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Battery Charging Routine Basics

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Short version:

How long will I have to run the engine to fully replenish a nearly depleted 600ah bank whilst also running the fridge, water pump and lights?

 

Long version

I'm not sure how long it should take to re-charge a almost completely depleted battery bank. I've read back through a bunch of alternator/charing/battery threads but they're all a bit over my head tbh.

 

My setup is barus shire 45 with domestic alternator (not sure of size right now - guess it's the 'default' one), starter altenator and travel power. I've got a 600ah battery bank (I think) that's about 2 years old but has only been used attached to shore power until I bought it a month ago. It's got a powermaster 1500 inverter charger that manages charging from the shore power (which I don't have)

 

It all seemed fine when I was cruising down from reading. Doing 8 hours a day left my batteries fully charged at 12.8 or so. Last week I left the boat and they were reading 12.2v according to the powermaster. I left the fridge on and a week later they were 11.8 but amazingly still powering the fridge.

 

So on saturday I moved it for three hours or so and left the engine idling for another two. Yesterday I moved it for another hour and left it idling for at least 6 (albeit with the travel power on and powering my stereo). I made it all the way back up to the heady heights of 12.2 again last night only to find it drained to 11.8 again before bed. All I'd been running was the 12v fridge, 12v lights and the laptop for an hour. Is this normal? Have I damaged my batteries by letting them get so low? Do I need to stick water in the top of them or something? Should my inverter measure the voltage as > 13 when the alternator is on (it's not) and the actual voltage when off?

 

How long should I expect to charge my bank given 12v fridge, 12v lights, pump and 1 hour laptop per day. I was guessing a couple hours a day max but I may be wrong here. How do I measure my usage?

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Sorry mate, I bet a few people will try to answer this by re stating what you have already identified as over your head; but there are just too many unstated variables to give a definative reply.

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Without knowing anything about your setup - if you've been away for a week and left the fridge on, no wonder your batteries are flat. I reckon you need to run the engine for at least six hours. (Just looked and you did, oh).

 

I never leave the fridge on if I'm away from the boat, even though I'm usually on mains hook up and I've got solar panels, if I'm away for more than a day or two, I empty and defrost it. That way, say if the mains supply on the marina trips or anything else happens, I know I'm not flattening my batteries.

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Sorry mate, I bet a few people will try to answer this by re stating what you have already identified as over your head; but there are just too many unstated variables to give a definative reply.

 

I'd love to learn this stuff if people ar e willing to indulge me, just one step at a time, starting with first principles and going on from there.

 

What details have I missed out?

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Sorry mate, I bet a few people will try to answer this by re stating what you have already identified as over your head; but there are just too many unstated variables to give a definative reply.

I think Sir Nibble is right it is very difficult to answer your question with any certainty other than it will be several hours and not at idle RPM. It may be too late for your batteries though since if they have been standing for a some days with such a low charge state you may find that they have sulfated up and will no longer accept much in the way of charge anyway no matter how long you run the engine.

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I think Sir Nibble is right it is very difficult to answer your question with any certainty other than it will be several hours and not at idle RPM. It may be too late for your batteries though since if they have been standing for a some days with such a low charge state you may find that they have sulfated up and will no longer accept much in the way of charge anyway no matter how long you run the engine.

 

So running the boat above idle charges the batteries faster? Ok, I didn't know that. It ties in with what I've observed but I thought I might have imagined it.

 

The boat was sat unused at a brokerage for 7 months, so I imagine the batteries might be knackered from that. Although they did seem to charge right up when I first had the boat.

Edited by deletedaccount

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So running the boat above idle charges the batteries faster? Ok, I didn't know that. It ties in with what I've observed but I thought I might have imagined it.

 

The boat was sat unused at a brokerage for 7 months, so I imagine the batteries might be knackered from that. Although they did seem to charge right up when I first had the boat.

Actually it's more to the point of how fast your alternator is spinning. Most modern engine set-ups will not spin the alternator fast enough at idle to reach peak/optimum amp output.

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Since you seem to have read the theory and found it a bit complex, I'll just miss that out and list a few things that I think are the most important.

 

First, to answer your direct question, it will take a long time to charge a set of batteries fully. As others have pointed out, there are too many unknowns to give a definitive answer, but it will probably be much longer than you would expect.

 

The lifespan of a lead acid battery is limited, and the more deeply you discharge them the shorter their life. The optimum lifespan is achieved by discharging down to no more than 50%. This gives the best tradeoff between the life of the batteries and how long you can use them between charges.

 

Another important point to bear in mind is that they should always be charged back to 100% after they have been discharged. As a battery charges, it becomes increasingly difficult to charge it further. This is part of the reason it takes so long to reach 100%.

 

The amount of power that you can get from a battery bank depends on how quickly you discharge it. If you draw a high current, it will discharge disproportionately more quickly than if you use it lightly. This means that it is not particularly clever to use batteries to run such things as microwave ovens, immersion heaters etc.

 

As regards measuring battery usage, if you haven't done so already, you might want to look at the Smartgauge site. This is a gadget that monitors the state of charge of your batteries, taking loads and charging into account. The site also contains masses of technical information about batteries and charging (if you want to confuse yourself even more...)

 

HTH

  • Greenie 1

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Since you seem to have read the theory and found it a bit complex, I'll just miss that out and list a few things that I think are the most important.

 

First, to answer your direct question, it will take a long time to charge a set of batteries fully. As others have pointed out, there are too many unknowns to give a definitive answer, but it will probably be much longer than you would expect.

 

The lifespan of a lead acid battery is limited, and the more deeply you discharge them the shorter their life. The optimum lifespan is achieved by discharging down to no more than 50%. This gives the best tradeoff between the life of the batteries and how long you can use them between charges.

 

Another important point to bear in mind is that they should always be charged back to 100% after they have been discharged. As a battery charges, it becomes increasingly difficult to charge it further. This is part of the reason it takes so long to reach 100%.

 

The amount of power that you can get from a battery bank depends on how quickly you discharge it. If you draw a high current, it will discharge disproportionately more quickly than if you use it lightly. This means that it is not particularly clever to use batteries to run such things as microwave ovens, immersion heaters etc.

 

As regards measuring battery usage, if you haven't done so already, you might want to look at the Smartgauge site. This is a gadget that monitors the state of charge of your batteries, taking loads and charging into account. The site also contains masses of technical information about batteries and charging (if you want to confuse yourself even more...)

 

HTH

 

Brilliant, thanks. I'm going off to read that site.

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From the smart gauge site (http://www.smartgauge.co.uk/chargesize.html):

Example 1 A 100 Ah battery with a 1 amp charger.

 

Assuming the battery has been discharged by 50% we have to replace 50 Ahrs. On the face it this will take 50 hours but it's not that simple. Generally (all other things being equal) we need to add 40% to this figure (this is because, as the battery becomes more fully charged it will accept less and less charge current) so this will give us a recharge time of 70 hours. Now in some installations this may be acceptable. In others (certainly on my liveaboard) it would be totally out of the question.

 

So if I have a 100a alternator and a 600ah battery bank that is 50% discharged it will take roughly (3x1.4) 4-5 hours to charge it up fully?

Is this roughly correct or does ther alternator have a lower than 100a effective charge rate?

Does this mean it would take 9-10 hours to charge up from empty?

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Hi DeletedAccount (is it Tom?)

 

You stopped and chatted to me this weekend at AvonCliff, as you were after a pair of bolt croppers.

 

I have a tavel power and MasterVolt charger, which is basically a mains charger and a 3.5kw gennie. I can normally charge my 500A/hr in 1.5/2 hours.

 

Bit of a woops this weekend as I left the inverter running and have flattened the batteries so I dont know how long it will take to charge them up.

 

If you remind me what your boat is called I might be able to stop by and give some amateur advice sometime this week.

 

Tim

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Hi DeletedAccount (is it Tom?)

 

You stopped and chatted to me this weekend at AvonCliff, as you were after a pair of bolt croppers.

 

I have a tavel power and MasterVolt charger, which is basically a mains charger and a 3.5kw gennie. I can normally charge my 500A/hr in 1.5/2 hours.

 

Bit of a woops this weekend as I left the inverter running and have flattened the batteries so I dont know how long it will take to charge them up.

 

If you remind me what your boat is called I might be able to stop by and give some amateur advice sometime this week.

 

Tim

 

Cheers Tim. Turned out I didn't need the boltcutters after all, cos I left the keys in the lock. duhh. (see blog)

 

I'm right in bath at the moment, just before bath narrowboats. I'll be there for a couple of weeks. The boat's got no signage at the moment, but If I'm in it's the green with white coach lines and black parasol on the back.

 

In the mean time I'm going to read up on what alternator I've got etc.

 

Tom.

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From the smart gauge site (http://www.smartgauge.co.uk/chargesize.html):

 

 

So if I have a 100a alternator and a 600ah battery bank that is 50% discharged it will take roughly (3x1.4) 4-5 hours to charge it up fully?

Is this roughly correct or does ther alternator have a lower than 100a effective charge rate?

Does this mean it would take 9-10 hours to charge up from empty?

 

Below 70% to 80% state of charge batts will take all that alternator can generate at that particular speed, after that, charge rate will drop gradually until it's just a few amps at 100%.

 

So in your example up to, say 80%, it will take approx 2hrs at sufficient alternator speed and then at least 6 hrs (at a gradually reducing speed if you wish) to replace the remaining 20%

Edited by nb Innisfree

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Below 70% to 80% state of charge batts will take all that alternator can generate at that particular speed, after that, charge rate will drop gradually until it's just a few amps at 100%.

 

So in your example up to, say 80%, it will take approx 2hrs at sufficient alternator speed and then at least 6 hrs (at a gradually reducing speed if you wish) to replace the remaining 20%

 

cheers. Then it sounds like my alternator isn't pumping out enough juice at tickover. Is there anything I can do about that (other than hooking my travelpower up to my charger)?

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As said, it will take a supprisingly long time.

- Batterys charge sort of exponetially slowly as they get to towards being full.

- When really slow you will get flatout bulk charging, but as the voltage comes up, the charge rate will decease. Espcailly if the cables are a bit small.

 

Leaving the fridge runing for a week will cane the batterys, particalarly if its a larger fridge, or if you have the temp turned down low. And running the batterys really flat wont do them any favours long term either. Although if the fridge was still running thats a lot better than if it wasnt. And 11.8 is a lot better than 10v! Espcailly if this was an underload voltage reading (with fridge running) rather than an relaxed open-terminal voltage reading.

- We read our battery volatage in the morning (fridge off over night) which means about 10hours off load before reading which give a fairly stable reading.

 

Do you have an ampmeter to see what the alternator is putting out? With low batterys you should get the rated output or close to it (have a look on the unit for stickers showing output rating). What is the voltage showing while under charge?

As said, you may also find a small increase from tickover (out of gear) will greatly increase the charging rate while moored. Hooking up the travel power to the inverter/charger may also indeed give a better charge.

 

How much are you using the laptop? A 50amp alternator at full whack is about 600watts, but if the fridge is drawing 50 most of the time, and the laptop 100 or so, and then you do some hoovering using the inverter and watch telly for four hours. the net input might not be huge!

 

ulitmaltly ofcause, if you really want to get the back up. Theres nothin quite like plugging into the mains for a 24hour spell.

 

 

 

 

 

Daniel

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As said, it will take a supprisingly long time.

- Batterys charge sort of exponetially slowly as they get to towards being full.

- When really slow you will get flatout bulk charging, but as the voltage comes up, the charge rate will decease. Espcailly if the cables are a bit small.

 

Leaving the fridge runing for a week will cane the batterys, particalarly if its a larger fridge, or if you have the temp turned down low. And running the batterys really flat wont do them any favours long term either. Although if the fridge was still running thats a lot better than if it wasnt. And 11.8 is a lot better than 10v! Espcailly if this was an underload voltage reading (with fridge running) rather than an relaxed open-terminal voltage reading.

- We read our battery volatage in the morning (fridge off over night) which means about 10hours off load before reading which give a fairly stable reading.

 

Do you have an ampmeter to see what the alternator is putting out? With low batterys you should get the rated output or close to it (have a look on the unit for stickers showing output rating). What is the voltage showing while under charge?

As said, you may also find a small increase from tickover (out of gear) will greatly increase the charging rate while moored. Hooking up the travel power to the inverter/charger may also indeed give a better charge.

 

How much are you using the laptop? A 50amp alternator at full whack is about 600watts, but if the fridge is drawing 50 most of the time, and the laptop 100 or so, and then you do some hoovering using the inverter and watch telly for four hours. the net input might not be huge!

 

ulitmaltly ofcause, if you really want to get the back up. Theres nothin quite like plugging into the mains for a 24hour spell.

 

 

 

 

 

Daniel

 

I'm going to get an ampmeter, but I'm not sure what type I need or even where to stick it in order to get readings :lol: While charging the voltmeter (on my inverter) doesn't change at all, except to slowly, everyso slowly creep up over 12 again. I suppose I just want to be able to keep my batteries around 75% full and not much less than 50%, but from what I read that means keeping them above 12.2v, which I can barely get to without cruising.

 

I'm also going to start turning the fridge off overnight, although it seems to be a pretty efficient beast without my fiddling. I'm also going to do something about the lights. They're set up in banks of 3 or 4 20w halogens, with at least on bank on at all times when it's dark.

Edited by deletedaccount

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If your voltage is creeping slowly up then that implies that the alternator is producing it's maximum. Now that may not be the maximum it says on the label, it may be the maximum it can produce at that speed and may benefit from more revs. It should go to something over 14V, at that point it will stop producing it's maximum and charge will tail off gently the closer you come to full charge. That is the bit that takes time. I was at Avoncliff this last weekend too, so I could have probably surveyed the system and given you some pointers on getting the best from what you have if I'd known. Perhaps we need a little list of who has troubles or queries and where they are so those with expertise can blow a whistle when it coincides with their cruising plans. Without wishing to be controversial, if your alternator is restricted by revs then you really are stuffed around there 'cos you cannot cruise above tickover for a day in either direction for all the moored boats!

 

cheers. Then it sounds like my alternator isn't pumping out enough juice at tickover. Is there anything I can do about that (other than hooking my travelpower up to my charger)?

Depends. I have had considerable success with an alternator modification which I posted ages ago but I'm damned if I could link to it now. DAN!

Something like that may help, I shall try and remember to pm you next time I'm heading that way and see if it's a sensible way forward. Basically it takes a 65 amp alternator that doesn't charge until you are doing towpath flooding speeds and alters it to give about 45A at all speeds.

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Do you have an ampmeter to see what the alternator is putting out?

Another vote from a great fan of ammeters.

 

Despite all the high tech gismos about actual state of battery charge, nothing beats an ammeter to see how fast you are actually managing to charge the batteries, (or to run them down).

 

It also allows you to see if increasing engine revs is helping charge rate, and the point at which doing so ceases to be helpful, and you are just trashing the engine and burning diesel pointlessly.

 

Despite what some will say, there is no right answer to "will my engine charge adequately on idle", because some will, and some will not. Despite having an old, low technology engine, and very standard alternator, ours charges almost as fast on tickover as when opened up.

 

The reason is to do with the speed the alternator is driven at, relative to the engine, which directly relates to the diameter of the pulley on the alternator, versus the driving one on the crankshaft. An engine/alternator combination that has a 3:1 step up ratio between engine and alternator will simply be spinning the alternator 50% faster than one where the ration is only 2:1, (i.e pulley on engine only twice the diameter of that on alterator). It is speed the alternator is driven at that determines how well it will charge, rather than the actual speed of the engine. A 50% increase in the RPM of the alternator can make a massive difference, so if your ratio were (say) somewhere closer to 2:1 than 3:1 it would be worth investigating repulleying.

 

It is of course easy to measure these pulleys and work out the ratio of how much faster your alternator spins than your engine, if you want to.

 

As I say, with an ammeter fitted, you can see how fast the amps are going into your battery. Do not be surprised if this is maybe only a third of the maximum number written on the alternator - sadly that's probably not a fault, just the way things work, particularly once the batteries have recouped some of their charge, and you are trying to get more in.

 

Good luck.

Edited by alan_fincher

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http://www.canalworld.net/forums/index.php...at+modification

 

There you go, see if that looks useful. Depends on what alternator you have of course, this is a particularly common type by Lucas. I wouldn't go messing with it tho' until the problem has been definately identified as speed based and the option of pullying up investigated.

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If your voltage is creeping slowly up then that implies that the alternator is producing it's maximum. Now that may not be the maximum it says on the label, it may be the maximum it can produce at that speed and may benefit from more revs. It should go to something over 14V, at that point it will stop producing it's maximum and charge will tail off gently the closer you come to full charge. That is the bit that takes time. I was at Avoncliff this last weekend too, so I could have probably surveyed the system and given you some pointers on getting the best from what you have if I'd known. Perhaps we need a little list of who has troubles or queries and where they are so those with expertise can blow a whistle when it coincides with their cruising plans. Without wishing to be controversial, if your alternator is restricted by revs then you really are stuffed around there 'cos you cannot cruise above tickover for a day in either direction for all the moored boats!

 

 

Depends. I have had considerable success with an alternator modification which I posted ages ago but I'm damned if I could link to it now. DAN!

Something like that may help, I shall try and remember to pm you next time I'm heading that way and see if it's a sensible way forward. Basically it takes a 65 amp alternator that doesn't charge until you are doing towpath flooding speeds and alters it to give about 45A at all speeds.

 

When I say creeps up, I mean I've seen it go from 11.8 to 11.9. Thinking about it, I have seen it climb above 13 (and drop right back down with the engine off) at some point (possibly when the travel power was on, which is weirder still). I think I'm going to have to experiment and see what is actually happening.

 

If you would pop by next time you're in the area that would be great!

Edited by deletedaccount

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There are two types on ammeter, those withs shunts and those without. The latter being avilable in both analog and digital form.

 

Ammeters have to go in series with the load, so the current flowing down the cable passed through them. This can be a pain in the current is large, or you wish to have the ammeter a distance away from the current carrying cables. Hence a 'shunt' which is a item of know resitance which is placed inline with the cables, from which two thin wires to the meter can be run so it can be place somewhere convenient. These type are usally more accurate also, if more expensive.

 

With a cheap automotive ammeter such as the one below you could wire it in to the alternators charge leads, proberbly near the alternator for easy, in a matter of minutes, and get readings on what the alternators doing. All be it prehaps with the deckboard raised. Which wouldnt be stupid.

Smiths_Cobra_Ammeter.jpg

 

If you then wanted to take it further, you could then fit a more expensive shunt-type meter near the batterys, to display the net current in and out of the batteries. Charge vs Load.

 

We have both and find them both usfull. Although with the main one, the alternators gauge gets used less. You will also have to be carfull with the inverter/charger as the inverter part will draw a lot of current (will need a large shunt) but if you run it in before the shunt as many do, you wont see the charge from the inverter/charger in your meter.

 

 

 

Daniel

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This looks simple to fit, any one fitted one http://cgi.ebay.co.uk/Clip-On-Ammeter-75-0...id=p3286.c0.m14

 

 

I have one of these. (Mine is 30-0-30 A) very useful as a diagnostic tool.

 

Note that is is a "Current indicator" and not to be relied on for any sort of accurate measurement.

 

No fitting involved, you just clip it to the insulation of the wire you need to check the current in.

 

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It is of course easy to measure these pulleys and work out the ratio of how much faster your alternator spins than your engine, if you want to.

Can you enlarge on this please Alan.

 

I'm planning on a single multi-vee (6xvee) belt. Do I measure inside belt or outside belt or does that not really make much difference.

 

Circumference or diameter i.e. Alternator pulley 50mm dia. crank pulley 150mm dia - is that a ratio of 3-1

 

Mike

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Can you enlarge on this please Alan.

 

I'm planning on a single multi-vee (6xvee) belt. Do I measure inside belt or outside belt or does that not really make much difference.

 

Circumference or diameter i.e. Alternator pulley 50mm dia. crank pulley 150mm dia - is that a ratio of 3-1

 

Mike

 

Correct.

 

Circ. or diam. doesn't make any difference.

 

If you can try to make alt pulley as large as you can, gives more 'wrap' so more grip

Edited by nb Innisfree

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