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Old 01-23-2007, 10:12 PM   #1
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Charging deep cycle batteries

Ok i have 6 napa deep cycle batts 105Ah i belive not the biggest but i got a fair deal on them i think 300 for all 6 new.
My question is, i have read that the 3 stage chargers (bulk, ???, float) from what i hear this is the prefered method of charging. If I were to either connect a cut off switch from my engine batts to the house batt, or add an alternator would this charging effect the longevity or completness of charge?
I have a genni for charging when stationary but would like to get the batts juiced up on the ride to where ever im headed
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Old 01-23-2007, 10:28 PM   #2
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Alternators are one of the smartest charging systems out there in that they will only provide as much current as the battery needs (up to their output limit, of course). You don't need to worry about overcharging your battereis with an alternator. You may well undercharge them, but that will be the case with any charger if it's not left on there. What I'm saying is don't expect the alternator to recharge the batteries fully in a half an hour. The one thing an alternator won't do that you will need to use a manual charger for is equalizing and knocking off any heavy sulphation on the plates. Regualr charging immediately after discharging will prevent the sulphasion for the most part, but a pulse type charger (such as a Deltran Battery Tender) will help break it up. An equalizing charge is basically an intentional overcharging or hot charging such that the electrolyte is forced to bubble, thus mixing the acid and water back up to provide a homogenious mix.
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Old 01-24-2007, 12:07 PM   #3
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Ok I just wanted to make sure that the output from the alternator would not harm deep cycle batteries.
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Old 01-24-2007, 02:26 PM   #4
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Deep cycles can be charged at about 20-25% of their rated output meaning if you have 400 amp hours, 100 amps would be the maximum rate I'd charge them at. You can get a good regulator for your alternator if you're worried, but I've never seen my batteries take that much at once. There is just too much other stuff drawing power off the alternator as well.
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Old 01-24-2007, 07:06 PM   #5
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if i did connect them to the bus batteries there would be 8 splitting up what ever current is available so I dont think the house batteries would get much more than 15A each assuming my alt output is 120A
I will set up a cutoff switch between the two banks this weekend.
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Old 01-24-2007, 09:38 PM   #6
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With 8, you should be just fine. You must be running 12 volt batteries, huh? If you have eight 6 volts, they would each be receiving 30 amps since they would have to be parallel-series'd into what is essentially four 12 volt batteries. Either way....you're more than safe.
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Old 01-24-2007, 11:13 PM   #7
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Yes 6 12v they are fit into a small woden box between the wheel wells next to the inverter. Id like to upgrade to some massive bank in the next few years but im not ready to spend 300 on one battery.
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Old 01-25-2007, 09:30 AM   #8
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A standard automotive alternator will not fully charge deep cycle batteries. The best it can do is about 85% to 90% and that's if the batteries are good, all the connections are good, and the alternator acts as it should. There are three stages to charging deep cycle batteries consisting of a bulk charge, an absorption charge, and a float charge. If your charging source can not accomodate all 3-stages (be it an alternator or a charger) then it can not fully charge deep cycle batteries.

The regulators in standard alternators are very simple switches and simply sense voltage; when the battery voltage is at some low point they turn "on" and send current to the batteries, when current is at some high point they turn "off" and stop the flow of current. Think of it as a light switch, it's either on or off. On the other hand multi-stage chargers are like a dimmer switch, not only does it have the on/off function but it can determine "how much" current to send; a stock alternator does NOT do this and always send all the current the alternator can produce to the batteries. Automotive alternators are voltage regulated and not current regulated (other than by maximum alternator output) which means they can not following the charging curve set by the battery manufacturer to fully charge deep cycle batteries. A good multi-stage charger (or 3-stage regulator on the alternator) will current regulate the charging and will bring the batteries back up to 100%. It does this by sensing the voltage of the battery and regulating current flow to the battery based on set points programmed in.

Automotive alternators are designed to charge starting batteries which have many thin plates with a lot of surface area that will quickly absorb the charging current and bring the battery voltage back up quickly. In this scenario the on/off switch operation of the regulator works fine. They are not designed to charge deep cycle batteries which have much thicker plates with a much reduced surface area that takes a comparitively long time to charge. When the simple on/off regulator senses that the voltage of a deep cycle battery is "ok" the battery still has another 15% or so to go before it's 100% charged (beacuse the interior of the thick plates hasn't reconverted) but the regulator doesn't know how to deal with that.

An OEM automotive alternator is not "hot rated"; it can not and should not put out it's rated amps for any length of time or it will die a quick death. You can not expect an alternator that's labeled 120-amps to put out more than perhaps 100-amps or so if you want it to last. If you add enough batteries to your bank to drive the required charging current higher than that you'll damage the alternator.

Charging deep cycle "house" batteries from an automotive alternator that has not been fitted with a 3-stage regulator is a "make do" situation. That is, you're only doing it to get the batteries at least partially charged between times when you can plug in a multi-stage charger. You can get your bus alternator to do the job correctly but it takes a rather expensive 3-stage regulator added to it to make that happen and you need to make sure the alternator is sized appropriately to the battery bank being charged. You can add enough batteries to limit current to any individual battery but you'll exceed the capacity of your alternator's continuous duty rating very quickly. Either way you'll end up replacing something.

I replace deep cycle batteries all the time that an automotive alternator has killed. The problem is that in what should be the the last two stages of charging (when you're trying to reconvert the interior of the plates) the alternator which only has one stage is sending way too much current to those thick plates and they're being damaged. That's specifically why a multi-stage charger is a multi-stage charger, for the life of the batteries the current must be reduced in the absorption and float stages and a standard alternator regulator can not do that. Limit the time you have your house batteries connected to the engine alternator.
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Old 01-25-2007, 01:24 PM   #9
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I have plenty of space under hood to mount a 3 stage alternator. Where does one puchase such an item.
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Old 01-27-2007, 07:07 AM   #10
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Originally Posted by the_experience03
Deep cycles can be charged at about 20-25% of their rated output meaning if you have 400 amp hours, 100 amps would be the maximum rate I'd charge them at. You can get a good regulator for your alternator if you're worried, but I've never seen my batteries take that much at once. There is just too much other stuff drawing power off the alternator as well.
The alternator on my bus is 190 amps, to drive my bus down the road now I am not using all the lights and crap as a school bus can be done with a 42 amp alternator like used on my 72 GMC pickup, I am powering a HEI ignition and the lights that total about the same as my pickup with a trailer how ever I am not including the heater fans I am not sure what they draw but I know they are way more than a regular truck, my point is they over sized the heck out of the charging system and their is alot of reserve their, I assume they over sized it so you can set at idle for hours with every thing on the bus running at the same time.

Is their any problem with charging with too low of a amp draw say using a 12 amp battery charger over using a 30 amp?
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