

 

[AndrewP]

If the panels are putting out fewer amps than your loads, then the voltage won't be 13 and the relay will be open. You're going to get about 5 amps in good conditions out of your system. SO if your loads exceed 5 amps, you are discharging the battery. You'll get some voltage sag below 12.8 volts, and that will open the relay. That's why in this type of use, it's smart to charge the battery that's supplying your loads. ie charge the aux battery with your solar, not the main engine battery.

Don't sweat this. In any given lead acid battery, anything above 12.7 or 12.8 is just surface charge. So as soon as you draw any power out of it, the voltage will read less than this. The ACR opens at 12.75 if I remember right, and this works perfectly in practice.

The ACR can only detect voltage not state of charge. If it's greater than 13.0 volts on either side, it closes after 2 minutes. If it's below 12.75 on both sides(It's closed, so both sides are equal), it opens immediately. Simple as that.

 

[DiploStrat]

AndrewP beat me to it.

Sub comment: If the wiring between your two battery banks is properly sized, it really doesn't matter where you apply the charge - get the voltage above the threshold and everybody benefits. Either battery drops below the threshold and the relay isolates the batteries. Really that simple.

 

[jeegro]

Pefect, thanks! I'm clear on this now.

Given this knowledge its time for me to select batteries. The last piece of the puzzle.

Starter Batt: whatever off the shelf CCA/CA rated battery compatible with my vehicle. No reason to overspend here

AUX Batt: If I could fit a Group 31 Deep Cycle (~100aH) I'd run with it. But I can't. After researching LiFePO4, I'm heavily considering this option. I know its relatively uncharted territory... but I could fit a 80+aH battery next to the starter (group 34 ish size), giving me way more usable energy than I could fit with a lead battery. The things I'm fuzzy on:

• Matching charging profiles as to not under/overcharge each battery
• temperature under the bonnet (LiFePO4 is max ~150F)

Since LiFePO4 has a built-in BMS, it sounds perfect and would be a dropin replacement for a lead batt. The Morningstar Sunsaver MPPT is compatible with LiFePO4.
Something like this:
batteryspace dot com/lifepo4-prismatic-battery-12-8v-100ah-1-28-kwh-10c-rate---un38-3-passed-3-2vx4-dgr.aspx

The other option is a rig up a lead AGM somewhere in the cabin. Not my preferred option

Thoughts?

 

[dwh]

The basic misconception you're working from jeegro, is that the solar "pumps out" a certain voltage.

That doesn't happen.

The charge controller might be set to whatever...say 14.5v. But that doesnt mean that is the actual working voltage. That is *the limit*.

The actual working voltage is whatever voltage the battery is at. The charge controller won't allow the solar to push the battery voltage above the limit. If the battery is at 12v, then the charging loop is at 12v and the charge controller is "putting out" 12v.

Power flows not because the solar is *at* a higher voltage. It flows because the solar has a higher *potential* voltage. But having a higher potential doesn't mean it is actually running at its full potential.

So until the battery/charging loop voltage reaches the limit programmed into the controller, the controller is basically just running wide open and waiting for the voltage on the battery side to rise - and that voltage will be whatever voltage the battery is at.

(Like putting your foot to the floor in a heavy vehicle...sure, the throttle is wide open, but it's gonna take a while for the engine revs to get to redline.)

So the solar doesn't fool the ACR, because the ACR does what it does based on battery voltage and the charge controller is always at battery voltage on the battery side of the controller.

 

[dwh]

Thoughts on the lithium?

If it has a built-in BMS, then ANY charge controller is compatible since all it has to do is supply enough voltage and then let the battery's built-in charge controller (the BMS) handle the details.

Matching charge profiles is irrelevant. Configure the solar for the lead-acid settings, and let the BMS do its thing for the lithium.

 

[DaveInDenver]

To dwh's point, the chance for the ML-ACR (which I use) to be 'fooled' is when the one battery in the bank is deeply discharged and I'm severely current limited.

Blue Sea set the combine at 13.5V for 30 seconds and isolate at 12.75V for 30, so it's possible under just the right conditions to have it cycle a few times. If the isolated-side battery (the starter if you're feeding the aux battery side, as I do) is significantly discharged and wants a lot more current than the charger has to source it can bang against the two set points.

The one time this has ever happened to me personally was trying to use a 6A charger (I use a Minn Kota MK-106D to charge and float the truck if it sits for long, that 6A would be similar to a 120Wish solar panel with MPPT) after winching our tractor out of the mud. This was on the old truck with it's 60A alternator, so the winch did actually draw down my starting battery pretty hard. The ACR cycled a couple of times before I noticed what was going on and in that case I just used my other battery charger, an Iota DLS-45/IQ4, which had no trouble holding up the bank.

An alternate option is to just lock the ACR into combine and let the charger run it's automated (presumably requiring a long bulk stage) profile. The potential downside to this is the aux battery will be drawn down making up the difference until the bank equalizes. Doing this once isn't really terrible, it's not unlike giving someone a jump start, but it's not something you want to rely on routinely since that's what will eventually bring a good battery down to the lowest common denominator with the poorer condition battery.

Also usually it's the house battery that's more discharged and when the voltage on it has risen enough to trip the combine with the starter battery you'll only droop the voltage for a few seconds, well within the hysteresis of the ACR. It's also extremely unlikely that you'd draw down a starting battery as far as I did. It was two winching pulls of a small tractor sunk axle deep in mud with an undersized alternator in my back yard. Had I even driven a few miles the alternator would have returned quite a bit of current so the charger wouldn't have had to work as hard.

In the future I don't think this can happen again. I still have my stock loads on my starter battery, which are all either ignition protected or have timers Toyota built in to prevent a dead battery. I don't yet have a bumper for the winch, though, so it's never had to do that. Anyway, all the potential issues, two-way radios and fridge, are on the house battery so it should always be the one that will need the charge. Plus the new truck has the tow package with a 130A alternator, so it should be less of an issue in the future.

 

[jeegro]

Got it. Thanks. The other thing I'm toying with is the ACR vs the DTEC 20a charger + smartpass. According to my research, the DTEC+smartpass is the right choice when 2 conditions are met: 1) you are using a larger deep cycle battery (like the Odyssey Group 31) and 2) the vehicle alternator does not pump out the needed 14.8+V

I don't know for sure what my alternator pumps out - as I'm currently in the middle of an engine rebuild - but I believe its ~13.8V at idle and 14.4V under high rpm. Its a 130A (stock Land Rover Discovery 2). So, it doesn't look like my vehicle is properly equipped to power a larger AGM deep cycle batt without the help of a DTEC DC-DC charger anyway. I realize that solar can help make up some of this.

And I want to run the ACR, not the DTEC. So... I think thats another vote for the LifePO4.

Is there any consensus on this forum as to quality Lithium batteries and suppliers? From searching, I can't seem to find much.

 

[DaveInDenver]

Vehicles are not optimized for charging anything, so don't concern yourself too much.

I consider periodic shore charge with a decent 3- or 4-stage charger, ideally designed for AGM voltages, to be SOP. I put my truck on the Minn Kota every few weeks. I usually pick the first Friday of the month, let it run a cycle for 24 hours (which is really floating for most of that) and let the batteries sit at rest another 24 hours until the next Monday.

I have Odyssey PC1400 batteries and their recommended reconditioning is to draw the battery down to 10V and recharge with a minimum of 40% of C/10 rate, so 22A in my case (40% of 55A-hr) at 14.7V and switch to a float of 13.6V when the current during bulk/absorption has gone to essentially zero. I use the Iota (45A) for this and just let it run it's normal profile. I'll do this maybe once a year after about 3 years of service. I got 7 years from the Red Top in my last truck and it was pretty abused, left partially discharged for a few months during an engine rebuild and run pretty low a few times.

There's also no consensus that you want such high voltages for AGM either. They are sensitive to temperature and car charging systems can charge them too hard. They can vent but there's no way to put back electrolyte that might boil off, so routinely charging with a higher than needed voltage will shorten their lives, too. So if your alternator is charging at 14.4V that's probably actually OK since it won't harm them with overcharging and assuming you do regular topping and occasional condition you should be fine.

 

[AndrewP]

My biased opinion of Lithiums...

They are way too expensive-A $80 lead acid battery can duplicate all of the functions of a Lithium. So $500 vs $80 to save a bit of weight. That's your call.

They are infant technology-Who knows how they'll hold up in a typical underhood environment. Maybe great, but if your $500 battery is dead in 4 years, you'll likely replace it with the $80 battery.

I'd say give the lithium batteries 5 years to be proven, get better and get cheaper. Then it will be time to invest in Lithium batteries. Until then, lead acid will do everything you need to travel around in your Land Rover.

 

[jeegro]

Andrew, I more or less agree with you. The problem is that I'm unable to fit a decent sized lead acid battery in my dual battery tray. The best I can do (I think) is a group 34 ~65aH battery. for a usable energy of 30aH. Not a lot. Now, compare that to a 60 or 80aH lithium battery.... most of that is usable energy. Space is my deciding factor here.

To get my feet wet in solar, maybe I start off with that 65 aH battery and go from there.

 

[DaveInDenver]

If LiFePO4 modules realize their potential life then the cost may work out. They have in theory several times longer life than a lead acid. No one really know since there's not a lot of real use data yet. They could last as much as 10X, but I have my doubts that will prove to be true. The cells may but the whole module, I dunno, so much can happen to a vehicle battery over its life, temperature extremes, vibration, charge/discharge abuse. Maybe it's true. Even if they last only 3x as long the price is getting close to competitive.

BTW, at what depth of discharge voltage are you basing the 30 A-hr usable energy? I use a 0% level as 10.5V and doing this gets me close to the 55 A-hr C/10 rating of my PC1400. Also depends I think on the manufacturer and how optimistic or conservative they were with their ratings.

This is a fairly standard measurement, to use 1.67 V/cell (10.05V) as the 0% level for capacity purposes but there's not really much energy to be extracted over that last 0.45V so practically speaking it doesn't matter and I use a low voltage of 11.0V as my absolute disconnect and I generally get roughly 80% of my capacity to that point.

 

[Rwhat]

Another possible issue you might have.

I'm currently running dual batteries. Both AGM group 31s. I have had a T-max dual battery controller with included dual sensing ACR. Been running this setup for 5years no issues. About a year ago I installed a 100watt solar panel and renogy view star charge controller.

The issue I have is on a cloudy day or when it's sunny and the clouds are moving my relay will open and close continuously. I ended up putting a spade male and female off the ACR so I can just unplug it so it won't sense the voltage changes and just charge the aux battery. I need to just need to run it into the cab on a switch. I don't think there's anyway around it unless the blue seas ACR might be better with it having to sit for 2 minutes and a continuous specified voltage before it either opens or closes.