How many amps does an engine add to batts

[DeanS]

If I have a 175amp alternator, and run the engine in tickover, how many amps will it feed back into the battery bank after 1 hour

[churchward]

If I have a 175amp alternator, and run the engine in tickover, how many amps will it feed back into the battery bank after 1 hour

The answer (for me at least!) is rather a complicated one. It depends on the state of the batteries when you start. If they are in a low SOC the more amps the alternator will be able to supply but it tails off quite steeply as the voltage rises and the SOC is higher.

 

So without knowing the SOC or battery capacity when you start running your batteries it is not possible to answer properly.

 

If you read this it may help.

 

http://www.smartgauge.co.uk/peukert2.html

Edited February 3, 2013 by churchward

[DeanS]

Lets assume I have 330AH of batteries.

Lets assume they are full in the morning and I run a desktop pc for a 3 hours, at about 40Amps per hour. = 120Amps used.

Now I run my boat engine to recharge batteries.

How many hours would I need to run my boat engine to recharge the batteries to 90%

 

This cycle may continue all week (Mon to Fri), and I could use a gennie to recharge the batteries from 90% to 100% over the weekend.

[nicknorman]

If I have a 175amp alternator, and run the engine in tickover, how many amps will it feed back into the battery bank after 1 hour

I think you have a Beta 43 or the like? Ours can just about produce full output at tickover of 850rpm. Depending on the size of your battery bank and voltage drop in the cabling, you may only get 175A for a few minutes until the surface charge effect raises the battery voltage. Minimising voltage drop in the cabling can make quite a difference here, although of course if you have battery sensing or a fancy alternator controller they can help with dodgy cabling. But when the batteries are moderately discharged, the charge effiency is 90 - 95% so most of the current going into the bats is adding Amphours directly to the SoC. Towards the end, the charge process gets much less efficient. After the bats reach around 80% SoC the charge current will drop off substantially and your 175A alternator will only be able to get a gradually reducing current into the batts.

 

One thing to bear in mind, the alternator is working very hard at 175A and will get very hot. If you run it a tickover, you are applying the maximum load to the belt, which will be on the point of slipping, and the cooling fan of the alternator will be going relatively slowly. Of course the alternator SHOULD be able to cope with this, but it is much kinder to belt and alternator to run the engine a little faster, say 1000 - 1200 rpm. That way you get less belt force / wear and a cooler alternator.

 

Lets assume I have 330AH of batteries.

Lets assume they are full in the morning and I run a desktop pc for a 3 hours, at about 40Amps per hour. = 120Amps used.

Now I run my boat engine to recharge batteries.

How many hours would I need to run my boat engine to recharge the batteries to 90%

 

This cycle may continue all week (Mon to Fri), and I could use a gennie to recharge the batteries from 90% to 100% over the weekend.

That's not a particularly big bank, so the surface charge effect will kick in quite soon. I suspect that the charge current will be below 100A within 30 mins. Too many variables such as cabling resistance, battery health etc, but 2-3 hours would probably get you close to 90%. Once past 80% the rate of increase of SoC is bound to be slow whatever system you use.

 

By the way, 40A for a desktop seems quite a lot - is it a monster? It will certainly help with heating the boat!

 

Of course, rather than running the PC to drain the batteries, then recharging them, its better to use the PC whilst the engine is running.

Edited February 3, 2013 by nicknorman

[DeanS]

I think you have a Beta 43 or the like? Ours can just about produce full output at tickover of 850rpm. Depending on the size of your battery bank and voltage drop in the cabling, you may only get 175A for a few minutes until the surface charge effect raises the battery voltage. Minimising voltage drop in the cabling can make quite a difference here, although of course if you have battery sensing or a fancy alternator controller they can help with dodgy cabling. But when the batteries are moderately discharged, the charge effiency is 90 - 95% so most of the current going into the bats is adding Amphours directly to the SoC. Towards the end, the charge process gets much less efficient. After the bats reach around 80% SoC the charge current will drop off substantially and your 175A alternator will only be able to get a gradually reducing current into the batts.

 

One thing to bear in mind, the alternator is working very hard at 175A and will get very hot. If you run it a tickover, you are applying the maximum load to the belt, which will be on the point of slipping, and the cooling fan of the alternator will be going relatively slowly. Of course the alternator SHOULD be able to cope with this, but it is much kinder to belt and alternator to run the engine a little faster, say 1000 - 1200 rpm. That way you get less belt wear and a cooler alternator.

 

ok, I'll run it a little faster.

It's a 52HP engine.

How long would it take batteries to go from 50% to 80%

 

ETA...perhaps I should be asking...how long would it take to replace what's used by a desktop pc..by running the boat engine.

 

Also...would the alternator put back 50-100Amps into the batteries...per hour. Would that be a good ballpark guess?

Edited February 3, 2013 by DeanS

[nicknorman]

ok, I'll run it a little faster.

It's a 52HP engine.

How long would it take batteries to go from 50% to 80%

Let me get my string and a tapemeasure! I'd say much the same, 2-3 hrs but its not an exact science. Do you have some sort of battery monitor? Or at least, get an idea of the current going into the bats (clamp meter from ebay if nothing else) and that will give you a better idea. In that region, you roughly have to put back what you took out plus say 10%, in terms of current.

Edited February 3, 2013 by nicknorman

[DeanS]

Let me get my string and a tapemeasure! I'd say much the same, 2-3 hrs but its not an exact science. Do you have some sort of battery monitor? Or at least, get an idea of the current going into the bats (clamp meter from ebay if nothing else) and that will give you a better idea. In that region, you roughly have to put back what you took out plus say 10%, in terms of current.

 

 

 

If it takes an hour to chew 40amps from the batteries (big desktop) (late at night), would a 175A alternator put more than that back into the batteries over the period of an hour. (daytime)

[Grace and Favour]

DeanS - - have you considered investing in a Smartguage?

[churchward]

ok, I'll run it a little faster.

It's a 52HP engine.

How long would it take batteries to go from 50% to 80%

 

ETA...perhaps I should be asking...how long would it take to replace what's used by a desktop pc..by running the boat engine.

 

Also...would the alternator put back 50-100Amps into the batteries...per hour. Would that be a good ballpark guess?

I would say aprox 3-4 hours possibly longer. You need to put back more than you take out around 20-30% extra I think.

 

One suggestion if you have the cash buy a smartgauge and you will have a easy SOC measurement. It will be able to tell you when you need to charge and when you can stop your engine and have reached a 90% or whatever state. It will probably help you take better care of you batteries and make them last longer (before you need to buy some new ones) so may give you some payback over a finger in the air approach.

Edited February 4, 2013 by churchward

[DeanS]

DeanS - - have you considered investing in a Smartguage?

 

Not really. I dont really want to watch the condition of batteries every day. If I understand how much an alternator puts back, compared to desktop pcs etc taking out...I'll be able to work out a routine to suit our needs.

[nicknorman]

If it takes an hour to chew 40amps from the batteries (big desktop) (late at night), would a 175A alternator put more than that back into the batteries over the period of an hour. (daytime)

You might think so, however if it was the top 40A from 100% SoC, to get back to 100% will probably take more than an hour. If it was 40A from 80% SoC, to get back to 80% would take less than an hour, more like 30 mins.

[DeanS]

I would say aprox 3-4 hours possibly longer. You need to put back more than you take out around 20-30% extra I think.

 

One suggestion if you have the cash buy a smartgauge and you will have a easy SOC measurement. It will be able to tell you when you need to charge and when you can stop your engine and have reached a 90% or whatever state. It will probably help you take better care of you batteries and make them last longer (before you need to buy so,me new ones) so may give you some payback over a finger in the air approach.

 

I understand that many use a Smartguage, but my situation is different. I am starting from the basis that every day we have flattened the batteries (from 80% to 50%), and every day I have to recharge them to 80% again. I would probably charge them to 100% occassionally, but not often. Due to this, I treat batteries as consumables....and more than happy to buy an extra new battery every 3 months.

 

I'm just trying to improve my basic understanding of how much current an alternator might put back each day.

 

You might think so, however if it was the top 40A from 100% SoC, to get back to 100% will probably take more than an hour. If it was 40A from 80% SoC, to get back to 80% would take less than an hour, more like 30 mins.

 

That's a very helpful post, thanks

[nicknorman]

You need to put back more than you take out around 20-30% extra I think.

 

The calculated charge efficiency of my system (according to my MasterShunt) is 94% - ie 94% of the current you put in goes to building Amphours. However, in terms of power, its discharging at 12 point something, but having to be charged at 14.something. Therefore in terms of power, I think your figure of 20-30 is right, but in terms of current, I think its too high. Of course if you are trying to get the last few % in to get to 100%, and in a hurry, the bats will be gassing and the charge efficiency in terms of current is a lot less, but its for a relatively small part of the charging process in terms of AH.

[DeanS]

I can say that in my last year of CC-ing, I probably kept the batteries bouncing from 50% to 70% to 50% to 70% for the whole year. I dont think they every charged fully to 100%. I got the boat with older batteries, and just kept using those...recharging daily with a gennie and lots of travelling (70Amp alternator then). I did eventually buy new batteries, (which were sold with the boat), but I envisage the same type of battery usage this year. I dont really have much interest in charging them to 100%....although I am cutting down a lot of heavy appliances (XBox) this year.

[churchward]

I understand that many use a Smartguage, but my situation is different. I am starting from the basis that every day we have flattened the batteries (from 80% to 50%), and every day I have to recharge them to 80% again. I would probably charge them to 100% occassionally, but not often. Due to this, I treat batteries as consumables....and more than happy to buy an extra new battery every 3 months.

 

I'm just trying to improve my basic understanding of how much current an alternator might put back each day.

 

 

 

That's a very helpful post, thanks

Well it will be interesting to see what happens but I think you will find it is a less happy outcome than you think it may be.

[nicknorman]

I can say that in my last year of CC-ing, I probably kept the batteries bouncing from 50% to 70% to 50% to 70% for the whole year. I dont think they every charged fully to 100%. I got the boat with older batteries, and just kept using those...recharging daily with a gennie and lots of travelling (70Amp alternator then). I did eventually buy new batteries, (which were sold with the boat), but I envisage the same type of battery usage this year. I dont really have much interest in charging them to 100%....although I am cutting down a lot of heavy appliances (XBox) this year.

Can't remember if you have solar? If so, running the engine in the morning to do the bulk of the charging, then letting the solar top up the last 20% is a good way to get the batts near 100% at least in the summer. Although that presupposes you are not using the 40A PC in the afternoon!

[DeanS]

Can't remember if you have solar? If so, running the engine in the morning to do the bulk of the charging, then letting the solar top up the last 20% is a good way to get the batts near 100% at least in the summer. Although that presupposes you are not using the 40A PC in the afternoon!

 

No solar this time round.

I may have exagerated a little. It is my intention to do a full charge on the batteries weekly via gennie.

[nb Innisfree]

That is called partial state of charging. If you discharge batts to below approx 80% SoC then when you start charging it will be a bulk charge untill approx 80% SoC is regained, bulk charging is quite efficient, very nearly 100%, that is if you put 100amps in for one hour you will get nearly 100 ah stored, but after bulk charge it gets progressively less efficient until you need 150% more power in to get batts fully charged. If you run your alternator fast enough while bulk charging it will charge at it's full output, i.e. in your case 175 amps. So if you use 120ah from a 330ah battery bank that will bring you down to about 64% SoC, from that point until approx 80% SoC will equal about 53 ah so it will take your alt about 20-30 mins to get there. After that the charge rate will progressively drop off and it will take much longer to replace the remaining 33 ah to get to 90% SoC, difficult to calculate but I would hazard a rough guess of about two to three hours in total. To get back to nearly 100% SoC about 8-10 hours in total. Assuming you are moored up it's probably better if you wait until you have dropped down to 50% SoC then do a do a regular bulk charge between 50% and 80% SoC (or until charge rate starts to reduce) then do a weekly absorption charge of at least 8 hrs.

 

That's my take on it.

 

ETA: You can never get back to a true 100% SoC but a weekly absorption is to reduce sulfate build up on batt plates, not totally successful but a reasonable compromise between fuel and batt cost.

Edited February 4, 2013 by nb Innisfree

[Bobbybass]

I can say that in my last year of CC-ing, I probably kept the batteries bouncing from 50% to 70% to 50% to 70% for the whole year. I dont think they every charged fully to 100%. I got the boat with older batteries, and just kept using those...recharging daily with a gennie and lots of travelling (70Amp alternator then). I did eventually buy new batteries, (which were sold with the boat), but I envisage the same type of battery usage this year. I dont really have much interest in charging them to 100%....although I am cutting down a lot of heavy appliances (XBox) this year.

I'm no expert...so I'm just posing this question to people who are more 'in the know'..

 

If you regularly only take the batteries 50% to 70% rather than 90+%... don't you knacker them ?

[nb Innisfree]

I'm no expert...so I'm just posing this question to people who are more 'in the know'..

 

If you regularly only take the batteries 50% to 70% rather than 90+%... don't you knacker them ?

 

Yes but you will knacker them anyhow in the long run it just happens faster if you don't get them back to full charge occasionaly.

[smileypete]

If I have a 175amp alternator, and run the engine in tickover, how many amps will it feed back into the battery bank after 1 hour

This is easy, wait for one hour then look at the ammeter.

 

cheers, Pete.

~smpt~

[nb Innisfree]

Our weekly absorption was done with our Victron and Kipor digital genny. Adaptive charging on the Victron is disabled and absorption is set at max of 8 hrs at 28.8v (AGM batts) I once put in half of the genny's tank capacity of 3.7lts and that lasted just over 8hrs on tickover which I consider to be a reasonable expense once a week to minimise sulfation.

[Biggles]

I have the BMV600s battery monitor. I think it's about £80 but can't remember. I'm not sure if its better or worse or what than a smart gauge but it tells me all I need to know and it logs historical data. One of these might be cheaper that replacing batteries as you are willing to do. I think a battery monitor of what ever flavour is almost an essential bit of kit as you have no other way of knowing your battery condition.

[nicknorman]

I understand that many use a Smartguage, but my situation is different. I am starting from the basis that every day we have flattened the batteries (from 80% to 50%), and every day I have to recharge them to 80% again. I would probably charge them to 100% occassionally, but not often. Due to this, I treat batteries as consumables....and more than happy to buy an extra new battery every 3 months.

 

I'm just trying to improve my basic understanding of how much current an alternator might put back each day.

 

 

 

That's a very helpful post, thanks

Just to mention you can't extrapolate that directly. If you take out 40AH and it takes 30 mins to replace, it will take longer than 90mins to put back 120AH. This is because a small, discharge comes from the plate surface and is easy to replace, but larger discharges include chemical reactions from below the surface and hence are less immediately able to be recharged. The battery will simply not accept a large current once the surface charge is replaced and whilst the deeper chemicals react.

[RLWP]

You might think so, however if it was the top 40A from 100% SoC, to get back to 100% will probably take more than an hour. If it was 40A from 80% SoC, to get back to 80% would take less than an hour, more like 30 mins.

 

 

 

That's a very helpful post, thanks

 

Just to mention you can't extrapolate that directly. If you take out 40AH and it takes 30 mins to replace, it will take longer than 90mins to put back 120AH. This is because a small, discharge comes from the plate surface and is easy to replace, but larger discharges include chemical reactions from below the surface and hence are less immediately able to be recharged. The battery will simply not accept a large current once the surface charge is replaced and whilst the deeper chemicals react.

 

Too late Nick, Dean has seen the answer he wants (less than an hour) and is off

 

This horse has bolted

 

Richard

Edited February 4, 2013 by RLWP