How long to recharge battery from alternator

[Yeoman_24]

A question for the mathematicians / scientists please.

• Engine is a 28hp Beta marine. Starts first time, every time in a maximum of 2 seconds
• Alternator is 65amp.
• Battery is a new 70amp, 450CCA starter lead calcium / battery.
  
  If the starter was battery fully charged, how long would it take to put a full alternator charge back into the battery? Thanks

 

[Minchsailor]

When starting, there will be a current draw of a lot of Amps (maybe c200), but only for a few seconds - so minimal Amphours. Probably similar to a couple of weeks of self-discharge.

My guess is that the battery will be recharged within a few minutes, depending on what the voltage regulator is doing.

 

[andsarkit]

Impossible to say with certainty but not very long is the best answer.
A quick sum of 100A for 2 seconds is 0.05Ah so would be replaced in about 4 seconds at 65A.
However the 70Ah battery is rated at the 20hr rate so would be rated at much less when drawing 100A.
Also the alternator would charge at a much lower rate into a nearly charged battery so would probably only charge at 20A max and rapidly reducing to very little.
If you check the charge current you can time how long it takes to reduce to an amp or so and that will tell you how long it takes to replace the starting charge.

 

[VicS]

I assume you mean if fully discharged?

It's impossible calculate .
The current will start high, maybe the full 65 amps alternator output, but fall exponentially to just a few amps

Estimate: possibly between 12 and 24 hours

 

[coopec]

A question for the mathematicians / scientists please.

• Engine is a 28hp Beta marine. Starts first time, every time in a maximum of 2 seconds
• Alternator is 65amp.
• Battery is a new 70amp, 450CCA starter lead calcium / battery.
  
  If the starter was battery fully charged, how long would it take to put a full alternator charge back into the battery? Thanks

I'll be interested to see the replies here.

If the Starter Battery was fully charged surely the Alternator would sense that and put no more charge into the battery.
Do I misunderstand the question?

"The alternator regulator must monitor and control the process to charge the battery. It must also ensure that all power loads in the vehicle have sufficient energy to function. The alternator control is usually a component of the alternator."

Alternator regulator | Mein Autolexikon

 

[Daydream believer]

I assume you mean if fully discharged?

Did not the Op state that the battery was fully charged before starting & that the engine started within a couple of seconds. If it had been fully discharged it would not have started the engine
That being said I do not believe that an alternator does fully charge a battery. The regulator adjusts the charging befor max is reached & i recall reading somewhere a cut off as low as 80%. Cannot be sure of that or what happens after. What i do know is that even if I run my engine for 25 hours & little current use, except autopilot & minimal nav lights, no other nav instruments,( which I have done on long trips with zero wind) the shore power smart charger still finds considerable space to add more charge immediately afterwards

 

[Tranona]

Minutes if the battery was fully charged to start with. The alternator capacity is largely irrelevant as the charge rate is governed by the acceptance rate of the battery. Best to make sure the battery is fully charged using a mains charger before you install it as getting the last bit in is very slow. That is what I did with my new batteries and as I will not be using the engine for a while. I shall top them up every so often with the mains charger. In fact I checked them yesterday. The house (3 months after installation) which has had some use testing the new wiring and which is not currently connected to the alternator was 12.7v and the start which has had less than an hour running from the engine was 12.9v.

 

[jackho]

I assume you mean if fully discharged?

It's impossible calculate .
The current will start high, maybe the full 65 amps alternator output, but fall exponentially to just a few amps

Estimate: possibly between 12 and 24 hours

Not and expert but from reading past articles on this subject I tend to agree with VisS. Stated in the past ,running engine at tickover revs. would take 24hrs. !!

 

[Neeves]

38' catamaran, sail. My experience would be (this all approximate) - if your battery has been discharged to 50% of its charge then the alternator will provide an initial 65 amps and it will quickly drop to to 35 amps. The charge will then drop as the charge in the battery increases. But getting the last 10 amps into the battery takes for ever. I don't recall how long it takes to get to near 95% charge.

That's how our regulator would manage your battery/alternator - yours will be different. We can alter how our regulator manages the re-charging.


We have split charge. that allows both battery banks to be charge together, or just one bank - if the other is full. We have a battery switch that allows us to isolate the house battery bank, the engine start battery or use both simultaneously (or switch the whole lot off).. We have never had the engine battery flat - so we have never had need to rely on the house to start the engine(s).

We have 400 amp-hrs house plus a single 60 amp/hr engine battery. I'll start one engine and after a few minutes start the second - this allows the first alternator to provide some power to start the second engine - rather than taking it all from the engine battery... Our alternators are 60 amps each. We only monitor the house bank but measure the voltage of the engine start. If we have used over 120 amps the alternators will provide 120 amps until the batteries reach 280 amps and the the regulator will manage the power from the alternators to provide less and less as the batteries are filled, so when we are at - 60 amps the alternators will provide, between the 2 of them, 60 amps. I will then switch one engine off. As the battery bank fills I'll turn down the revs so that the single alternator provides what the regulator will accept.

The alternative to turning the revs down, to allow the alternator to match what the regulator will accept, is to use the excess power, charge up the lap top, turn the thermostat down on the fridge to make it colder (remember to reverse this when you turn the engine off!), use the desalinator, lift anchor with the electric windlass, bake bread - or motor out of the anchorage, on one engine.

I forget what our monitor and regulator were made by.....'Xantrex' for the Mon, 'Next Step' for the regulator...??

Running the engine out of gear flat chat will not increase the output. You need to run the engine such that is providing 60 amps, or 35 amps - whatever has been used. Running the engine flat chat just uses fuel (and is said to glaze the cylinders).

In addition to a good regulator you also need a good visual monitor, voltage, amps in, amps remaining.

If you have a visual display, ours simply gives the numerical data I list - others might be more clever - simply write down how many amps you have used, monitor the amps being used the charge the batter, record the time - then plot it. The curve is exponential (I think that is the term) it never quite reaches zero input - or it does but you will have lost interest by then.

Not quite an answer to your query but the best I can offer.

The best way to recharge the final amps is to use solar, rather than your engine/alternator. But this demands your solar have the ability to produce more power than you use )for the fridge and instruments etc. But drip feeding with solar is better than running an engine to pump in the last few amps - unless you are motoring.

Someone will come along and turn my script into something professional.

Jonathan

 

[Richard10002]

I assume you mean if fully discharged?

A confusing question but, what he wants to know is how long it takes to charge the battery up after using it to start the engine.

The answer is of the order of "not many minutes". If the OP wants more precision than that, we could ask why?

 

[coopec]

Here are a couple of explanations I've found on the internet which may be of interest, (They have clarified the matter for me)

If a car has a 12V 65Ah battery, how can it throw 300 amps to the starter motor?

Amp hours is a rating of the capacity of a battery. Specifically it says that this battery can deliver 65 amps for a period of 1 hour before dying.

In theory, this battery could deliver 130 amps for 30 minutes. ((65 amps X 2)X (1 hour / 2)).

This would also mean 260 amps for 15 minutes, or in your example 300 amps for 13 minutes.

Going the other direction, the battery could deliver 1 amp for 65 hours.

I said in theory, because at higher or lower rates than the 1 hour rate, there can be a small deviation.(Husband, Father, Engineer, Courier, Uber driver, CDL A )


So what do these ratings mean?

CCA (Cold Cranking Amperes) is the most popular industry rating and is a measurement of the current a fully charged battery can deliver for 30 seconds and maintain a voltage of 7.2 volts (12 volt battery) at a temperature of -18°C. CCA’s are important, but they are not the best measure for warmer or Australian conditions. They are the ratings used by the battery industry to define a new batteries ability to start an engine in extremely cold temperatures and such are more relevant for the North American and European markets.

MCA (Marine Cranking Amperes) or CA (Cranking Amps) is a measurement of the current a fully charged battery can deliver for 30 seconds and maintain a voltage of 7.2 volts (12 volt battery) at a temperature of 0°C. It is a more relevant method for rating batteries in warmer or marine conditions where -18°C (Freezing) temperatures are rarely encountered. Calculating at this temperature increases the cranking capacity of the battery by approximately 20%. For example the Century Marine Pro 730 battery has a rating of 730CCA and 875MCA. With this in mind it is important to ensure you are comparing apples for apples.

 

[andsarkit]

Amp hours is a rating of the capacity of a battery. Specifically it says that this battery can deliver 65 amps for a period of 1 hour before dying.

In theory, this battery could deliver 130 amps for 30 minutes. ((65 amps X 2)X (1 hour / 2)).

This would also mean 260 amps for 15 minutes, or in your example 300 amps for 13 minutes.

Going the other direction, the battery could deliver 1 amp for 65 hours.

This is not quite true as capacity decreases with increasing discharge current.

 

[Yeoman_24]

Thanks for everyone’s input which is much appreciated . I have another post about battery monitors which has encouraged me to switch to a VSR rather than combined 1, both, 2 battery switch as a little job for this winter.
In the mean time, sticking with the old battery switch, rather than chugging along with it on ‘both’ I now know that after 5 mins max I can switch from ‘starter’ or ‘both’ to just ‘house’ so they are getting maximum charge from the alternator.
I hope I understand that correctly?

 

[andsarkit]

Thanks for everyone’s input which is much appreciated . I have another post about battery monitors which has encouraged me to switch to a VSR rather than combined 1, both, 2 battery switch as a little job for this winter.
In the mean time, sticking with the old battery switch, rather than chugging along with it on ‘both’ I now know that after 5 mins max I can switch from ‘starter’ or ‘both’ to just ‘house’ so they are getting maximum charge from the alternator.
I hope I understand that correctly?

The switch should be make before break so that the alternator does not see an open circuit while you are switching. Theoretically an open circuit could cause a voltage spike that destroys the alternator diodes but I think modern alternators are fairly robust and usually survive. Unless you have the spec of the switch it is difficult to know if it is make before break.
I have put a couple of diodes across the switch so that even if it is switched off accidentally,the current will have somewhere to go.

 

[Neeves]

I think you will one day need both batteries, together (but maybe that's what you intend). Eventually your batteries will get old. My guess is one will fail before the other. You may not realise its about to fail - until it does. If yo cannot start your engine it will be, possibly, embarrassing. Being able to switch to both batteries will allow you to start your engine - and over the next week you can take the questionable battery off - have it tested and then take appropriate action.

In the meantime - simply measure voltage of each battery, independently, when not being charged - if they are below (someone will correct me) 12.75v - you need to have them, or it, recharged and/or checked. Voltmeters are as cheap as chips.

Jonathan

 

[William_H]

Thanks for everyone’s input which is much appreciated . I have another post about battery monitors which has encouraged me to switch to a VSR rather than combined 1, both, 2 battery switch as a little job for this winter.
In the mean time, sticking with the old battery switch, rather than chugging along with it on ‘both’ I now know that after 5 mins max I can switch from ‘starter’ or ‘both’ to just ‘house’ so they are getting maximum charge from the alternator.
I hope I understand that correctly?

The concept of switching to just one battery to get max charge does not work in practice. The alternator will have a max current capability depending on size of alternator and RPM of the alternator (engine). However in practice an alternator especially one simply regulated to say 14v, when charging a battery is not often running at full capacity. Or if it does it is only for a short period. This is because the battery in taking a charge will exhibit a rising voltage which in effect opposes the charge voltage so limits charge current. A very flat battery will take a lot of current for longer than a near charged battery which will take a decent charge for a short time then taper off to a small charge current. That means that in fact the more batteries you have connected to an alternator the greater total charge current you will get collectively into the batteries. You will still be taking a current way less than capacity of alternator.
So for charging you may as well just charge both at once. Engine battery presumably well charged will take less and house battery well discharged will take most while both have the same charge voltage across their terminals. Incidentally this is also how a VSR charges. Albeit it does put in charge into engine battery until voltage rises to switch on voltage before connecting house battery. (often less than a minute) ol'will

 

[coopec]

I think you will one day need both batteries, together (but maybe that's what you intend). Eventually your batteries will get old. My guess is one will fail before the other. You may not realise its about to fail - until it does. If yo cannot start your engine it will be, possibly, embarrassing. Being able to switch to both batteries will allow you to start your engine - and over the next week you can take the questionable battery off - have it tested and then take appropriate action.

In the meantime - simply measure voltage of each battery, independently, when not being charged - if they are below (someone will correct me) 12.75v - you need to have them, or it, recharged and/or checked. Voltmeters are as cheap as chips.

Jonathan

Why not a very heavy set of jumper leads to cover that situation? (I can understand and use jumper leads without stuffing up)

 

[Neeves]

Why not a very heavy set of jumper leads to cover that situation? (I can understand and use jumper leads without stuffing up)

No problem - once its wired up - a switch is easier (and, admittedly, more expensive.

Jonathan

 

[coopec]

No problem - once its wired up - a switch is easier (and, admittedly, more expensive.

Jonathan

Yes, a H/D set of jumper leads can cost.......$37 (I suppose that is cheaper than H/D switches and cable)

 

[ChromeDome]

The part about the last bit of charging battery taking a long time is supported by the issue of range/charging of electric cars.

To make the facts look acceptable marketing is concentrated on charging time to 80%. That's not optimal for batteries and these cars need to be fully charged whenever possible, most likely at home, at night.

Different technology, but same charging picture and even though the cars charge with all kinds of computer control, the last part is disproportionately long.