Oddball question about combining agm bank and lead acid

[Bear in NM]

I did search for this specific question, but most I can find is the obvious caution about putting an agm and lead acid into a single bank. My situation is that I have a pair of lead acid starter batteries in my new van. I just installed a pair of agm batteries for a house bank. I installed a 12v electrical panel with two independent sides, one for house, one for start, each with blue sea breaker panels, both with switches, fuses etc. Between the two, they are connected with a Samlex ACR, which is voltage sensing to allow the system to connect, based on voltage. The bi-directional switch is factory set at 12.8 volts, both sides. Once the charging (solar/charger/alternator) at either end reaches 12.8v, the switch closes, and combines the banks. I have confirmed everything is working.

My problem is that the resting voltage of the starter bank is 12.7v. Good. The resting voltage of the house agm bank is more like 13.1v. With my van just sitting with no outside source charging on either end, my switch is remaining closed due to the 13.1v resting agm side. This effectively combines the two banks, while just resting.

I think that this is a bad situation, but am not sure? I do have a 3-way control switch installed (per DWH"s help in another thread), so manual control is not a problem. Thinking that I should keep the banks separated when not actively charging?

Thanks,

Craig

 

[john61ct]

Lead acid includes AGM, they're just sealed not FLA.

The resting voltage of the house agm bank is more like 13.1v

Resting means with surface charge removed and batt isolated for some time, usually 24-72 hours.

> combines the two banks, while just resting

Not a problem

When any significant load starts up, the voltage will drop.

Main point is to keep Starter from getting pulled down too much, and that's taken care of.

 

[john61ct]

FYI only:

Many setups don't want the extra dead lead weight of dedicated Starter batts.

A single much larger bank gives much higher AH capacity and thus much greater longevity. Lower weight and costs are a bonus.

Say you have three 12V strings, obviously all the same model and age.

One string can be protected from running down too far* with a VSR, ideally one with adjustable setpoints.

With a very large AH capacity and an appropriate chemistry, that string may have no problem cranking starters even at say 60% SoC. Temperature is of course a factor, but find the right Voltage setpoint, and

You maximize your AH, cost and efficiency, rarely if ever "split" your bank, and make sure you can always start your engines.

If you do find the bank gets split more than occasionally, then rotate which string gets isolated for even wear & tear.

For belt & suspenders when off the beaten track, carry (and keep charged!) a little lithium jumpstart powerpack in your glovebox, they make ones can even start huge diesel engines now.

 

[Bear in NM]

John,

Thanks. Yes, dead nuts resting voltages. I have a couple of years experience with these AGM batteries, and know what they do. The van is new to me, but a couple of weeks of monitoring and resting is at 12.7 in the morning for the starter bats. And yes, I do understand that AGM and lead acid have the same chemistry, just different containers, if you will. I was just concerned about the different voltage potential, and whether any undo "wear' would occur with combination due to the different voltage potential.

That is a good point about the concept of only one main bank to minimize overall cost. Obviously folks who spend time in the out and about, want to know that they will have start, no matter the use during vehicle shutdown. The jump pack certainly gives you the "spare", if you are not monitoring your single bank during use. I guess it comes down to ones comfort level, attention to detail and ability to stay on top of things.

I did not note another "issue" with my system that would have probably cluttered my main question (resting combination) but does cross your point about starting and combining. My van is a 6L diesel. The system uses a FICM (control module) that bucks the 12v starting to 48V for the start. Upon start, my start batteries will draw down to 11.4v for crank and initial run, until the alternator kicks in which is some number of many seconds after start. For my dual batteries, I do not want them combined for start. All my primary battery wiring is 1/0, so the wires can handle, I think. But my ACR is rated at 160 amps. I am not up to snuff on what starting amp pulls are like, but guessing more than 160. My plan was to not ever connect at start. Only connect if for some reason I let my starts get weak. In that scenario, I would probably combine for a while before start.

Craig

 

[john61ct]

When that combiner breaks, replace with this

https://www.bluesea.com/products/7620/ML-ACR_Automatic_Charging_Relay_-_12V_DC_500A

And it never will.

Ignition sense is a challenge to wire correctly, even for experienced pro's.

 

[Bear in NM]

John,

Thanks, I think. I do not plan on breaking, but one never knows. Used 1/0 for all my wiring, as I had not planned on starter combining. That Blue Sea relay calls for 4/0. I have a ton of Blue Sea stuff, and really prefer their "goodies", but that ACR is more than I think I need.

Craig

 

[DiploStrat]

Advice is worth what you pay for it, but here goes: I wouldn't worry about things.

-- Intelligent relays often take hours to open. Between 12.7v and 13.1v you probably don't have enough difference to worry about. If/when you add solar, you will see that the relay only opens when you put a real load (e.g. microwave on the camper battery.)

-- Start isolation. Scholars differ, but again, with 1/0 AWG wire I simply would not worry about it. Starting your engine isn't that much of a load and it has been my experience that simply turning on the ignition (glow plugs, other loads) is enough to drop the starter battery voltage below the break point and you will generally hear the relay open even before you crank. If not, I would be surprised if it didn't open at the first crank.

My experience was with a Duramax diesel, OEM GM batteries and Lifeline AGM. YMMV

 

[Bear in NM]

Diplo,

The advice I get here is always pretty solid.

I have the led on my relay, and a 3 position switch. Normal operation, switched off, and switched on. The led is on and the relay clunks closed with the switch in Normal (voltage sensing) or On, with the led glowing. It is instantaneous. I had originally thought that my switch wiring was bad (Blue Sea on-off-on), but realized my resting AGM voltage was over the 12.8 ACR threshold, and closing or leaving the relay closed (connected). It was another duh moment. Yes, my glow plug initialization drops my start battery to 11.5 which would be switch open from the start side. But the agm house side at 13.1 keeps it closed, essentially back feeding with switch closed voltage.

I am fine with the switch operation, now that I know what is happening. I was just uncertain whether leaving the batteries combined while parked or camped might cause issues with the start batteries, at lower voltage than my house battery bank. I definitely plan to do more testing on the starting load parameters on the start side. I would really like to know what kind of draw I get from the start batteries on crank.

I am still on the fence about whether to use my SS Duo with the two systems isolated, or just dump the solar via a regular controller on either end. I am being a bit anal here, as both of my lead acid start batteries are on my frame rail, and dropping the batteries to check electrolyte is not simple nor easy. I like the concept of isolating the two systems, and letting the charge controller 90% charge my house side, while 10%ing the start side. I will be checking those numbers via voltage and amps to make sure whatever I decide is working like I suspect.

Thanks,

Craig

 

[DiploStrat]

… Yes, my glow plug initialization drops my start battery to 11.5 which would be switch open from the start side. But the agm house side at 13.1 keeps it closed, essentially back feeding with switch closed voltage.

That surprises me. I used a Magnum Smart Battery Combiner and a low voltage on either side would open the circuit immediately. I think that the Blue Sea ACR tests a bit - that is, it has to see a solid 13.2v or higher for an extended period. Might the Samlex be single sensing?

… I was just uncertain whether leaving the batteries combined while parked or camped might cause issues with the start batteries, at lower voltage than my house battery bank. ...

No, they should love it. I am a bit surprised at the difference in voltages

I am still on the fence about whether to use my SS Duo with the two systems isolated, or just dump the solar via a regular controller on either end. I am being a bit anal here, as both of my lead acid start batteries are on my frame rail, and dropping the batteries to check electrolyte is not simple nor easy. I like the concept of isolating the two systems, and letting the charge controller 90% charge my house side, while 10%ing the start side. I will be checking those numbers via voltage and amps to make sure whatever I decide is working like I suspect.
...

I would suggest that you are worrying too much. Two batteries connected together will try to reach a common voltage - that voltage being determined by the differences in their voltages and their respective amperes. In this case, neither is going to be hurt by being connected together. This might not be the case with lead acid being combined with LiFePO4.

Awaiting dwh's take on all of this.

 

[dwh]

Hrmm...

Wel first of all, this "start side/house side" idea only applies when the batteries are untied. When the batteries are tied, there is only one side. Even though the ACR is a "dual-sensing" unit, that only happens when the batteries are untied.

According to the manual, if the batteries are untied and either side rises to 13.2v for 60 seconds, it will tie the batteries. If the voltage of the combined bank drops to 12.8v (or less) for 60 seconds it will untie. It will untie immediately if the voltage spikes up to 16v, or drops to <11.8v for 4 seconds.


In normal operation, the batteries get charged up to 14v+ and the charging stops (engine shutdown). Assume both batteries are fully saturated/absorbed and 100% charged. Both batteries will have a surface charge and it will take some time for the surface charges to dissipate. (Which takes a bit longer when the electrolyte is already saturated.)

Finally the surface charge is dissipated and the batteries are resting. But because they are tied, the resting voltage of the combined bank will be 13.1v because one of the batteries rests at 13.1.

Now what?

Well...nothing. or not much anyway. The idea is that current will trickle from the higher voltage battery to the lower voltage battery until they equalize.

Really? How? Both batteries have a very high resistance, and are saturated. Especially, the lower voltage battery is saturated, so it's going to take a pretty good voltage difference potential to overcome that and cause current to flow. 0.3v difference potential probably won't do it.

Basically, they just sit there and nothing happens. So the idea that the lower voltage battery is going to drag down the higher voltage battery and somehow be bad for the higher voltage battery is pretty much false.

Oh sure, in a full-time combined battery bank (solar off-grid house), over a period of months or years, you'd have a measurable problem. But that's a different situation than a part-time bank in a truck.

So they are sitting there and then house loads are applied, pulling down the voltage of the combined bank. Eventually it gets to 12.8v or less for 60 seconds and the ACR disconnects. No problem - it's the house battery that got pulled down to 12.8v. After disconnect, the engine battery is still sitting there at 12.8v.

So far, the entire system is working as intended.


But what about this "starting the truck" deal? It's a diesel. Heating up the glow plugs doesn't draw down the voltage far enough to untie the batteries, so there is a worry that engaging the starter will exceed the 160a rating of the ACR.

Unlikely.

First of all the ACR is rated to handle spikes uo to 480a for 1/4 of a second. Which means it can handle lesser spikes for longer. Like say 240a for 1/2 second.

Second, the cranking battery will flow more current faster than a deep cycle. So the majority of the current needed by the starter is going to flow from the cranking battery.


But it really depends on the size/cranking amps of the engine battery and the size of the engine/starter. Most gas engines have starters that max out around 250a and normally pull maybe 100. Big Rig diesels can hit 500a, or more when it's really cold out.

I think the rule of thumb for diesel starters is like 60a per litre is average. Something like that. Don't quote me. So a 4 litre might draw 240a.

 

[john61ct]

Cranking currents are very often well over 200A, but for very short duration.

If you think that's going to harm your combiner, flash your headlights for a while, it'll isolate quick enough.

The sitting full & combined is no issue at all.

 

[dwh]

Even at 200a probably 150a (or more) of it will come from the cranking battery. Which leaves maybe 50a flowing from the aux through the ACR.

 

[Bear in NM]

Whew, busy weekend with the van electrical. Have the main cover for the electrical installed, and Blue Sea Contura switch panels installed for switches and usb. The solar is not hooked up, but I am trying not to stress over what gets done, just that what is done is done correctly. King Starboard is some really nice material to work with.

dwh, thank you for the detailed reply. I know you do your homework on specific parts/components for these type of questions, and that is not lost on me. Sincerely, thank you.

It seems like the consensus is that the batteries tied at slightly different voltages should not be problematic. That was my main question, and one for which there is not a lot of discussion out there. Yes, combining lead acid and agm is a no no in a single bank. That info is out there, and clear to me.

The ACR is absolutely working correctly. I disconnected the house agm bank for my work this weekend, and the surface/resting charge finally hit 13.1, and my 3 way switch was finally disconnecting in the middle/auto sensing position, as it should. Switch down, ground to ground works. Switch up manual connect clunks the relay closed.

dwj and John, I appreciate the input on tied cranking. When I found the thread mentioning the Samlex ACR, it sounded like a good idea, but I did not give thought to cranking amps, nor the fact that the switch's trigger voltages were better applied to two lead acid banks. I have a PACT 500 in my Avalanche, and when connected it has no issues. I do like the ACR's auto sensing, as it lets me tie things together in the back of the van, without the need to connect to switched power, at the front of the van. It is a very clean install, with a minimal footprint.

John, the headlight trick will not help if the house bank is over 13.2, as the headlight draw down would lower the crank side, not the house side. Back to back feeding the switch from the house side. It does look like the house side might finally rest below the 13.2 given a full charge and a couple of days of sitting. I think in actual use, the house side is going to be lower than my threshold of 13.2 after any kind of camping. Real world v. theoretical.

The more serious concern is the auto tied position and cranking, when driving the van while not camping, or perhaps when leaving for a trip after charging during the night. Or once I have things tied into a roof mounted solar. The obvious solution is to just leave the two banks disconnected until the situation arises where I want them tied.

dwh, mine is a 6l diesel. When I key to glow plugs, per my Scangauge II, the start battery goes to about 11.4v. This is with 2 new batteries, and it starts just fine. No issues on the start side, and the voltatge would certainly untie, if it were not for the fact that the house side could be above the 13.2. I really do not want to have the house auto tied, unless I really know for sure I would have less than 150 amps pulled from the house batteries. I really need to test the start draw, but this is all a bit new to me. If your 60a/liter is even ball park, that would be a lot of amps drawn to start for my van.

I guess the bottom line is I will not worry about the dissimilar resting voltages, but I am going to keep the the two banks separated unless I make the decision to combine, either after starting, or perhaps when in use, when I know the house side is low. The led on the ACR is very visible, I'll just need to make sure to look. One help is that I have to key to fire up the glow plugs, so I have time to verify I am not tied.

Craig

 

[john61ct]

the headlight trick will not help if the house bank is over 13.2, as the headlight draw down would lower the crank side, not the house side.

Reminder: when combined, it's all one circuit. Headlights, or whatever large-enough load engaged before cranking will indeed result in the relay opening, it's just a question of how long.

That bit of AH used up should get replaced pretty quickly once alt is charging, I reckon most of what you see is just surface charge anyway.

> The obvious solution is to just leave the two banks disconnected until the situation arises where I want them tied.

If you resort to manual switches, you defeat the purpose of an ACR, and risk being stranded if you forget to isolate.

But maybe your memory's better than mine. . .

 

[Bear in NM]

John,

Good point about being one bank. I was a little tired when I posted last evening.

The point of installing the ACR was to tie, but have switching to charge the house battery when headed out to camp, or when moving around while camping. I saw the switch noted in one of the DC-DC charging threads. I did not want to commit the funds or footprint/wiring to DC-DC yet, as my house charging needs while traveling are minimal. And the new van's charging system is new to me.

The only possible tie stranding scenario is not possible me thinks. It seems like the resting AGM side is right at the disconnect threshold. Any house loads running should cause disconnect from the house side. Worst case switching will be auto or off. On will be only when charging, or if a situation happened where the start batteries got a bit drained, and I want to connect the banks for a while, prior to starting.

I think things will work as I had hoped, I just had not thought about auto connecting for crank, which I really do not want, just yet, and I was not expecting the fact that the house bank when topped off will auto connect.

My next test will be to charge both sides fully, and let things rest. Then I will key to glow plug start for the diesel engine, and see if this draw will disconnect. Easier than trying to remember to run a load first, as I have to do this anyway, to start the van. And this glow plug initialization hits the starter bank pretty hard.

Craig