Using Alternators to Charge House Batteries: Not sure where to start (a pun there)

[comptiger5000]

The benefit of a big alternator isn't necessarily for getting more max current into the battery, but for being able to get max charge rate under a wider variety of conditions.

Using my Jeep as an example, with what the basic vehicle systems draw, plus a night drive with the high beams and driving lights on, I could easily pull a steady 80 - 90 amps that are now unavailable for battery charging. If I only had, say, a 110 amp alternator, that would only leave 20 - 30 amps for charging. While with a 200a alternator, that same situation would leave enough excess that the batteries will likely be able to charge as fast as they can accept current (100+ amps if they're drained enough).

 

[DiploStrat]

Magic Number(s)

You can certainly pay more for the ACR with the manual switch. The switch is electro-mechanical, not pure mechanical and, in two years of use; I have never found any need for it.

As I mentioned, Beloved Spouse and I have used the Tiger for months on end over the last two years. Cooking basically all electrically, running the refrigerator, and running diesel heat as needed, we average 125Ah from engine stop in the evening through to engine start the next morning after breakfast. Grill dinner outside and we use a bit less, make extra toast in the morning and we use a bit more.

With 600Ah of battery I have 300Ah usable, so that gives me a minimum autonomy of two days without any attempt to conserve.

Similarly, dropping 125Ah off of 600Ah means that I am only using about 20% of the capacity, which means that when I start charging I am already near the end of the bulk stage, so I can't really get the full benefit of the 250A of alternators. This is a long winded way of saying that my 500w of solar are the real secret weapon for charging. On a typical day of driving between three and six hours, I am normally fully recharged by noon. But even if we stop for coffee and a grocery store run, we are still getting 15-30A off of the solar so even in the absorb stage, the batteries will take just about all I can get from solar.

In other words, the rule of thumb, 100w for every 100Ah of lead acid battery is a good one. Thus I would focus on upping my solar as fast as possible.

N.B. When wild camping in rainy weather, e.g. Overland Expo East 2015, we simply start the engine BEFORE we unleash any big loads. It is much quieter than a propane or diesel genset.

Best wishes!

 

[deminimis]

Good points all. The reason for the switch (approx $25 more) is we often don't have the camper on the truck and we're using the truck bed for other pursuits (fortunately, hauling hay is no longer one of them with the departure of the wife's horses). Just a little more insurance during those times.

Yea, my wallet is screaming at me, so two more panels (and the MPPT controller to handle it) will have to wait. I could install the two additional panels and disconnect the 110 panel (480 watts) and be safe w/ my current controller, but Baby needs new shoes (or rather, The Boy outgrew his ski boots). We'll be up to speed with panels by Spring and our next Baja trip. You may think, well, just toss one additional panel up there in the interim. It's a symmetry thing. The two panels will go on the slight down slope towards the front of the camper and one would look, well, asymmetrical (it will be visible).

 

[DiploStrat]

I know the pain of $$. But, look at it this way; you don't have to spring for a genset!

 

[deminimis]

That's true. I need to wrap this up and get back to working on our Chenowth.

 

[deminimis]

Well, seems the panels I use are going up in price. With that, I found them still on the cheap from one supplier. Although I wasn't wanting to do this now, I ended up ordering the final two panels (for 590w max total). Finally got the second bank in for 420 AH (that was a process installing a second exterior battery box). Anyway, wired up with 4/0. Inverter works and panels (the remaining 320) arrive today (install this weekend). As discussed in another thread, I'm going to install a simple 30 auto reset breaker before my 30 amp controller just to play it safe until I can spring for a 45 amp mppt (Theoretically, I could be around 34 amps max, but I doubt I'd ever see near 30 for a few reasons). The Blue Sea ACR is next on the hit list.

Yesterday's progress:

 

[barkerco]

I installed my battery bank in a compartment below my flatbed. 315Ah bank, using the 440A alternators for charging, CTEK DC-DC chargers.

 

[DiploStrat]

????

Laying aside, for the moment, the question of why you would want to spend hundreds of dollars to reduce your 440A alternator to a maximum of 100A, I would suggest that you reread the installation manual for the D250S/SmartPass combination. If I have read your diagram correctly, you will not get the full benefit of the D250S/SmartPass combo the way you have it wired now.

The D250S is a 20A DC-DC charger; the SmartPass is not a charger, but rather an 80A intelligent relay that integrates nicely with the D250S. Basically, the SmartPass closes its relay during the bulk stage to allow up to 80A to flow directly from the alternator to the batteries. When the charge rate drops to below 20A, the relay opens and the D250S completes the absorb stage. But they only achieve their mojo when cross connected as outlined on page 20 of the CTEK manual.

You may also want to review your wire sizes; when in doubt, go larger.

See page 20 in the CTEK Manual for the correct wiring for your application: http://smartercharger.com/wp-content/uploads/2010/11/D250S-DUAL_EN.pdf

 

[barkerco]

The primary reason for dual alternators was that one was not enough. With the engine grid heater, UREA heating, aux lighting, compressor, winch, camper, etc. 440A was what I wanted. The winch alone could use 507A at full load. I also have a Duramax work van with only an 180A and it can barely keep up with power demands with just the grid heater/plugs.

I hooked up the CTEK combo the way it is supposed to be, just made my own wiring link, instead of using theirs provided as shown in the manual. That way if one unit failed, I could easily swap it out and if they changed the design.

The Smartpass can "charge" 80A and also provide bypass power from the truck alternator directly to the camper while doing so. The D250S will put out 20A max with 5-step charging and complete the charge. With my amp meter, I have seen it charge at over 100A. Which is plenty given that I don't discharge my bank that low (for cycle life). Discharging an average AGM battery to 50% has a 370 cycle life, 25% would give you 925 cycles. Even if I had just a relay and used "all" 440A of alternator, the batteries really wouldn't use that much anyway- not for long as they would absorb that quickly. Also recharging at high rates could cause the battery to gas, which is not good for AGM.

A rule of thumb for gel and AGM batteries states that the minimum charge current should be 15 to 25% of the battery capacity. Connected equipment usually also needs to be powered during charging, so include the power used for that purpose in the abovementioned figure. This means that, with my battery bank of 315 Ah and a connected load of 10 amps, battery charger capacity has to be between 57 and 89 amps in order to charge the battery in reasonable time. The maximum charge current is 50% for a gel battery and 30% for an AGM battery. So, the CTEK combo was perfect for my application.

Wiring gauges are good. I'm an electrician by trade and they are sized correctly. Voltage drops are within critical 3%. Larger wiring than needed just adds weight and cost.

 

[DiploStrat]

Partian Shot

Your system will work, if only because you have an 80A relay to do the heavy lifting. I don't doubt that you have seen 100A or more. 3% is great for a 110v system; it is an enormous drop in a 12v system, but the D250S will probably compensate for some 20A or so of that. In any case, 1 AWG will have very little voltage loss by the time the amperage drops to 20A. It is not that you cannot get the two components to work as you have wired them, it is rather that you are giving up the synergy that CTEK designed into them.

You may want to recheck the charge specs for your AGM batteries. My Lifeline brand AGM call for a charge rate of up to 5C. Do your AGM batteries really limit the charge to C/30?

Out of curiosity, what is the charging voltage of your RAM truck at 20C/70F? How does that compare with the charging spec of your AGM camper battery?

There is no question but that the D250S can save the cost of a solar controller as long as your solar array isn't that large.