How to charge new agm house batteries

We are using a Blue Sea VSR along with our Blue Sky 3024 solar controller to provide reasonably complete charging. The van alternator is the 230A unit supplied to charge the car dual AGMs. No additional inverter is used.

The idea is that IF the house is depleted to some lowish voltage AND the car is running, the relay engages, and the car alternator will automatically charge the house. Once the house gets over some threshold, the relay disconnects. This will be before full float. At that point, the solar controller will continue to charge the house to 100% based on its AGM profile. I don't have the VSR transition points in front of me, but they are in the Blue Sea documents.

Even if this method does not completely match a theoretically ideal charge curve, the battery life impact should be minimal..at least from what I've read. Probably long enough for lithiums to become a mature viable, and affordable alternative.

It has few other advantages:
It easily allows for the house to be used to jump start the car.
It is far less lossy than an extra inverter scheme.so house charging can go as fast as the alternator can handle.
It all happens in the background. Normally, no need to flip switches (unless you are doing a jump start). The VSR come with a remote switch for handling the different scenarios.
When in storage, shore power can be used to charge & maintain both the house and car AGMs via the MS2012 inverter/charger.
It's practical to set the system to use the solar to keep the car topped off as well as the house. For example if we are out camping someplace sunny.

Stan

orton said:

Do you have a diesel that should not be idled? With gas engine you do not have to drive the vehicle. Just run the engine.

Doubt that the charge time will be much different. As the house battery gets charged the rate of charge will be reduced well below the limitation caused by the 30% inefficiency.

My 1000 watt Magnum MMS1012 has a max charger rate of 50 amps so that is my limiting factor with the two inverter system. Not the capability of the alternator or the vehicle inverter.

It is very important to me to charge the battery correctly with a 3 stage charger with temperature compensation. Shorter battery life is expensive.

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I was pointing out that efficiency is not just measured by fuel costs. For my application, other inefficiencies of the extra inverter scheme come into play.

But to address your points:

Idling the engine is not a desirable option for me.
There is no high idle option with gas engines, and the alternator output is much lower at idle than when driving, so charge time is extended.
Since I would not leave a vehicle running if I'm not there, I would need to be sitting in the (exhaust laden) campsite, instead of going off hiking, fishing, etc.

As the charge rate drops (with whatever engine driven charging scheme), and the system drops out of bulk mode, my solar can (mostly) handle the absorption and float modes...along with most of the day-to-day loads (LEDs, H20 pump, MaxxAir, etc.). Except on the very darkest days, it now does that. What will cause my system to need a boost is the use of stuff like the microwave, lots of house heater fan on a cold night, and possibly the fridge (TBD).
So, bulk mode mainly sets the time I must drive. Unless the batteries are completely flat, bulk mode charging rate...even without temperature compensation, is likely to be well under what the batteries can accept safely.

With the VSR, my charge rate isn't limited by either side of the extra inverter design. My MS2012 isn't in the circuit at this point. The only time it will be used as a charger is when I'm on shore power (rare if ever when not in storage).

Yes, decent and large AGM batteries are expensive. But amortized over the lifetime of the batteries, the cost of a (possibly) somewhat shortened battery life is acceptable to me. I don't think I've ever had a car, motor-home, or trailer camper battery die in under 5 years. All charged by alternator alone. Many without any modern 3 stage charger available, even for storage. So for my use, possibly extended battery life doesn't outweigh the trade-offs I'd make with the extra inverter design.

skagitstan said:

We are using a Blue Sea VSR along with our Blue Sky 3024 solar controller to provide reasonably complete charging. The van alternator is the 230A unit supplied to charge the car dual AGMs. No additional inverter is used.

The idea is that IF the house is depleted to some lowish voltage AND the car is running, the relay engages, and the car alternator will automatically charge the house. Once the house gets over some threshold, the relay disconnects. ....
Stan

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I've just re-read the Blue Sea docs, and this red part of my statement is inaccurate. Here is what the documents say:

Relay Contact Position:
-Combine (30 sec.) 13.5V
-Combine (90 sec.) 13.0V
-Open Low (10 sec.) 12.35V
-Open Low (30 sec.) 12.75V
-Open High 16.2V 32.4V

So the true operation is if one of the battery banks is >13V for some period of time, it combines the banks.
In my setup, this state happens if the engine is running, or if the solar/shore power controllers are providing enough oomph.
If both banks are less than 12.75V, the implication is that there are no active charging sources, so the relay releases.

This does leave the possibility that the alternator can hold the house batteries at bulk longer than we'd prefer. I will need to run some tests.

Sorry for any misleading....:blush:
Stan

morey said:

Sounds like a good plan. Do you have a schematic of your system?

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Nothing in a post-able format that's real current. I will work on getting what I have updated. However, here's what I put in my blog back in October.

Here are the major differences from current reality:
There is now a 200A breaker in series with the 300A house battery fuse.
There is a 200A fuse between the car battery and the VSR
I have remotes for both the 3024 and MS2012 chargers. They both have sense lines (in parallel) monitoring the shunt.
The ML-ACR VSR's remote switch is not shown.
Car and house (-) tie point is not shown.

Stan

swellguy said:

...

I got my electrical system up and attached a fancy battery monitor to it. It turns out that if you simply wire the house battery to the starter battery, that the current being charged wildly fluctuates, and my guess is, it's charging both as a unit. This is wrong. What you want is for each of the batteries to be charged independently according to their state of charge.

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I am going to disagree somewhat with this.
It may not be the best scenario, but I doubt that it would cause much (if any) damage...unless one of the battery banks had a shorted battery. And in that case, you would find out very quickly with any charging scheme.

True, the higher voltage battery, along with your charger, will attempt to charge the weaker battery. This will cause wide current swings. But, unless you exceed the charge/discharge current spec of either battery, I don't see this as a problem. Any more than starting the engine, running a house based high current device, or using a high current charger (like the alternator), should cause a problem. The higher voltage battery is just another charging source. The weaker is just another load. The voltage on the cabling is a function of the input impedance of the weaker battery, and the output impedance of the stronger (+ the output impedance of your alternator/ sterling, MPPT, etc.).

If your charging device is bulk charging, and thus possibly at higher voltage than either battery, the higher voltage battery will present a higher impedance to the charger, and thus suck less current. So most of the charge will go to the weaker battery. The charger should know enough not to let voltage rise to the point of outgassing.

If you have reasonably deep cycle/heavy charge batteries OF THE SAME TYPE (say AGM), on house and car side, the charging characteristics will be similar. Once the batteries approach the same voltage, the alternator, or solar, or whatever, will take over the finishing charging.

If you are not blowing properly rated fuses/breakers, or regularly running one battery completely flat, I don't see a real world problem here.

Stan

tthrsn said:

Using a Sterling battery to battery charger is about getting as much charge into the auxiliary battery as possible. I believe that issue with the fluctuations is inefficiency, not damage.

Take a look at the Sterling literature to see how the Sterling charger eliminates this inefficiency.

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As I read it (could be wrong), the purpose of the Sterling is to allow a "proper" charge profile to top off the house batteries, one that the alternator doesn't provide.

Inefficiencies might be time to bulk charge, charge top off ability, and energy wasted as heat.

I'm guessing that folks are concerned mainly about top-off, and secondarily at time to bulk charge. Does the Sterling literature talk about how far off full charge the alternator leaves you?

I suppose this could be a concern if you don't have solar or some other non alternator charging source. Or you expect to do much of your driving at night, where solar won't help.

I've been looking at this from the perspective of someone with solar, and who mostly sleeps at night. My alternator charges the banks to whatever it's going to do. Then the MPPT enters absorb and float as appropriate. Since absorb and float are low current activities, even on cloudy days that keeps my system topped off very nicely. In fact, I usually let the solar keep the car topped off as well.

Stan

I don't think anyone here is saying not to properly charge, as possible.

My sense is that it's not all that essential for reasonable battery life. For example, my wife's car is now 7years old and still on it's OEM flooded battery. She doesn't drive much...once or sometimes twice a week into town to food shop. The rest of the time it just sits there drawing whatever parasitic current it likes. A few times, she's left a door open, which leaves puddle lamps on and kills the battery. A quick boost, and she's good to go.

I'd be interested to know what the amortized cost of the batteries are, vs. the hoped for amount of extended life. That would help drive a decision about how much cost I'd put into a specialized charging system.

I have been unable to find good data about this, so I made my WAG decision based on past battery life experience. If I get an extra 1/2 year of usable life over a 5 year span, I figure the extra cost, tradeoffs (heavy gauge cable vs. extra electronics, importance of easily using the house to jump the car, etc.), and technical complexity of a dual charger system (or DC-DC converter) isn't worth the expense. Dave and I have balanced these trade-offs differently in several previous threads.

The only way to get a spec perfect profile is if your charger can provide enough current to both charge and source enough current for any heavy loads that may be on the system (fridge, etc.). To some extent, this is dependent on how much your alternator can provide. I suspect that the available current can be wildly variable. For example, if you are in stop and go traffic, you may not be able to rev high enough to provide optimal/spec bulk charging rates.

In short, I'm not convinced that a 100% optimum charge profile is real world practical in a vehicle. I don't have any data to suggest that the battery lifetime is extended significantly. Finally, there's a pretty good chance that I'll be moving to a different battery tech (lithium?) when my house AGMs die.
So, I decided not to worry about it, and designed to let the solar MPPT (or MS2012) handle the majority of my house bank charging needs. If, for some reason, it can't keep up with my loads, with the ACR I can always drive around for a while to get some bulk mode done.