Alternator Outputs; Battery Charging; DC Power Loads

Thread Starter

Wirefiche

Joined Nov 30, 2020
3
Greetings!

I'm a layman, so please accept my apologies for what could be considered an astounding degree of ignorance on these matters.

My setup:
My offroad vehicle alternator is rated from the manufacturer at 40 amps. I'm not sure how much voltage, other than that I know it charges a pair of 12v batteries, in parallel. I'm guessing it's probably charging at around 13 volts, minimum. A friend of mine says the alternator probably has a higher charging voltage applied to the battery when the battery is at a lower discharge state, which then automatically adjusts lower and lower as the battery approaches a full charge. He's says that's because most alternators these days are "smart", but I'm not sure if I should trust that is true. Not because I don't trust my friend, but because I can't be sure the vehicle wasn't just made with cheaper components. I'm assuming a "dumb" alternator would be cheaper than a "smart" alternator.

I recently installed a new lightbar on said vehicle. The lightbar mentions a certain minimum fusing requirement, I believe it's something like 20 or 30 amps. This isn't terribly important to me as the lightbar came with a 40 amp fuse. This worried me slightly though, as it suggests to me the lightbar could pull almost that much current under certain circumstances.

I have a multimeter. The vehicle has an on-board voltmeter as well.

My questions:
How do I go about verifying my alternator output?
How do I check to see if my alternator is "smart" or not?
Should I be concerned that running my vehicle with the light bar on constantly is going to prevent the battery from charging?
 

geekoftheweek

Joined Oct 6, 2013
641
What kind of vehicle / engine? Is it a normal automotive type alternator, or flywheel and stator combination?
A typical automotive alternator should be at least 13 Volts... usually 13.5 to 14 Volts.
Chances are a 40 Amp alternator isn't going to have much in the way of brains to worry about. They aren't used for much these days as modern engine electronics and creature comforts need a bit more power to run.

Verifying your output could be as simple as connecting a volt gauge, turn on your light, and see if the voltage drops. If you have to give it more than a little engine speed to get the voltage back up then it's going to be too much.

Usually fuses are specified as the maximum value. Recheck your directions. It may have a minimum listed, but will for sure have a maximum. Usually the maximum is a bit more than what you will normally need, but not enough to cause a fire if something goes wrong.

As long as your voltage is above 13 volts with the light bar on your battery will still charge. It just won't charge as fast. If you only run the engine for a few minutes each time it may eventually run down the battery.
 

Irving

Joined Jan 30, 2016
1,491
Your alternator wll generate around 13.8 - 14.4v, typically at or just above idle. Alternators generally have some form of voltage regulator to stop the voltage going above 14.4v at higher revs. There's nothing smart about any alternator, they're pretty much all the same. An alternator generates 3-phase AC which is then rectified by a diode stack to give DC (unlike a generator which generates DC directly), and so requires no additional support when the revs drop too low to maintain output voltage as the diodes block and prevent the battery trying to drive the alternator. The important thing to understand is that the alternator only generates the current actually demanded by the vehicle's systems, up to its capability.

A typical car alternator generates around 80 - 100A at full output, a truck alternator may generate more. Your alternator should be stamped on the casing with numbers like "40/80A 14.4v". The first number is the output current available at idle, the second is the current available at 6000rpm and the voltage is the max/regulated output voltage.

If your lightbar is rated for 40A and its switched on at idle (which I'm guessing is your manufacturer's 40A rating else its a pretty poor alternator), its going to drain the batteries - how fast depends on what else is drawing current from the system.
 

Thread Starter

Wirefiche

Joined Nov 30, 2020
3
What kind of vehicle / engine? Is it a normal automotive type alternator, or flywheel and stator combination?
Vehicle is a 2019 Sherp Pro with a Kubota V1505-T engine. I believe the alternator would be a typical automotive alternator, but I'm not sure how to tell or how to look that up.

I'll be having a look at my alternator soon. I hadn't considered it would have any ratings stamped on the side. I just know from the published data that the stock Sherp Pro alternator is 40 amp.
 

geekoftheweek

Joined Oct 6, 2013
641
Judging from what I could find in a quick search of the V1505 a 40 Amp alternator sounds about right. With the mechanical injection pump I'm guessing the engine itself doesn't make much of a draw electrical wise. Other than the main fuel valve the only other thing that may use power is any sort of pump timing controls if it has any. Glow plugs or grid heaters on the other hand will probably have a decent draw to account for. I'm assuming it has one or the other from the super glow reference on the page https://kubota.com.au/product/v1505/.

The engine picture also looks like it has a normal automotive alternator mounted to it. I don't know what Kubota uses for alternators, but I'm willing to bet there is a higher capacity one of the same dimensions bolted on a car engine somewhere. I'll guess either Mitsubishi or Denso. The trick will be finding it. You may be able to get a part number from Kubota that will cross reference to an automotive part number and start from there.

Even considering the presumed small load from the engine controls I'd try to upgrade the alternator. Considering blower motors for heat / ventilation (if any), windshield fans and wipers, factory installed lighting, and any other loads you may already be drawing say 15 Amps on a normal day, then adding another say 20 Amp light bar and you'll be pushing the limits for sure.
 

MaxHeadRoom

Joined Jul 18, 2013
23,085
He's says that's because most alternators these days are "smart", but I'm not sure if I should trust that is true. Not because I don't trust my friend, but because I can't be sure the vehicle wasn't just made with cheaper components. I'm assuming a "dumb" alternator would be cheaper than a "smart" alternator.
Since inception, alternators and even the previous, DC generators had a degree of smartness.
IOW, a regulator has always been used to control the level of output
The first type were not that elegant and was done with an electro-mechanical method, the second was an electronic version of the mechanical one.
The problem was they were electrically noisy and caused radio interference.
This led to the present PWM 'smart' versions. ;)
Max.
 

Thread Starter

Wirefiche

Joined Nov 30, 2020
3
I'm willing to bet there is a higher capacity [alternator]
There definitely is. Kubota appears to offer up to 90 amps for various incarnations of the V1505-T. Not to say the 90 amp would be compatible with my engine, it seems there's some variance based on serial number. But the Sherp manufacturer already sells a 60 amp version. Likely I'll just buy a compatible 60 amp version from a Kubota vendor and call it good enough. The 40 amp I can keep as a spare, should the 60 ever fail.

Fortunately, even in these months of 24 hour night, the lightbar is not always needed or even beneficial. During times when blown snow is in the air, the reflected light in the air renders any additional vision at distance meaningless. So it's not as if I drive with it on constantly.

I'm happy to report the vehicle's voltmeter stays well above 13v while driving whether the lightbar is on or off, including standard accessories, so there's that at least. At idle, the voltmeter drops well below 12v(down to somewhere near 11v, but it's hard to tell as the voltmeter's markings aren't very precise between 8v and 12v. So definitely no leaving it on when parked.

Just wanted to thank you all for the excellent advice. I still have some more homework to do, but this has pointed me firmly in the right direction.
 

geekoftheweek

Joined Oct 6, 2013
641
Glad to hear the news. One small detail you may want to also look in to for the upgrade is cable size between the alternator and the battery. If it is a pretty short distance the factory cable may be good enough, but it also one of those "since it's already apart" sort of upgrades.

Know all to well about headlights and blowing snow. We just had our first blast for the season and wile it didn't amount to much it did come down hard and is still on the ground three days later.
 
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