charging batteries with a car alternator.

[tigger24]

Hi just been sitting in the sun after finding that my single cylinder diesel generator has died again ! This time the rotor has a fault and needs to be rewound or replaced, replacing would be with complete alternator or whole gen as my diesel engine has seen better days, so that would be the chosen route, how ever I don't and cant spend £1300 on this. While contemplating my next move and after talking to the gen people, who said the single cylinder engine is really meant as back up power and as Iam off grid need something better suited to longer running. They suggested a 3 cylinder water cooled diesel unit at about 3 times the price, doh ! So as said, while sitting in the sun, I thought I have a 2007 diesel dumper parked out the back, now not used much. If I was to change the alternator from what I guess is a small 50amp to, for example, a 130amp unit could I charge my battery bank with this. My battery bank ( I know should not be 12v but it is ) consists off 10x 115ah at 12v. Might be the sun on my head but would this work ?
Any help full advise needed.

[lkruper]

What kind of batteries are they? Also, how was your generator charging them, directly from DC or AC to a charger?

[tigger24]

Hi, There heavy duty leisure batteris, sealed and charged from generator to battery charger AC-DC

[lkruper]

Hi, There heavy duty leisure batteris, sealed and charged from generator to battery charger AC-DC

It sounds like your generator produces AC to you charger, is that correct? If you want to duplicate what you were doing you would need a separate inverter to take the DC from your alternator to AC.

Perhaps you are wondering if you can charge your batteries directly from the DC output of your alternator? It is possible, depending on the technical specs on your batteries, but won't be as good for your batteries as a three-stage charger. Your battery bank is pretty large. If you could get 100A out of the alternator that would be 10%, a good charge rate for FLA batteries and a bit low for AGM.

How many amps is your charger? You could acquire an inverter that could handle it and save your batteries until you come up with a better solution.

[tigger24]

yes thats correct, I just want to be able to charge the batteries directly from the car alternator. The battery charger I use at the moment is 100amps with 3 stage charging. Dose the car alternator not have 3 stage charging, if not dose it have any way of telling the current required so as that not to over charge.

[lkruper]

yes thats correct, I just want to be able to charge the batteries directly from the car alternator. The battery charger I use at the moment is 100amps with 3 stage charging. Dose the car alternator not have 3 stage charging, if not dose it have any way of telling the current required so as that not to over charge.

Car alternators do not have the same capability as smart chargers. They also probably don't have the same voltage. A smart charger will start with a bulk charge at a particular amperage until it is 80% and then switch to absorb which is a different setting and then to float which is designed to put only enough power into the battery to overcome what it would be losing.

The Odyssey battery manual has a good write-up on how they like their batteries to be charged, how to use a vehicle alternator and the cons of doing so. It says in part:

All ODYSSEY batteries can be quickly charged. Figure 7
on the next page shows their exceptional fast charge
characteristics at a constant 14.7V for three levels of inrush
current. These current levels are similar to the output
currents of modern automotive alternators. Table 6 and
Figure 7 show the capacity returned as a function of the
magnitude of the inrush3 current.

Standard internal combustion engine alternators with an
output voltage of 14.2V can also charge these batteries.
The inrush current does not need to be limited under
constant voltage charge. However, because the typical
alternator voltage is only 14.2V instead of 14.7V, the
charge times will be longer than those shown in Table 5.

However, to be safe you need to find the instructions for your particular battery or contact the manufacturer and also find out what your alternator puts out, under load. AGM batteries have a narrower tolerance for input voltage than do FLA batteries.

[tigger24]

Thats really helpful, thanks for this. I will try to find the info for my batteries and try and work out the out put from the car alternator. But as far as using my diesel engine and a car alternator, it would seam possible but not as good as it could be.

[inetdog]

Thats really helpful, thanks for this. I will try to find the info for my batteries and try and work out the out put from the car alternator. But as far as using my diesel engine and a car alternator, it would seam possible but not as good as it could be.

Probably the biggest problem would be the wear and tear on the diesel engine running at very low power but high RPM, along with the relatively poor fuel economy.

[tigger24]

Thanks Inetdog, I could decrease the engine speed by changing the the pulley sizes which should limit the the engine strain ? I think my main problem might be controlling the charge to the battery by not having any stage control. I can use an amp meter for the out put from alternator, but how can I tell how full the battery's are. Is there an easy way to tell how full there are at anytime ie a meter of some kind. At the moment I can only use my Morningstar mppt for a rough guide.

[inetdog]

Thanks Inetdog, I could decrease the engine speed by changing the the pulley sizes which should limit the the engine strain ? I think my main problem might be controlling the charge to the battery by not having any stage control. I can use an amp meter for the out put from alternator, but how can I tell how full the battery's are. Is there an easy way to tell how full there are at anytime ie a meter of some kind. At the moment I can only use my Morningstar mppt for a rough guide.

Once you have it calibrated for the increase in terminal voltage with current, you can get a rough idea of SOC from the battery voltage.
Note that running the alternator at a constant regulated voltage, with current limiting to protect the battery during the low SOC part of the charge cycle gives you essentially a Float charge profile, which Sunking often recommends for FLA when three stage chargers are not doing the job for one reason or another.

But the only reliable gauge of SOC are SG of the electrolyte and the resting voltage of the battery (no charge or discharge for at least three hours before reading.)

[Wy_White_Wolf]

Car/truck alternators are regulated to 13.6~13.8 volts. Using one to charge you bank will only get it to about 80~85% charged. WQhile that may not sound good it will work. If I did this I'd charge them with this in the morning. That way you could use solar to absorb charge them for the rest of the day to bring them up to 100%.

WWW

[tigger24]

Thanks Wy_White_Wolf, I have just checked my Nissan truck and that is putting out 14.32v, but as you say still short of FLA of 14.5v. However as sun is short in the winter here it would help out the mppt by using in the morning as you suggested.

[Willy T]

Here is a product that could be used for 3 stage charging from your alternator. They also have bigger models to 400 amps. Do a google search to find a USA retailer.

http://sterling-power.com/collections/alternator-to-battery-chargers/products/alternator-to-battery-chargers-up-to-130a

On the cheap, I use a old Xantrex c-35 controller to give me 3 stage with any fixed DC input >30 amps. They will give you adjustable Bulk and float voltage, but only have 1 hour fixed absorb.

[tigger24]

Thanks Willy T, looked up the price of this charger, quite high but will do the job. You say about your unit, how do you use yours, is it as a back up to pv or to boost during the winter?

[lkruper]

Here is a product that could be used for 3 stage charging from your alternator. They also have bigger models to 400 amps. Do a google search to find a USA retailer.

http://sterling-power.com/collections/alternator-to-battery-chargers/products/alternator-to-battery-chargers-up-to-130a

On the cheap, I use a old Xantrex c-35 controller to give me 3 stage with any fixed DC input >30 amps. They will give you adjustable Bulk and float voltage, but only have 1 hour fixed absorb.

I found it on Amazon: http://www.amazon.com/Pro-Alternator...g+power+AB1280

Great tip.