Battery Charging - Via Alternator - Stationary Vehicle

[tweetyduck]

A simple question with a no doubt difficult answer.

How much effect would running my car engine for say 30mins have on charging up a couple of batteries ? I have in a Exxide pair, one DC and one starter on a split charge. My alternator is the larger one which i think is 100amp.

I'm not really asking if it will keep me going from a power point of view but is it bad for the batteries and how much effect with 30mins actully have. If i remember a training course from may years ago then most of the charge is dumped in to the battery in the initial stages and the charge rate is reverse-exponential. To reach full charge can take a long time but to get to 80% doesn't take that long.

the reason i'm asking is the normal one. In a campsite for 4 days and need to top up charge the leisure battery up every 48 hours max. So i have the question, 30 mins or so make any difference and will it screw up the battery ?

I might post this on Addict as well as some of those guys do a lot of travel.

[FridgeFreezer]

You can't in this scenario accidentally put "too much" charge into a battery, 30 min should be fine. Worth noting that a 100A alternator won't put out 100A at tickover and may only manage 30A.

If you want to get a bit serious you could calculate how much power you use in 48H and how long you need to run the alternator for to recharge it.

[tweetyduck]

So you think i should be fine doing this. I can work out the power consumption and probably use a clamp to test the Alternator output before we go. I just worried that doing this is in some way detrimental ?

I'm starting to panic about all sorts of stuff........

[Bowie69]

I seem to remember, in an automotive situation, a battery won't take more than about 25A charge, but as above, that will still need the engine above tickover, maybe a brick on the throttle gently to bring up to around 12-1300rpm and it will make a big difference.

It won't wreck the battery what you are proposing, think about most short-ish car journeys.... they describe exactly what you propose.

4 days, 48 max before recharge.... 1 charge

I'd just monitor the battery level and then charge as and when it needs be. Try and conserve power as well of course

[Superpants]

On most alternators you will find a big difference in the output at idle and at a 'high idle'- the curve usually gives a much better output at around 1200 RPM.

[Ed Poore]

You can't in this scenario accidentally put "too much" charge into a battery, 30 min should be fine. Worth noting that a 100A alternator won't put out 100A at tickover and may only manage 30A.

Don't think that's correct for the majority of vehicles (I say majority because there are no two Land Rovers the same and therefore what normally applies doesn't normally apply if you get my drift ). Most alternators are designed such that they can generate maximum power at more or less idle speed.

Basically* the alternator is an induction machine and therefore one fundamental property of it is that as you increase the rotational speed then the output voltage also increases, the current that it outputs depends on the electrical load attached to it. So to prevent damage to batteries, lights etc then this output voltage must be regulated. The method for doing this is to disconnect and reconnect the circuit rapidly. The duty cycle (i.e. the ratio of time the circuit is on to off) therefore controls the average voltage seen at the other end. The reason you see a DC (smoothish) waveform on the alternator is due to the filtering properties of having a massive inductance (the alternator) and a massive capacitance (the battery) - they're literally a low-pass filter. To simplify matters the field current rather than the stator current (i.e. the little rather than big cables) are chopped since it's cheaper to deal with low currents (MOSFETs that can deal with ~10A are pennies whereas MOSFETs that can deal with ~100A are more like £50 a pop).

So the underlying fact of the matter is that as the alternator speeds up the input current (and therefore output current) is chopped and therefore some power is "lost", or rather not generated in the first place. Therefore as rotational speed increases the power that can be generated decreases.

That's a rather simplified view but hopefully justifies why they're tuned for lower speeds. There's some pretty clever electronics / physics going on in the physical design of the alternators to make changes in rotational speed matter less and the like.

The reason alternators may not output maximum power at idle is due to the electronics controlling them - basically if you wallop a 100A load onto the alternator then it in turn will load the engine - this may not be desirable as it gives a rather rough lumpy response. In fact one of the alternators I've worked closely with actually stalls some amateur rally cars when they switch on all the floodlights and heated windows! During my Master's project I built a test-rig for work on alternators and this was a 4.5kW DC motor driving said alternator (150A tiny Denso thingy) and if I switched on my load-bank (~1.5kW of halogen bulbs ) then smoke would start appearing from the belts and invariably blew several fuses on the motor-driver and reached the limits of a single-phase supply.

At the moment I'm tidying up this project etc for the Uni so they can use it as a demonstration in the labs and will be wiring into a three-phase supply to make it a little more reliable.

Going back to the original comment - if the batteries are severely discharged then the alternator will try it's best to deliver the power in order to recharge them because the lead-acid batteries will draw a substantial current initially. I haven't noticed it on my 110 because of the torque that engine produces but certainly on the little <1L Suzuki I used to drive if you switched on the head-lights while the engine was ticking over you could hear the engine note drop as the alternator produced more power and therefore loaded the engine.

*because there's a lot of mathematical gubbins behind the actual operation of them which I can't be bothered to explain and more importantly because I can't remember it off the top of my head...

[tweetyduck]

Thanks all. I will do some testing. I might try leaving the fridge on few a few days and then start the car to see what current i get. Now i need to borrow a clamp from someone. The fridge i have runs in turbo mode if it gets more than 13.1 volts so starting the car will also help the fridge get to temp if its not there already.

The fridge draws between 1.5 and 2.6 amps so i need to get this back into the battery. It will be at the higher end if its hot. the fridge is ellegedly tested in hot climates and made in South Africa. Its a National Luna.

[Bowie69]

How big is the battery, in Ah?

[tweetyduck]

its only 50 amp hour. they are both 50amp hours. Trouble with these Optima and Exide batteries is the low Ah

[Retroanaconda]

I would have thought a big (100+ Ah) caravan battery would have been the best bet.

I have a 90Ah battery in my 90 and I'm hoping that will run an Engel fridge for 50 or so hours

[tweetyduck]

Yes but they are very BIG....and i've got these fitted now and they wern't cheap. I could buy 4 x 100Ah for the same as these !

It runs the fridge easily but i would say in 35C ambient the duty cycle will be a good deal more....so i will need to charge it up more. The reserve capacity is pretty good. IIRC its 90mins. So thats 25amps for 90mins if i understand reserve capacity. So thats 15 hours at 2.5A which is a hell of a duty cycle. Although the battery would be completely flat at that. So i need to watch it and charge it up every day i think.

[Gremlin]

A little silent gen set could work, some have a 12v output, that could keep your battery topped up.

G

[zardos]

A little silent gen set could work, some have a 12v output, that could keep your battery topped up.

G

I would say solar power might be much better, might only give you 0.5Amp from a cheapy setup but it will do that for at least 10+ hours for free.

[Deej]

That was a good description of how an alternator works. However, as I'm feeling particularly pedantic today there are a couple of things which are definitely not the case:

• Basically* the alternator is an induction machine
• Therefore as rotational speed increases the power that can be generated decreases

An alternator is a synchronous machine much like the large generators you would get in a power station. An induction machine does not have a separately excited field (i.e. windings or magnets) and has a rotor which moves at a different speed to the stator field (slower for a motor or faster for a generator).

If anything the faster the alternator spins the more power can be generated. At low speeds the regulator will struggle to supply enough field current to provide 14V at full load, whereas at high speeds only a small field current is required to produce the required output voltage. Also as the alternator input power = Torque x Angular velocity, the faster it spins the more power is available. Trying to produce a lot of power at low speed requires a large amount of torque, which is when the drive belt starts squealing as there is insufficient friction between the pulley and belt.

*Pedantic mode off*

@ tweetyduck: If you don't want to buy a larger battery I think your best bet is to do the test you described and look at the idle charge current using a clamp meter. Of course you will have to bear in mind the duty cycle will have been much lower seeing as the ambient temperature in Yorkshire is rarely 35 deg C!

Dave

[tweetyduck]

LOL... its a balmy 35 all the time in Yorkshire as the Ferrets keep my legs warm.

If i could get a 100Ah AGM with the same spec as the Optimas or Exide then i might be tempted. Its also got to fit in the battery box !

[Ed Poore]

That was a good description of how an alternator works. However, as I'm feeling particularly pedantic today there are a couple of things which are definitely not the case:

An alternator is a synchronous machine much like the large generators you would get in a power station. An induction machine does not have a separately excited field (i.e. windings or magnets) and has a rotor which moves at a different speed to the stator field (slower for a motor or faster for a generator).

If anything the faster the alternator spins the more power can be generated. At low speeds the regulator will struggle to supply enough field current to provide 14V at full load, whereas at high speeds only a small field current is required to produce the required output voltage. Also as the alternator input power = Torque x Angular velocity, the faster it spins the more power is available. Trying to produce a lot of power at low speed requires a large amount of torque, which is when the drive belt starts squealing as there is insufficient friction between the pulley and belt.

*Pedantic mode off*

@ tweetyduck: If you don't want to buy a larger battery I think your best bet is to do the test you described and look at the idle charge current using a clamp meter. Of course you will have to bear in mind the duty cycle will have been much lower seeing as the ambient temperature in Yorkshire is rarely 35 deg C!

Dave

Even more pedantic mode on - an induction machine is anything that operates by the process of electro-magnetic induction. A synchronous machine is a special case of an induction machine and an alternator is a special case of a synchronous machine.

I'm not denying that you can't get more power from alternators by spinning them faster (in a non-automotive application) it's just that with all the extra electronics built into the ones installed on vehicles the way that the voltage is regulated is by PWM regulation of the field current. Therefore they are tuned to produce maximum power at a particular rotational speed, which is usually a fast-idle as mentioned in other posts.

[Deej]

To prevent this thread going completely off topic, I think we are going to have to agree to disagree about this point!

Even more pedantic mode on - an induction machine is anything that operates by the process of electro-magnetic induction. A synchronous machine is a special case of an induction machine and an alternator is a special case of a synchronous machine.

I do agree with your second point though, I think I was being a bit too theoretical in my explanation as in practice alternators are tuned to a particular operating point which will be below the maximum speed.

Apologies to tweetyduck for letting my geekery corrupt your thread

[tweetyduck]

No problem here. I normally start arguments that i can't win and did this in spectacular fashion when i first joined the forum. I think i've now been forgiven now.

Since i'm going on my trip (in not many weeks time) and this trip is the very reason i joined here i think all the helping members can be proud i went from knowing nothing to something is a short two years ! I now know slightly less than nothing about Alternators and this coming from someone who did a HND in Electronics and specialised in "Heavy Power", as they called it then, all focussed on Generators and Motors I've stayed out of it for the previously mentioned reason.

one all....its a draw.

[Ed Poore]

No problem here. I normally start arguments that i can't win and did this in spectacular fashion when i first joined the forum. I think i've now been forgiven now.

Since i'm going on my trip (in not many weeks time) and this trip is the very reason i joined here i think all the helping members can be proud i went from knowing nothing to something is a short two years ! I now know slightly less than nothing about Alternators and this coming from someone who did a HND in Electronics and specialised in "Heavy Power", as they called it then, all focussed on Generators and Motors I've stayed out of it for the previously mentioned reason.

one all....its a draw.

I wouldn't worry about it - I spent the last year of my degree heavily immersed in automotive alternators and still know bugger all about them. Worrying since that's what my Master's project was about , although I'm quite interested in an aspect that Si (simonr) pointed out about the project which basically means it could be adapted to improve battery life...

[robhybrid]

I seem to remember (from ag college)that an average tractor had to run for 2min to recharge the batteries to the state they were in before before cranking the engine.

what I am getting at is, would several 5min engine runs be better than a few 30min engine runs?

[discomikey]

i reckon that a wagon battery, the sort that is 18" long would do for a weekend. and what about a dynamo, arent they meant to fully charge a battery quicker in this sort of situation?

[Ed Poore]

I seem to remember (from ag college)that an average tractor had to run for 2min to recharge the batteries to the state they were in before before cranking the engine.

what I am getting at is, would several 5min engine runs be better than a few 30min engine runs?

Depends on what kind of battery it is, broadly speaking there are two kinds: starter and leisure, however as with everything there are ones which combine features of both (Optima springs to mind). I'm sure most know the difference but essentially they can deliver the same amount of energy it's the manner in which they deliver it that varies. Starter batteries can dump massive (talking >1000A in many cases) currents but don't survive deep discharge well. Leisure batteries can't provide the wallop a starter does but deliver the same amount of energy over a longer period - one side-effect of their design is that they can be deeply discharged without suffering too much.

If you're powering auxiliary equipment off a starter battery then I'd have thought (purely from an electrical point of view) that more frequent but shorter duration charges would be better to prevent deep discharge. If you're running leisure batteries then longer periods between recharges (and subsequently longer charging periods) may be more advisable. I'd just do some readings (or getting someone friendly with electronics to rig up a little microcontroller or something (Arduino springs to mind since readily available and easy to use)) to determine what the charge / discharge profiles of the batteries would be. From these you can make more informed decisions about what would be the best solution.

There's also a decent write-up on how to determine the state-of-charge of lead-acid batteries here: http://www.mpoweruk.com/soc.htm which might be worth a glance.

E