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Old 09-07-2015, 02:56 AM   #1
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Default AGM battery charging from alternator

So my husband and I are proud new owners or a 2015 Sprinter with a 200Amp alternator. We are in the process of outfitting it as a camper, and I am in charge of the electrical system.

We have decided that we would like to charge our house batteries from the Alternator and forgo solar panels at this time. Our one main reason is that we are very serious ice climbers and the first 3 months of travel will consist of snow and ice in the Rockies. Short days and difficulties cleaning off the top of a high roof Sprinter doesn't seem to make sense for solar.

I have been reading a lot of threads on this forum and elsewhere about charging a house battery from your alternator. I am an electrical engineer who has spent her career designing control circuits for industrial settings. The concepts are not that hard (appropriate awg wire, isolation relays, etc), but one thing I have very little experience with are batteries.

Seems that many people are charging their AGM batteries directly from their alternator (with an automatic charging relay) as per this thread: http://sprinter-source.com/forum/sho...ght=alternator

But I have also read quite a bit on charging batteries and multistage charging requirements. Will the system in the thread posted about fully charge a AGM battery without hours of driving? I assume that I would also need a some sort of multi stage regulator to charge an AGM battery to 100%?
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Old 09-07-2015, 09:45 AM   #2
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Default Re: AGM battery charging from alternator

Short answer - no. A simple relay won't allow you to bulk charge as the stock voltage regulator will taper off the voltage way too fast. Your instincts are correct, you need a separate multistage alternator controller.

However there is an issue with using the Sprinter diesel for battery charging you may not be aware of - you are not supposed to idle the motor for long periods of time. You might want to consider a portable generator combined with a high-amp charger as a better alternative.
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Old 09-07-2015, 01:59 PM   #3
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Default Re: AGM battery charging from alternator

As climbers we will likely be driving from our camping area to our climbing area most days. Also, every second day is usually a rest day, which will include a trip into town for restocking/boredom.

So in a basic alternator charging circuit I would have my isolator and then have a voltage regulator befor the house battery.

Now all I need to do is figure is where I can order all the bits and pieces I need being in Canada. Hoping to visit a marine store or two tomorrow and see what might be avalible through them.
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Old 09-07-2015, 02:21 PM   #4
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Default Re: AGM battery charging from alternator

If you are planning on installing an inverter I use a set up very similar to this one in post 29. http://sprinter-source.com/forum/sho...ht=Evil&page=3

I use a 600 watt inverter to power a 120v 3 stage battery charger powered off the starter battery while driving. You can charge laptops or other things as well while your house battery is charging. I get the charge my battery Manf. Recommends. I can also use the inverter powered off the house battery while camped but I rarely camp without shore power or use of a generator.
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Old 09-07-2015, 02:38 PM   #5
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Default Re: AGM battery charging from alternator

I agree with Chromisdesigns that your proposed configuration will likely not be very satisfactory. Systems with isolation relays essentially NEVER fully charge the house batteries because the vehicle charger will not behave like a 3-stage charger for those batteries. Most RV systems essentially depend on the 3-stage solar charger to at least occasionally top-off the battery, which is important for battery health. It is best to think of a modest solar system more as as battery-maintenance device than as a source of operating power.

Moreover, late-model Sprinters have highly-optimized charging systems that appear to closely monitor the chassis battery and don't seem to provide many amps to the house, even though the capacity is there. Our 2014 I4-based camper came with an isolation relay. We were very dissatisfied with the performance--took forever to charge the battery while driving, even with some solar to supplement. (Our old T1N-based van did much better in this regard, even though it had a smaller alternator).

Adding a "regulator" between the vehicle electric system and the battery will not help with proper charging unless it is capable of boosting the voltage provided to it in order to maintain the proper 3-stage charging profile. Such devices are normally referred to as DC-DC chargers. And even then, I think you will find you will get very slow charging by taking this route.

We finally bit the bullet and added a second engine alternator and a balmar 3-stage regulator (which, unlike the devices discussed above, directly controls the alternator output and so is far more effective.)

You can read about our install here:

http://www.sprinter-source.com/forum...758#post394758

This upgrade was one of the best investments we have made in our vehicle. Not that difficult. I would seriously consider this approach.
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Old 09-07-2015, 02:45 PM   #6
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Default Re: AGM battery charging from alternator

Most likely the answer is no. As with most real life problems, its really a trade off. What kind of reduction in battery life is acceptable to you?

To start with some information would be helpful. Please provide the following.
  • Daily Ah usage
  • Average daily driving time
  • Battery bank size
  • Battery temp (inside or outside cabin?)

The difficulty with lead acid (LA) batteries is that they do not have a linear charge curve. The charge rate in amps drops off quickly as the batteries go above 80% state of charge. This means getting a ful charge directly from the alternator is difficult. Unfortunately in order to avoid life losses, LA batteries should also be charged to 100% regularly. Some posters her have reported good results while only getting a 100% charge once a week with a shore power charger. Note that if your AUX bank varies much in temperature from the chassis battery you are not going to get optimal charging. A good charger will have a temp sensor on or near the batteries to adjust the charge voltage appropriately. See page 19 of the pdf below for a table. This same page gives a formula for calculating charge times.
http://www.lifelinebatteries.com/manual.pdf


A more basic question, can you get enough amps (kWh) into your batteries during your driving to account for daily uses? The alternator is a constant voltage source (assuming you don't hit its current limit). Charge current to the batteries is a function of SOC (internal resistance) alternator voltage, and wiring losses.

If your power usage is light enough or if you drive enough hours, you may be able to get by with just alternator charging. To enhance the charging ability there may be additional equipment you could use. Battery to Battery chargers (Sterling makes some?) and external regulators for the alternator may be available for your application. Another option is the "graphite dave" method, which uses an inverter powering a charger.

Once you get down to crunching numbers you may discover that it is more reasonable to use the alternator to supply the bulk of your needs, and install a small solar system. A system around 100-150 watts could be done for ~$300+battery monitor. Just make sure you have a quality battery monitor, and a temperature compensated solar controller.

Simple right?
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Old 09-07-2015, 02:51 PM   #7
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Default Re: AGM battery charging from alternator

I do not believe that controlling the alternator of a modern Sprinter with an external regulator is a viable option. Do not underestimate the complexity and level of integration within the 2014+ Sprinter electrical system.
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Old 09-07-2015, 02:54 PM   #8
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Default Re: AGM battery charging from alternator

Another thing to take into account is max charging rate, AGM batteries typically shouldn't be charged at more than %15 of their capacity, so a 100 AH battery should not be charged at more than a 15 amps charge rate. Just connecting the battery to the alternator with a solenoid would significantly exceed this charge rate depending on how charged the house battery is, the lower the charge in the battery the higher the charge rate will be off the alternator.
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Old 09-07-2015, 03:03 PM   #9
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Default Re: AGM battery charging from alternator

Another choice is to have two inverters. A "house" and a "vehicle" inverter.

The "house" inverter provides 120 volt power from the house battery. The "vehicle" inverter provides 120 volt power from the alternator while driving. The 120 volt power from the alternator is another source of "shore power". The "vehicle" inverter output powers a proper 3 stage charger that has bulk/absorb/float so that you charge the house battery with a charge profile that matches the house batteries requirements. A selector switch is used to select real shore power or vehicle produced "shore power". My system utilizes a Magnum MSS1012 1000 watt "house" inverter and a pure sine "vehicle" inverter. The Magnum is a inverter/charger/transfer switch. If 120 volt shore power is available, the transfer switch simply passes the 120 volt power through the inverter to the duplex plugs with inverter off.

Attached is a post that includes a electrical diagram. I have a newer slightly revised drawing for my new Transit build if you have any interest. System works very well for my application.

http://sprinter-source.com/forum/sho...7&postcount=14

Understand your roof solar decision. Maybe a portable panel that you set up outside when weather conditions permit? That could extend your dry camping time without idling the diesel engine. I regularly city camp and in Monterey my favorite spot during the day is along side the bay. A guy with a VW van is there almost every day. He pulls out what looks like a 100 watt panel and sets it up outside each day.
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Old 09-07-2015, 03:06 PM   #10
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Default Re: AGM battery charging from alternator

Quote:
Originally Posted by jackbombay View Post
Another thing to take into account is max charging rate, AGM batteries typically shouldn't be charged at more than %15 of their capacity, so a 100 AH battery should not be charged at more than a 15 amps charge rate.
I do not believe the above information to be correct. Fast charging of AGM batteries (as long as you are not OVERcharging them) is perfectly OK. See, for example:

http://www.powerstream.com/SLA-fast-charge.htm
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