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Old 10-13-2009, 10:08 AM   #1
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Re: Using the alternator to charge deep cell battery

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Originally Posted by anfarber
I am curious whether it is possible/safe/advisable to use the alternator on my DT466 to charge a deep cell battery to be used in a battery bank for ac power.

First off, I plan to charge the batteries off the alternator from the engine - one at a time - because there is room for a third in the battery compartment. I understand it is not good to mix types of batteries but the 8Ds for the bus are expensive ($150) so I plan to go with a cheaper option. My first question is an automotive question. Is it safe to use the alternator as a charger?

Second, I am planning on a battery bank with an inverter for the AC power. Using two deep cells to power the AC outlets at a time, one battery on standby, and the fourth in charging. The standby battery will be switched to be charged when the vehicle is shut off - say midday - then restarted for more travel. I am not sure on my power usage for now (1 laptop, 1 computer, various battery chargers for electronics, 3 reading lights, and 1 RV air conditioner unit.) My second question is whether or not two batteries can supply enough power for my needs? I figure the four battery system (charging, standby, and 2 battery bank) will be manageable on a daily routine for charging as well as cycling out at a 50% DOD.

Third, I have a concern that this system may be detrimental to the cranking batteries since the "AC battery" will be drawing a greater charge. I could use some advice on this subject.

Thats a bunch of info man. Here are my thoughts. First off, you are not gonna run any air conditioner off of batteries and an inverter. At least not any longer than a few moments. You are gonna have to have either shore power or a genny to give enough power for you to run your A/C.

I am confused by what you are planning on doing. Are you gonna move batteries under the hood and let the alternator charge them all day? Are you gonna be driving during that time or parked? You could put in a battery isolator and have them all charged at the same time from the alternator. But in all reality your gonna need to have another way to charge the batteries like a converter/charger, solar, or both. An alternator is made to keep batteries topped off, not to do continuous deep charges. If you really want you can add a second alternator and have the line run just to your battery bank, that way you would never have to move batteries. But I wouldnt rely on that unless you are gonna be keeping them charged from shore/genny/solar power in between and the alternator is just to keep them topped off.

If you are gonna have a battery isolator that takes care of any problems you would have with mixing batteries though. And it also fixes any problems you would have with cranking batteries. It keeps your batteries separate. You need to have two different systems. Your battery bank that runs the cabin area needs to be completely separate from your engine cranking batteries. This stuff took me a while to understand too. Here is a copy of my wiring diagram so you can see some of what I am talking about.
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Old 10-13-2009, 11:53 PM   #2
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Re: Using the alternator to charge deep cell battery

First - the alternator is a charger. It should be sized to run all your loads plus refill the batteries when the engine is cruising. Being able to do it when idling is even better.

Second - I don't understand what you mean by a standby battery that is charged when the vehicle is off.

Third - an alternator is safe for hydrate batteries, the kind where you can open the cell tops and check the distilled water and acid mix. It doesn't matter if they are starting batteries (high current, short duration) or deep-cycle batteries (low current, long duration).

Fourth - some sealed deep-cycle (gel cell or AGM) batteries don't like the 14+ volts an alternator can put out. Others may not mind. I work with some batteries that don't like more than 13.6 volts. If you overcharge them and the cells vent acid, capacity is lost forever. Read the data sheet from the manufacturer if you are using specialized batteries, and take the numbers seriously. They may require a special charging arrangement for longest life.

Fifth - don't mix different batteries in a bank. This means different sizes, but it also means different ages of the same type, because the old cells may not store as much electricity as fresh ones. If you connect a battery that has discharged down to 11.9 volts together with a fresh one storing 12.6 volts, even with no load the fresh one will discharge in an attempt to bring up the low one to match it. If you intended to switch your charged standby battery on TOGETHER with one or both of the inverter pair, don't. On the other hand, if you intended to switch to the standby battery INSTEAD of the pair, no problem.

Sixth - you can generally charge the batteries while they are running the inverter. To do so, your charging source must power the load drawn and have power left over for charging. In most cases, there is no need to take the batteries off line, unless you have an under-sized charger, or an unregulated mechanic's boost charger that might cook the inverter.

Seventh - if you are not always traveling, it will cost you less to buy a small generator and charge the batteries from that, instead of running the bus motor. You will need a generator if you run air conditioning, anyway.

Finally - if you install an isolator as BFL Johnny suggests, it will charge the lowest batteries until they are equal, and then charge both. The isolators have rectifiers (aka diodes) in them, which are electric one-way valves. The rectifiers are mounted back-to-back. They flow electricity toward a battery bank when the pressure (voltage) is higher on the alternator stud than on a battery stud. If the battery voltage is higher than the common point, the rectifiers shut off. The idea is to prevent your inverter loads from drawing down the starting batteries through the alternator connection when the house bank runs down.

If both banks are discharged to 12.2 volts, an alternator capable of around 14.5 volts will push open both valves, and fill both banks. If the inverter bank is 100% discharged to 10.0 volts, and a low-capacity alternator is pulled down to 12 volts to fill it, then the valve to the starting battery will shut and the bus will run on its battery. This will continue until either the inverter bank comes up or the starting battery runs down to the point of matching voltages, and both valves open. Can this affect your starting battery? If the alternator does not have a high enough output to run all the loads in both systems plus fill both batteries, the battery can run down even when the engine is running.
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Old 10-14-2009, 01:38 AM   #3
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Re: Using the alternator to charge deep cell battery

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Originally Posted by macgyver
Go up to your local hardware store. One that sells lawnmowers... They'll usually have a mower come through that's been destroyed for whatever reason, but the engine still runs fine... If possible, pick up one of those two or three cylinder lawn tractor engines... Diesel, of course.

-Mac
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Old 10-14-2009, 10:32 AM   #4
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Re: Using the alternator to charge deep cell battery

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Originally Posted by macgyver
I have another suggestion for you...

This is something I had originally planned on doing in lieu of having a generator before I had a generator...

Go up to your local hardware store. One that sells lawnmowers... They'll usually have a mower come through that's been destroyed for whatever reason, but the engine still runs fine... If possible, pick up one of those two or three cylinder lawn tractor engines... Diesel, of course.

Then, you can have a bracket fabricated to put on whatever sized alternator you'd like. Basically, it becomes a 12v generator instead of a standard shore-power like generator. This would allow you to run all your 12v equipment, keep your batteries topped off, etc... And these tiny engines use a fraction of the fuel a full sized genny uses.

Just something to think about....

-Mac
although it will work to charge batteries, if you plan to use it a lot, it does not make cents ($) Automotive alternators are great at charging batteries, and can be used from speeds from around 800 to 8000 rpm, but they are not particularly efficient. A lawn mower at full load (which is equal to an alternator on a lawn mower engine) probably consumes a gallon of fuel every 2 or 3 hours. Plus, an overworked alternator will get hot and eventually need replaced, which equals more money, down time etc. This is a fun experiment, and a suitable way to produce good power for charging batteries, but will not save you money in the long run unless you have free fuel and an alternator with a lifetime guarantee. Another problem with lawn mower engines is noise. not too many of us would enjoy listening to a lawn mower running full tilt right outside our window.

If you plan to spend hundreds of hours running an engine to power things in your stationary bus, you should consider buying a honda eu or similar inverter equipped generator. Super quiet, and effiecient. They can run up to 15 hours on a single gallon of fuel. Howerver, It will take more fuel, and maybe 2 generators to run an A/C unit.

for reference: 105 amp alternator is about 1500 watts.

if you want to charge deep cycle batteries when you drive, i would suggest adding a 2nd alternator to you bus rather than an isolator. You can buy an alternator for about the same price as a good isolator. Redundancy is good! I have a reasonably high failure rate with alternators in my buses. The new alternator can charge just the house batteries, and the stock alternator can run the stock bus systems. If you go to a 24 volt system that is even better. A 100 amp 24 volt alternator produces twice as many watts as a 100 amp 12 volt alternator.

A diesel bus at idle consumes between 0.5-1 gph of fuel. An alternator at idle won't produce much power, so you'll really have to run your engine at high idle, which will use even more fuel. Although it works, it's not a very efficient way to make power either.
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Old 10-14-2009, 12:05 PM   #5
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Re: Using the alternator to charge deep cell battery

I have to agree with Jason. My tractornator is allowed to consume 5 gallons of fuel per day which equates out to about 8 hours of runtime. It is only producing about 50 amps while running which is terribly inefficient. The 12 horse Briggs along with the pulley ratio is able to do that at just above idle which is nice from a fuel and noise standpoint, but that 1 wire alternator just really isn't up to snuff for full on charging. I built the apparatus as an experiment from stuff I already had so I can afford a lot of fuel for it to offset the cost of a real generator...and I like tinkering. But would I actually build one with parts I bought and expect it to fulfill my charging needs? Not unless I had a giant 24 volt alternator on it and a 24 volt inverter system.
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