Can I use the alternator to charge batteries?

[Matencio]

Hello all,
I was wondering if I can use the alternator to charge multiple 12v batteries? Would I have to get a bigger or higher charging alternator or would the standard one work?

I want to install a 3000-watt inverter so I have power for my sleep apnea unit, LED lights, a computer, monitor, and TV until I can install an electrical system. I'm just needing basic electrical, for now, to make it through the night off-grid. Can anyone tell me if this will work? I'm in the planning stages and working with a minimal budget. No bus yet. I want to have a plan before I start building it.

Thanks,
Mike

[john61ct]

Depends.

Many modern vehicles with everything computer controlled, you may need a DCDC even with lead.

Usually needed with LFP.

Old school trucks with lots of room, can fit a firetruck style large frame 160A up to 200A output 24*7 if you can keep rpm up.

But requires serpentine / multi Vee belts soon as you try for over 80A.

Some allow for multiple alts as well.

Most stock alts can't put their rated amps out for more than a few minutes without the circuitry overheating, maybe 30-40% continuously.

But a lead bank may not accept more than that, really want LFP to draw the big amps.

An external VR where you can regulate V setpoint, de-rate current based on temperatures etc really helps, may not need the DCDC then.

Also get the diodes etc remote from the main alt body help with the heat factor.

[dbsoundman]

I'm purely guessing in my case, not knowing any of the specs on my alternator I have purchased a 40 amp DC-DC charger. I feel like this is the best option anyway because with a DC-DC charger you know the charging current you're pulling off the alternator is limited. 40 amps seemed "safe" to me given that I'm not operating any of the school bus lighting anymore which no doubt pulled some decent current (5 bright incandescent lamps on at a time).

[flattracker]

Alternators are not battery chargers, they are battery maintainers.

[magnakansas]

I have Bosch alternator rated at 200 amps 12v. This is intended to be run continuous, but is supposed to be internally regulated to less amperage output if it starts to get too hot. I run a battery isolator and split house and engine batteries. The problem is a 250 hp battery charger is not very efficient. I also have a 110v ac battery charger, and an on board generator. I do not have solar, but I am rarely using a bunch of power either. But I can do it if I need to use that many angry pixies.

William

[Iceni John]

SLA vehicle batteries require a different charging regimen than deep-cycle golfcart-type batteries, so a vehicle alternator won't correctly charge the latter that ideally need three-stage (Bulk, Absorption, Float) charging. Even different makes of similar batteries can sometimes need slightly different charge voltages, e.g. Trojan deep-cycle batteries need 14.8V compared to other such batteries that are happy with 14.7V. I suggest you charge strictly according to the battery manufacturer's specific recommendations.

John

[john61ct]

Originally Posted by flattracker

Alternators are not battery chargers, they are battery maintainers.

Yes, but if you read my post, that can be corrected and becomes a gross overgeneralization.

I met a guy who rigged an ex-Hummer that output 13kW, even a 15-min drive refilled his big LTO bank to last four days of boondocking.

Solar becomes a trivial contributor, can park in the middle of a cool forest instead of out in the hot sun.

160-200A at the right voltage isn't even hard so long as you have the room in the engine compartment.

[flattracker]

13 Kw - wow. at the normal operating voltage of a HMMWV that is 541 amps.
My HMMWV has the dual voltage alternator (24 VDC and 12 VDC) I use that setup with two red top optima batteries to start the engine, and two yellow top optima batteries in the right rear foot well (where a rear seat passenger would put their feet if it had four seats). I run a 1000 watt sine wave inverter from the yellow top optimas.


The total output current from that alternator is about 300 amps.


I know there were larger alternators than mine that were put in HMMWVs.

[Booyah45828]

13kw

That is a massive amount of power. Any specs or write ups on that? I've got a 10kw auragen unit under the shop bench, and it's a fairly large unit, so I can only imagine what 13kw looks like, especially under the hood of a hmmwv.

In my experience, most vehicle alternators aren't built to withstand max output for lengthy amounts of time. Some can, but not most. That's where the battery maintainer not charger comes from. I agree with him on that.

The reason for an alternator in the first place is to recharge the batteries back up after starting, and cover the originally designed vehicle electric demands. Charging dead batteries isn't in the design criteria, and can overwork the alternator, which will lead to an untimely failure.

Moral of the story is that if you change the system to increase demands, you should reevaluate your alternator situation. It's why a standard f250 might have a single 100 amp alternator, whereas the plow/ambulance package will have dual units creating double(or more) the amperage then a standard truck.

Certain loads can be added and not thought about. But a 3000 watt inverter has the ability to consume a couple hundred amps of 12vdc. So unless your stock alternator is in the 3-400 amp range, I wouldn't run a 3000 watt inverter off it. Granted, you likely won't be consuming all 3000 watts all the time, but it's still not good practice to have things undersized IMO.

I don't know how much your cpap machine will consume, but the lights/tv/laptop likely won't require 3000 watts. Do an energy audit and see how much power you really need, and then go from there.

[flattracker]

I agree.
My electrical design includes a 300 watt inverter to operate my cpap and the fridge. When outside power or generator power is applied the cpap and fridge are automatically switched over to that power.


The cpap uses 90 watts and the fridge uses 165 watts.

[john61ct]

Yes that was a full on military alt, comms unit most likely.

But 200A alts for ambulances firetruck are common, Leece Neville, and as stated put out rated 24*7 without breaking a sweat

Often quite cheap on eBay, also getting reconditioned.

Full size frame, does not fit in most modern consumer trucks these days

If you need a custom pulley set, special work on the mounting, that's often more than all the rest of it.

Then the right VR, can be a grand or more

[Rwnielsen]

I think your best bet would be a small(ish) inverter generator and a few deep cycle lead acid batteries with a battery charger. Some people here use lead acid and do just fine. Harborfreight.com has a 4500 watt generator for under $900. You might find a deal on an used one on Facebook or eBay. I personally keep my bus electrical completely separate from any of the "coach". If you smoke an alternator in the middle of nowhere it'll probably be expensive and without a backup plan, nothing you mentioned will work for long.

[john61ct]

Yes the new inverter gensets are nice and quiet.

Just size the AC-DC (grid power) charger a fair bit smaller than the gennie's maximum output

maybe even small enough so that it can purr along nicely in ECO mode.

With a lead bank, it takes minimum 7 hours to get back to 100% Full, and depending on initial C-rate, the last 5 hours is likely to be at a very low current rate.

So FF sources in general are very inefficient and expensive and annoying to use for the whole cycle.

Run the FF genset in the early AM to get to say 80-85% then let the silent "free" solar finish the job the rest of the day.

True for using the alt as well, a morning roadtrip for supplies rather than afternoon.

Of course if you consider your lead bank a cheap consumable, to be replaced every few years

or when you convert to LFP

then no need to bother paying attention to any of that.

[Oldyeller]

LOL, you guys used so many acronyms and jargon I'm afraid the OP got scared away. I know it scared me and I have to do a solar system this summer. This is the thing I'm most afraid of.

[RamRod4]

I have the line sheet from Navistar for my bus and it came with a 200A alternator from the factory. I was surprised by this but I'm not complaining. Pairing it with a Victron Orion DC-DC charger.

[Booyah45828]

Aren't those helpful?! Mine was taped to the inner radiator support and I've used it a handful of times to order stuff, like the king pins and front brake chambers. I've also used it for general knowledge, like having a 65 gallon fuel tank, 5.57 axle ratio, or 100 amp alternator.

[john61ct]

Originally Posted by Oldyeller

LOL, you guys used so many acronyms and jargon

If a quick google doesn't clear one up just ask

None of this is rocket science, just another learning curve

[BigPaul367]

Originally Posted by Matencio

Hello all,
I was wondering if I can use the alternator to charge multiple 12v batteries? Would I have to get a bigger or higher charging alternator or would the standard one work?

I want to install a 3000-watt inverter so I have power for my sleep apnea unit, LED lights, a computer, monitor, and TV until I can install an electrical system. I'm just needing basic electrical, for now, to make it through the night off-grid. Can anyone tell me if this will work? I'm in the planning stages and working with a minimal budget. No bus yet. I want to have a plan before I start building it.

Thanks,
Mike

I have a much better idea. Determine what you use while sleeping in total and have a separate inverter just a little larger than that demand. I don't understand it when people turn on a 3000 watt inverter to charge their cell phones and run a diesel heater when the total draw to operate both is less than 400 watts. When you install and run an inverter, it draws the rated input as soon as you turn it on. It is NOT linear. If you power a 30 watt charger with your 3000 watt inverter you are drawing 3000 watts to charge your cell phone. I have a great idea, you need to watch a few videos from people who know better. Go to YouTube and and watch 'Everlanders', this dude has a tiny system that he uses to power everything in his Overland RV. His system charges and never seems to go dead because he only draws what he is using and not what he thinks he MIGHT use. There are very large alternators that will charge up to twenty batteries and deep charge them if you travel more than 100 miles at a time. The average solar system will charge dozens of batteries, to capacity, before noon. Then you either need to shut them off or dump the extra energy into another type of storage. With a dozen Solar Cells
it is possible to run for months without running short of power. As an example!
I have a Tool Truck Vendor tell me his truck/sales batteries went dead between every stop and he had to run his Generator. He had a 4000 watt inverter. His Point Of Sale (POS) system needed 300 watts maximum, (A Monitor, Laptop and Mini Dot Matrix Receipt Printer!) After I advised him, he shut off the large inverter and installed a 350 watt inverter for his POS system. Now his battery lasts all day and is recharged by the time he reaches his security parking spot every night. He uses the large inverter to test large equipment, MAYBE three time a year. MOST Inverters are Overkill! If your CPAP Needs 200 watts and you want to charge a few small electronics while you sleep. Sounds like you only need 400 TO 500 watts, WHILE YOU SLEEP. SAVE THE 'BIG BAD DOG' FOR WHEN YOU NEED IT!

[HamSkoolie]

Originally Posted by BigPaul367

When you install and run an inverter, it draws the rated input as soon as you turn it on.

As someone who teaches electrical theory, circuitry, and such, I don't buy this. With no load there is no draw.

Do you have a definitive source for your claim?
There IS an amount of power pulled just to run the inverter and that is pulled at any time it's on but it's a SMALL amount. The Victron 24/3000 MultiPlus all in one that we're going with is a 15 watt draw when there is no load.

[TheHubbardBus]

Originally Posted by BigPaul367

I don't understand it when people turn on a 3000 watt inverter to charge their cell phones and run a diesel heater when the total draw to operate both is less than 400 watts. When you install and run an inverter, it draws the rated input as soon as you turn it on. It is NOT linear. If you power a 30 watt charger with your 3000 watt inverter you are drawing 3000 watts to charge your cell phone.

Like Ham said, this isn't correct. An inverter is going to draw the cost of the load, + efficiency losses, + its no-load current (what it pulls just to do its thing). So if you had a 30w load, at 80% efficiency (low-balling for purposes of example), + a no-load consumption of 30w (using our actual inverter/charger as an example), that's ~ 66 watts. Your point still stands... paying twice the cost to charge a small device that could be charged off DC isn't smart... but you're mistaken in how much power an inverter draws.

An inverter can draw a large current when first installed/connected to a battery bank, as it charges its caps, but that's a different matter lasting a matter of milliseconds.

Originally Posted by BigPaul367

The average solar system will charge dozens of batteries, to capacity, before noon.

I guess you'd have to define average, as well as the type / capacity of the batteries in question, before I could say for certain this was a nonfactual statement. But in the context of even the most impressive of solar arrays found on buses, and assuming a battery other than a rechargeable D-cell... no. That's not true.