Hi All

As part of my electrical overhaul, I now have a high-quality lithium house battery that can charge very quickly (small cylindrical cell, 125Ah that can charge at something like 125A).

I've matched that with a dual-input charger (i.e. solar and alternator) that has a maximum output of 30A. I'd like a more powerful one, but the prices really shoot up, and the cabling starts to get thicker than my anchor rode. So 30A charging it is.

My question - and I'm looking for practical experience as much as electrical theory - is whether a 30A draw will harm my alternator at idle speed. I have an almost new Yanmar 2ym15 engine with whatever the standard alternator for that model is, and I believe it can output 50-80A.

But at idle, will it output 30A? And will the damn thing heat up and cook itself? I have read quite a few posts over the years on this topic, but I wasn't paying attention - so any advice would be appreciated.

I've heard that idling this engine with no load on the prop will glaze up the cylinders (or something like that), so I recently charged my old AGM battery by running the engine for 20 mins with the prop engaged in reverse, but at the lowest revs possible - gently chugging away and pulling against my mooring.

So given all of that, if I use the same technique to charge my lithium house bank, will I be threatening the health of my alternator? I'm hoping not, because at low revs the engine is super quiet - not like the infernal petrol generators some people decide to run when they've just anchored next to you of an evening!

Tips and wisdom much appreciated ...

You say "I believe it can output 50-80A" but "believing" is not good enough when dealing with electrics. You need facts and to refer to the spec sheet for the alternator model on your engine. That may be in your engine manual. The output of 80A is no doubt at full revs of the engine. The output of the alternator depends on how fast it is spinning and this will be shown on a graph on the spec sheet. It may be that at idle speed your alternator can only supply 10A, or 5A or 30A. Whatever it can supply will not affect it's health. It can only do what it can do, so to speak.
Consider allowing the engine alternator to charge the battery whenever the engine is running and rely more on a solar panel to keep it topped up (and maybe supplemented by a separate stand alone charger when on shore power).

50 amp alternator is ample. The alternator will output 50 amps for a very short time after start, probably 5 minutes or so depending on the state of the battery. You have solar so the battery will be close to fully charged anyway so the alternator will be outputting very little to the battery on a sunny day. The best bet is to balance the size of the solar panels, batteries, regulators and just let the alternator replace the amps from the starting battery after a start.
Engines do glaze up when idling with no load but there is no need to ever idle the engine to charge a battery. This is extremely inefficient as the alternator will be outputting something like 4 amps. The best bet is to use a portable [Honda suitcase] generator or shore power to charge those batteries. With modern cheap solar panels and regulators, this will probably never be required.
Take care with that lithium battery. They are a bit delicate. I was let down by the lithium battery in my race car a fortnight ago. The positive post came loose and separated from the circuit board under the top. I was able to operate the battery by pushing the post back down and completing the day's event. I will be replacing it with a quality AGM. More expensive than lithium and 4 times the weight but bulletproof.

As Ramona pointed out the alternator is designed to recharge the start battery, for a constant say 30A you would have to be able to manipulate the field to manage constant output, it's an old way of getting around the issue, a balance of engine revs and adjustment of a reostat that you use to manipulate the field and a hand on the alternator to indicate to much heat, apparently too hot to touch is too hot and you will need to back off the reostat or the revs, it's old school. There's Aussie made petrol battery chargers that use the same principle, predominantly used for remote expeditions. Lazz put us onto the red arc dc to dc charger with mppt reg, they have the ability to choose which charging source to use, which is what you've got ? How much solar do you have ?

Just a reminder if you decide to force the alternator to output 45 to 50 amps or so it will take about 5 hp to do so. That might be too much for a 20hp motor!

Hi Bluewaterbound, to answer the question you actually asked, if the alternator is rated at 50A to 80A then a 30A load won't bother it at any time!

The load is from the battery.

I spoke to an electrician about putting lithium batteries in. He said its better to put a control system in. Cant remember what they are called sorry. Lithium batteries can demand a high load that can make your alternator overheat. I think they should be charged at a lower voltage than normal batteries as well.
I think Victron do one.
You have solar to keep things topped up but if your start battery and lithium battery are low its going to demand a lot from the alternator.
You really need to find out for sure the alternators size. Make sure the wires are big enough all the terminals are in good condition etc.

A quick Google search and I found this. Havnt read it but think it explains things.

www.victronenergy.com/blog/2019/10/07/careful-alternator-charging-lithium/

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tarquin1 said..
A quick Google search and I found this. Havnt read it but think it explains things.

www.victronenergy.com/blog/2019/10/07/careful-alternator-charging-lithium/

This is a good video if your flogging their system. In reality you should never be recharging your battery with an a automotive alternator. That's what solar and shore power or a generator plant is for. The alternator is there to replace the charge in a starter battery after a start.
Lithium batteries have a charge circuitry built in under the top cover and they like to be recharged at a higher voltage than lead acid batteries. My race car has a 35 amp Suzuki alternator {similar to what ride on lawn mowers use} and is wired in the same as on a normal car. The car starts and the fields wire supplies 12v in the alternator [dash lights goes out] and the battery gets recharged. In my case the lithium battery is recharged in under a minute. On the occasions I recharged with a battery charger I use a 4 amp charger from a garage sale [$5].
My yacht has lead acid batteries mounted alongside the keel where the weight helps. The Kubota has a 35amp alternator. The batteries however are mostly charged by solar panels via a Renogy 30amp solar controller. I have two of them and run two solar panels to two batteries. www.ebay.com.au/itm/292808405581?epid=17034370369&hash=item442cbdb24d:g:CiQAAOSw9E5b5APd
Mine are set up to charge lead acid batteries but there is just a couple of changes to the digital system to have them charge lithium or any other battery type. There are temperature probes that just sit on top of the batteries that feed back info to the controllers.
If you want to go down the route of running your main engine to recharge your battery bank which would be OK for a motor sailor at night if you don't have a generator plant then I suggest reading this forum.

www.cruisersforum.com/forums/f166/

www.cruisersforum.com/forums/f14/

Bluewaterbound, do you still have a normal lead acid start battery?
What I was told is its not good to charge a lead acid battery and lithium from an alternator at the same time as they like to be charged at different voltage. Apparently lithium is higher not lower as I remembered.

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tarquin1 said..
Bluewaterbound, do you still have a normal lead acid start battery?
What I was told is its not good to charge a lead acid battery and lithium from an alternator at the same time as they like to be charged at different voltage. Apparently lithium is higher not lower as I remembered.

There are some very useful responses here ... I've had a look at a 'typical' alternator curve (see lower-most curve on chart below) and it looks like a fairly low rev range would spin the alternator at the required speed.

I've bought a MiniBoost Pro dual input charger, which is designed specifically for people who are combining different battery types - used in RVs it's usually with a lead-acid cranking battery, and a lithium house battery. So I'm pretty confident the charge controller will deliver the right charge to the lithium.

If this web site is accurate (shop.pkys.com/Installing-a-High-Power-marine-alternator-on-your-boat_b_111.html ), then the engine revs of 1000rpm would produce about 2500rpm at the alternator (using the lowest curve below as the comparison) - meaning it would have about 50 useable amps. Now that I've read my charger handbook, it will only draw 22A (30A is for the solar side of the charger), which means in theory my alternator would only need to spin at about 700 rpm.

I still don't have the specific power-curve for my alternator, so it's a bit of guesswork - but it will be pretty easy to test the amps produced at different revs with an inline ammeter.

I should have said at the start that I'm unlikely to use shore power. I have 80w of solar now, but that will be bumped up to about 200w eventually for longer term cruising. The engine charging is just a back-up for when things get cloudy. My hope with the current install is that is 1) very simple 2) very reliable and 3) able to get me through cloudy days with, say, half an hour of motor sailing to provide basic power.

Yes after looking at the link Ramona posted it looks like battery to battery charger is a good option. Thanks for that.
Let us know how it works.
We run 2 alternators one for start and one for house. It works OK but not a great setup for longer periods. So looking at changing to Lithium.

I won't sweat too much about alternators.
They smart enough to supply current and voltage proportionally to load and revs.
Because charging voltage goes over 14v, its OK for Lit.bat too. Sure RVs have dc- dc convertor, to limit voltage, ones Li.bat changed they don't need over 14v, but voltage should drop to 13.6.
The same applies for solar controller, should be selected to Li.bat .
I recently installed Li.bat to 2 RVs, but for my van I purchased 2Giant bat.
Not quite convinced that constant charging from solar does any good to Li.bat consider specifications Li.bat amount of chages.

For those who like circuit diagrams, the sketch below is my first attempt at understanding where all my wires need to go. I've marked the heavy battery cables in orange, and the other wires marked in green are a combination of 50A wires with a couple of lighter 10A wires (e.g. for the shunt sensor connections). I'll try to explain the 'before' and 'after' of this system upgrade just to get my own thoughts in order...

BEFORE
Alternator with built-in charge regulator sends current to the positive terminal of the cranking battery, with a second battery cable running back to the engine/alternator to complete the circuit. Simple!

AFTER
Alternator sends current to cranking battery. When voltage across the cranking battery terminals is higher than a certain level (my charger gives me a choice of 12v or 12.6v, so I'll probably use the latter), the dual-input charger knows to start taking current to put into the lithium house battery (with the engine running this should be at just over 14V). The charger I've picked (I chose BMPro's Miniboost Pro based on a combination of price and features - there are better units out there but they cost a lot more) 'blends' alternator current with solar current, and if the solar is really pumping it favours the solar power.

Meanwhile, all 'in' or 'out' net-flows of power are measured by the battery monitor (this works by measuring tiny voltage changes across the length of the 175A shunt). If I've got this right, an hour of 'in' flow of 10A offset by a 2A draw from my chart-plotter and stereo, say, would be recorded by the battery monitor as an net change of 8Ah in the battery level. The monitor I'm using was $50 from Altronics in Melbourne and can measure flows up to 200A. (I've seen monitors ranging from $50 up to about $400, but as far as I can tell they all work the same way and do the same thing).

One more thing - the thermal breaker in the diagram is just something I found wired into the previous solar charge circuit. There will never be enough current to trip it, so I'm just re-using the component as a handy way to isolate the panels. It has a push-button to break the circuit and a swing down lever that when pushed back into place reconnects everything. With my new set-up I can wire in a maximum of 360W of panels, though for reasons of space I'll probably only have about 200W.

Many 'common earth' wiring set-ups would connect the negative wires at various places, but as all of my wires will be in the same small cupboard I'm literally running all common earth wires to a big, chunky brass bus bar with two 8mm posts that I can bolt all the ring-terminals to. That keeps things simple for the future (i.e. when I've forgotten how all this works) and means only one place to investigate bad earthing connections.

It's taken me months to get my head around all of this. Fingers crossed it works. I'd welcome any thoughts on whether or not it will!

A quick one, that broken lithium battery of yours Ramona sounds bad. What sort was it? I just put lithiums on my boat

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Charriot said..
I won't sweat too much about alternators.
They smart enough to supply current and voltage proportionally to load and revs.
Because charging voltage goes over 14v, its OK for Lit.bat too. Sure RVs have dc- dc convertor, to limit voltage, ones Li.bat changed they don't need over 14v, but voltage should drop to 13.6.
The same applies for solar controller, should be selected to Li.bat .
I recently installed Li.bat to 2 RVs, but for my van I purchased 2Giant bat.
Not quite convinced that constant charging from solar does any good to Li.bat consider specifications Li.bat amount of chages.

I agree Charriot. It would be a very poor alternator that got hurt no matter what you did to it except sudden dead short. They are (should be) voltage and current limited. I installed a buck/boost solar lithium regulator for my new lithiums in my boat. My very old 1985 alternator puts out 14.5V when engine is running fast and its marked as "40A". The solar regulator is designed to acccept 0 - 50V so alternator is connected to input of that. It thinks its driven by a solar panel putting out 14.5V day and night! Its a 30A current limited regulator so cant overload the alternator which is current limited anyway.

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BluewaterBound said..
For those who like circuit diagrams, the sketch below is my first attempt at understanding where all my wires need to go. I've marked the heavy battery cables in orange, and the other wires marked in green are a combination of 50A wires with a couple of lighter 10A wires (e.g. for the shunt sensor connections). I'll try to explain the 'before' and 'after' of this system upgrade just to get my own thoughts in order...

BEFORE
Alternator with built-in charge regulator sends current to the positive terminal of the cranking battery, with a second battery cable running back to the engine/alternator to complete the circuit. Simple!

AFTER
Alternator sends current to cranking battery. When voltage across the cranking battery terminals is higher than a certain level (my charger gives me a choice of 12v or 12.6v, so I'll probably use the latter), the dual-input charger knows to start taking current to put into the lithium house battery (with the engine running this should be at just over 14V). The charger I've picked (I chose BMPro's Miniboost Pro based on a combination of price and features - there are better units out there but they cost a lot more) 'blends' alternator current with solar current, and if the solar is really pumping it favours the solar power.

Meanwhile, all 'in' or 'out' net-flows of power are measured by the battery monitor (this works by measuring tiny voltage changes across the length of the 175A shunt). If I've got this right, an hour of 'in' flow of 10A offset by a 2A draw from my chart-plotter and stereo, say, would be recorded by the battery monitor as an net change of 8Ah in the battery level. The monitor I'm using was $50 from Altronics in Melbourne and can measure flows up to 200A. (I've seen monitors ranging from $50 up to about $400, but as far as I can tell they all work the same way and do the same thing).

One more thing - the thermal breaker in the diagram is just something I found wired into the previous solar charge circuit. There will never be enough current to trip it, so I'm just re-using the component as a handy way to isolate the panels. It has a push-button to break the circuit and a swing down lever that when pushed back into place reconnects everything. With my new set-up I can wire in a maximum of 360W of panels, though for reasons of space I'll probably only have about 200W.

Many 'common earth' wiring set-ups would connect the negative wires at various places, but as all of my wires will be in the same small cupboard I'm literally running all common earth wires to a big, chunky brass bus bar with two 8mm posts that I can bolt all the ring-terminals to. That keeps things simple for the future (i.e. when I've forgotten how all this works) and means only one place to investigate bad earthing connections.

It's taken me months to get my head around all of this. Fingers crossed it works. I'd welcome any thoughts on whether or not it will!

Circuit looks OK to me, except I wonder why the thermal breaker in series with solar panel?

Trek
Your set up, and measurements are spot on, you have it covered.

Thermal breaker in line of solar, I have seen it a few times. Got answer, its cheaper than switch.

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Trek said..

Charriot said..
I won't sweat too much about alternators.
They smart enough to supply current and voltage proportionally to load and revs.
Because charging voltage goes over 14v, its OK for Lit.bat too. Sure RVs have dc- dc convertor, to limit voltage, ones Li.bat changed they don't need over 14v, but voltage should drop to 13.6.
The same applies for solar controller, should be selected to Li.bat .
I recently installed Li.bat to 2 RVs, but for my van I purchased 2Giant bat.
Not quite convinced that constant charging from solar does any good to Li.bat consider specifications Li.bat amount of chages.

I agree Charriot. It would be a very poor alternator that got hurt no matter what you did to it except sudden dead short. They are (should be) voltage and current limited. I installed a buck/boost solar lithium regulator for my new lithiums in my boat. My very old 1985 alternator puts out 14.5V when engine is running fast and its marked as "40A". The solar regulator is designed to acccept 0 - 50V so alternator is connected to input of that. It thinks its driven by a solar panel putting out 14.5V day and night! Its a 30A current limited regulator so cant overload the alternator which is current limited anyway.

That's an interesting concept, hooking an alternator to a pv reg. In the usual arrangement the problem with standard alternators is they stop producing 14.5 v when the battery reaches 12v & with deep cycle batteries 12v is reached but more amps could be accommodated but the higher voltage is required to oblige the battery to accept, hence the old school work around.

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Trek said..
A quick one, that broken lithium battery of yours Ramona sounds bad. What sort was it? I just put lithiums on my boat

I have not taken it out yet. Can't recall the brand. It still works fine just has a loose positive terminal. I will use it as a jump start battery.