How much current does an alternator supply in this situation?

[markccc]

The van's alternator is rated at 125A.

The battery is 130Ah (RC 190 mins 1000CCA).

I think in theory the battery should be able to deliver 115A for 40 mins (less in reality due to heat build-up). When I ask it to do that the voltage drops to 11.7V & it holds there for at least a few mins (I haven't let it go to for longer).

With the same load with the engine idling, the alternator manages to keep the voltage at the battery terminals above 12V (can't remember what exactly) but how do I work out how much current it's contributing? I guess it can't make 115A at those revs so I'd like to estimate the draw on the battery so I know how long I've got (roughly!)

All cables & contacts are sturdy (don't get too hot).

Cheers

 

[IRstuff]

Actually, WAY less. A battery's rating is based on a current draw equal to C/20, i.e., 6.5 A draw. The battery's internal resistance is therefore optimized for that current, with the exception of the CCA, but that's only for a VERY short while

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[markccc]

Thanks for that IRStuff, good advice.

'RC 190' on the battery means it's rated to deliver 25A for 190 minutes, so let's say it can do 50A for 60 minutes & let's say the alternator can make 50A at idle revs, that means I've got a stable situation for an hour.

That's a lot of guessing & I haven't got an ammeter big enough to measure the alternator current so I suppose I'll just have to test it for longer watch the voltage to work out what's going on...

 

[GregLocock]

If you attempt to pull C1.2 (ie 115 A) for 40 minutes you'll damage the battery quite quickly, if it bears any resemblance to a normal automotive battery.

Alternator characteristics are:

at 1000 rpm (alternator, not engine) 0A
at 2000 rpm 60%rated output
at 6000 rpm rated output
at 15000 rpm 110%

for a couple of different designs I looked at those curves don't change much.

It is not unusual, although it is annoying, to run with the battery discharging slightly at idle if you have any of the usual loads running.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[LiteYear]

You might have already thought of this, but if you can't measure current directly you could just load the battery until it falls to the "above 12V" level and calculate the current based on the load and voltage - call the result I. Then, the ammeter current will be roughly 115 - I. Theoretically this works because the alternator is supplying part of the 115A load and the battery is supplying the rest - that is, the ammeter is not directly propping up the battery voltage, instead it is lowering the voltage drop experienced by the battery.

My second potentially useless comment is to Greg: that "C1.2" doesn't look right. Is that supposed to mean 1.2 * C? C is 130A, so 115A is actually 0.88C and (Mark) the perfect case capacity is 53 minutes, not 40.

 

[GregLocock]

oops yes you are right.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[IRstuff]

Based on the numbers provided, the battery's Peukert coefficient is 1.3682, which results in 23.5 mins at 115A

As for the "50A for 60 minutes & let's say the alternator can make 50A at idle revs" At idle, a typical car electrical load is only the order of 20A, so the alternator would not supply more than that. Additionally, it 's not exactly a "stable" condition. Assuming a battery starts at full charge, and 50A is drawn, the battery continues to discharge until the threshold of the alternator kicks in, at which point, the alternator supplies 50A. But, since the load is 50A, the alternator will not be charging the battery until the load goes away. If the system is stopped completely, the battery is still down the energy drawn before the alternator kicked in.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[markccc]

Thanks guys, really handy info. Peukert's coefficient is a new one for me. I don't want to load test the battery for long (without the alternator) for fear of battery damage, & I realise that kind of current draw will ultimately deplete the battery unless the engine idles all the time (the 115 load is intermittent, albeit big). I think I'll measure the voltage drop across an inexpensive calibrated shunt resistor wired inline at different locations to give actual amps from each power source - better than theorising, much as I love it! Will report back when I have some results.

 

[GregLocock]

You may find the battery lead is in itself a suitable resistor!

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[markccc]

Ha! True

 

[DanEE]

Much easier to get a Clamp on DC Current probe (uses Hall effect) to measure the DC Current..

I've found a quick and easy way to measure max alternator output is to simply place a standard 100amp battery tester load on the battery and, with the current probe on the alternator output lead. The load tester will pull the battery voltage down low enough that the alternator should go to max current-limited output..

A 100 amp current load is within range or in many cases much less that the typical engine starter current load, depending on whether the starter motor is a reduction gear permanent magnet starter motor (far more efficient) or the older generation wound field and armature starter motor.

I've measured PM motors starting Ford 460cu size industrial generator engines only pulling about 70amps to start and the old wound field and armature motors sometimes having an initial current surge approaching 10x this value for same size engines.

Regarding final charge voltage levels, remember that alternators have (now a days many internal to the alternator using a hybrid chip regulator) regulate battery charge voltage levels to follow a battery temperature compensation curve, or done by external regulator. You can find systems for typically marine applications that do this far more precisely with temperature probes that mount on the battery positive terminal for the regulator to more precisely follow the temperature compensation recommended by the battery manufacturer. Many automotive applications consistently undercharge batteries due to not using more precise temperature compensation control and I guess to cheaply hedge against very high battery temperatures in an under hood environment.

 

[markccc]

Got some numbers!

Wired a 200A 75mV shunt resistor in between the van's electrics & the battery positive terminal. 1mV drop across the shunt with engine idling & no extra electrical load (14.3V at battery).

With a 1150W 240VAC load on the 85% efficient inverter wired to the battery, the drop across the shunt was 36mV (13.3V at battery).

Looks like the alternator is supplying most of the current I need?

(After 5 minutes the shunt was 34mV & battery was 13.2V)

Interesting about under-charged OE batteries & a current probe would have been great but not cheap! A friend has a couple but they're AC only :-(

 

[IRstuff]

1150W/240V is barely 5A, but 36mV would represent nearly 100A

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[markccc]

Yes, I'm impressed - to make 1150W 240VAC the inverter is pulling 102A from the van & the shunt says the alternator is contributing 96A to the mix. Not bad for an idling engine! Worth noting that the van engine's max power is at 3500rpm so I expect the alternator is geared-up quite a lot, perhaps by a factor of 3 or more looking at 'alternator characteristics' above.

After 5 mins the alternator's contribution is down to 90A - I guess that is due to heat build-up?

Will do more testing but it's looking good